Avoid synchronized this in Java

Question

Whenever a question pops up on SO about Java synchronization  some people are very eager to point out that synchronized this  should be avoided  Instead  they claim  a lock on a private reference is to be preferred   Some of the given reasons are    some evil code may steal your lock  very popular this one  also has an  accidentally  variant  all synchronized methods within the same class use the exact same lock  which reduces throughput you are  unnecessarily  exposing too much information   Other people  including me  argue that synchronized this  is an idiom that is used a lot  also in Java libraries   is safe and well understood  It should not be avoided because you have a bug and you don t have a clue of what is going on in your multithreaded program  In other words  if it is applicable  then use it   I am interested in seeing some real-world examples  no foobar stuff  where avoiding a lock on this is preferable when synchronized this  would also do the job   Therefore  should you always avoid synchronized this  and replace it with a lock on a private reference       Some further info  updated as answers are given     we are talking about instance synchronization both implicit  synchronized methods  and explicit form of synchronized this  are considered if you quote Bloch or other authorities on the subject  don t leave out the parts you don t like  e g  Effective Java  item on Thread Safety  Typically it is the lock on the instance itself  but there are exceptions   if you need granularity in your locking other than synchronized this  provides  then synchronized this  is not applicable so that s not the issue

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The java util concurrent package has vastly reduced the complexity of my thread safe code  I only have anecdotal evidence to go on  but most work I have seen with synchronized x  appears to be re-implementing a Lock  Semaphore  or Latch  but using the lower-level monitors   With this in mind  synchronizing using any of these mechanisms is analogous to synchronizing on an internal object  rather than leaking a lock  This is beneficial in that you have absolute certainty that you control the entry into the monitor by two or more threads

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It depends on the task you want to do  but I wouldn t use it  Also  check if the thread-save-ness you want to accompish couldn t be done by synchronize this  in the first place  There are also some nice locks in the API that might help you

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While I agree about not adhering blindly to dogmatic rules  does the  lock stealing  scenario seem so eccentric to you  A thread could indeed acquire the lock on your object  externally  synchronized theObject          blocking other threads waiting on synchronized instance methods   If you don t believe in malicious code  consider that this code could come from third parties  for instance if you develop some sort of application server    The  accidental  version seems less likely  but as they say   make something idiot-proof and someone will invent a better idiot    So I agree with the it-depends-on-what-the-class-does school of thought     Edit following eljenso s first 3 comments   I ve never experienced the lock stealing problem but here is an imaginary scenario   Let s say your system is a servlet container  and the object we re considering is the ServletContext implementation  Its getAttribute method must be thread-safe  as context attributes are shared data  so you declare it as synchronized  Let s also imagine that you provide a public hosting service based on your container implementation   I m your customer and deploy my  good  servlet on your site  It happens that my code contains a call to getAttribute   A hacker  disguised as another customer  deploys his malicious servlet on your site  It contains the following code in the init method    synchronized  this getServletConfig   getServletContext         while  true         Assuming we share the same servlet context  allowed by the spec as long as the two servlets are on the same virtual host   my call on getAttribute is locked forever  The hacker has achieved a DoS on my servlet   This attack is not possible if getAttribute is synchronized on a private lock  because 3rd-party code cannot acquire this lock   I admit that the example is contrived and an oversimplistic view of how a servlet container works  but IMHO it proves the point   So I would make my design choice based on security consideration  will I have complete control over the code that has access to the instances  What would be the consequence of a thread s holding a lock on an instance indefinitely

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While you are using synchronized this  you are using the class instance as a lock itself  This means that while lock is acquired by thread 1  the thread 2 should wait   Suppose the following code   public void method1            do something         synchronized this            a                                             public void method2            do something         synchronized this            b                                             Method 1 modifying the variable a and method 2 modifying the variable b  the concurrent modification of the same variable by two threads should be avoided and it is  BUT while thread1 modifying a and thread2 modifying b it can be performed without any race condition   Unfortunately  the above code will not allow this since we are using the same reference for a lock  This means that threads even if they are not in a race condition should wait and obviously the code sacrifices concurrency of the program   The solution is to use 2 different locks for two different variables   public class Test        private Object lockA   new Object        private Object lockB   new Object         public void method1                do something             synchronized lockA                a                                                             public void method2                do something             synchronized lockB                b                                                            The above example uses more fine grained locks  2 locks instead one  lockA  and lockB for variables a and b respectively  and as a result allows better concurrency  on the other hand it became more complex than the first example

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While I agree about not adhering blindly to dogmatic rules  does the  lock stealing  scenario seem so eccentric to you  A thread could indeed acquire the lock on your object  externally  synchronized theObject          blocking other threads waiting on synchronized instance methods   If you don t believe in malicious code  consider that this code could come from third parties  for instance if you develop some sort of application server    The  accidental  version seems less likely  but as they say   make something idiot-proof and someone will invent a better idiot    So I agree with the it-depends-on-what-the-class-does school of thought     Edit following eljenso s first 3 comments   I ve never experienced the lock stealing problem but here is an imaginary scenario   Let s say your system is a servlet container  and the object we re considering is the ServletContext implementation  Its getAttribute method must be thread-safe  as context attributes are shared data  so you declare it as synchronized  Let s also imagine that you provide a public hosting service based on your container implementation   I m your customer and deploy my  good  servlet on your site  It happens that my code contains a call to getAttribute   A hacker  disguised as another customer  deploys his malicious servlet on your site  It contains the following code in the init method    synchronized  this getServletConfig   getServletContext         while  true         Assuming we share the same servlet context  allowed by the spec as long as the two servlets are on the same virtual host   my call on getAttribute is locked forever  The hacker has achieved a DoS on my servlet   This attack is not possible if getAttribute is synchronized on a private lock  because 3rd-party code cannot acquire this lock   I admit that the example is contrived and an oversimplistic view of how a servlet container works  but IMHO it proves the point   So I would make my design choice based on security consideration  will I have complete control over the code that has access to the instances  What would be the consequence of a thread s holding a lock on an instance indefinitely

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I think points one  somebody else using your lock  and two  all methods using the same lock needlessly  can happen in any fairly large application  Especially when there s no good communication between developers   It s not cast in stone  it s mostly an issue of good practice and preventing errors

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There seems a different consensus in the C  and Java camps on this   The majority of Java code I have seen uses      apply mutex to this instance synchronized this           do work here     whereas the majority of C  code opts for the arguably safer      instance level lock object private readonly object  syncObj   new object             apply mutex to private instance level field  a System Object usually  lock  syncObj           do work here     The C  idiom is certainly safer   As mentioned previously  no malicious   accidental access to the lock can be made from outside the instance   Java code has this risk too  but it seems that the Java community has gravitated over time to the slightly less safe  but slightly more terse version     That s not meant as a dig against Java  just a reflection of my experience working on both languages

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A good example for use synchronized this       add listener public final synchronized void addListener IListener l   listeners add l       remove listener public final synchronized void removeListener IListener l   listeners remove l       routine that raise events public void run           some code here       Set ls     synchronized this          ls   listeners clone            for  IListener l   ls    l processEvent event           some code here        As you can see here  we use synchronize on this to easy cooperate of lengthly  possibly infinite loop of run method  with some synchronized methods there   Of course it can be very easily rewritten with using synchronized on private field  But sometimes  when we already have some design with synchronized methods  i e  legacy class  we derive from  synchronized this  can be the only solution

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I ll cover each point separately       Some evil code may steal your lock  very popular this one  also has an    accidentally  variant    I m more worried about accidentally   What it amounts to is that this use of this is part of your class  exposed interface  and should be documented   Sometimes the ability of other code to use your lock is desired   This is true of things like Collections synchronizedMap  see the javadoc      All synchronized methods within the same class use the exact same   lock  which reduces throughput   This is overly simplistic thinking  just getting rid of synchronized this  won t solve the problem   Proper synchronization for throughput will take more thought     You are  unnecessarily  exposing too much information   This is a variant of  1   Use of synchronized this  is part of your interface   If you don t want need this exposed  don t do it

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Short answer  You have to understand the difference and make choice depending on the code   Long answer  In general I would rather try to avoid synchronize this  to reduce contention but private locks add complexity you have to be aware of  So use the right synchronization for the right job  If you are not so experienced with multi-threaded programming I would rather stick to instance locking and read up on this topic   That said  just using synchronize this  does not automatically make your class fully thread-safe   This is a not an easy topic but once you get used to it  the answer whether to use synchronize this  or not comes naturally

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If you ve decided that    the thing you need to do is lock on the current object  and you want to lock it with granularity smaller than a whole method    then I don t see the a taboo over synchronizezd this    Some people deliberately use synchronized this   instead of marking the method synchronized  inside the whole contents of a method because they think it s  clearer to the reader  which object is actually being synchronized on  So long as people are making an informed choice  e g  understand that by doing so they re actually inserting extra bytecodes into the method and this could have a knock-on effect on potential optimisations   I don t particularly see a problem with this  You should always document the concurrent behaviour of your program  so I don t see the   synchronized  publishes the behaviour  argument as being so compelling   As to the question of which object s lock you should use  I think there s nothing wrong with synchronizing on the current object if this would be expected by the logic of what you re doing and how your class would typically be used  For example  with a collection  the object that you would logically expect to lock is generally the collection itself

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My two cents in 2019 even though this question could have been settled already   Locking on  this  is not bad if you know what you are doing but behind the scene locking on  this  is  which unfortunately what synchronized keyword in method definition allows    If you actually want users of your class to be able to  steal  your lock  i e  prevent other threads from dealing with it   you actually want all the synchronized methods to wait while another sync method is running and so on   It should be intentional and well thought off  and hence documented to help your users understand it    To further elaborate  in the reverse you must know what you are  gaining   or  losing  out on  if you lock on a non accessible lock  nobody can  steal  your lock  you are in total control and so on        The problem for me is that synchronized keyword in the method definition signature makes it just too easy for programmers not to think about what to lock on which is a mighty important thing to think about if you don t want to run into problems in a multi-threaded program   One can t argue that  typically  you don t want users of your class to be able to do these stuff or that  typically  you want   It depends on what functionality you are coding  You can t make a thumb rule as you can t predict all the use cases   Consider for e g  the printwriter which uses an internal lock but then people struggle to use it from multiple threads if they don t want their output to interleave   Should your lock be accessible outside of the class or not is your decision as a programmer on the basis of what functionality the class has  It is part of the api  You can t move away for instance from synchronized this  to synchronized provateObjet  without risking breaking changes in the code using it   Note 1  I know you can achieve whatever synchronized this   achieves  by using a explicit lock object and exposing it but I think it is unnecessary if your behaviour is well documented and you actually know what locking on  this  means   Note 2  I don t concur with the argument that if some code is accidentally stealing your lock its a bug and you have to solve it  This in a way is same argument as saying I can make all my methods public even if they are not meant to be public  If someone is  accidentally  calling my intended to be private method its a bug  Why enable this accident in the first place    If ability to steal your lock is a problem for your class don t allow it  As simple as that

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It depends on the situation  If There is only one sharing entity or more than one      See full working example here              A small introduction     Threads and shareable entities It is possible for multiple threads to access same entity  for eg multiple connectionThreads sharing a single messageQueue  Since the threads run concurrently there may be a chance of overriding one s data by another which may be a messed up situation  So we need some way to ensure that shareable entity is accessed only by one thread at a time   CONCURRENCY      Synchronized block synchronized   block is a way to ensure concurrent access of shareable entity  First  a small analogy Suppose There are two-person P1  P2  threads  a Washbasin  shareable entity  inside a washroom and there is a door  lock   Now we want one person to use washbasin at a time  An approach is to lock the door by P1 when the door is locked P2 waits until p1 completes his work P1 unlocks the door then only p1 can use washbasin      syntax       synchronized this      SHARED ENTITY           this  provided the intrinsic lock associated with the class  Java developer designed Object class in such a way that each object can work as monitor   Above approach works fine when there are only one shared entity and multiple threads  1  N      N shareable entities-M threads Now think of a situation when there is two washbasin inside a washroom and only one door  If we are using the previous approach  only p1 can use one washbasin at a time while p2 will wait outside  It is wastage of resource as no one is using B2  washbasin   A wiser approach would be to create a smaller room inside washroom and provide them one door per washbasin  In this way  P1 can access B1 and P2 can access B2 and vice-versa     washbasin1    washbasin2   Object lock1 new Object    Object lock2 new Object       synchronized lock1          washbasin1         synchronized lock2          washbasin2           See more on Threads----  here

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I ll cover each point separately       Some evil code may steal your lock  very popular this one  also has an    accidentally  variant    I m more worried about accidentally   What it amounts to is that this use of this is part of your class  exposed interface  and should be documented   Sometimes the ability of other code to use your lock is desired   This is true of things like Collections synchronizedMap  see the javadoc      All synchronized methods within the same class use the exact same   lock  which reduces throughput   This is overly simplistic thinking  just getting rid of synchronized this  won t solve the problem   Proper synchronization for throughput will take more thought     You are  unnecessarily  exposing too much information   This is a variant of  1   Use of synchronized this  is part of your interface   If you don t want need this exposed  don t do it

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While you are using synchronized this  you are using the class instance as a lock itself  This means that while lock is acquired by thread 1  the thread 2 should wait   Suppose the following code   public void method1            do something         synchronized this            a                                             public void method2            do something         synchronized this            b                                             Method 1 modifying the variable a and method 2 modifying the variable b  the concurrent modification of the same variable by two threads should be avoided and it is  BUT while thread1 modifying a and thread2 modifying b it can be performed without any race condition   Unfortunately  the above code will not allow this since we are using the same reference for a lock  This means that threads even if they are not in a race condition should wait and obviously the code sacrifices concurrency of the program   The solution is to use 2 different locks for two different variables   public class Test        private Object lockA   new Object        private Object lockB   new Object         public void method1                do something             synchronized lockA                a                                                             public void method2                do something             synchronized lockB                b                                                            The above example uses more fine grained locks  2 locks instead one  lockA  and lockB for variables a and b respectively  and as a result allows better concurrency  on the other hand it became more complex than the first example

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I think there is a good explanation on why each of these are vital techniques under your belt in a book called Java Concurrency In Practice by Brian Goetz  He makes one point very clear - you must use the same lock  EVERYWHERE  to protect the state of your object  Synchronised method and synchronising on an object often go hand in hand  E g  Vector synchronises all its methods  If you have a handle to a vector object and are going to do  put if absent  then merely Vector synchronising its own individual methods isn t going to protect you from corruption of state  You need to synchronise using synchronised  vectorHandle   This will result in the SAME lock being acquired by every thread which has a handle to the vector and will protect overall state of the vector  This is called client side locking  We do know as a matter of fact vector does synchronised  this    synchronises all its methods and hence synchronising on the object vectorHandle will result in proper synchronisation of vector objects state  Its foolish to believe that you are thread safe just because you are using a thread safe collection  This is precisely the reason ConcurrentHashMap explicitly introduced putIfAbsent method - to make such operations atomic   In summary   Synchronising at method level allows client side locking  If you have a private lock object - it makes client side locking impossible  This is fine if you know that your class doesn t have  put if absent  type of functionality  If you are designing a library - then synchronising on this or synchronising the method is often wiser  Because you are rarely in a position to decide how your class is going to be used  Had Vector used a private lock object - it would have been impossible to get  put if absent  right  The client code will never gain a handle to the private lock thus breaking the fundamental rule of using the EXACT SAME LOCK to protect its state  Synchronising on this or synchronised methods do have a problem as others have pointed out - someone could get a lock and never release it  All other threads would keep waiting for the lock to be released  So know what you are doing and adopt the one that s correct  Someone argued that having a private lock object gives you better granularity - e g  if two operations are unrelated - they could be guarded by different locks resulting in better throughput  But this i think is design smell and not code smell - if two operations are completely unrelated why are they part of the SAME class  Why should a class club unrelated functionalities at all  May be a utility class  Hmmmm - some util providing string manipulation and calendar date formatting through the same instance       doesn t make any sense to me at least

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I only want to mention a possible solution for unique private references in atomic parts of code without dependencies  You can use a static Hashmap with locks and a simple static method named atomic   that creates required references  automatically using stack information  full class name and line number   Then you can use this method in synchronize statements without writing new lock object      Synchronization objects  locks  private static HashMap lt String  Object gt  locks   new HashMap lt String  Object gt        Simple method private static Object atomic         StackTraceElement    stack   Thread currentThread   getStackTrace       get execution point      StackTraceElement exepoint   stack 2          creates unique key from class name and line number using execution point     String key   String format   s  d   exepoint getClassName    exepoint getLineNumber          Object lock   locks get key      use old or create new lock     if  lock    null            lock   new Object            locks put key  lock             return lock     return reference to lock      Synchronized code void dosomething1            start commands     synchronized  atomic                 atomic commands 1                          other command      Synchronized code void dosomething2            start commands     synchronized  atomic                 atomic commands 2                          other command

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I ll cover each point separately       Some evil code may steal your lock  very popular this one  also has an    accidentally  variant    I m more worried about accidentally   What it amounts to is that this use of this is part of your class  exposed interface  and should be documented   Sometimes the ability of other code to use your lock is desired   This is true of things like Collections synchronizedMap  see the javadoc      All synchronized methods within the same class use the exact same   lock  which reduces throughput   This is overly simplistic thinking  just getting rid of synchronized this  won t solve the problem   Proper synchronization for throughput will take more thought     You are  unnecessarily  exposing too much information   This is a variant of  1   Use of synchronized this  is part of your interface   If you don t want need this exposed  don t do it

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There seems a different consensus in the C  and Java camps on this   The majority of Java code I have seen uses      apply mutex to this instance synchronized this           do work here     whereas the majority of C  code opts for the arguably safer      instance level lock object private readonly object  syncObj   new object             apply mutex to private instance level field  a System Object usually  lock  syncObj           do work here     The C  idiom is certainly safer   As mentioned previously  no malicious   accidental access to the lock can be made from outside the instance   Java code has this risk too  but it seems that the Java community has gravitated over time to the slightly less safe  but slightly more terse version     That s not meant as a dig against Java  just a reflection of my experience working on both languages

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I think points one  somebody else using your lock  and two  all methods using the same lock needlessly  can happen in any fairly large application  Especially when there s no good communication between developers   It s not cast in stone  it s mostly an issue of good practice and preventing errors

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The java util concurrent package has vastly reduced the complexity of my thread safe code  I only have anecdotal evidence to go on  but most work I have seen with synchronized x  appears to be re-implementing a Lock  Semaphore  or Latch  but using the lower-level monitors   With this in mind  synchronizing using any of these mechanisms is analogous to synchronizing on an internal object  rather than leaking a lock  This is beneficial in that you have absolute certainty that you control the entry into the monitor by two or more threads

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Avoid using synchronized this  as a locking mechanism  This locks the whole class instance and can cause deadlocks  In such cases  refactor the code to lock only a specific method or variable  that way whole class doesn t get locked  Synchronised can be used inside method level  Instead of using synchronized this   below code shows how you could just lock a method        public void foo     if operation   null        synchronized foo     if  operation    null        enter your code that this method has to handle

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Short answer  You have to understand the difference and make choice depending on the code   Long answer  In general I would rather try to avoid synchronize this  to reduce contention but private locks add complexity you have to be aware of  So use the right synchronization for the right job  If you are not so experienced with multi-threaded programming I would rather stick to instance locking and read up on this topic   That said  just using synchronize this  does not automatically make your class fully thread-safe   This is a not an easy topic but once you get used to it  the answer whether to use synchronize this  or not comes naturally

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Well  firstly it should be pointed out that   public void blah       synchronized  this           do stuff         is semantically equivalent to   public synchronized void blah          do stuff     which is one reason not to use synchronized this    You might argue that you can do stuff around the synchronized this  block   The usual reason is to try and avoid having to do the synchronized check at all  which leads to all sorts of concurrency problems  specifically the double checked-locking problem  which just goes to show how difficult it can be to make a relatively simple check threadsafe   A private lock is a defensive mechanism  which is never a bad idea   Also  as you alluded to  private locks can control granularity   One set of operations on an object might be totally unrelated to another but synchronized this  will mutually exclude access to all of them   synchronized this  just really doesn t give you anything

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A lock is used for either visibility or for protecting some data from concurrent modification which may lead to race   When you need to just make primitive type operations to be atomic there are available options like AtomicInteger and the likes   But suppose you have two integers which are related to each other like x and y co-ordinates  which are related to each other and should be changed in an atomic manner  Then you would protect them using a same lock   A lock should only protect the state that is related to each other  No less and no more  If you use synchronized this  in each method then even if the state of the class is unrelated all the threads will face contention even if updating unrelated state   class Point     private int x     private int y      public Point int x  int y          this x   x         this y   y             mutating methods should be guarded by same lock    public synchronized void changeCoordinates int x  int y          this x   x         this y   y           In the above example I have only one method which mutates both x and y and not two different methods as x and y are related and if I had given two different methods for mutating x and y separately then it would not have been thread safe    This example is just to demonstrate and not necessarily the way it should be implemented  The best way to do it would be to make it IMMUTABLE   Now in opposition to Point example  there is an example of TwoCounters already provided by  Andreas where the state which is being protected by two different locks as the state is unrelated to each other   The process of using different locks to protect unrelated states is called Lock Striping or Lock Splitting

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Short answer  You have to understand the difference and make choice depending on the code   Long answer  In general I would rather try to avoid synchronize this  to reduce contention but private locks add complexity you have to be aware of  So use the right synchronization for the right job  If you are not so experienced with multi-threaded programming I would rather stick to instance locking and read up on this topic   That said  just using synchronize this  does not automatically make your class fully thread-safe   This is a not an easy topic but once you get used to it  the answer whether to use synchronize this  or not comes naturally

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It depends on the situation  If There is only one sharing entity or more than one      See full working example here              A small introduction     Threads and shareable entities It is possible for multiple threads to access same entity  for eg multiple connectionThreads sharing a single messageQueue  Since the threads run concurrently there may be a chance of overriding one s data by another which may be a messed up situation  So we need some way to ensure that shareable entity is accessed only by one thread at a time   CONCURRENCY      Synchronized block synchronized   block is a way to ensure concurrent access of shareable entity  First  a small analogy Suppose There are two-person P1  P2  threads  a Washbasin  shareable entity  inside a washroom and there is a door  lock   Now we want one person to use washbasin at a time  An approach is to lock the door by P1 when the door is locked P2 waits until p1 completes his work P1 unlocks the door then only p1 can use washbasin      syntax       synchronized this      SHARED ENTITY           this  provided the intrinsic lock associated with the class  Java developer designed Object class in such a way that each object can work as monitor   Above approach works fine when there are only one shared entity and multiple threads  1  N      N shareable entities-M threads Now think of a situation when there is two washbasin inside a washroom and only one door  If we are using the previous approach  only p1 can use one washbasin at a time while p2 will wait outside  It is wastage of resource as no one is using B2  washbasin   A wiser approach would be to create a smaller room inside washroom and provide them one door per washbasin  In this way  P1 can access B1 and P2 can access B2 and vice-versa     washbasin1    washbasin2   Object lock1 new Object    Object lock2 new Object       synchronized lock1          washbasin1         synchronized lock2          washbasin2           See more on Threads----  here

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A good example for use synchronized this       add listener public final synchronized void addListener IListener l   listeners add l       remove listener public final synchronized void removeListener IListener l   listeners remove l       routine that raise events public void run           some code here       Set ls     synchronized this          ls   listeners clone            for  IListener l   ls    l processEvent event           some code here        As you can see here  we use synchronize on this to easy cooperate of lengthly  possibly infinite loop of run method  with some synchronized methods there   Of course it can be very easily rewritten with using synchronized on private field  But sometimes  when we already have some design with synchronized methods  i e  legacy class  we derive from  synchronized this  can be the only solution

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Avoid using synchronized this  as a locking mechanism  This locks the whole class instance and can cause deadlocks  In such cases  refactor the code to lock only a specific method or variable  that way whole class doesn t get locked  Synchronised can be used inside method level  Instead of using synchronized this   below code shows how you could just lock a method        public void foo     if operation   null        synchronized foo     if  operation    null        enter your code that this method has to handle

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The reason not to synchronize on this is that sometimes you need more than one lock  the second lock often gets removed after some additional thinking  but you still need it in the intermediate state   If you lock on this  you always have to remember which one of the two locks is this  if you lock on a private Object  the variable name tells you that   From the reader s viewpoint  if you see locking on this  you always have to answer the two questions     what kind of access is protected by this  is one lock really enough  didn t someone introduce a bug    An example   class BadObject       private Something mStuff      synchronized setStuff Something stuff            mStuff   stuff            synchronized getStuff Something stuff            return mStuff            private MyListener myListener   new MyListener             public void onMyEvent                    setStuff                           synchronized void longOperation MyListener l                        l onMyEvent                             If two threads begin longOperation   on two different instances of BadObject  they acquire  their locks  when it s time to invoke l onMyEvent       we have a deadlock because neither of the threads may acquire the other object s lock   In this example we may eliminate the deadlock by using two locks  one for short operations and one for long ones

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I ll cover each point separately       Some evil code may steal your lock  very popular this one  also has an    accidentally  variant    I m more worried about accidentally   What it amounts to is that this use of this is part of your class  exposed interface  and should be documented   Sometimes the ability of other code to use your lock is desired   This is true of things like Collections synchronizedMap  see the javadoc      All synchronized methods within the same class use the exact same   lock  which reduces throughput   This is overly simplistic thinking  just getting rid of synchronized this  won t solve the problem   Proper synchronization for throughput will take more thought     You are  unnecessarily  exposing too much information   This is a variant of  1   Use of synchronized this  is part of your interface   If you don t want need this exposed  don t do it

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I think there is a good explanation on why each of these are vital techniques under your belt in a book called Java Concurrency In Practice by Brian Goetz  He makes one point very clear - you must use the same lock  EVERYWHERE  to protect the state of your object  Synchronised method and synchronising on an object often go hand in hand  E g  Vector synchronises all its methods  If you have a handle to a vector object and are going to do  put if absent  then merely Vector synchronising its own individual methods isn t going to protect you from corruption of state  You need to synchronise using synchronised  vectorHandle   This will result in the SAME lock being acquired by every thread which has a handle to the vector and will protect overall state of the vector  This is called client side locking  We do know as a matter of fact vector does synchronised  this    synchronises all its methods and hence synchronising on the object vectorHandle will result in proper synchronisation of vector objects state  Its foolish to believe that you are thread safe just because you are using a thread safe collection  This is precisely the reason ConcurrentHashMap explicitly introduced putIfAbsent method - to make such operations atomic   In summary   Synchronising at method level allows client side locking  If you have a private lock object - it makes client side locking impossible  This is fine if you know that your class doesn t have  put if absent  type of functionality  If you are designing a library - then synchronising on this or synchronising the method is often wiser  Because you are rarely in a position to decide how your class is going to be used  Had Vector used a private lock object - it would have been impossible to get  put if absent  right  The client code will never gain a handle to the private lock thus breaking the fundamental rule of using the EXACT SAME LOCK to protect its state  Synchronising on this or synchronised methods do have a problem as others have pointed out - someone could get a lock and never release it  All other threads would keep waiting for the lock to be released  So know what you are doing and adopt the one that s correct  Someone argued that having a private lock object gives you better granularity - e g  if two operations are unrelated - they could be guarded by different locks resulting in better throughput  But this i think is design smell and not code smell - if two operations are completely unrelated why are they part of the SAME class  Why should a class club unrelated functionalities at all  May be a utility class  Hmmmm - some util providing string manipulation and calendar date formatting through the same instance       doesn t make any sense to me at least

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The java util concurrent package has vastly reduced the complexity of my thread safe code  I only have anecdotal evidence to go on  but most work I have seen with synchronized x  appears to be re-implementing a Lock  Semaphore  or Latch  but using the lower-level monitors   With this in mind  synchronizing using any of these mechanisms is analogous to synchronizing on an internal object  rather than leaking a lock  This is beneficial in that you have absolute certainty that you control the entry into the monitor by two or more threads

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This is really just supplementary to the other answers  but if your main objection to using private objects for locking is that it clutters your class with fields that are not related to the business logic then Project Lombok has  Synchronized to generate the boilerplate at compile-time    Synchronized public int foo         return 0      compiles to  private final Object  lock   new Object 0    public int foo         synchronized  lock            return 0

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This is really just supplementary to the other answers  but if your main objection to using private objects for locking is that it clutters your class with fields that are not related to the business logic then Project Lombok has  Synchronized to generate the boilerplate at compile-time    Synchronized public int foo         return 0      compiles to  private final Object  lock   new Object 0    public int foo         synchronized  lock            return 0

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There seems a different consensus in the C  and Java camps on this   The majority of Java code I have seen uses      apply mutex to this instance synchronized this           do work here     whereas the majority of C  code opts for the arguably safer      instance level lock object private readonly object  syncObj   new object             apply mutex to private instance level field  a System Object usually  lock  syncObj           do work here     The C  idiom is certainly safer   As mentioned previously  no malicious   accidental access to the lock can be made from outside the instance   Java code has this risk too  but it seems that the Java community has gravitated over time to the slightly less safe  but slightly more terse version     That s not meant as a dig against Java  just a reflection of my experience working on both languages

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While I agree about not adhering blindly to dogmatic rules  does the  lock stealing  scenario seem so eccentric to you  A thread could indeed acquire the lock on your object  externally  synchronized theObject          blocking other threads waiting on synchronized instance methods   If you don t believe in malicious code  consider that this code could come from third parties  for instance if you develop some sort of application server    The  accidental  version seems less likely  but as they say   make something idiot-proof and someone will invent a better idiot    So I agree with the it-depends-on-what-the-class-does school of thought     Edit following eljenso s first 3 comments   I ve never experienced the lock stealing problem but here is an imaginary scenario   Let s say your system is a servlet container  and the object we re considering is the ServletContext implementation  Its getAttribute method must be thread-safe  as context attributes are shared data  so you declare it as synchronized  Let s also imagine that you provide a public hosting service based on your container implementation   I m your customer and deploy my  good  servlet on your site  It happens that my code contains a call to getAttribute   A hacker  disguised as another customer  deploys his malicious servlet on your site  It contains the following code in the init method    synchronized  this getServletConfig   getServletContext         while  true         Assuming we share the same servlet context  allowed by the spec as long as the two servlets are on the same virtual host   my call on getAttribute is locked forever  The hacker has achieved a DoS on my servlet   This attack is not possible if getAttribute is synchronized on a private lock  because 3rd-party code cannot acquire this lock   I admit that the example is contrived and an oversimplistic view of how a servlet container works  but IMHO it proves the point   So I would make my design choice based on security consideration  will I have complete control over the code that has access to the instances  What would be the consequence of a thread s holding a lock on an instance indefinitely

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No  you shouldn t always   However  I tend to avoid it when there are multiple concerns on a particular object that only need to be threadsafe in respect to themselves   For example  you might have a mutable data object that has  label  and  parent  fields  these need to be threadsafe  but changing one need not block the other from being written read    In practice I would avoid this by declaring the fields volatile and or using java util concurrent s AtomicFoo wrappers    Synchronization in general is a bit clumsy  as it slaps a big lock down rather than thinking exactly how threads might be allowed to work around each other   Using synchronized this  is even clumsier and anti-social  as it s saying  no-one may change anything on this class while I hold the lock    How often do you actually need to do that   I would much rather have more granular locks  even if you do want to stop everything from changing  perhaps you re serialising the object   you can just acquire all of the locks to achieve the same thing  plus it s more explicit that way   When you use synchronized this   it s not clear exactly why you re synchronizing  or what the side effects might be   If you use synchronized labelMonitor   or even better labelLock getWriteLock   lock    it s clear what you are doing and what the effects of your critical section are limited to

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I think points one  somebody else using your lock  and two  all methods using the same lock needlessly  can happen in any fairly large application  Especially when there s no good communication between developers   It s not cast in stone  it s mostly an issue of good practice and preventing errors

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It depends on the task you want to do  but I wouldn t use it  Also  check if the thread-save-ness you want to accompish couldn t be done by synchronize this  in the first place  There are also some nice locks in the API that might help you

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My two cents in 2019 even though this question could have been settled already   Locking on  this  is not bad if you know what you are doing but behind the scene locking on  this  is  which unfortunately what synchronized keyword in method definition allows    If you actually want users of your class to be able to  steal  your lock  i e  prevent other threads from dealing with it   you actually want all the synchronized methods to wait while another sync method is running and so on   It should be intentional and well thought off  and hence documented to help your users understand it    To further elaborate  in the reverse you must know what you are  gaining   or  losing  out on  if you lock on a non accessible lock  nobody can  steal  your lock  you are in total control and so on        The problem for me is that synchronized keyword in the method definition signature makes it just too easy for programmers not to think about what to lock on which is a mighty important thing to think about if you don t want to run into problems in a multi-threaded program   One can t argue that  typically  you don t want users of your class to be able to do these stuff or that  typically  you want   It depends on what functionality you are coding  You can t make a thumb rule as you can t predict all the use cases   Consider for e g  the printwriter which uses an internal lock but then people struggle to use it from multiple threads if they don t want their output to interleave   Should your lock be accessible outside of the class or not is your decision as a programmer on the basis of what functionality the class has  It is part of the api  You can t move away for instance from synchronized this  to synchronized provateObjet  without risking breaking changes in the code using it   Note 1  I know you can achieve whatever synchronized this   achieves  by using a explicit lock object and exposing it but I think it is unnecessary if your behaviour is well documented and you actually know what locking on  this  means   Note 2  I don t concur with the argument that if some code is accidentally stealing your lock its a bug and you have to solve it  This in a way is same argument as saying I can make all my methods public even if they are not meant to be public  If someone is  accidentally  calling my intended to be private method its a bug  Why enable this accident in the first place    If ability to steal your lock is a problem for your class don t allow it  As simple as that

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Make your data immutable if it is possible   final variables  If you can t avoid mutation of shared data across multiple threads  use high level programming constructs  e g  granular  Lock API        A Lock provides exclusive access to a shared resource  only one thread at a time can acquire the lock and all access to the shared resource requires that the lock be acquired first    Sample code to use ReentrantLock which implements Lock interface   class X      private final ReentrantLock lock   new ReentrantLock                  public void m          lock lock        block until condition holds      try                 method body        finally          lock unlock                    Advantages of Lock over Synchronized this    The use of synchronized methods or statements forces all lock acquisition and release to occur in a block-structured way  Lock implementations provide additional functionality over the use of synchronized methods and statements by providing    A non-blocking attempt to acquire a lock  tryLock    An attempt to acquire the lock that can be interrupted  lockInterruptibly    An attempt to acquire the lock that can timeout  tryLock long  TimeUnit     A Lock class can also provide behavior and semantics that is quite different from that of the implicit monitor lock  such as    guaranteed ordering non-re entrant usage  Deadlock detection    Have a look at this SE question regarding various type of Locks   Synchronization vs Lock  You can achieve thread safety by using advanced concurrency API instead of Synchronied blocks  This documentation page provides good programming constructs to achieve thread safety   Lock Objects support locking idioms that simplify many concurrent applications   Executors define a high-level API for launching and managing threads  Executor implementations provided by java util concurrent provide thread pool management suitable for large-scale applications   Concurrent Collections make it easier to manage large collections of data  and can greatly reduce the need for synchronization   Atomic Variables have features that minimize synchronization and help avoid memory consistency errors   ThreadLocalRandom  in JDK 7  provides efficient generation of pseudorandom numbers from multiple threads   Refer to java util concurrent and java util concurrent atomic packages too for other programming constructs

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A lock is used for either visibility or for protecting some data from concurrent modification which may lead to race   When you need to just make primitive type operations to be atomic there are available options like AtomicInteger and the likes   But suppose you have two integers which are related to each other like x and y co-ordinates  which are related to each other and should be changed in an atomic manner  Then you would protect them using a same lock   A lock should only protect the state that is related to each other  No less and no more  If you use synchronized this  in each method then even if the state of the class is unrelated all the threads will face contention even if updating unrelated state   class Point     private int x     private int y      public Point int x  int y          this x   x         this y   y             mutating methods should be guarded by same lock    public synchronized void changeCoordinates int x  int y          this x   x         this y   y           In the above example I have only one method which mutates both x and y and not two different methods as x and y are related and if I had given two different methods for mutating x and y separately then it would not have been thread safe    This example is just to demonstrate and not necessarily the way it should be implemented  The best way to do it would be to make it IMMUTABLE   Now in opposition to Point example  there is an example of TwoCounters already provided by  Andreas where the state which is being protected by two different locks as the state is unrelated to each other   The process of using different locks to protect unrelated states is called Lock Striping or Lock Splitting

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No  you shouldn t always   However  I tend to avoid it when there are multiple concerns on a particular object that only need to be threadsafe in respect to themselves   For example  you might have a mutable data object that has  label  and  parent  fields  these need to be threadsafe  but changing one need not block the other from being written read    In practice I would avoid this by declaring the fields volatile and or using java util concurrent s AtomicFoo wrappers    Synchronization in general is a bit clumsy  as it slaps a big lock down rather than thinking exactly how threads might be allowed to work around each other   Using synchronized this  is even clumsier and anti-social  as it s saying  no-one may change anything on this class while I hold the lock    How often do you actually need to do that   I would much rather have more granular locks  even if you do want to stop everything from changing  perhaps you re serialising the object   you can just acquire all of the locks to achieve the same thing  plus it s more explicit that way   When you use synchronized this   it s not clear exactly why you re synchronizing  or what the side effects might be   If you use synchronized labelMonitor   or even better labelLock getWriteLock   lock    it s clear what you are doing and what the effects of your critical section are limited to

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It depends on the task you want to do  but I wouldn t use it  Also  check if the thread-save-ness you want to accompish couldn t be done by synchronize this  in the first place  There are also some nice locks in the API that might help you

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Make your data immutable if it is possible   final variables  If you can t avoid mutation of shared data across multiple threads  use high level programming constructs  e g  granular  Lock API        A Lock provides exclusive access to a shared resource  only one thread at a time can acquire the lock and all access to the shared resource requires that the lock be acquired first    Sample code to use ReentrantLock which implements Lock interface   class X      private final ReentrantLock lock   new ReentrantLock                  public void m          lock lock        block until condition holds      try                 method body        finally          lock unlock                    Advantages of Lock over Synchronized this    The use of synchronized methods or statements forces all lock acquisition and release to occur in a block-structured way  Lock implementations provide additional functionality over the use of synchronized methods and statements by providing    A non-blocking attempt to acquire a lock  tryLock    An attempt to acquire the lock that can be interrupted  lockInterruptibly    An attempt to acquire the lock that can timeout  tryLock long  TimeUnit     A Lock class can also provide behavior and semantics that is quite different from that of the implicit monitor lock  such as    guaranteed ordering non-re entrant usage  Deadlock detection    Have a look at this SE question regarding various type of Locks   Synchronization vs Lock  You can achieve thread safety by using advanced concurrency API instead of Synchronied blocks  This documentation page provides good programming constructs to achieve thread safety   Lock Objects support locking idioms that simplify many concurrent applications   Executors define a high-level API for launching and managing threads  Executor implementations provided by java util concurrent provide thread pool management suitable for large-scale applications   Concurrent Collections make it easier to manage large collections of data  and can greatly reduce the need for synchronization   Atomic Variables have features that minimize synchronization and help avoid memory consistency errors   ThreadLocalRandom  in JDK 7  provides efficient generation of pseudorandom numbers from multiple threads   Refer to java util concurrent and java util concurrent atomic packages too for other programming constructs

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I think points one  somebody else using your lock  and two  all methods using the same lock needlessly  can happen in any fairly large application  Especially when there s no good communication between developers   It s not cast in stone  it s mostly an issue of good practice and preventing errors

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As already said here synchronized block can use user-defined variable as lock object  when synchronized function uses only  this   And of course you can manipulate with areas of your function which should be synchronized and so on   But everyone says that no difference between synchronized function and block which covers whole function using  this  as lock object  That is not true  difference is in byte code which will be generated in both situations  In case of synchronized block usage should be allocated local variable which holds reference to  this   And as result we will have a little bit larger size of function  not relevant if you have only few number of functions    More detailed explanation of the difference you can find here  http   www artima com insidejvm ed2 threadsynchP html  Also usage of synchronized block is not good due to following point of view      The synchronized keyword is very limited in one area  when exiting a synchronized block  all threads that are waiting for that lock must be unblocked  but only one of those threads gets to take the lock  all the others see that the lock is taken and go back to the blocked state  That s not just a lot of wasted processing cycles  often the context switch to unblock a thread also involves paging memory off the disk  and that s very  very  expensive    For more details in this area I would recommend you read this article  http   java dzone com articles synchronized-considered

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As already said here synchronized block can use user-defined variable as lock object  when synchronized function uses only  this   And of course you can manipulate with areas of your function which should be synchronized and so on   But everyone says that no difference between synchronized function and block which covers whole function using  this  as lock object  That is not true  difference is in byte code which will be generated in both situations  In case of synchronized block usage should be allocated local variable which holds reference to  this   And as result we will have a little bit larger size of function  not relevant if you have only few number of functions    More detailed explanation of the difference you can find here  http   www artima com insidejvm ed2 threadsynchP html  Also usage of synchronized block is not good due to following point of view      The synchronized keyword is very limited in one area  when exiting a synchronized block  all threads that are waiting for that lock must be unblocked  but only one of those threads gets to take the lock  all the others see that the lock is taken and go back to the blocked state  That s not just a lot of wasted processing cycles  often the context switch to unblock a thread also involves paging memory off the disk  and that s very  very  expensive    For more details in this area I would recommend you read this article  http   java dzone com articles synchronized-considered

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While you are using synchronized this  you are using the class instance as a lock itself  This means that while lock is acquired by thread 1  the thread 2 should wait   Suppose the following code   public void method1            do something         synchronized this            a                                             public void method2            do something         synchronized this            b                                             Method 1 modifying the variable a and method 2 modifying the variable b  the concurrent modification of the same variable by two threads should be avoided and it is  BUT while thread1 modifying a and thread2 modifying b it can be performed without any race condition   Unfortunately  the above code will not allow this since we are using the same reference for a lock  This means that threads even if they are not in a race condition should wait and obviously the code sacrifices concurrency of the program   The solution is to use 2 different locks for two different variables   public class Test        private Object lockA   new Object        private Object lockB   new Object         public void method1                do something             synchronized lockA                a                                                             public void method2                do something             synchronized lockB                b                                                            The above example uses more fine grained locks  2 locks instead one  lockA  and lockB for variables a and b respectively  and as a result allows better concurrency  on the other hand it became more complex than the first example

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Well  firstly it should be pointed out that   public void blah       synchronized  this           do stuff         is semantically equivalent to   public synchronized void blah          do stuff     which is one reason not to use synchronized this    You might argue that you can do stuff around the synchronized this  block   The usual reason is to try and avoid having to do the synchronized check at all  which leads to all sorts of concurrency problems  specifically the double checked-locking problem  which just goes to show how difficult it can be to make a relatively simple check threadsafe   A private lock is a defensive mechanism  which is never a bad idea   Also  as you alluded to  private locks can control granularity   One set of operations on an object might be totally unrelated to another but synchronized this  will mutually exclude access to all of them   synchronized this  just really doesn t give you anything

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I only want to mention a possible solution for unique private references in atomic parts of code without dependencies  You can use a static Hashmap with locks and a simple static method named atomic   that creates required references  automatically using stack information  full class name and line number   Then you can use this method in synchronize statements without writing new lock object      Synchronization objects  locks  private static HashMap lt String  Object gt  locks   new HashMap lt String  Object gt        Simple method private static Object atomic         StackTraceElement    stack   Thread currentThread   getStackTrace       get execution point      StackTraceElement exepoint   stack 2          creates unique key from class name and line number using execution point     String key   String format   s  d   exepoint getClassName    exepoint getLineNumber          Object lock   locks get key      use old or create new lock     if  lock    null            lock   new Object            locks put key  lock             return lock     return reference to lock      Synchronized code void dosomething1            start commands     synchronized  atomic                 atomic commands 1                          other command      Synchronized code void dosomething2            start commands     synchronized  atomic                 atomic commands 2                          other command

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The java util concurrent package has vastly reduced the complexity of my thread safe code  I only have anecdotal evidence to go on  but most work I have seen with synchronized x  appears to be re-implementing a Lock  Semaphore  or Latch  but using the lower-level monitors   With this in mind  synchronizing using any of these mechanisms is analogous to synchronizing on an internal object  rather than leaking a lock  This is beneficial in that you have absolute certainty that you control the entry into the monitor by two or more threads

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While you are using synchronized this  you are using the class instance as a lock itself  This means that while lock is acquired by thread 1  the thread 2 should wait   Suppose the following code   public void method1            do something         synchronized this            a                                             public void method2            do something         synchronized this            b                                             Method 1 modifying the variable a and method 2 modifying the variable b  the concurrent modification of the same variable by two threads should be avoided and it is  BUT while thread1 modifying a and thread2 modifying b it can be performed without any race condition   Unfortunately  the above code will not allow this since we are using the same reference for a lock  This means that threads even if they are not in a race condition should wait and obviously the code sacrifices concurrency of the program   The solution is to use 2 different locks for two different variables   public class Test        private Object lockA   new Object        private Object lockB   new Object         public void method1                do something             synchronized lockA                a                                                             public void method2                do something             synchronized lockB                b                                                            The above example uses more fine grained locks  2 locks instead one  lockA  and lockB for variables a and b respectively  and as a result allows better concurrency  on the other hand it became more complex than the first example

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There seems a different consensus in the C  and Java camps on this   The majority of Java code I have seen uses      apply mutex to this instance synchronized this           do work here     whereas the majority of C  code opts for the arguably safer      instance level lock object private readonly object  syncObj   new object             apply mutex to private instance level field  a System Object usually  lock  syncObj           do work here     The C  idiom is certainly safer   As mentioned previously  no malicious   accidental access to the lock can be made from outside the instance   Java code has this risk too  but it seems that the Java community has gravitated over time to the slightly less safe  but slightly more terse version     That s not meant as a dig against Java  just a reflection of my experience working on both languages

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It depends on the task you want to do  but I wouldn t use it  Also  check if the thread-save-ness you want to accompish couldn t be done by synchronize this  in the first place  There are also some nice locks in the API that might help you

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If you ve decided that    the thing you need to do is lock on the current object  and you want to lock it with granularity smaller than a whole method    then I don t see the a taboo over synchronizezd this    Some people deliberately use synchronized this   instead of marking the method synchronized  inside the whole contents of a method because they think it s  clearer to the reader  which object is actually being synchronized on  So long as people are making an informed choice  e g  understand that by doing so they re actually inserting extra bytecodes into the method and this could have a knock-on effect on potential optimisations   I don t particularly see a problem with this  You should always document the concurrent behaviour of your program  so I don t see the   synchronized  publishes the behaviour  argument as being so compelling   As to the question of which object s lock you should use  I think there s nothing wrong with synchronizing on the current object if this would be expected by the logic of what you re doing and how your class would typically be used  For example  with a collection  the object that you would logically expect to lock is generally the collection itself

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No  you shouldn t always   However  I tend to avoid it when there are multiple concerns on a particular object that only need to be threadsafe in respect to themselves   For example  you might have a mutable data object that has  label  and  parent  fields  these need to be threadsafe  but changing one need not block the other from being written read    In practice I would avoid this by declaring the fields volatile and or using java util concurrent s AtomicFoo wrappers    Synchronization in general is a bit clumsy  as it slaps a big lock down rather than thinking exactly how threads might be allowed to work around each other   Using synchronized this  is even clumsier and anti-social  as it s saying  no-one may change anything on this class while I hold the lock    How often do you actually need to do that   I would much rather have more granular locks  even if you do want to stop everything from changing  perhaps you re serialising the object   you can just acquire all of the locks to achieve the same thing  plus it s more explicit that way   When you use synchronized this   it s not clear exactly why you re synchronizing  or what the side effects might be   If you use synchronized labelMonitor   or even better labelLock getWriteLock   lock    it s clear what you are doing and what the effects of your critical section are limited to

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If you ve decided that    the thing you need to do is lock on the current object  and you want to lock it with granularity smaller than a whole method    then I don t see the a taboo over synchronizezd this    Some people deliberately use synchronized this   instead of marking the method synchronized  inside the whole contents of a method because they think it s  clearer to the reader  which object is actually being synchronized on  So long as people are making an informed choice  e g  understand that by doing so they re actually inserting extra bytecodes into the method and this could have a knock-on effect on potential optimisations   I don t particularly see a problem with this  You should always document the concurrent behaviour of your program  so I don t see the   synchronized  publishes the behaviour  argument as being so compelling   As to the question of which object s lock you should use  I think there s nothing wrong with synchronizing on the current object if this would be expected by the logic of what you re doing and how your class would typically be used  For example  with a collection  the object that you would logically expect to lock is generally the collection itself

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Well  firstly it should be pointed out that   public void blah       synchronized  this           do stuff         is semantically equivalent to   public synchronized void blah          do stuff     which is one reason not to use synchronized this    You might argue that you can do stuff around the synchronized this  block   The usual reason is to try and avoid having to do the synchronized check at all  which leads to all sorts of concurrency problems  specifically the double checked-locking problem  which just goes to show how difficult it can be to make a relatively simple check threadsafe   A private lock is a defensive mechanism  which is never a bad idea   Also  as you alluded to  private locks can control granularity   One set of operations on an object might be totally unrelated to another but synchronized this  will mutually exclude access to all of them   synchronized this  just really doesn t give you anything

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Well  firstly it should be pointed out that   public void blah       synchronized  this           do stuff         is semantically equivalent to   public synchronized void blah          do stuff     which is one reason not to use synchronized this    You might argue that you can do stuff around the synchronized this  block   The usual reason is to try and avoid having to do the synchronized check at all  which leads to all sorts of concurrency problems  specifically the double checked-locking problem  which just goes to show how difficult it can be to make a relatively simple check threadsafe   A private lock is a defensive mechanism  which is never a bad idea   Also  as you alluded to  private locks can control granularity   One set of operations on an object might be totally unrelated to another but synchronized this  will mutually exclude access to all of them   synchronized this  just really doesn t give you anything

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While I agree about not adhering blindly to dogmatic rules  does the  lock stealing  scenario seem so eccentric to you  A thread could indeed acquire the lock on your object  externally  synchronized theObject          blocking other threads waiting on synchronized instance methods   If you don t believe in malicious code  consider that this code could come from third parties  for instance if you develop some sort of application server    The  accidental  version seems less likely  but as they say   make something idiot-proof and someone will invent a better idiot    So I agree with the it-depends-on-what-the-class-does school of thought     Edit following eljenso s first 3 comments   I ve never experienced the lock stealing problem but here is an imaginary scenario   Let s say your system is a servlet container  and the object we re considering is the ServletContext implementation  Its getAttribute method must be thread-safe  as context attributes are shared data  so you declare it as synchronized  Let s also imagine that you provide a public hosting service based on your container implementation   I m your customer and deploy my  good  servlet on your site  It happens that my code contains a call to getAttribute   A hacker  disguised as another customer  deploys his malicious servlet on your site  It contains the following code in the init method    synchronized  this getServletConfig   getServletContext         while  true         Assuming we share the same servlet context  allowed by the spec as long as the two servlets are on the same virtual host   my call on getAttribute is locked forever  The hacker has achieved a DoS on my servlet   This attack is not possible if getAttribute is synchronized on a private lock  because 3rd-party code cannot acquire this lock   I admit that the example is contrived and an oversimplistic view of how a servlet container works  but IMHO it proves the point   So I would make my design choice based on security consideration  will I have complete control over the code that has access to the instances  What would be the consequence of a thread s holding a lock on an instance indefinitely

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Short answer  You have to understand the difference and make choice depending on the code   Long answer  In general I would rather try to avoid synchronize this  to reduce contention but private locks add complexity you have to be aware of  So use the right synchronization for the right job  If you are not so experienced with multi-threaded programming I would rather stick to instance locking and read up on this topic   That said  just using synchronize this  does not automatically make your class fully thread-safe   This is a not an easy topic but once you get used to it  the answer whether to use synchronize this  or not comes naturally

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If you ve decided that    the thing you need to do is lock on the current object  and you want to lock it with granularity smaller than a whole method    then I don t see the a taboo over synchronizezd this    Some people deliberately use synchronized this   instead of marking the method synchronized  inside the whole contents of a method because they think it s  clearer to the reader  which object is actually being synchronized on  So long as people are making an informed choice  e g  understand that by doing so they re actually inserting extra bytecodes into the method and this could have a knock-on effect on potential optimisations   I don t particularly see a problem with this  You should always document the concurrent behaviour of your program  so I don t see the   synchronized  publishes the behaviour  argument as being so compelling   As to the question of which object s lock you should use  I think there s nothing wrong with synchronizing on the current object if this would be expected by the logic of what you re doing and how your class would typically be used  For example  with a collection  the object that you would logically expect to lock is generally the collection itself

User · Answer

No  you shouldn t always   However  I tend to avoid it when there are multiple concerns on a particular object that only need to be threadsafe in respect to themselves   For example  you might have a mutable data object that has  label  and  parent  fields  these need to be threadsafe  but changing one need not block the other from being written read    In practice I would avoid this by declaring the fields volatile and or using java util concurrent s AtomicFoo wrappers    Synchronization in general is a bit clumsy  as it slaps a big lock down rather than thinking exactly how threads might be allowed to work around each other   Using synchronized this  is even clumsier and anti-social  as it s saying  no-one may change anything on this class while I hold the lock    How often do you actually need to do that   I would much rather have more granular locks  even if you do want to stop everything from changing  perhaps you re serialising the object   you can just acquire all of the locks to achieve the same thing  plus it s more explicit that way   When you use synchronized this   it s not clear exactly why you re synchronizing  or what the side effects might be   If you use synchronized labelMonitor   or even better labelLock getWriteLock   lock    it s clear what you are doing and what the effects of your critical section are limited to

User · Answer

The reason not to synchronize on this is that sometimes you need more than one lock  the second lock often gets removed after some additional thinking  but you still need it in the intermediate state   If you lock on this  you always have to remember which one of the two locks is this  if you lock on a private Object  the variable name tells you that   From the reader s viewpoint  if you see locking on this  you always have to answer the two questions     what kind of access is protected by this  is one lock really enough  didn t someone introduce a bug    An example   class BadObject       private Something mStuff      synchronized setStuff Something stuff            mStuff   stuff            synchronized getStuff Something stuff            return mStuff            private MyListener myListener   new MyListener             public void onMyEvent                    setStuff                           synchronized void longOperation MyListener l                        l onMyEvent                             If two threads begin longOperation   on two different instances of BadObject  they acquire  their locks  when it s time to invoke l onMyEvent       we have a deadlock because neither of the threads may acquire the other object s lock   In this example we may eliminate the deadlock by using two locks  one for short operations and one for long ones

[java] Avoid synchronized(this) in Java?

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