链接地址：

    上一篇中，我们简单的介绍了一下Guava Concurrency Monitor监控类，并对Monitor的源码结构进行了分析，对Monitor中的一些重要的方法做了简单的整理，那么在实际的工作中我们又该怎样去应用Monitor呢？接下来，我们首先通过分析GitHub上面的开源代码，来继续进行Guava Concurrency Monitor的学习。

    下面的开源代码：包含Monitor的实例代码MonitorExample和对应的测试类MonitorExampleTest，由于针对的是Guava较早的版本，所以其中的一些API已经舍弃，不过仍然具有很高的学习价值，代码如下：

package guava; import com.google.common.util.concurrent.Monitor; import java.util.concurrent.atomic.AtomicInteger; /** * 原文地址：https://gist.github.com/bbejeck/1369371 * User: bbejeck */public class MonitorExample {     private final Monitor monitor = new Monitor();    private volatile boolean condition = true;    private int taskDoneCounter;    //AtomicInteger：线程安全的加减操作    private AtomicInteger taskSkippedCounter = new AtomicInteger(0);    private int stopTaskCount;     private Monitor.Guard conditionGuard = new Monitor.Guard(monitor) {        @Override        public boolean isSatisfied() {            return condition;        }    };     public void demoTryEnterIf() throws InterruptedException {        if (monitor.tryEnterIf(conditionGuard)) {            try {                simulatedWork();                taskDoneCounter++;            } finally {                monitor.leave();            }        } else {            //自增加1            taskSkippedCounter.incrementAndGet();        }    }     public void demoEnterIf() throws InterruptedException {        if (monitor.enterIf(conditionGuard)) {            try {                taskDoneCounter++;                if (taskDoneCounter == stopTaskCount) {                    condition = false;                }            } finally {                monitor.leave();            }        } else {            taskSkippedCounter.incrementAndGet();        }     }     public void demoEnterWhen() throws InterruptedException {        monitor.enterWhen(conditionGuard);        try {            taskDoneCounter++;            if (taskDoneCounter == stopTaskCount) {                condition = false;            }        } finally {            monitor.leave();        }    }     private void simulatedWork() throws InterruptedException {        Thread.sleep(250);    } //    public void reEvaluateGuardCondition() {//        monitor.reevaluateGuards();//    }     public int getStopTaskCount() {        return stopTaskCount;    }     public void setStopTaskCount(int stopTaskCount) {        this.stopTaskCount = stopTaskCount;    }     public void setCondition(boolean condition) {        this.condition = condition;    }     public int getTaskSkippedCounter() {        return taskSkippedCounter.get();    }     public int getTaskDoneCounter() {        return taskDoneCounter;    }}

package guava; import org.junit.After;import org.junit.Before;import org.junit.Test; import java.lang.reflect.Method;import java.util.concurrent.*; import static org.hamcrest.CoreMatchers.is;import static org.junit.Assert.assertThat; /** * 原文地址：https://gist.github.com/bbejeck/1369371 * User: bbejeck */public class MonitorExampleTest {     private MonitorExample monitorExample;    private ExecutorService executorService;    private int numberThreads = 10;    // CountDownLatch：同步辅助类，允许一个或多个线程等待其他线程所执行的一组操作完成    private CountDownLatch startSignal;    private CountDownLatch doneSignal;     @Before    public void setUp() throws Exception {        monitorExample = new MonitorExample();        executorService = Executors.newFixedThreadPool(numberThreads);        startSignal = new CountDownLatch(1);        doneSignal = new CountDownLatch(numberThreads);    }     @After    public void tearDown() {        executorService.shutdownNow();    }     /**     * 第一个线程会进入Monitor调用simulatedWork()后线程等待     * 其余9个线程则会进入else，对taskSkippedCounter自增     *     * @throws Exception     */    @Test    public void testDemoTryEnterIf() throws Exception {        setUpThreadsForTestingMethod("demoTryEnterIf");        startAllThreadsForTest();        waitForTestThreadsToFinish();        int expectedTaskCount = 1;        int expectedSkippedTasks = 9;        assertThat(monitorExample.getTaskDoneCounter(), is(expectedTaskCount));        assertThat(monitorExample.getTaskSkippedCounter(), is(expectedSkippedTasks));    }     /**     * 前5个线程会等待Monitor，因为Guard的isSatisfied()为true     * 但是一旦isSatisfied()变为false，剩余的线程会进入else，     * 对taskSkippedCounter自增     *     * @throws Exception     */    @Test    public void testDemoEnterIfOnlyFiveTasksComplete() throws Exception {        monitorExample.setStopTaskCount(5);        setUpThreadsForTestingMethod("demoEnterIf");         startAllThreadsForTest();        waitForTestThreadsToFinish();        int expectedTaskCount = 5;        int expectedSkippedTasks = 5;         assertThat(monitorExample.getTaskDoneCounter(), is(expectedTaskCount));        assertThat(monitorExample.getTaskSkippedCounter(), is(expectedSkippedTasks));     }     /**     * 所有10个线程都会进入Monitor，因为在整个时间内Guard的isSatisfied()为true     *     * @throws Exception     */    @Test    public void testDemoEnterIfAllTasksComplete() throws Exception {        monitorExample.setStopTaskCount(Integer.MAX_VALUE);        setUpThreadsForTestingMethod("demoEnterIf");         startAllThreadsForTest();        waitForTestThreadsToFinish();        int expectedTaskCount = 10;        int expectedSkippedTasks = 0;         assertThat(monitorExample.getTaskDoneCounter(), is(expectedTaskCount));        assertThat(monitorExample.getTaskSkippedCounter(), is(expectedSkippedTasks));     }     /**     * Guard的isSatisfied()初始化为true，但是所有10个线程会进入Monitor     *     * @throws Exception     */    @Test    public void testDemoEnterWhen() throws Exception {        monitorExample.setStopTaskCount(Integer.MAX_VALUE);        monitorExample.setCondition(false);        setUpThreadsForTestingMethod("demoEnterWhen");        startAllThreadsForTest();        int expectedCompletedCount = 0;        int completedCount = monitorExample.getTaskDoneCounter();        assertThat(completedCount, is(expectedCompletedCount));         monitorExample.setCondition(true);         waitForTestThreadsToFinish();        expectedCompletedCount = 10;        completedCount = monitorExample.getTaskDoneCounter();        assertThat(completedCount, is(expectedCompletedCount));    }     /**     * 在3个线程完成工作后，人为的设置Guard的isSatisfied()为false     * 以证明剩余的7个线程将等待，直到isSatisfied()变为true     * 然后才会进入Monitor.     *     * @throws Exception     */    @Test    public void testDemoEnterWhenAllTasksCompleteEvenWhenConditionChanges() throws Exception {        monitorExample.setCondition(true);        monitorExample.setStopTaskCount(3);        setUpThreadsForTestingMethod("demoEnterWhen");        startAllThreadsForTest();         //验证最初只有3个线程工作, 重新设定Guard的isSatisfied()为true        FutureTask
    
      checkInitialTasksCompleted = new FutureTask
     
      (                new Callable
      
       () {                    public Integer call() {                        int initialCompletedTasks = monitorExample.getTaskDoneCounter();                        monitorExample.setCondition(true);//                        monitorExample.reEvaluateGuardCondition();                        return initialCompletedTasks;                     }                });         new Thread(checkInitialTasksCompleted).start();         int expectedCompletedCount = 3;        int completedCount = checkInitialTasksCompleted.get();        assertThat(completedCount, is(expectedCompletedCount));         waitForTestThreadsToFinish();        assertThat(completedCount, is(expectedCompletedCount));        expectedCompletedCount = 10;        completedCount = monitorExample.getTaskDoneCounter();        assertThat(completedCount, is(expectedCompletedCount));    }     private void waitForTestThreadsToFinish() throws InterruptedException {        doneSignal.await(1000l, TimeUnit.MILLISECONDS);    }     private void startAllThreadsForTest() {        startSignal.countDown();    }     private Method getMethodUnderTest(String methodName) throws Exception {        return monitorExample.getClass().getDeclaredMethod(methodName);    }      private void setUpThreadsForTestingMethod(String methodName) throws Exception {        final Method testMethod = getMethodUnderTest(methodName);        for (int i = 0; i < numberThreads; i++) {            executorService.execute(new Runnable() {                @Override                public void run() {                    try {                        startSignal.await();                        testMethod.invoke(monitorExample);                    } catch (Exception e) {                        //异常无须处理                    } finally {                        doneSignal.countDown();                    }                }            });        }    } }