为什么synced关键字每次都不创建监视器输入

Question

为什么synced关键字每次使用时都不会在字节码级别创建监视器输入？

Answer 1

The synchronized keyword can be used in two ways:

1. When you use synchronized(obj) inside the body of a function, the compiler will emit monitorenter / monitorexit bytecodes for the relevant monitor.

2. If the entire method is declared synchronized , in the bytecodes the method will be marked as ACC_SYNCHRONIZED . The JVM will implicitly enter and exit the monitor when entering/exiting the method. No monitorenter / monitorexit bytecodes are emitted, nor indeed required.

Consider the following two methods:

public class Sync {

    public void f() {
        synchronized (this) {
        }
    }

    public synchronized void g() {
    }

}

They compile to:

public void f();
  Code:
   0:   aload_0
   1:   dup
   2:   monitorenter
   3:   monitorexit
   4:   return

public synchronized void g();
  Code:
   0:   return

As you can see, g() is still marked as synchronized in the bytecodes, so the JVM knows what to do.

Disclaimer: This is what my compiler does. It seems possible that a different compiler might chose to emit monitorenter / monitorexit instead of using ACC_SYNCHRONIZED . Whether any existing compiler does that, I don't know.

Answer 2

synchronized keyword does not create monitor

The mutex or monitor is the object itself. In Java the synchronized keyword is implicit locking ie it uses the objects iternal lock for sunchronization