Why is synchronized block better than synchronized method

Question

I have started learning synchronization in threading   Synchronized method    public class Counter       private static int count   0      public static synchronized int getCount           return count           public synchronized setCount int count          this count   count            Synchronized block   public class Singleton       private static volatile Singleton  instance      public static Singleton getInstance           if   instance    null             synchronized Singleton class                if   instance    null                  instance   new Singleton                             return  instance           When should I use synchronized method and synchronized block   Why is synchronized block better than synchronized method

User · Answer

Because lock is expensive  when you are using synchronized block you lock only if  instance    null  and after  instance finally initialized you ll never lock  But when you synchronize on method you lock unconditionally  even after the  instance is initialized  This is the idea behind double-checked locking optimization pattern http   en wikipedia org wiki Double-checked locking

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In your case both are equivalent   Synchronizing a static method is equivalent to a synchronized block on corresponding Class object   In fact when you declare a synchronized static method lock is obtained on the monitor corresponding to the Class object   public static synchronized int getCount                    is same as  public int getCount         synchronized  ClassName class

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Although not usually a concern  from a security perspective  it is better to use synchronized on a private object  rather than putting it on a method   Putting it on the method means you are using the lock of the object itself to provide thread safety  With this kind of mechanism  it is possible for a malicious user of your code to also obtain the lock on your object  and hold it forever  effectively blocking other threads  A non-malicious user can effectively do the same thing inadvertently    If you use the lock of a private data member  you can prevent this  since it is impossible for a malicious user to obtain the lock on your private object   private final Object lockObject   new Object     public void getCount         synchronized  lockObject                           This technique is mentioned in Bloch s Effective Java  2nd Ed   Item  70

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One classic difference between Synchronized block and Synchronized method is that Synchronized method locks the entire object  Synchronized block just locks the code within the block   Synchronized method  Basically these 2 sync methods disable multithreading  So one thread completes the method1   and the another thread waits for the Thread1 completion   class SyncExerciseWithSyncMethod    public synchronized void method1         try           System out println  In Method 1            Thread sleep 5000         catch  Exception e            System out println  Catch of method 1          finally           System out println  Finally of method 1              public synchronized void method2         try           for  int i   1  i  lt  10  i                  System out println  Method 2     i               Thread sleep 1000                   catch  Exception e            System out println  Catch of method 2          finally           System out println  Finally of method 2                 Output  In Method 1  Finally of method 1  Method 2 1  Method 2 2  Method 2 3  Method 2 4  Method 2 5  Method 2 6  Method 2 7  Method 2 8  Method 2 9  Finally of method 2    Synchronized block  Enables multiple threads to access the same object at same time  Enables multi-threading     class SyncExerciseWithSyncBlock    public Object lock1   new Object    public Object lock2   new Object     public void method1         synchronized  lock1            try               System out println  In Method 1                Thread sleep 5000             catch  Exception e                System out println  Catch of method 1              finally               System out println  Finally of method 1                        public void method2          synchronized  lock2            try               for  int i   1  i  lt  10  i                      System out println  Method 2     i                   Thread sleep 1000                           catch  Exception e                System out println  Catch of method 2              finally               System out println  Finally of method 2                           Output  In Method 1  Method 2 1  Method 2 2  Method 2 3  Method 2 4  Method 2 5  Finally of method 1  Method 2 6  Method 2 7  Method 2 8  Method 2 9  Finally of method 2

User · Answer

Define  better   A synchronized block is only better because it allows you to    Synchronize on a different object Limit the scope of synchronization   Now your specific example is an example of the double-checked locking pattern which is suspect  in older Java versions it was broken  and it is easy to do it wrong    If your initialization is cheap  it might be better to initialize immediately with a final field  and not on the first request  it would also remove the need for synchronization

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Difference between synchronized block and synchronized method are following     synchronized block reduce scope of lock  but synchronized method s scope of lock is whole method  synchronized block has better performance as only the critical section is locked but synchronized method has poor performance than block  synchronized block provide granular control over lock but synchronized method lock either on current object represented by this or class level lock  synchronized block can throw NullPointerException but synchronized method doesn t throw  synchronized block   synchronized this     synchronized method  public synchronized void fun

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It s not a matter of better  just different   When you synchronize a method  you are effectively synchronizing to the object itself   In the case of a static method  you re synchronizing to the class of the object   So the following two pieces of code execute the same way   public synchronized int getCount                    This is just like you wrote this   public int getCount         synchronized  this                             If you want to control synchronization to a specific object  or you only want part of a method to be synchronized to the object  then specify a synchronized block   If you use the synchronized keyword on the method declaration  it will synchronize the whole method to the object or class

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The difference is in which lock is being acquired    synchronized method acquires a lock on the whole object  This means no other thread can use any synchronized method in the whole object while the method is being run by one thread  synchronized blocks acquires a lock in the object between parentheses after the synchronized keyword  Meaning no other thread can acquire a lock on the locked object until the synchronized block exits    So if you want to lock the whole object  use a synchronized method  If you want to keep other parts of the object accessible to other threads  use synchronized block   If you choose the locked object carefully  synchronized blocks will lead to less contention  because the whole object class is not blocked   This applies similarly to static methods  a synchronized static method will acquire a lock in the whole class object  while a synchronized block inside a static method will acquire a lock in the object between parentheses

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synchronized should only be used when you want your class to be Thread safe  In fact most of the classes should not use synchronized anyways  synchronized method would only provide a lock on this object and only for the duration of its execution  if you really wanna to make your classes thread safe  you should consider making your variables volatile or synchronize the access   one of the issues of using synchronized method is that all of the members of the class would use the same lock which will make your program slower  In your case synchronized method and block would execute no different  what I d would recommend is to use a dedicated lock and use a synchronized block something like this   public class AClass   private int x  private final Object lock   new Object          it must be final    public void setX         synchronized lock            x

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It should not be considered as a question of best for usage  but it really depends on the use case or the scenario   Synchronized Methods  An entire method can be marked as synchronized  resulting an implicit lock on the this reference  instance methods  or class  static methods   This is very convenient mechanism to achieve synchronization   Steps A thread access the synchronized  method  It implicitly acquires the lock and execute the code  If other thread want to access the above method  it has to wait  The thread can t get the lock  will be blocked and has to wait till the lock is released    Synchronized Blocks  To acquire a lock on an object for a specific set of code block  synchronized blocks are the best fit  As a block is sufficient  using a synchronized method will be a waste    More specifically with Synchronized Block   it is possible to define the object reference on which are want to acquire a lock

[java] Why is synchronized block better than synchronized method?

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