java multithreaded application: getting thread blocked time

Question

I am calling threadInfo.getBlockedCount() and getBlockedTime() just before the worker threads die. I get a blocked count of 1, but a blocked time of 0. Does this mean that the thread was blocked but it the blocked time was less than a millisecond?

If the above is true, is there another way to get accurate time for which a thread was blocked?

Answer 1

Can test it out, with something like:

import java.lang.management.ManagementFactory;
import java.lang.management.ThreadInfo;
import java.lang.management.ThreadMXBean;

public class BlockedTimeMain {
    public static void main(String[] _) throws InterruptedException  {
        ThreadMXBean mbean = ManagementFactory.getThreadMXBean();
        mbean.setThreadContentionMonitoringEnabled(true);
        final Object lock = new Object();

        Thread t = new Thread("Foo") {
            @Override public void run() {
                // This will block forever
                synchronized(lock) {
                    // Will never get here
                    System.out.println("Got the lock from " + Thread.currentThread());
                }
            }
        };
        synchronized(lock) {
            t.start();
            for (;;) {
                ThreadInfo[] tis = mbean.getThreadInfo(new long[]{t.getId()}, true, true);
                ThreadInfo ti = tis[0];

                if (ti.getThreadId() != t.getId())
                    throw new AssertionError("Unexpected " + t.getId() + " vs " + tis[0].getThreadId());

                System.out.println(t + " " + ti.getThreadState() 
                        + ": blockedTime=" + ti.getBlockedTime() + "/" + ti.getBlockedCount() 
                        + ", waitTime" + ti.getWaitedTime() + "/" + ti.getWaitedCount());
                Thread.sleep(1000);
            }
        }
    }
}

Sample Output:

Thread[Foo,5,main] BLOCKED: blockedTime=2/1, waitTime0/0
Thread[Foo,5,main] BLOCKED: blockedTime=1007/1, waitTime0/0
Thread[Foo,5,main] BLOCKED: blockedTime=2012/1, waitTime0/0
Thread[Foo,5,main] BLOCKED: blockedTime=3016/1, waitTime0/0
Thread[Foo,5,main] BLOCKED: blockedTime=4021/1, waitTime0/0
Thread[Foo,5,main] BLOCKED: blockedTime=5025/1, waitTime0/0
Thread[Foo,5,main] BLOCKED: blockedTime=6028/1, waitTime0/0
Thread[Foo,5,main] BLOCKED: blockedTime=7032/1, waitTime0/0
Thread[Foo,5,main] BLOCKED: blockedTime=8035/1, waitTime0/0