转载：https://my.oschina.net/realfighter/blog/349924
https://my.oschina.net/realfighter/blog/349926
Monitor类是作为ReentrantLock的一个替代，代码中使用 Monitor比使用ReentrantLock更不易出错，可读性也更强，并且也没有显著的性能损失，使用Monitor甚至有潜在的性能得到优化。
public abstract static class Guard：一个标识线程是否等待的布尔条件，Guard类总是与单一的Monitor相关联，Monitor可以在任意时间从任意占用Monitor的线程检查Guard，这样代码的编写将不在关心Guard是否被检查的频率。
public abstract boolean isSatisfied()：Guard内部提供的抽象方法，isSatisfied()，当被关联的Monitor被占用时，Guard的此方法会被调用，该方法的实现必须取决于被关联Monitor保护的状态，并且状态不可修改。
Monitor有几个常用的方法
- enter()：进入到当前Monitor，无限期阻塞，等待锁。(这里没有Guard)
- enter(long time, TimeUnit unit)：进入到当前Monitor，最多阻塞给定的时间，返回是否进入Monitor。
- tryEnter()：如果可以的话立即进入Monitor，不阻塞，返回是否进入Monitor。
- enterWhen(Guard guard)：进入当前Monitor，等待Guard的isSatisfied()为true后，继续往下执行 ，但可能会被打断; 为false，会阻塞。
- enterIf(Guard guard)：如果Guard的isSatisfied()为true，进入当前Monitor。等待获得锁(这里会等待获取锁)，不需要等待Guard satisfied。
- tryEnterIf(Guard guard)：如果Guard的isSatisfied()为true并且可以的话立即进入Monitor，不等待获取锁(这里不等待获取锁)，也不等待Guard satisfied。
- 感觉 enterWhen enterIf的用的区别就是前者无返回值，后者返回Boolean值。
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;
}
}
Test类：
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<Integer> checkInitialTasksCompleted = new FutureTask<Integer>(
new Callable<Integer>() {
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();
}
}
});
}
}
}