Multiple threads hold the same lock?

Question

  "dashboardRefreshContainer-8" - Thread t@1384
   java.lang.Thread.State: RUNNABLE
    at sun.util.calendar.ZoneInfo.getLastRule(ZoneInfo.java:638)
    - locked <4d70153e> (a sun.util.calendar.ZoneInfo)
    at sun.util.calendar.ZoneInfo.getOffsets(ZoneInfo.java:275)
    at sun.util.calendar.ZoneInfo.getOffsets(ZoneInfo.java:225)
    at java.util.GregorianCalendar.computeFields(GregorianCalendar.java:2024)
    at java.util.GregorianCalendar.computeFields(GregorianCalendar.java:1996)
    at java.util.Calendar.setTimeInMillis(Calendar.java:1109)
    at java.util.Calendar.setTime(Calendar.java:1075)

"TP-Processor38" - Thread t@158
   java.lang.Thread.State: RUNNABLE
    at sun.util.calendar.ZoneInfo.getLastRule(ZoneInfo.java:638)
    - locked <4d70153e> (a sun.util.calendar.ZoneInfo)
    at sun.util.calendar.ZoneInfo.getOffsets(ZoneInfo.java:275)
    at sun.util.calendar.ZoneInfo.getOffsets(ZoneInfo.java:225)
    at java.util.GregorianCalendar.computeFields(GregorianCalendar.java:2024)
    at java.util.GregorianCalendar.computeFields(GregorianCalendar.java:1996)
    at java.util.Calendar.setTimeInMillis(Calendar.java:1109)
    at java.util.Calendar.setTime(Calendar.java:1075)

The threads are both Runnable, and they hold the same lock. Can both threads lock the same address while they are both RUNNABLE? Is that a JRE bug?

What are we looking at? Please add more context to the question. — Paul Grime, Apr 27 '13 at 08:26
What JVM is this? (Thread dump formats are not standardized, are they?) — meriton, Apr 27 '13 at 09:14

nosid · Accepted Answer · 2013-04-27T09:59:43.087

11

The problem exists only in the thread dump. In fact, at any point in time, the lock is held by only one thread. However, the thread dump shows two different threads with the same lock, because it is not atomic.

The behavior can easily reproduced with the following program:

public class Test {
    public static void main(String[] args) {
        Runnable runnable = new Runnable() {
            public void run() {
                for (;;) {
                    synchronized (this) { }
                }
            }
        };
        new Thread(runnable).start();
        new Thread(runnable).start();
    }
}

edited Apr 27 '13 at 09:59

answered Apr 27 '13 at 09:19

nosid

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I used this as a test case to file a bug with Oracle: https://bugs.openjdk.java.net/browse/JDK-8036823 – rxg Apr 07 '14 at 12:04

Multiple threads hold the same lock?

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