Java多线程--让主线程等待所有子线程执行完毕【转】

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Java多线程--让主线程等待所有子线程执行完毕【转】已有 73 次阅读 2010-06-08 15:45 http://3ccoder.javaeye.com/blog/581476 朋友让我帮忙写个程序从文本文档中导入数据到oracle数据库中,技术上没有什么难度,文档的格式都是固定的只要对应数据库中的字段解析就行了,关键在于性能。数据量很大百万条记录,因此考虑到要用多线程并发执行,在写的过程中又遇到问题,我想统计所有子进程执行完毕总共的耗时,在第一个子进程创建前记录当前时间用System.currentTimeMillis()在最后一个子进程结束后记录当前时间,两次一减得到的时间差即为总共的用时,代码如下 Java代码long tStart = System.currentTimeMillis(); System.out.println(Thread.currentThread().getName() + "开始");//打印开始标记 for (int ii = 0; ii < threadNum; ii++) {//开threadNum个线程 Runnable r = new Runnable(){ @Override public void run(){ System.out.println(Thread.currentThread().getName() + "开始"); //做一些事情... ... System.out.println(Thread.currentThread().getName() + "结束."); } } Thread t = new Thread(r); t.start(); } System.out.println(Thread.currentThread().getName() + "结束.");//打印结束标记 long tEnd = System.currentTimeMillis(); System.out.println("总共用时:"+ (tEnd - tStart) + "millions"); long tStart = System.currentTimeMillis(); System.out.println(Thread.currentThread().getName() + "开始");//打印开始标记 for (int ii = 0; ii < threadNum; ii++) {//开threadNum个线程 Runnable r = new Runnable(){ @Override public void run(){ System.out.println(Thread.currentThread().getName() + "开始"); //做一些事情... ... System.out.println(Thread.currentThread().getName() + "结束."); } } Thread t = new Thread(r); t.start(); } System.out.println(Thread.currentThread().getName() + "结束.");//打印结束标记 long tEnd = System.currentTimeMillis(); System.out.println("总共用时:"+ (tEnd - tStart) + "millions"); 结果是几乎在for循环结束的瞬间就执行了主线程打印总共用时的语句,原因是所有的子线程是并发执行的,它们运行时主线程也在运行,这就引出了一个问题即本文标题如何"让主线程等待所有子线程执行完毕"。试过在每个子线程开始后加上t.join(),结果是所有线程都顺序执行,这就失去了并发的意义了,显然不是我想要的。网上Google了很久也没有找到解决方案,难道就没有人遇到过这种需求吗?还是这个问题太简单了?无耐只得自己想办法了... 最后我的解决办法是,自定义一个ImportThread类继承自java.lang.Thread,重载run()方法,用一个List属性保存所有产生的线程,这样只要判断这个List是否为空就知道还有没有子线程没有执行完了,类代码如下： Java代码public class ImportThread extends Thread { private static List runningThreads = new ArrayList(); public ImportThread() { } @Override public void run() { regist(this);//线程开始时注册 System.out.println(Thread.currentThread().getName() + "开始...");//打印开始标记 //做一些事情... ... unRegist(this);//线程结束时取消注册 System.out.println(Thread.currentThread().getName() + "结束.");//打印结束标记 } public void regist(Thread t){ synchronized(runningThreads){ runningThreads.add(t); } } public void unRegist(Thread t){ synchronized(runningThreads){ runningThreads.remove(t); } } public static boolean hasThreadRunning() { return (runningThreads.size() > 0);//通过判断runningThreads是否为空就能知道是否还有线程未执行完 } } public class ImportThread extends Thread { private static List runningThreads = new ArrayList(); public ImportThread() { } @Override public void run() { regist(this);//线程开始时注册 System.out.println(Thread.currentThread().getName() + "开始...");//打印开始标记 //做一些事情... ... unRegist(this);//线程结束时取消注册 System.out.println(Thread.currentThread().getName() + "结束.");//打印结束标记 } public void regist(Thread t){ synchronized(runningThreads){ runningThreads.add(t); } } public void unRegist(Thread t){ synchronized(runningThreads){ runningThreads.remove(t); } } public static boolean hasThreadRunning() { return (runningThreads.size() > 0);//通过判断runningThreads是否为空就能知道是否还有线程未执行完 } } 主线程中代码： Java代码long tStart = System.currentTimeMillis(); System.out.println(Thread.currentThread().getName() + "开始");//打印开始标记 for (int ii = 0; ii < threadNum; ii++) {//开threadNum个线程 Thread t = new ImportThread(); t.start(); } while(true){//等待所有子线程执行完 if(!ImportThread.hasThreadRunning()){ break; } Thread.sleep(500); } System.out.println(Thread.currentThread().getName() + "结束.");//打印结束标记 long tEnd = System.currentTimeMillis(); System.out.println("总共用时:"+ (tEnd - tStart) + "millions"); long tStart = System.currentTimeMillis(); System.out.println(Thread.currentThread().getName() + "开始");//打印开始标记 for (int ii = 0; ii < threadNum; ii++) {//开threadNum个线程 Thread t = new ImportThread(); t.start(); } while(true){//等待所有子线程执行完 if(!ImportThread.hasThreadRunning()){ break; } Thread.sleep(500); } System.out.println(Thread.currentThread().getName() + "结束.");//打印结束标记 long tEnd = System.currentTimeMillis(); System.out.println("总共用时:"+ (tEnd - tStart) + "millions"); 打印的结果是： main开始 Thread-1开始... Thread-5开始... Thread-0开始... Thread-2开始... Thread-3开始... Thread-4开始... Thread-5结束. Thread-4结束. Thread-2结束. Thread-0结束. Thread-3结束. Thread-1结束. main结束. 总共用时:20860millions 可以看到main线程是等所有子线程全部执行完后才开始执行的。===============================================以下为第二次编辑=============================================== 上面的方法有一个隐患:如果线程1开始并且结束了,而其他线程还没有开始此时runningThreads的size也为0,主线程会以为所有线程都执行完了。解决办法是用一个非简单类型的计数器来取代List型的runningThreads,并且在线程创建之前就应该设定好计数器的值。 MyCountDown类 Java代码public class MyCountDown { private int count; public MyCountDown(int count){ this.count = count; } public synchronized void countDown(){ count--; } public synchronized boolean hasNext(){ return (count > 0); } public int getCount() { return count; } public void setCount(int count) { this.count = count; } } public class MyCountDown { private int count; public MyCountDown(int count){ this.count = count; } public synchronized void countDown(){ count--; } public synchronized boolean hasNext(){ return (count > 0); } public int getCount() { return count; } public void setCount(int count) { this.count = count; } } ImportThread类 Java代码public class ImportThread extends Thread { private MyCountDown c; public ImportThread(MyCountDown c) { this.c = c; } @Override public void run() { System.out.println(Thread.currentThread().getName() + "开始...");//打印开始标记 //Do something c.countDown();//计时器减1 System.out.println(Thread.currentThread().getName() + "结束. 还有" + c.getCount() + " 个线程");//打印结束标记 } } public class ImportThread extends Thread { private MyCountDown c; public ImportThread(MyCountDown c) { this.c = c; } @Override public void run() { System.out.println(Thread.currentThread().getName() + "开始...");//打印开始标记 //Do something c.countDown();//计时器减1 System.out.println(Thread.currentThread().getName() + "结束. 还有" + c.getCount() + " 个线程");//打印结束标记 } } 主线程中 Java代码System.out.println(Thread.currentThread().getName() + "开始");//打印开始标记 MyCountDown c = new MyCountDown(threadNum);//初始化countDown for (int ii = 0; ii < threadNum; ii++) {//开threadNum个线程 Thread t = new ImportThread(c); t.start(); } while(true){//等待所有子线程执行完 if(!c.hasNext()) break; } System.out.println(Thread.currentThread().getName() + "结束.");//打印结束标记 System.out.println(Thread.currentThread().getName() + "开始");//打印开始标记 MyCountDown c = new MyCountDown(threadNum);//初始化countDown for (int ii = 0; ii < threadNum; ii++) {//开threadNum个线程 Thread t = new ImportThread(c); t.start(); } while(true){//等待所有子线程执行完 if(!c.hasNext()) break; } System.out.println(Thread.currentThread().getName() + "结束.");//打印结束标记打印结果: main开始 Thread-2开始... Thread-1开始... Thread-0开始... Thread-3开始... Thread-5开始... Thread-4开始... Thread-5结束. 还有5 个线程 Thread-1结束. 还有4 个线程 Thread-4结束. 还有3 个线程 Thread-2结束. 还有2 个线程 Thread-3结束. 还有1 个线程 Thread-0结束. 还有0 个线程 main结束.更简单的方法:使用java.util.concurrent.CountDownLatch代替MyCountDown,用await()方法代替while(true){...} ImportThread类 Java代码public class ImportThread extends Thread { private CountDownLatch threadsSignal; public ImportThread(CountDownLatch threadsSignal) { this.threadsSignal = threadsSignal; } @Override public void run() { System.out.println(Thread.currentThread().getName() + "开始..."); //Do somethings threadsSignal.countDown();//线程结束时计数器减1 System.out.println(Thread.currentThread().getName() + "结束. 还有" + threadsSignal.getCount() + " 个线程"); } } public class ImportThread extends Thread { private CountDownLatch threadsSignal; public ImportThread(CountDownLatch threadsSignal) { this.threadsSignal = threadsSignal; } @Override public void run() { System.out.println(Thread.currentThread().getName() + "开始..."); //Do somethings threadsSignal.countDown();//线程结束时计数器减1 System.out.println(Thread.currentThread().getName() + "结束. 还有" + threadsSignal.getCount() + " 个线程"); } } 主线程中 Java代码CountDownLatch threadSignal = new CountDownLatch(threadNum);//初始化countDown for (int ii = 0; ii < threadNum; ii++) {//开threadNum个线程 final Iterator itt = it.get(ii); Thread t = new ImportThread(itt,sql,threadSignal); t.start(); } threadSignal.await();//等待所有子线程执行完 System.out.println(Thread.currentThread().getName() + "结束.");//打印结束标记 CountDownLatch threadSignal = new CountDownLatch(threadNum);//初始化countDown for (int ii = 0; ii < threadNum; ii++) {//开threadNum个线程 final Iterator itt = it.get(ii); Thread t = new ImportThread(itt,sql,threadSignal); t.start(); } threadSignal.await();//等待所有子线程执行完 System.out.println(Thread.currentThread().getName() + "结束.");//打印结束标记打印结果: main开始 Thread-1开始... Thread-0开始... Thread-2开始... Thread-3开始... Thread-4开始... Thread-5开始... Thread-0结束. 还有5 个线程 Thread-1结束. 还有4 个线程 Thread-4结束. 还有3 个线程 Thread-2结束. 还有2 个线程 Thread-5结束. 还有1 个线程 Thread-3结束. 还有0 个线程 main结束.