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10 Feb 2019

Points to remenber about synchronization in java?

Synchronization in java is a strategy or a method to avoid thread interference and hence protecting the data from inconsistency. synchronization is also one of the way to make code thread safe. Through synchronization, we can make the threads to execute particular method or block in sync not simultaneously.

When a method or block is declared as synchronized, only one thread can enter into that method or block. When one thread is executing synchronized method or block, the other threads which wants to execute that method or block wait or suspend their execution until first thread is done with that method or block. Thus avoiding the thread interference and achieving thread safeness.

Whenever an object is created to any class, an object lock is created and is stored inside the object.

One object will have only one object lock associated with it.

Any thread wants to enter into synchronized methods or blocks of any object, they must acquire object lock associated with that object and release the lock after they are done with the execution.

The other threads which wants to enter into synchronized methods of that object have to wait until the currently executing thread releases the object lock.

To enter into static synchronized methods or blocks, threads have to acquire class lock associated with that class as static members are stored inside the class memory.

Important points about Synchronization:

1) You can use synchronized keyword only with methods but not with variables, constructors, static initializer and instance initializers.

2) Constructors, Static initializer and instance initializer can’t be declared with synchronized keyword, but they can contain synchronized blocks.

3) Both static and non-static methods can use synchronized keyword. For static methods, thread need class level lock and for non-static methods, thread need object level lock.

4) It is possible that both static synchronized and non-static synchronized methods can run simultaneously. Because, static methods need class level lock and non-static methods need object level lock.

5) synchronized method or block is very slow. They decrease the performance of an application. So, special care need to be taken while using synchronization. Use synchronization only when you needed it the most.

6) Synchronization blocks can be nested.