Java之多线程之线程池之线程重复使用

一、问题背景

在使用多线程时，如果要开启一个任务，则就需要新建一个线程。
线程也是对象，那么是否可以不用新建，而使用原来的呢？

试试下面的方法：

        Thread incT = new Thread(new Inc(c));
        Thread decT = new Thread(new Dec(c));
        
        
        for(int i= 0; i < 200; i++){
            incT.start();
            System.out.println("incT:" + incT.getName());
            decT.start();
            System.out.println("decT:" + decT.getName());
        }

结果报错了：
Exception in thread "main" java.lang.IllegalThreadStateException

原因：
线程在执行完 start() 方法后，就会自动销毁。

二、如何解决线程重用

1、分析
每一个 Thread 的类都有一个 start 方法。
Thread 的 start 方法是这样描述的：

Causes this thread to begin execution;
the Java Virtual Machine calls the run method of this thread.
启动线程。Java虚拟机会调用该类的 run 方法。

那么该类的 run() 方法中就是调用了 Runnable 对象的 run() 方法。

2、解决
我们可以继承重写 Thread 类，在其 start 方法中添加不断循环调用传递过来的 Runnable 对象。
这就是线程池的实现原理。循环方法中不断获取 Runnable 是用 Queue 实现的，在获取下一个 Runnable 之前可以是阻塞的。


三、问题出处

Question: 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?


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Answer:

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.


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Thread pool threads are basically running loops that pull submitted tasks off of a queue. The threads do not stop executing when they service a task, they just wait for the next one to be submitted to the queue. They never get 'rerun' as asked in the question, as they are just constantly running.

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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.

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:         }

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It's not actually unnecessary - the (strange) test for task!=null in the loop makes it necessary to prevent continually processing the same task. Even if the loop were more conventional nulling task would be good because otherwise if getTask() blocks for a long time, the GC of task would otherwise be delayed for the same length of time.








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http://stackoverflow.com/questions/2324030/java-thread-reuse