线程重用——线程池的基本原理

为简单起见，线程池中只有一个线程：

package com.xs.concurrent;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;

public class ThreadPool {
	
	private final BlockingQueue blockingQueue = new LinkedBlockingQueue();
	
	private final Thread thread;
	
	public ThreadPool() {
		thread = ThreadFactory.newThread(blockingQueue);
		thread.start();
	}
	
	public void execute(Runnable runnable){
		try {
			blockingQueue.put(runnable);
		} catch (InterruptedException e) {
			throw new RuntimeException(e);
		}
	}
	
	public static void main(String[] args) throws Exception {
		ThreadPool pool = new ThreadPool();
		Runnable runnable = new Runnable() {
			
			@Override
			public void run() {
				System.out.println("Hello world!");
			}
		};
		for (int i = 0; i < 10; i++) {
			pool.execute(runnable);
			Thread.sleep(1000);
		}
	}
}

ThreadFactory：

package com.xs.concurrent;

import java.util.concurrent.BlockingQueue;

public class ThreadFactory {
	
	public static Thread newThread(final BlockingQueue tasks){
		Thread t = new Thread(new Runnable() {
			
			@Override
			public void run() {
				for (;;) {
					try {
						Runnable task = tasks.take(); // 阻塞方法，直到取到任务为止
						task.run();
					} catch (InterruptedException e) {
						throw new RuntimeException(e);
					}
				}
			}
		});
		return t;
	}
	
	private ThreadFactory(){}
}

Thread.start()不能重复调用，所以要重用Thread，就不能让Thread执行完一个任务后终止，因此就必须阻塞Thread.run()方法，让该方法不停地从任务队列中获取任务并执行。

下面看看JDK的源码。

ThreadPoolExecutor.execute()方法：

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
            if (runState == RUNNING && workQueue.offer(command)) {
                if (runState != RUNNING || poolSize == 0)
                    ensureQueuedTaskHandled(command);
            }
            else if (!addIfUnderMaximumPoolSize(command))
                reject(command); // is shutdown or saturated
        }
    }

addIfUnderCorePoolSize()：

private boolean addIfUnderCorePoolSize(Runnable firstTask) {
        Thread t = null;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (poolSize < corePoolSize && runState == RUNNING)
                t = addThread(firstTask);
        } finally {
            mainLock.unlock();
        }
        if (t == null)
            return false;
        t.start();
        return true;
    }

这个方法启动了一个线程。

addThread()：

private Thread addThread(Runnable firstTask) {
        Worker w = new Worker(firstTask);
        Thread t = threadFactory.newThread(w);
        if (t != null) {
            w.thread = t;
            workers.add(w);
            int nt = ++poolSize;
            if (nt > largestPoolSize)
                largestPoolSize = nt;
        }
        return t;
    }

这个方法将任务交给了Worker。Worker是ThreadPoolExecutor的内部类。

Worker.run()：

public void run() {
            try {
                Runnable task = firstTask;
                firstTask = null;
                while (task != null || (task = getTask()) != null) {
                    runTask(task); // 执行任务
                    task = null;
                }
            } finally {
                workerDone(this);
            }
        }

getTask()方法属于ThreadPoolExecutor类：

Runnable getTask() {
        for (;;) {
            try {
                int state = runState;
                if (state > SHUTDOWN)
                    return null;
                Runnable r;
                if (state == SHUTDOWN)  // Help drain queue
                    r = workQueue.poll();
                else if (poolSize > corePoolSize || allowCoreThreadTimeOut)
                    r = workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS);
                else
                    r = workQueue.take();
                if (r != null)
                    return r;
                if (workerCanExit()) {
                    if (runState >= SHUTDOWN) // Wake up others
                        interruptIdleWorkers();
                    return null;
                }
                // Else retry
            } catch (InterruptedException ie) {
                // On interruption, re-check runState
            }
        }

该方法不停地从工作队列中获取任务。