Which object's monitor does a synchronized method acquire?

Question

from the Java Tutorial for concurrency programming:
" When a thread invokes a synchronized method, it automatically acquires the intrinsic lock _for that method's object_ and releases it when the method returns. The lock release occurs even if the return was caused by an uncaught exception. "
what exactly does this mean?
do synchronized methods acquire the monitors for objects of type: java.lang.reflection.Method
please consider this code:
public class Foo {
  private int counter = 0;
  public synchronized void incriment() { counter++; }
  public synchronized void decriment() { counter--; }
Foo f = new Foo();
Class[] sig = new Class[0];
Method m = f.getClass().getMethod("incriment", sig);
// ok. so "m" is the relevant method object.
f.incriment(); // <-- is the monitor for "m" ,
                      // or the monitor for "f", acquired?
.......my reading of the Concurrency Tutorial is that synchronized methods use the monitors of java.lang.reflection.Method objects?
and thus, Foo is not thread safe, right?
however, this simple change makes Foo thread-safe?
public class Foo {
  private volatile int counter = 0; // "volatile"
  public void incriment() { counter++; }
  public void decriment() { counter--; }
}thanks.
Edited by: kogose on Feb 23, 2009 7:13 PM

Answer 1

tensorfield wrote:
jverd wrote:
tensorfield wrote:
kogose wrote:
what exactly does this mean?It means you're complicating things.
If a method is synchronized, it is. You don't need to go beyond that. The method is synchronized.Not true. You have to know what it means for a method to be synchronized. Often people come in with the erroneous impression that it somehow prevents you from using or accessing the object in any other thread.It's very simple. If a synchronized method is called at the same time from many threads only one call will be executed at a time. The calls will be lined up and performed one after the other in sequence.
AND because synchronization is on a per object basis, when one synchronized method is being called from one thread, all synchronized methods of that same object are blocked for calling from other threads.
Simple as that.No, it's not that simple, and as stated, that is not correct. In particular, you didn't mention that for an instance method, all the various threads have to be trying to call instance methods on the same object in order for execution to be sequential.
You really can't understand Java's syncing without understanding how it relates to locks, and what it means for a method to be synchronized in terms of which lock it acquires.
Edited by: jverd on Feb 25, 2009 2:47 PM