Java thread dump: BLOCKED thread without “waiting to lock …”

javajstackthread-dump

I'm having difficulties in understanding the thread dump I got from jstack for a Spring MVC web application running on Tomcat 6 (java 1.6.0_22, Linux).

I see blocking threads (that cause other threads to wait) which are blocked themselves, however the thread dump doesn't tell me why or for which monitor they are waiting.

Example:

"TP-Processor75" daemon prio=10 tid=0x00007f3e88448800 nid=0x56f5 waiting for monitor entry [0x00000000472bc000]
    java.lang.Thread.State: BLOCKED (on object monitor)
        at java.lang.Class.initAnnotationsIfNecessary(Class.java:3067)
        - locked <0x00007f3e9a0b3830> (a java.lang.Class for org.catapultframework.resource.ResourceObject)
        at java.lang.Class.getAnnotation(Class.java:3029)
        ...

I.e. I am missing the "waiting to lock …" line in the stack trace. Apparently the thread locks a Class object, but I don't see why the thread itself is blocked.

The thread-dump does not contain any hints for deadlocks.

What can I do to identify the locking monitor?

Thanks,
Oliver

Best Answer

Apparently the situation where we observed these kinds of blocked threads were related to heavy memory consumption and therefore massive garbage collection.

This question Java blocking issue: Why would JVM block threads in many different classes/methods? describes a similar situation, so I believe these threads were simply blocked by the garbage collector.

(Anyway, after solving the memory issue this problem with the blocking threads was gone.)

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