Question

Why synchronized keyword does not create monitor enter at byte code level every time I use it?

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.