Difference between synchronized and volatile modifier in java

The following are the differences between synchronized and volatile  modifier which is one of the core java interview question..

1. Synchronization ensures  both   visibilty   and  mutual exclusion  whereas volatile  ensures  only visibilty  ( one thread reads the most up-to-date value written by another thread)

2. The volatile modifier is applied  to a field variable  whereas synchronized is applied to methods and block of codes.

3. When using  synchronized modifier, an unlock on monitor  happens before / synchronizes with all subsequent lock  on that monitor.When using volatile ,  a write to a volatile variable  happens before  with all subsequent reads of that variable by any thread

4. Volatile  does not guarantee the atomicity of composite operations such as incrementing a variable where as synchronized modifier  guarantees the atomicity of composite operations.
for example

private static volatile int sno = 0;
public static int getNextSno() {
return sno++;
    }

The above method won’t work properly without synchronization. Every invocation may not return unique value because increment operator performs  three operations read, add, store.   first it reads the value then increments  plus one  then it stores  back a new value. When a thread reads the field , another thread may enter and read the same value before the first thread writes the incremented value back . Now both thread returns the same sno. Use synchronized on the method getNextSno(),   to ensure that the increment operation is atomic and remove volatile modifier from sno.

4. A thread acquires or releases a lock on a  monitor to enter or exit  a synchronized method or  block of code  where as a thread does not need to aquire / release a monitor to  read or write a volatile variable. But both have the same memory effects.

5.   final variable can not be a volatile where as final method can be synchronized

6.  volatile variable  can be null where as synchronization can not be done on null object .

7. In some situations , code is simplified  using volatile  than  using synchronized. At the same time,  we have to be careful when using volatile , code using volatile is weaker (easily broken)  than synchronized  as shown in the above example (4th point).

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