How can one Java thread check the state of another one, e.g. whether the other one is blocked?

Question

This is a question asked to one of my friends during an interview.

How do you know whether a thread is blocked inside a synchronized method, from another thread?

Can anybody please explain this using an example?

Answer 1

Using Thread.getState():

Thread.State state = getThreadInQuestion().getState();
if(state == Thread.State.BLOCKED) {
    System.out.println("Blocked");
} else {
    System.out.println("Not blocked");
}

Outside of a VM, you can use the jstack tool to get full thread information for every thread, or connect to JMX and explore the Thread MBeans.

Answer 2

My short answer would be "no, not reliably".

Somebody mentioned checking for getState() == Thread.State.BLOCKED. However, by the time you get the answer it may already be obsolete if the blocked thread is waiting on a monitor locked by a third thread, and the monitor gets released just as getState is about to return.