Thread.getId() global uniqueness question

Question

If multiple Java applications are running on a system, is each Thread ID unique relative to all other Java threads, regardless of what application they are running in?

J

Answer 1

Well, let me check the source.

In the Thread's init method (which is called by every constructor):

/* Set thread ID */
tid = nextThreadID();

In nextThreadID():

private static synchronized long nextThreadID() {
    return ++threadSeqNumber;
}

And:

/* For generating thread ID */
private static long threadSeqNumber;

It is never set, and thus defaults to 0.

So apparently thread ID numbers always start at 0 and increment by 1. In other words, the answer to your question is that they are not globally unique.

Answer 2

According to the JDK source, a thread ID is unique in a given JVM - in fact, it's simply implemented as a running sequence.

Here's the nextThreadID() method from 1.6.0_10:

private static synchronized long nextThreadID() {
    return ++threadSeqNumber;
}

(there's probably actually a long overflow bug in there, presumably it's never actually happened)