怎样使用mock object测试一个启动新线程的类

本文是在jmock的网站上发现的，很有实际意义，因为一直用easymock，试了一下jmock，觉得很别扭，方法名以字符串的方式自己输入，容易写错，而且还要继承它自己的基类，不爽。
所以本文的程序样例用easymock重写了。

在下面的例子中，Guard持有一个Alarm的引用，在必要的时候进行报警。

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publicinterfaceAlarm{
publicvoidring();
}

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publicclassGuard{
privateAlarmalarm;

publicGuard(Alarmalarm){
this.alarm=alarm;
}

publicvoidgetBored(){
startRingingTheAlarm();
}

privatevoidstartRingingTheAlarm(){
RunnableringAlarmTask=newRunnable(){
publicvoidrun(){
alarm.ring();
}
};
ThreadringAlarmThread=newThread(ringAlarmTask);
ringAlarmThread.start();
}
}

Guard.getBored()的测试代码如下：

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publicvoidtestGuardDoesNotRingTheAlarmWhenHeGetsBored(){
Alarmalarm=EasyMock.createMock(Alarm.class);
Guardguard=newGuard(alarm);
guard.getBored();
}

在此例中，预期的异常并没有发生，测试通过了。这是因为alarm抛出的异常是在ringAlarm线程中，而不是在测试主线程中。此问题的根源是试图使用mock object来进行集成测试。用mock object来进行单元测试是希望将测试的单元与系统其他单元相隔离。然而，线程从其特性来说，是属于集成测试的范畴。并发和同步都要涉及到全局范围，线程的创建也用到了操作系统底层的特性。

一种解决方案是将要执行任务的对象与任务的细节相隔离，在它们之间引入一个接口。这样，可以用mock object来测试要执行任务的对象，在集成测试中测试任务的执行。

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publicinterfaceTaskRunner{
publicvoidstart(Runnabletask);
}

测试方案如下：

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publicvoidtestGuardDoesNotRingTheAlarmWhenHeGetsBored(){
Alarmalarm=EasyMock.createMock(Alarm.class);
TaskRunnertaskRunner=newImmediateTaskRunner();
Guardguard=newGuard(alarm,taskRunner);
guard.getBored();
}

<!-- = END of automatically generated HTML code = --><!-- ======================================================== --> 在TaskRunner的实现中，如果是启用一个新线程来执行任务，那么又回到了问题的开始，测试还是不能得到希望的异常。我们需要将任务的执行放在TaskRunner相同的线程中。最简单的方法就是立即执行该任务，而不是启线程来执行。在单元测试中，可以直接实现TaskRunner接口，得到如下的任务执行器。

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publicclassImmediateTaskRunnerimplementsTaskRunner{
publicvoidstart(Runnabletask){
task.run();
}
}

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Guard代码更新如下：

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publicclassGuard{
privateAlarmalarm;

privateTaskRunnertaskRunner;

publicGuard(Alarmalarm,TaskRunnertaskRunner){
this.alarm=alarm;
this.taskRunner=taskRunner;
}

publicvoidgetBored(){
startRingingTheAlarm();
}

privatevoidstartRingingTheAlarm(){
RunnableringAlarmTask=newRunnable(){
publicvoidrun(){
alarm.ring();
}
};
taskRunner.start(ringAlarmTask);
}
}

在实际项目中使用的TaskRunner

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publicclassConcurrentTaskRunnerimplementsTaskRunner{
publicvoidstart(Runnabletask){
(newThread(task)).start();
}
}

<!-- = END of automatically generated HTML code = --><!-- ======================================================== --> 另一种方案是在Guard.getBored()执行结束后，在测试所在的线程中执行任务。如果Guard中的try/finally 掩盖了任务引起的测试错误，应用此方案则特别适合。
实现的TaskRunner如下：<!--CTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dt--> <style type="text/css"> <!--code { font-family: Courier New, Courier; font-size: 10pt; margin: 0px; }--> </style> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <!-- ======================================================== --><!-- = Java Sourcecode to HTML automatically converted code = --><!-- = Java2Html Converter 5.0 [2006-02-26] by Markus Gebhard [e-mail] = --><!-- = Further information: http://www.java2html.de = -->

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publicclassDelayedTaskRunnerimplementsTaskRunner{
privateList<Runnable>delayedTasks=newArrayList<Runnable>();

publicvoidstart(Runnabletask){
delayedTasks.add(task);
}

publicvoidrunTasks(){
for(Iterator<Runnable>i=delayedTasks.iterator();i.hasNext();){
i.next().run();
i.remove();
}
}
}

<!-- = END of automatically generated HTML code = --><!-- ======================================================== -->对应的测试代码为：<!--CTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dt--> <style type="text/css"> <!--code { font-family: Courier New, Courier; font-size: 10pt; margin: 0px; }--> </style> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <!-- ======================================================== --><!-- = Java Sourcecode to HTML automatically converted code = --><!-- = Java2Html Converter 5.0 [2006-02-26] by Markus Gebhard [e-mail] = --><!-- = Further information: http://www.java2html.de = -->

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publicvoidtestGuardDoesNotRingTheAlarmWhenHeGetsBored(){
Alarmalarm=EasyMock.createMock(Alarm.class);
DelayedTaskRunnertaskRunner=newDelayedTaskRunner();
Guardguard=newGuard(alarm,taskRunner);
guard.getBored();
taskRunner.runTasks();
}

<!-- = END of automatically generated HTML code = --><!-- ======================================================== --> 将对象中运行多线程任务的机制提取出来，不仅方便单元测试，而且还能使得程序之间的耦合更松，扩展性更好。比如可以毫不费力的将现在的并发任务处理器替换成线程池。