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.