线程同步

1、在线程上调用wait()方法，并使用while循环；

while(someCondition){try {wait();} catch (InterruptedException e) {//In this case we don't care, but we may want//to propagate with Thread.interrupt()}
}

2、notifyAll()唤醒阻塞的线程；

JDK1.5引入ReentrantLock

1、使用ReentrantLock.newCondition()，来实现更精细粒度的控制；

2、Condition.signalAll()

while(list.isEmpty()){Condition.await();
}

Monitor

Guava的Monitor类为我们提供了一个允许多种条件的解决方案，并且完全消除了显式notify线程的方式。

public class MonitorSample {private List<String> list = new ArrayList<String>();private static final int MAX_SIZE = 10;private Monitor monitor = new Monitor();private Monitor.Guard listBelowCapacity = new Monitor.Guard(monitor) {@Overridepublic boolean isSatisfied() {return list.size() < MAX_SIZE;}};public void addToList(String item) throws InterruptedException {monitor.enterWhen(listBelowCapacity);try {list.add(item);} finally {monitor.leave();}}
}

任何时候都只有一个Thread进入Monitor块，语义类似synchronized和ReentrantLock

最佳实践

// 条件判断是否进入阻塞块
if (monitor.enterIf(guardCondition)) {try {doWork();} finally {monitor.leave();}
}// 进入阻塞&满足condition
monitor.enterWhen(guardCondition);
try {doWork();
} finally {monitor.leave()
}

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;public class MonitorExampleTest {private MonitorExample monitorExample;private ExecutorService executorService;private int numberThreads = 10;private CountDownLatch startSignal;private CountDownLatch doneSignal;@Beforepublic void setUp() throws Exception {monitorExample = new MonitorExample();executorService = Executors.newFixedThreadPool(numberThreads);startSignal = new CountDownLatch(1);doneSignal = new CountDownLatch(numberThreads);}@Afterpublic void tearDown() {executorService.shutdownNow();}/** First thread does some simulated work and the following* 9 threads will move on.*/@Testpublic void testDemoTryEnterIf() throws Exception {setUpThreadsForTestingMethod("demoTryEnterIf");startAllThreadsForTest();waitForTestThreadsToFinish();int expectedTaskCount = 1;int expectedSkippedTasks = 9;assertThat(monitorExample.getTaskDoneCounter(), is(expectedTaskCount));assertThat(monitorExample.getTaskSkippedCounter(), is(expectedSkippedTasks));}/*The first 5 threads will wait for the monitor becausethe guard condition is true, but once it turns false therest of the threads drop off*/@Testpublic 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));}/*All 10 threads enter the monitor as the guard conditionremains true the entire time.*/@Testpublic 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 condition is initially false, but all 10 threadsenter the monitor.*/@Testpublic 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));}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() {@Overridepublic void run() {try {startSignal.await();testMethod.invoke(monitorExample);} catch (Exception e) {//Don't care} finally {doneSignal.countDown();}}});}}}