Java一个简单的线程池实现
线程池代码
import java.util.List; import java.util.Vector; public class ThreadPool { private static ThreadPool instance_ = null; //定义优先级别常数，空闲的线程按照优先级不同分别存放在三个vector中 public static final int LOW_PRIORITY = 0; public static final int NORMAL_PRIORITY = 1; public static final int HIGH_PRIORITY = 2; //保存空闲线程的List，或者说它是"池" private List<PooledThread>[] idleThreads_; private boolean shutDown_ = false; private int threadCreationCounter_; //以创建的线程的个数 private boolean debug_ = false; //是否输出调试信息 //构造函数，因为这个类视作为singleton实现的，因此构造函数为私有 private ThreadPool() { // 产生空闲线程.三个vector分别存放分别处在三个优先级的线程的引用 List[] idleThreads = {new Vector(5), new Vector(5), new Vector(5)}; idleThreads_ = idleThreads; threadCreationCounter_ = 0; } public int getCreatedThreadsCount() { return threadCreationCounter_; } //通过这个函数得到线程池类的实例 public static ThreadPool instance() { if (instance_ == null) instance_ = new ThreadPool(); return instance_; } public boolean isDebug() { return debug_; } //将线程repoolingThread从新放回到池中，这个方式是同步方法。 //这个方法会在多线程的环境中调用，设计这个方法的目的是让工作者线程 //在执行完target中的任务后，调用池类的repool()方法， //将线程自身从新放回到池中。只所以这么做是因为线程池并不能预见到 //工作者线程何时会完成任务。参考PooledThread的相关代码。 protected synchronized void repool(PooledThread repoolingThread) { if (!shutDown_) { if (debug_) { System.out.println("ThreadPool.repool() : repooling "); } switch (repoolingThread.getPriority()) { case Thread.MIN_PRIORITY : { idleThreads_[LOW_PRIORITY].add(repoolingThread); break; } case Thread.NORM_PRIORITY : { idleThreads_[NORMAL_PRIORITY].add(repoolingThread); break; } case Thread.MAX_PRIORITY : { idleThreads_[HIGH_PRIORITY].add(repoolingThread); break; } default : throw new IllegalStateException("Illegal priority found while repooling a Thread!"); } notifyAll();//通知所有的线程 } else { if (debug_) { System.out.println("ThreadPool.repool() : Destroying incoming thread."); } repoolingThread.shutDown();//关闭线程 } if (debug_) { System.out.println("ThreadPool.recycle() : done."); } } public void setDebug(boolean newDebug) { debug_ = newDebug; } //停止池中所有线程 public synchronized void shutdown() { shutDown_ = true; if (debug_) { System.out.println("ThreadPool : shutting down "); } for (int prioIndex = 0; prioIndex <= HIGH_PRIORITY; prioIndex++) { List prioThreads = idleThreads_[prioIndex]; for (int threadIndex = 0; threadIndex < prioThreads.size(); threadIndex++) { PooledThread idleThread = (PooledThread) prioThreads.get(threadIndex); idleThread.shutDown(); } } notifyAll(); if (debug_) { System.out.println("ThreadPool : shutdown done."); } } //以Runnable为target，从池中选择一个优先级为priority的线程创建线程 //并让线程运行。 public synchronized void start(Runnable target, int priority) { PooledThread thread = null; //被选出来执行target的线程 List idleList = idleThreads_[priority]; if (idleList.size() > 0) { //如果池中相应优先级的线程有空闲的，那么从中取出一个 //设置它的target，并唤醒它 //从空闲的线程队列中获取 int lastIndex = idleList.size() - 1; thread = (PooledThread) idleList.get(lastIndex); idleList.remove(lastIndex); thread.setTarget(target); } //池中没有相应优先级的线程 else { threadCreationCounter_++; // 创建新线程， thread = new PooledThread(target, "PooledThread #" + threadCreationCounter_, this); // 新线程放入池中 switch (priority) { case LOW_PRIORITY : { thread.setPriority(Thread.MIN_PRIORITY); break; } case NORMAL_PRIORITY : { thread.setPriority(Thread.NORM_PRIORITY); break; } case HIGH_PRIORITY : { thread.setPriority(Thread.MAX_PRIORITY); break; } default : { thread.setPriority(Thread.NORM_PRIORITY); break; } } //启动这个线程 thread.start(); } } } 工作者线程代码：
public class PooledThread extends Thread { private ThreadPool pool_; // 池中线程需要知道自己所在的池 private Runnable target_; // 线程的任务 private boolean shutDown_ = false; private boolean idle_ = false;//设置是否让线程处于等待状态 private PooledThread() { super(); } private PooledThread(Runnable target) { super(target); //初始化父类 } private PooledThread(Runnable target, String name) { super(target, name); } public PooledThread(Runnable target, String name, ThreadPool pool) { super(name); pool_ = pool; target_ = target; } private PooledThread(String name) { super(name);//初始化父类 } private PooledThread(ThreadGroup group, Runnable target) { super(group, target); } private PooledThread(ThreadGroup group, Runnable target, String name) { super(group, target, name); } private PooledThread(ThreadGroup group, String name) { super(group, name); } public java.lang.Runnable getTarget() { return target_; } public boolean isIdle() { return idle_;//返回当前的状态 } //工作者线程与通常线程不同之处在于run()方法的不同。通常的线程， //完成线程应该执行的代码后，自然退出，线程结束。 //虚拟机在线程结束后收回分配给线程的资源，线程对象被垃圾回收。] //而这在池化的工作者线程中是应该避免的，否则线程池就失去了意义。 //作为可以被放入池中并重新利用的工作者线程，它的run()方法不应该结束， //随意，在随后可以看到的实现中，run()方法执行完target对象的代码后， //就将自身repool()，然后调用wait()方法，使自己睡眠而不是退出循环和run()。 //这就使线程池实现的要点。 public void run() { // 这个循环不能结束，除非池类要求线程结束 // 每一次循环都会执行一次池类分配给的任务target while (!shutDown_) { idle_ = false; if (target_ != null) { target_.run(); // 运行target中的代码 } idle_ = true; try { //线程通知池重新将自己放回到池中 pool_.repool(this); // //进入池中后睡眠，等待被唤醒执行新的任务， //这里是线程池中线程于普通线程的run()不同的地方。 synchronized (this) { wait(); } } catch (InterruptedException ie) { } idle_ = false; } //循环这里不能结束，否则线程结束，资源被VM收回， //就无法起到线程池的作用了 } public synchronized void setTarget(java.lang.Runnable newTarget) {//设置新的target，并唤醒睡眠中的线程 target_ = newTarget; // 新任务 notifyAll(); // 唤醒睡眠的线程 } public synchronized void shutDown() { shutDown_ = true; notifyAll(); } } 测试代码：
public static void main(String[] args) { System.out.println("Testing ThreadPool "); System.out.println("Creating ThreadPool "); ThreadPool pool = ThreadPool.instance(); pool.setDebug(true); class TestRunner implements Runnable { public int count = 0; public void run() { System.out.println("Testrunner sleeping 5 seconds "); //此方法使本线程睡眠5秒 synchronized (this) { try { wait(5000);//等待5秒时间 } catch (InterruptedException ioe) { } } System.out.println("Testrunner leaving "); count++; } } System.out.println("Starting a new thread "); TestRunner runner = new TestRunner(); pool.start(runner, pool.HIGH_PRIORITY); System.out.println("count : " + runner.count); System.out.println("Thread count : " + pool.getCreatedThreadsCount()); pool.shutdown(); } } 结果
Testing ThreadPool 
Creating ThreadPool 
Starting a new thread Testrunner sleeping 5 seconds count : 0Thread count : 1ThreadPool : shutting down ThreadPool : shutdown done.Testrunner leaving ThreadPool.repool() : Destroying incoming thread.ThreadPool.recycle() : done.