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<thread> runningthreads = new arraylist<thread>(); 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<thread> runningthreads = new arraylist<thread>(); 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<string> 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<string> 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.本文章为转载内容，我们尊重原作者对文章享有的著作权。如有内容错误或侵权问题，欢迎原作者联系我们进行内容更正或删除文章。
