Re: Unsafe: removing the monitorEnter/monitorExit/tryMonitorEnter methods

[Paul Sandoz]

On Feb 27, 2014, at 12:52 PM, Peter Levart <peter.lev...@gmail.com> wrote:
> 
> Ok, then what about the following:
> 
>    static boolean isLocked(final Object monitor, final long millis) throws 
> InterruptedException {
>        return new Thread() {
>            volatile boolean unlocked;
> 
>            @Override
>            public void run() {
>                synchronized (monitor) { unlocked = true; }
>            }
> 
>            boolean isLocked() throws InterruptedException {
>                start();
>                join(millis);
>                return !unlocked;
>            }
>        }.isLocked();
>    }
> 
> 
> // and usage it like:
> 
>            while (!isLocked(s, 100L)) {
>                Thread.sleep(100L);
>            }
> 
> 
> This will allocate about 10 Thread objects per second until the other thread 
> finally reaches the synchronized read() method in BufferedInputStream...
> 

I like it.

I do have a slight preference for Mandy's solution, see below, but i am not 
sure we can guarantee it is supported on all test platforms.

Paul.

        try {
            while (!isMonitorLockHeld(monitor)) {
                Thread.sleep(1);
            }
        } catch (UnsupportedOperationException e) {
            // Back up plan?
            Thread.sleep(TimeUnit.SECONDS.toMillis(1));
        }

    /**
     * Returns {@code true} if and only if the a thread holds the
     * monitor lock on the specified object.
     *
     * @throws java.lang.UnsupportedOperationException if monitoring of
     * object monitor use is not supported
     */
    public static boolean isMonitorLockHeld(Object o) {
        int oihc = System.identityHashCode(o);
        ThreadMXBean tmxbean = ManagementFactory.getThreadMXBean();
        ThreadInfo[] tinfos = tmxbean.dumpAllThreads(true, false);

        for (ThreadInfo ti : tinfos) {
            for (MonitorInfo mi : ti.getLockedMonitors()) {
                if (mi.getIdentityHashCode() == oihc)
                    return true;
            }
        }
        return false;
    }