How more than one thread is "locked" on the same object (as shown in the thread dump)

In this context, blocking means that your executable Java code has entered a block synchronous

, but has not yet exited that block.

As your thread dump shows, you are calling wait()

which internally unlocks the monitor associated with the block synchronous

. However, since you are blocking the wait and have not exited the synchronous block, the thread dump shows locked

. Thus, it is possible that multiple threads are displayed in the thread dump locked

even though the main monitor is unlocked.

This can be easily demonstrated with a simple test:

public class TestMonitor {

    synchronized public void lockAndWait() {
        try {
            wait();
        } catch ( InterruptedException ex ) {
            // Stifle
        }
    }

    public static void main( String args[] ) {
        TestMonitor tm = new TestMonitor();
        tm.lockAndWait();
    }
}

      

        
        
        
      

    

which dumps the following dump thread on startup:

"main" prio=10 tid=0x00007f86c4008000 nid=0x5d35 in Object.wait() [0x00007f86cbae2000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000759055df8> (a TestMonitor) at java.lang.Object.wait(Object.java:503) at TestMonitor.lockAndWait(TestMonitor.java:5) - locked <0x0000000759055df8> (a TestMonitor) at TestMonitor.main(TestMonitor.java:13

Please note that the monitor is still in locked

, even though it is in wait

.

UPDATE

If the single thread case is not convincing, you can run the above example, slightly modified, in which case you will see multiple threads locked

on the same monitor in the thread dump:

public static void main( String args[] ) {
    final TestMonitor tm = new TestMonitor();

    Thread thread1 = new Thread( new Runnable() { public void run() { tm.lockAndWait(); } } );
    Thread thread2 = new Thread( new Runnable() { public void run() { tm.lockAndWait(); } } );
    thread1.start();
    thread2.start();
}