java thread reuse

I have always read that creating threads is expensive.
I also know that you cannot rerun a thread.

I see in the doc of Executors class:

Creates a thread pool that creates new threads as needed, but will reuse previously constructed threads when they are available.

Mind the word 'reuse'.

How do thread pools 'reuse' threads?

I think I understood what is confuzzabling you so here's my longer answer: the terminology is a tiny bit misleading (obviously, or you wouldn't ask that question specifically putting the emphasis on 'reuse'):

How do thread pools 'reuse' threads?

What is happening is that a single thread can be used to process several tasks (typically passed as Runnable, but this depend on your 'executor' framework: the default executors accepts Runnable, but you could write your own "executor" / thread-pool accepting something more complex than a Runnable [like, say, a CancellableRunnable]).

Now in the default ExecutorService implementation if a thread is somehow terminated while still in use, it is automatically replaced with a new thread, but this is not the 'reuse' they're talking about. There is no "reuse" in this case.

So it is true that you cannot call start() on a Java Thread twice but you can pass as many Runnable as you want to an executor and each Runnable's run() method shall be called once.

You can pass 30 Runnable to 5 Java Thread and each worker thread may be calling, for example, run() 6 times (practically there's not guarantee that you'll be executing exactly 6 Runnable per Thread but that is a detail).

In this example start() would have been called 6 times. Each one these 6 start() will call exactly once the run() method of each Thread:

From Thread.start() Javadoc:

 * Causes this thread to begin execution; the Java Virtual Machine 
 * calls the <code>run</code> method of this thread.

BUT then inside each Thread's run() method Runnable shall be dequeued and the run() method of each Runnable is going to be called. So each thread can process several Runnable. That's what they refer to by "thread reuse".

One way to do your own thread pool is to use a blocking queue on to which you enqueue runnables and have each of your thread, once it's done processing the run() method of a Runnable, dequeue the next Runnable (or block) and run its run() method, then rinse and repeat.

I guess part of the confusion (and it is a bit confusing) comes from the fact that a Thread takes a Runnable and upon calling start() the Runnable 's run() method is called while the default thread pools also take Runnable.

The run method of threads in a thread pool does not consist only of running a single task. The run method of a thread in a thread pool contains a loop. It pulls a task off of a queue, executes the task (which returns back to the loop when it is complete), and then gets the next task. The run method doesn't complete until the thread is no longer needed.

Edited to add:

Here is the run method of the Worker inner class in ThreadPoolExecutor.

696:         /**
697:          * Main run loop
698:          */
699:         public void run() {
700:             try {
701:                 Runnable task = firstTask;
702:                 firstTask = null;
703:                 while (task != null || (task = getTask()) != null) {
704:                     runTask(task);
705:                     task = null; // unnecessary but can help GC
706:                 }
707:             } finally {
708:                 workerDone(this);
709:             }
710:         }

The thread pool consists of a number of fixed worker threads that can take tasks from an internal task queue. So if one task ends, the thread does not end but waits for the next task. If you abort a thread, it is automatically replaced.

Look at the documentation for more details.