Is there an advantage to use a Synchronized Method instead of a Synchronized Block

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Can any one tell me the advantage of synchronized method over synchronized block with an example

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With synchronized blocks  you can have multiple synchronizers  so that multiple simultaneous but non-conflicting things can go on at the same time

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Synchronized methods can be checked using reflection API  This can be useful for testing some contracts  such as all methods in model are synchronized   The following snippet prints all the synchronized methods of Hashtable   for  Method m   Hashtable class getMethods              if  Modifier isSynchronized m getModifiers                   System out println m

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When java compiler converts your source code to byte code  it handles synchronized methods and synchronized blocks very differently   When the JVM executes a synchronized method  the executing thread identifies that the method s method info structure has the ACC SYNCHRONIZED flag set  then it automatically acquires the object s lock  calls the method  and releases the lock  If an exception occurs  the thread automatically releases the lock   Synchronizing a method block  on the other hand  bypasses the JVM s built-in support for acquiring an object s lock and exception handling and requires that the functionality be explicitly written in byte code  If you read the byte code for a method with a synchronized block  you will see more than a dozen additional operations to manage this functionality    This shows calls to generate both a synchronized method and a synchronized block   public class SynchronizationExample       private int i       public synchronized int synchronizedMethodGet             return i             public int synchronizedBlockGet             synchronized  this                 return i                      The synchronizedMethodGet   method generates the following byte code   0   aload 0 1   getfield 2   nop 3   iconst m1 4   ireturn   And here s the byte code from the synchronizedBlockGet   method   0   aload 0 1   dup 2   astore 1 3   monitorenter 4   aload 0 5   getfield 6   nop 7   iconst m1 8   aload 1 9   monitorexit 10  ireturn 11  astore 2 12  aload 1 13  monitorexit 14  aload 2 15  athrow     One significant difference between synchronized method and block is that  Synchronized block generally reduce scope of lock  As scope of lock is inversely proportional to performance  its always better to lock only critical section of code  One of the best example of using synchronized block is double checked locking in Singleton pattern where instead of locking whole getInstance   method we only lock critical section of code which is used to create Singleton instance  This improves performance drastically because locking is only required one or two times   While using synchronized methods  you will need to take extra care if you mix both static synchronized and non-static synchronized methods

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Important note on using the synchronized block  careful what you use as lock object   The code snippet from user2277816 above illustrates this point in that a reference to a string literal is used as locking object  Realize that string literals are automatically interned in Java and you should begin to see the problem  every piece of code that synchronizes on the literal  lock   shares the same lock  This can easily lead to deadlocks with completely unrelated pieces of code   It is not just String objects that you need to be careful with  Boxed primitives are also a danger  since autoboxing and the valueOf methods can reuse the same objects  depending on the value   For more information see  https   www securecoding cert org confluence display java LCK01-J  Do not synchronize on objects that may be reused

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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  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 for 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

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In general these are mostly the same other than being explicit about the object s monitor that s being used vs the implicit this object   One downside of synchronized methods that I think is sometimes overlooked is that in using the  this  reference to synchronize on you are leaving open the possibility of external objects locking on the same object   That can be a very subtle bug if you run into it   Synchronizing on an internal explicit Object or other existing field can avoid this issue  completely encapsulating the synchronization

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Synchronized Method  Pros    Your IDE can indicate the synchronized methods  The syntax is more compact  Forces to split the synchronized blocks to separate methods    Cons    Synchronizes to this and so makes it possible to outsiders to synchronize to it too  It is harder to move code outside the synchronized block    Synchronized block  Pros    Allows using a private variable for the lock and so forcing the lock to stay inside the class  Synchronized blocks can be found by searching references to the variable    Cons    The syntax is more complicated and so makes the code harder to read      Personally I prefer using synchronized methods with classes focused only to the thing needing synchronization  Such class should be as small as possible and so it should be easy to review the synchronization  Others shouldn t need to care about synchronization

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Note  static synchronized methods and blocks work on the Class object   public class MyClass         locks MyClass class    public static synchronized void foo        do something             similar    public static void foo           synchronized MyClass class       do something

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The main difference is that if you use a synchronized block you may lock on an object other than this which allows to be much more flexible   Assume you have a message queue and multiple message producers and consumers  We don t want producers to interfere with each other  but the consumers should be able to retrieve messages without having to wait for the producers  So we just create an object  Object writeLock   new Object      And from now on every time a producers wants to add a new message we just lock on that   synchronized writeLock        do something     So consumers may still read  and producers will be locked

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Most often I use this to synchronize access to a list or map but I don t want to block access to all methods of the object   In the following code one thread modifying the list will not block waiting for a thread that is modifying the map   If the methods were synchronized on the object then each method would have to wait even though the modifications they are making would not conflict   private List lt Foo gt  myList   new ArrayList lt Foo gt     private Map lt String Bar  myMap   new HashMap lt String Bar gt      public void put  String s  Bar b       synchronized  myMap         myMap put  s b           then some thing that may take a while like a database access or RPC or notifying listeners        public void hasKey  String s        synchronized  myMap         myMap hasKey  s           public void add  Foo f       synchronized  myList         myList add  f       then some thing that may take a while like a database access or RPC or notifying listeners        public Thing getMedianFoo       Foo med   null    synchronized  myList         Collections sort myList       med   myList get myList size   2          return med

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As a practical matter  the advantage of synchronized methods over synchronized blocks is that they are more idiot-resistant  because you can t choose an arbitrary object to lock on  you can t misuse the synchronized method syntax to do stupid things like locking on a string literal or locking on the contents of a mutable field that gets changed out from under the threads   On the other hand  with synchronized methods you can t protect the lock from getting acquired by any thread that can get a reference to the object   So using synchronized as a modifier on methods is better at protecting your cow-orkers from hurting themselves  while using synchronized blocks in conjunction with private final lock objects is better at protecting your own code from the cow-orkers

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In case of synchronized methods  lock will be acquired on an Object  But if you go with synchronized block you have an option to specify an object on which the lock will be acquired   Example        Class Example       String test    abc          lock will be acquired on String  test object      synchronized  test               do something           lock will be acquired on Example Object    public synchronized void testMethod             do some thing

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The only difference   synchronized blocks allows granular locking unlike synchronized method  Basically synchronized block or methods have been used to write thread safe code by avoiding memory inconsistency errors    This question is very old and many things have been changed during last 7 years   New programming constructs have been introduced for thread safety   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   Better replacement for synchronized is ReentrantLock  which uses Lock API     A reentrant mutual exclusion Lock with the same basic behavior and semantics as the implicit monitor lock accessed using synchronized methods and statements  but with extended capabilities    Example with locks   class X      private final ReentrantLock lock   new ReentrantLock                  public void m          lock lock        block until condition holds      try                 method body        finally          lock unlock                    Refer to java util concurrent and java util concurrent atomic packages too for other programming constructs    Refer to this related question too   Synchronization vs Lock

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The only real difference is that a synchronized block can choose which object it synchronizes on   A synchronized method can only use  this   or the corresponding Class instance for a synchronized class method    For example  these are semantically equivalent   synchronized void foo              void foo         synchronized  this                        The latter is more flexible since it can compete for the associated lock of any object  often a member variable   It s also more granular because you could have concurrent code executing before and after the block but still within the method   Of course  you could just as easily use a synchronized method by refactoring the concurrent code into separate non-synchronized methods   Use whichever makes the code more comprehensible

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Often using a lock on a method level is too rude  Why lock up a piece of code that does not access any shared resources by locking up an entire method  Since each object has a lock  you can create dummy objects to implement block level synchronization  The block level is more efficient because it does not lock the whole method   Here some example  Method Level  class MethodLevel        shared among threads SharedResource x  y    public void synchronized method1          multiple threads can t access   public void synchronized method2         multiple threads can t access     public void method3         not synchronized     multiple threads can access        Block Level  class BlockLevel       shared among threads   SharedResource x  y        dummy objects for locking   Object xLock   new Object      Object yLock   new Object         public void method1          synchronized xLock         access x here  thread safe              do something here but don t use SharedResource x  y        because will not be thread-safe      synchronized xLock           synchronized yLock            access x y here  thread safe                        do something here but don t use SharedResource x  y        because will not be thread-safe        end of method1       Edit   For Collection like Vector and Hashtable they are synchronized when ArrayList or HashMap are not and you need set synchronized keyword or invoke Collections synchronized method   Map myMap   Collections synchronizedMap  myMap      single lock for the entire map List myList   Collections synchronizedList  myList      single lock for the entire list

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I suppose this question is about the difference between Thread Safe Singleton and Lazy initialization with Double check locking  I always refer to this article when I need to implement some specific singleton   Well  this is a Thread Safe Singleton      Java program to create Thread Safe     Singleton class  public class GFG           private instance  so that it can be       accessed by only by getInstance   method    private static GFG instance      private GFG                 private constructor           synchronized method to control simultaneous access    synchronized public static GFG getInstance              if  instance    null                    if instance is null  initialize        instance   new GFG                return instance                Pros          Lazy initialization is possible    It is thread safe          Cons          getInstance   method is synchronized so it causes slow performance as multiple threads can   t access it simultaneously       This is a Lazy initialization with Double check locking      Java code to explain double check locking  public class GFG           private instance  so that it can be       accessed by only by getInstance   method    private static GFG instance      private GFG                 private constructor          public static GFG getInstance             if  instance    null                   synchronized block to remove overhead        synchronized  GFG class                    if instance  null                           if instance is null  initialize            instance   new GFG                                     return instance                Pros          Lazy initialization is possible    It is also thread safe    Performance reduced because of synchronized keyword is overcome          Cons          First time  it can affect performance    As cons  of double check locking method is bearable so it can be   used for high performance multi-threaded applications       Please refer to this article for more details   https   www geeksforgeeks org java-singleton-design-pattern-practices-examples

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I know this is an old question  but with my quick read of the responses here  I didn t really see anyone mention that at times a synchronized method may be the wrong lock  From Java Concurrency In Practice  pg  72    public class ListHelper lt E gt      public List lt E gt  list   Collections syncrhonizedList new ArrayList lt  gt           public syncrhonized boolean putIfAbsent E x     boolean absent    list contains x   if absent     list add x     return absent      The above code has the appearance of being thread-safe   However  in reality it is not   In this case the lock is obtained on the instance of the class   However  it is possible for the list to be modified by another thread not using that method  The correct approach would be to use   public boolean putIfAbsent E x     synchronized list      boolean absent    list contains x     if absent        list add x         return absent        The above code would block all threads trying to modify list from modifying the list until the synchronized block has completed

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Synchronizing with threads   1  NEVER use synchronized this  in a thread it doesn t work  Synchronizing with  this  uses the current thread as the locking thread object  Since each thread is independent of other threads  there is NO coordination of synchronization  2  Tests of code show that in Java 1 6 on a Mac the method synchronization does not work  3  synchronized lockObj  where lockObj is a common shared object of all threads synchronizing on it will work  4  ReenterantLock lock   and  unlock   work  See Java tutorials for this   The following code shows these points  It also contains the thread-safe Vector which would be substituted for the ArrayList  to show that many threads adding to a Vector do not lose any information  while the same with an ArrayList can lose information   0  Current code shows loss of information due to race conditions A  Comment the current labeled A line  and uncomment the A line above it  then run  method loses data but it shouldn t  B  Reverse step A  uncomment B and    end block    Then run to see results no loss of data C  Comment out B  uncomment C  Run  see synchronizing on  this  loses data  as expected  Don t have time to complete all the variations  hope this helps  If synchronizing on  this   or the method synchronization works  please state what version of Java and OS you tested   Thank you   import java util         RaceCondition - Shows that when multiple threads compete for resources       thread one may grab the resource expecting to update a particular       area but is removed from the CPU before finishing   Thread one still       points to that resource   Then thread two grabs that resource and       completes the update   Then thread one gets to complete the update        which over writes thread two s work       DEMO   1  Run as is - see missing counts from race condition  Run severa times  values change               2  Uncomment  synchronized countLock      - see counts work             Synchronized creates a lock on that block of code  no other threads can              execute code within a block that another thread has a lock          3  Comment ArrayList  unComment Vector - See no loss in collection             Vectors work like ArrayList  but Vectors are  Thread Safe           May use this code as long as attribution to the author remains intact        mf      public class RaceCondition       private ArrayList lt Integer gt  raceList   new ArrayList lt Integer gt        simple add        private Vector lt Integer gt  raceList   new Vector lt Integer gt        simple add         private String countLock  lock         Object use for locking the raceCount     private int raceCount   0            simple add 1 to this counter     private int MAX   10000            Do this 10 000 times     private int NUM THREADS   100        Create 100 threads      public static void main String    args        new RaceCondition               public RaceCondition         ArrayList lt Thread gt  arT   new ArrayList lt Thread gt             Create thread objects  add them to an array list     for  int i 0  i lt NUM THREADS  i             Thread rt   new RaceThread        i            arT add  rt                  Start all object at once      for  Thread rt   arT            rt start                  Wait for all threads to finish before we can print totals created by threads     for  int i 0  i lt NUM THREADS  i             try   arT get i  join              catch  InterruptedException ie     System out println  Interrupted thread   i                   All threads finished  print the summary information          Try to print this informaiton without the join loop above      System out printf   nRace condition  should have   d  Really have   d in array  and count of   d  n                   MAX NUM THREADS  raceList size    raceCount        System out printf  Array lost   d  Count lost   d n                MAX NUM THREADS-raceList size    MAX NUM THREADS-raceCount               end RaceCondition constructor        class RaceThread extends Thread       public void run             for   int i 0  i lt MAX  i             try               update  i                            These  catches show when one thread steps on another s values         catch  ArrayIndexOutOfBoundsException ai    System out print  A              catch  OutOfMemoryError oome     System out print  O                              so we don t lose counts  need to synchronize on some object  not primitive        Created  countLock  to show how this can work         Comment out the synchronized and ending    see that we lose counts         public synchronized void update int i        use A     public void update int i                       remove this when adding A         synchronized countLock                 or B         synchronized this                  or C         raceCount   raceCount   1          raceList add  i            use Vector                              end block for B or C            end update             end RaceThread inner class        end RaceCondition outter class

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TLDR  Neither use the synchronized modifier nor the synchronized this       expression but synchronized myLock       where myLock is a final instance field holding a private object     The difference between using the synchronized modifier on the method declaration  and the synchronized        expression in the method body are this    The synchronized modifier specified on the method s signature   is visible in the generated JavaDoc   is programmatically determinable via reflection when testing a method s modifier for Modifier SYNCHRONIZED  requires less typing and indention compared to synchronized this            and  depending on your IDE  is visible in the class outline and code completion  uses the this object as lock when declared on non-static method or the enclosing class when declared on a static method   The synchronized           expression allows you   to only synchronize the execution of parts of a method s body  to be used within a constructor or a  static  initialization block  to choose the lock object which controls the synchronized access     However  using the synchronized modifier or synchronized            with this as the lock object  as in synchronized this          have the same disadvantage  Both use it s own instance as the lock object to synchronize on  This is dangerous because not only the object itself but any other external object code that holds a reference to that object can also use it as a synchronization lock with potentially severe side effects  performance degradation and deadlocks    Therefore best practice is to neither use the synchronized modifier nor the synchronized      expression in conjunction with this as lock object but a lock object private to this object  For example   public class MyService       private final lock   new Object         public void doThis            synchronized lock                 do code that requires synchronous execution                      public void doThat            synchronized lock                 do code that requires synchronous execution                     You can also use multiple lock objects but special care needs to be taken to ensure this does not result in deadlocks when used nested   public class MyService       private final lock1   new Object        private final lock2   new Object         public void doThis            synchronized lock1              synchronized lock2                     code here is guaranteed not to be executes at the same time                  as the synchronized code in doThat   and doMore                           public void doThat            synchronized lock1                     code here is guaranteed not to be executes at the same time                  as the synchronized code in doThis                     doMore   may execute concurrently                      public void doMore            synchronized lock2                     code here is guaranteed not to be executes at the same time                  as the synchronized code in doThis                     doThat   may execute concurrently

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Can anyone tell me the advantage of the synchronized method over the synchronized block with an example  Thanks    There is not a clear advantage of using synchronized method over the block    Perhaps the only one   but I wouldn t call it an advantage   is you don t need to include the object reference this   Method   public synchronized void method        blocks  this  from here                                   to here   Block   public void method          synchronized  this        blocks  this  from here                                                        to here        See  No advantage at all    Blocks do have advantages over methods though  mostly in flexibility because you can use another object as lock whereas syncing the method would lock the entire object   Compare       locks the whole object      private synchronized void someInputRelatedWork                private synchronized void someOutputRelatedWork                  vs       Using specific locks Object inputLock   new Object    Object outputLock   new Object     private void someInputRelatedWork         synchronized inputLock                           private void someOutputRelatedWork         synchronized outputLock                            Also if the method grows you can still keep the synchronized section separated    private void method              code here          code here          code here     synchronized  lock                  very few lines of code here                code here          code here          code here          code here

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Synchronized method  Synchronized methods have two effects  First  when one thread is executing a synchronized method for an object  all other threads that invoke synchronized methods for the same object block  suspend execution  until the first thread is done with the object    Second  when a synchronized method exits  it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object  This guarantees that changes to the state of the object are visible to all threads    Note that constructors cannot be synchronized     using the synchronized keyword with a constructor is a syntax error  Synchronizing constructors doesn t make sense  because only the thread that creates an object should have access to it while it is being constructed    Synchronized Statement  Unlike synchronized methods  synchronized statements must specify the object that provides the intrinsic lock  Most often I use this to synchronize access to a list or map but I don t want to block access to all methods of the object    Q  Intrinsic Locks and Synchronization Synchronization is built around an internal entity known as the intrinsic lock or monitor lock   The API specification often refers to this entity simply as a  monitor    Intrinsic locks play a role in both aspects of synchronization  enforcing exclusive access to an object s state and establishing happens-before relationships that are essential to visibility    Every object has an intrinsic lock associated with it  By convention  a thread that needs exclusive and consistent access to an object s fields has to acquire the object s intrinsic lock before accessing them  and then release the intrinsic lock when it s done with them  A thread is said to own the intrinsic lock between the time it has acquired the lock and released the lock  As long as a thread owns an intrinsic lock  no other thread can acquire the same lock  The other thread will block when it attempts to acquire the lock    package test   public class SynchTest implements Runnable         private int c   0       public static void main String   args            new SynchTest   test               public void test                Create the object with the run   method         Runnable runnable   new SynchTest            Runnable runnable2   new SynchTest               Create the thread supplying it with the runnable object         Thread thread   new Thread runnable  thread-1            Thread thread2   new Thread runnable  thread-2            Here the key point is passing same object  if you pass runnable2 for thread2          then its not applicable for synchronization test and that wont give expected         output Synchronization method means  it is not possible for two invocations         of synchronized methods on the same object to interleave              Start the thread         thread start            thread2 start               public synchronized  void increment             System out println  Begin thread     Thread currentThread   getName             System out println this hashCode      Value of C       c           If we uncomment this for synchronized block  then the result would be different         synchronized this                for  int i   0  i  lt  9999999  i                      c    i                                  System out println  End thread     Thread currentThread   getName                  public synchronized void decrement               System out println  Decrement     Thread currentThread   getName                  public int value             return c              Override     public void run             this increment              Cross check different outputs with synchronized method  block and without synchronization

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Synchronized method is used for lock all the objects Synchronized block is used to lock specific object

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From a Java specification summary  http   www cs cornell edu andru javaspec 17 doc html  The synchronized statement    14 17  computes a reference to an object  it then attempts to perform a lock action on that object and does not proceed further until the lock action has successfully completed      A synchronized method    8 4 3 5  automatically performs a lock action when it is invoked  its body is not executed until the lock action has successfully completed  If the method is an instance method  it locks the lock associated with the instance for which it was invoked  that is  the object that will be known as this during execution of the body of the method   If the method is static  it locks the lock associated with the Class object that represents the class in which the method is defined       Based on these descriptions  I would say most previous answers are correct  and a synchronized method might be particularly useful for static methods  where you would otherwise have to figure out how to get the  quot Class object that represents the class in which the method was defined  quot  Edit  I originally thought these were quotes of the actual Java spec   Clarified that this page is just a summary explanation of the spec

[java] Is there an advantage to use a Synchronized Method instead of a Synchronized Block?

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