之前写了一篇关于FutureTask的Blog: http://rainbow702.iteye.com/admin/blogs/2206301
里面的源码如下(不包含之前写好的main方法):
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class="java" name="code">public class Preloader { private final FutureTask<Long> future = new FutureTask<Long>(new Callable<Long>() { @Override public Long call() throws Exception { Thread.currentThread().setName("Thread(3)"); System.out.println(Thread.currentThread().getName() + ": simulate a latency "); try { Thread.sleep(5000); } catch (Exception e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + ": the latency is over"); return Math.round(Math.random() * 1000); } }); private final Thread loader = new Thread(future); public void start() { System.out.println(Thread.currentThread().getName() + ": start the loader"); loader.start(); System.out.println(Thread.currentThread().getName() + ": loader started"); } public Long get() { try { System.out.println(Thread.currentThread().getName() + ": begin to get"); long start = System.currentTimeMillis(); Long result = future.get(); System.out.println(Thread.currentThread().getName() + ": got result: " + result); System.out.println(Thread.currentThread().getName() + ": spent time: " + (System.currentTimeMillis() - start)); return result; } catch (InterruptedException e) { e.printStackTrace(); } catch (ExecutionException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + ": got nothing"); return null; } }
?当时写好的JUnit测试代码如下:
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public class PreloaderTest { /** * Test method for {@link com.rainbow.util.futuretask.Preloader#get()}. */ @Test public void testGet() { Thread.currentThread().setName("Thread(main)"); final Preloader pl = new Preloader(); pl.start(); // try to get the result before the latency is over new Thread(new Runnable() { @Override public void run() { Thread.currentThread().setName("Thread(1)"); pl.get(); } }).start(); // try to get the result after the latency is over new Thread(new Runnable() { @Override public void run() { Thread.currentThread().setName("Thread(2)"); try { Thread.sleep(6000); pl.get(); } catch (InterruptedException e) { e.printStackTrace(); } } }).start(); } }
?可是这段测试代码,不管怎么运行,运行多少次,它的运行结果却怎么也不是想像中的结果,却是如下的结果:
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Thread(main): start the loader Thread(main): loader started Thread(3): simulate a latency Thread(1): begin to get
?从结果看来,Thread(1) ?Thread(2) Thread(3) 都没有执行完了就被结束掉了。
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对这个结果很是意外,于是上网查了一下,找到如下的一段说明:
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在多线程环境下,程序退出的条件是,所有的非Daemon线程都正常结束或者某个线程条用了system.exit方法,导致进程强行退出。在eclipse下运行Junit的类是org.eclipse.jdt.internal.junit.runner.RemoteTestRunner。通过查看这个类的main方法。如下: public static void main(String [] args) { try { RemoteTestRunner testRunServer= new RemoteTestRunner(); testRunServer.init(args); testRunServer.run(); } catch (Throwable e) { e.printStackTrace(); // don't allow System.exit(0) to swallow exceptions } finally { // fix for 14434 System.exit(0); } } 显然,只要主线程结束,整个程序将会退出,这就是采用junit的时候奇怪退出程序的原因。
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看到这段,也就明白了上面的结果了。
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为了避免上面的这种情况,我想出来的两种方法:
① 不使用JUnit来测试,而是自己写个main方法来进行测试,就如http://rainbow702.iteye.com/admin/blogs/2206301?中那样
② 还使用JUnit,但在测试代码的最后加上sleep语句,即如下代码的最后的 sleep那样:
/** * */ package com.rainbow.util.futuretask; import org.junit.Test; /** * @author Rainbow */ public class PreloaderTest { /** * Test method for {@link com.rainbow.util.futuretask.Preloader#get()}. */ @Test public void testGet() { Thread.currentThread().setName("Thread(main)"); final Preloader pl = new Preloader(); pl.start(); // try to get the result before the latency is over new Thread(new Runnable() { @Override public void run() { Thread.currentThread().setName("Thread(1)"); pl.get(); } }).start(); // try to get the result after the latency is over new Thread(new Runnable() { @Override public void run() { Thread.currentThread().setName("Thread(2)"); try { Thread.sleep(6000); pl.get(); } catch (InterruptedException e) { e.printStackTrace(); } } }).start(); try { Thread.sleep(10000); } catch (InterruptedException e) { e.printStackTrace(); } } }
?但是这样的话,必须要正确估算出需要进行sleep的时间。 ?或者干脆使用不指定时间的 ?sleep() ?。