The glazier canвЂ™t continue until the carpenterвЂ™s finished. For example, they could have done everything that they usefully can until some other thread has completed some task that would allow them to carry on. For example, they could be waiting for IO, or they might have voluntarily exempted themselves from the pool of runnable threads. Just like real life, really.Д«ut of course, in all this, we have only been talking about threads that are available to run. Threads that have a higher priority will, of course, always take precedence. So what can we say for certain? We can say that threads that are available to be run and that have the same priority will be selected by the native operating system on the basis of some criterion or mixture of criteria that aim to give each thread an equal chance in life. It just depends on the native operating system. Is the queue simply ordered on a first-come, first-served basis? Or are the threads queued on the basis of how much time theyвЂ™ve spent on the processor, with the least-used thread at the head? Or might they be queued on the basis of how long it has been since the last time they were given a chance to run? The Java specification has nothing to say about any of this, as far as I can tell. Now, therefore, we have to worry about how threads are put into the queue. In other words, the scheduler might view all the threads as being in some sort of queue and simply pick the next one in the queue.

For example, the next thread might be picked on a round-robin basis. So now, either because of a thread yielding volutarily or because of a thread exhausting its time slice, we have a situation where the scheduler has to pick another thread to run amongst many, all with the same priority. I discuss each of these below, after dealing with the way that threads are scheduled. Threads stop running voluntarily due to calls to sleep(), wait() or join(). Or it can terminate, of course, but weвЂ™re not concerned with that case here. This latter prevents вЂgreedyвЂ™ threads from hogging the processor and is the way that nearly all current operating systems work. Well, the thread can stop running either voluntarily or it can be pre-empted by a thread of higher priority (which weвЂ™ll ignore for the purposes of this article) or it can come to the end of its time slice. It doesnвЂ™t really matter, because the important question is: вЂњWhat stops it running and when it does stop, how does the scheduler pick from amongst the remaining threads all of which have the same priority?вЂќ WeвЂ™ll not bother about how this first thread gets to run. Native operating system schedulers are anything but simple.ДЄs a first cut, letвЂ™s look at the situation where you have a number of threads all with the same priority. And if it were that simple, I wouldnвЂ™t be writing an article about it!Д«ecause, in fact, what Java does is to hand off these decisions to the native operating system scheduler. The scheduler, when called to make a decision, simply picks the thread with the highest priority and lets it run.

The first thing to note about scheduling threads in Java is that it appears to use a very simple algorithm. Now, before I go any further, I should state quite clearly that this is not my sphere of expertise but I am perfectly capable of using Google and that being the case, the following is a summary of what I found. In particular, they wanted to know how they were scheduled. Somebody asked me this week about Java threads.

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