Java thread dump: BLOCKED thread without "waiting to lock ..."

Question

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

Answer 1

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.)