multithreading - What is the difference between a synchronized method and synchronized block in Java?

Question

(This question already has answers here): question from:https://stackoverflow.com/questions/1149928/what-is-the-difference-between-a-synchronized-method-and-synchronized-block-in-j

Answer 1

A synchronized method uses the method receiver as a lock (i.e. this for non static methods, and the enclosing class for static methods). Synchronized blocks uses the expression as a lock.

So the following two methods are equivalent from locking prospective:

synchronized void mymethod() { ... }

void mymethod() {
  synchronized (this) { ... }
}

For static methods, the class will be locked:

class MyClass {
  synchronized static mystatic() { ... }

  static mystaticeq() {
    syncrhonized (MyClass.class) { ... }
  }
}

For synchronized blocks, you can use any non-null object as a lock:

synchronized (mymap) {
  mymap.put(..., ...);
}

(Lock scope)

For synchronized methods, the lock will be held throughout the method scope, while in the synchronized block, the lock is held only during that block scope (otherwise known as critical section). In practice, the JVM is permitted to optimize by removing some operations out of the synchronized block execution if it can prove that it can be done safely.