Java: waiting on synchronized block, who goes first?

Question

This question is inspired by this other question.

If multiple threads are waiting on a synchronized block, and the lock becomes available, who goes firs

Answer 1

for your second Question

one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. A thread waits on an object's monitor by calling one of the wait methods.

From http://download.oracle.com/javase/1.4.2/docs/api/java/lang/Object.html#notify()

Answer 2

Someone else mentioned the availability of fair locks. If you really care who goes first, then you may have a real-time problem. In that case, you can make use of RTSJ, wherein the ordering and other semantics of lock acquisition is specified. The specifics are available in the RTSJ Spec under Synchronization. Quoting from the rationale section:

Java's rules for synchronized code provide a means for mutual exclusion but do not prevent unbounded priority inversions and thus are insufficient for real-time applications. This specification strengthens the semantics for synchronized code by mandating priority inversion control, in particular by furnishing classes for priority inheritance and priority ceiling emulation. Priority inheritance is more widely implemented in real-time operating systems and thus is required and is the initial default mechanism in this specification.

Answer 3

It depends on thread priority and thread scheduling algorithm and also the lock on the synchronized block is not "fair". This means that if there are 2 waiting threads with the same priority and the first thread waited more than the second thread that doesn't necessarily mean that the first thread will be executed first.

Answer 4

According to this guy: http://tutorials.jenkov.com/java-concurrency/starvation-and-fairness.html

Java issues no guarantees about the sequence. So I guess it is not based on thread priority

I'll try to look further for an explanation on how Java actually decides who goes first.