What are differences between synchronized method and synchronized block (statement)?

The synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared resource while another thread is in the process of using or updating that resource.

There two synchronization syntax in Java Language. The practical differences are in controlling scope and the monitor. With a synchronized method, the lock is obtained for the duration of the entire method. With synchronized blocks you can specify exactly when the lock is needed.

Basically, synchronized blocks are more general, and synchronized methods can be rewritten to use synchronized blocks:

class Program {
  public synchronized void f() {
   .........
  }
}

is equivalent to

class Program {
  public void f() {
     synchronized(this){
       ...
     }
  }
}

For example, You have a method with some parts that need synchronized and others don't. The synchronized block lets you synchronize only the partial line codes that really need it.

public class Program {
  private static Object locker1 = new Object();
  private static Object locker1 = new Object();
  public void doSomething1() {
    ...
    synchronized(locker1) {
      ......... //do something protected;
    }
    ....
  }
  public void doSomething2() {
    ...
    synchronized(locker2) {
      ......... //do something protected;
    }
    ....
  }
}

The synchronized block can only be executed after a thread has acquired the lock for the object or class referenced, for example the "locker1" or "locker2" in above code, in the synchronized statement.

The above code shows that synchronized block can be holding different object monitors. Maybe it's necessary to protect doSomething1() method and doSomething2() method from multiple threads, but it's fine if one thread is in the doSomething1() method and another is in the doSomething2() method. But the synchronized method can not do it.

A synchronized method synchronizes on the object instance or the class. A thread only executes a synchronized method after it has acquired the lock for the method's object or class.

• static synchronized methods synchronize on the class object. If one thread is executing a static synchronized method, all other threads trying to execute any static synchronized methods, in the same class, will block.
• non-static synchronized methods synchronize on "this" (the instance object). If one thread is executing a synchronized method, all other threads trying to execute any synchronized methods, in the same class, will block.

These are very public monitors, meaning some other thread could synchronize on them for the wrong reason, leading to slowdowns or deadlocks.