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‎Java相关/Multithread/AQS.md

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> 常见问题：AQS 原理？;CountDownLatch和CyclicBarrier了解吗,两者的区别是什么？用过Semaphore吗？
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> 常见问题：AQS原理？;CountDownLatch和CyclicBarrier了解吗,两者的区别是什么？用过Semaphore吗？
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**本节思维导图：**
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![并发编程面试必备：AQS 原理以及 AQS 同步组件总结](http://my-blog-to-use.oss-cn-beijing.aliyuncs.com/18-10-31/61115865.jpg)
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<!-- MarkdownTOC -->
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- [1 AQS 简单介绍](#1-aqs-简单介绍)
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- [2 AQS 原理](#2-aqs-原理)
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- [2.1 AQS 原理概览](#21-aqs-原理概览)
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- [2.2 AQS 对资源的共享方式](#22-aqs-对资源的共享方式)
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- [2.3 AQS底层使用了模板方法模式](#23-aqs底层使用了模板方法模式)
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- [3 Semaphore\(信号量\)-允许多个线程同时访问](#3-semaphore信号量-允许多个线程同时访问)
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- [4 CountDownLatch （倒计时器）](#4-countdownlatch-（倒计时器）)
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- [4.1 CountDownLatch 的三种典型用法](#41-countdownlatch-的三种典型用法)
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- [4.2 CountDownLatch 的使用示例](#42-countdownlatch-的使用示例)
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- [4.3 CountDownLatch 的不足](#43-countdownlatch-的不足)
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- [4.4 CountDownLatch相常见面试题：](#44-countdownlatch相常见面试题：)
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- [5 CyclicBarrier\(循环栅栏\)](#5-cyclicbarrier循环栅栏)
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- [5.1 CyclicBarrier 的应用场景](#51-cyclicbarrier-的应用场景)
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- [5.2 CyclicBarrier 的使用示例](#52-cyclicbarrier-的使用示例)
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- [5.3 CyclicBarrier和CountDownLatch的区别](#53-cyclicbarrier和countdownlatch的区别)
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- [6 ReentrantLock 和 ReentrantReadWriteLock](#6-reentrantlock-和-reentrantreadwritelock)
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<!-- /MarkdownTOC -->
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### 1 AQS 简单介绍
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AQS的全称为（AbstractQueuedSynchronizer），这个类在java.util.concurrent.locks包下面。
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@@ -115,38 +138,38 @@ tryReleaseShared(int)//共享方式。尝试释放资源，成功则返回true
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* @Description: 需要一次性拿一个许可的情况
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*/
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public class SemaphoreExample1 {
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// 请求的数量
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private static final int threadCount = 550;
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public static void main(String[] args) throws InterruptedException {
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// 创建一个具有固定线程数量的线程池对象（如果这里线程池的线程数量给太少的话你会发现执行的很慢）
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ExecutorService threadPool = Executors.newFixedThreadPool(300);
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// 一次只能允许执行的线程数量。
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final Semaphore semaphore = new Semaphore(20);
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for (int i = 0; i < threadCount; i++) {
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final int threadnum = i;
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threadPool.execute(() -> {// Lambda 表达式的运用
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try {
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semaphore.acquire();// 获取一个许可，所以可运行线程数量为20/1=20
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test(threadnum);
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semaphore.release();// 释放一个许可
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} catch (InterruptedException e) {
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// TODO Auto-generated catch block
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e.printStackTrace();
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}
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});
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}
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threadPool.shutdown();
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System.out.println("finish");
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}
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public static void test(int threadnum) throws InterruptedException {
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Thread.sleep(1000);// 模拟请求的耗时操作
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System.out.println("threadnum:" + threadnum);
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Thread.sleep(1000);// 模拟请求的耗时操作
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}
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// 请求的数量
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private static final int threadCount = 550;
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public static void main(String[] args) throws InterruptedException {
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// 创建一个具有固定线程数量的线程池对象（如果这里线程池的线程数量给太少的话你会发现执行的很慢）
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ExecutorService threadPool = Executors.newFixedThreadPool(300);
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// 一次只能允许执行的线程数量。
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final Semaphore semaphore = new Semaphore(20);
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for (int i = 0; i < threadCount; i++) {
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final int threadnum = i;
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threadPool.execute(() -> {// Lambda 表达式的运用
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try {
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semaphore.acquire();// 获取一个许可，所以可运行线程数量为20/1=20
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test(threadnum);
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semaphore.release();// 释放一个许可
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} catch (InterruptedException e) {
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// TODO Auto-generated catch block
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e.printStackTrace();
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}
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});
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}
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threadPool.shutdown();
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System.out.println("finish");
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}
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public static void test(int threadnum) throws InterruptedException {
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Thread.sleep(1000);// 模拟请求的耗时操作
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System.out.println("threadnum:" + threadnum);
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Thread.sleep(1000);// 模拟请求的耗时操作
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}
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}
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```
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@@ -155,9 +178,9 @@ public class SemaphoreExample1 {
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当然一次也可以一次拿取和释放多个许可，不过一般没有必要这样做：
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```java
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semaphore.acquire(5);// 获取5个许可，所以可运行线程数量为20/5=4
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test(threadnum);
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semaphore.release(5);// 获取5个许可，所以可运行线程数量为20/5=4
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semaphore.acquire(5);// 获取5个许可，所以可运行线程数量为20/5=4
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test(threadnum);
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semaphore.release(5);// 获取5个许可，所以可运行线程数量为20/5=4
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```
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除了 `acquire`方法之外，另一个比较常用的与之对应的方法是`tryAcquire`方法，该方法如果获取不到许可就立即返回false。
@@ -207,37 +230,37 @@ CountDownLatch是一个同步工具类，它允许一个或多个线程一直等
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* @Description: CountDownLatch 使用方法示例
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*/
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public class CountDownLatchExample1 {
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// 请求的数量
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private static final int threadCount = 550;
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public static void main(String[] args) throws InterruptedException {
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// 创建一个具有固定线程数量的线程池对象（如果这里线程池的线程数量给太少的话你会发现执行的很慢）
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ExecutorService threadPool = Executors.newFixedThreadPool(300);
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final CountDownLatch countDownLatch = new CountDownLatch(threadCount);
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for (int i = 0; i < threadCount; i++) {
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final int threadnum = i;
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threadPool.execute(() -> {// Lambda 表达式的运用
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try {
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test(threadnum);
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} catch (InterruptedException e) {
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// TODO Auto-generated catch block
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e.printStackTrace();
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} finally {
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countDownLatch.countDown();// 表示一个请求已经被完成
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}
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});
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}
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countDownLatch.await();
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threadPool.shutdown();
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System.out.println("finish");
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}
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public static void test(int threadnum) throws InterruptedException {
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Thread.sleep(1000);// 模拟请求的耗时操作
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System.out.println("threadnum:" + threadnum);
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Thread.sleep(1000);// 模拟请求的耗时操作
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}
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// 请求的数量
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private static final int threadCount = 550;
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public static void main(String[] args) throws InterruptedException {
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// 创建一个具有固定线程数量的线程池对象（如果这里线程池的线程数量给太少的话你会发现执行的很慢）
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ExecutorService threadPool = Executors.newFixedThreadPool(300);
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final CountDownLatch countDownLatch = new CountDownLatch(threadCount);
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for (int i = 0; i < threadCount; i++) {
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final int threadnum = i;
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threadPool.execute(() -> {// Lambda 表达式的运用
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try {
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test(threadnum);
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} catch (InterruptedException e) {
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// TODO Auto-generated catch block
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e.printStackTrace();
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} finally {
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countDownLatch.countDown();// 表示一个请求已经被完成
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}
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});
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}
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countDownLatch.await();
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threadPool.shutdown();
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System.out.println("finish");
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}
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public static void test(int threadnum) throws InterruptedException {
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Thread.sleep(1000);// 模拟请求的耗时操作
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System.out.println("threadnum:" + threadnum);
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Thread.sleep(1000);// 模拟请求的耗时操作
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}
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}
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```
@@ -283,42 +306,42 @@ CyclicBarrier 可以用于多线程计算数据，最后合并计算结果的应
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* @Description: 测试 CyclicBarrier 类中带参数的 await() 方法
284307
*/
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public class CyclicBarrierExample2 {
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// 请求的数量
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private static final int threadCount = 550;
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// 需要同步的线程数量
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private static final CyclicBarrier cyclicBarrier = new CyclicBarrier(5);
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public static void main(String[] args) throws InterruptedException {
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// 创建线程池
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ExecutorService threadPool = Executors.newFixedThreadPool(10);
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for (int i = 0; i < threadCount; i++) {
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final int threadNum = i;
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Thread.sleep(1000);
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threadPool.execute(() -> {
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try {
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test(threadNum);
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} catch (InterruptedException e) {
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// TODO Auto-generated catch block
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e.printStackTrace();
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} catch (BrokenBarrierException e) {
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// TODO Auto-generated catch block
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e.printStackTrace();
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}
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});
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}
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threadPool.shutdown();
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}
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public static void test(int threadnum) throws InterruptedException, BrokenBarrierException {
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System.out.println("threadnum:" + threadnum + "is ready");
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try {
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cyclicBarrier.await(2000, TimeUnit.MILLISECONDS);
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} catch (Exception e) {
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System.out.println("-----CyclicBarrierException------");
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}
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System.out.println("threadnum:" + threadnum + "is finish");
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}
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// 请求的数量
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private static final int threadCount = 550;
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// 需要同步的线程数量
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private static final CyclicBarrier cyclicBarrier = new CyclicBarrier(5);
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public static void main(String[] args) throws InterruptedException {
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// 创建线程池
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ExecutorService threadPool = Executors.newFixedThreadPool(10);
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for (int i = 0; i < threadCount; i++) {
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final int threadNum = i;
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Thread.sleep(1000);
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threadPool.execute(() -> {
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try {
323+
test(threadNum);
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} catch (InterruptedException e) {
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// TODO Auto-generated catch block
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e.printStackTrace();
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} catch (BrokenBarrierException e) {
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// TODO Auto-generated catch block
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e.printStackTrace();
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}
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});
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}
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threadPool.shutdown();
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}
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public static void test(int threadnum) throws InterruptedException, BrokenBarrierException {
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System.out.println("threadnum:" + threadnum + "is ready");
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try {
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cyclicBarrier.await(2000, TimeUnit.MILLISECONDS);
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} catch (Exception e) {
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System.out.println("-----CyclicBarrierException------");
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}
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System.out.println("threadnum:" + threadnum + "is finish");
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}
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}
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```
@@ -360,40 +383,40 @@ threadnum:6is finish
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* @Description: 新建 CyclicBarrier 的时候指定一个 Runnable
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*/
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public class CyclicBarrierExample3 {
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// 请求的数量
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private static final int threadCount = 550;
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// 需要同步的线程数量
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private static final CyclicBarrier cyclicBarrier = new CyclicBarrier(5, () -> {
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System.out.println("------当线程数达到之后，优先执行------");
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});
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public static void main(String[] args) throws InterruptedException {
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// 创建线程池
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ExecutorService threadPool = Executors.newFixedThreadPool(10);
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for (int i = 0; i < threadCount; i++) {
375-
final int threadNum = i;
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Thread.sleep(1000);
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threadPool.execute(() -> {
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try {
379-
test(threadNum);
380-
} catch (InterruptedException e) {
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// TODO Auto-generated catch block
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e.printStackTrace();
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} catch (BrokenBarrierException e) {
384-
// TODO Auto-generated catch block
385-
e.printStackTrace();
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}
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});
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}
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threadPool.shutdown();
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}
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392-
public static void test(int threadnum) throws InterruptedException, BrokenBarrierException {
393-
System.out.println("threadnum:" + threadnum + "is ready");
394-
cyclicBarrier.await();
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System.out.println("threadnum:" + threadnum + "is finish");
396-
}
386+
// 请求的数量
387+
private static final int threadCount = 550;
388+
// 需要同步的线程数量
389+
private static final CyclicBarrier cyclicBarrier = new CyclicBarrier(5, () -> {
390+
System.out.println("------当线程数达到之后，优先执行------");
391+
});
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public static void main(String[] args) throws InterruptedException {
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// 创建线程池
395+
ExecutorService threadPool = Executors.newFixedThreadPool(10);
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for (int i = 0; i < threadCount; i++) {
398+
final int threadNum = i;
399+
Thread.sleep(1000);
400+
threadPool.execute(() -> {
401+
try {
402+
test(threadNum);
403+
} catch (InterruptedException e) {
404+
// TODO Auto-generated catch block
405+
e.printStackTrace();
406+
} catch (BrokenBarrierException e) {
407+
// TODO Auto-generated catch block
408+
e.printStackTrace();
409+
}
410+
});
411+
}
412+
threadPool.shutdown();
413+
}
414+
415+
public static void test(int threadnum) throws InterruptedException, BrokenBarrierException {
416+
System.out.println("threadnum:" + threadnum + "is ready");
417+
cyclicBarrier.await();
418+
System.out.println("threadnum:" + threadnum + "is finish");
419+
}
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398421
}
399422
```
@@ -449,4 +472,4 @@ ReentrantLock 和 synchronized 的区别在上面已经讲过了这里就不多
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由于篇幅问题，关于 ReentrantLock 和 ReentrantReadWriteLock 详细内容可以查看我的这篇原创文章。
451474

452-
- [ReentrantLock 和 ReentrantReadWriteLock](https://mp.weixin.qq.com/s?__biz=MzU4NDQ4MzU5OA==&mid=2247483745&idx=2&sn=6778ee954a19816310df54ef9a3c2f8a&chksm=fd985700caefde16b9970f5e093b0c140d3121fb3a8458b11871e5e9723c5fd1b5a961fd2228&token=1829606453&lang=zh_CN#rd)
475+
- [ReentrantLock 和 ReentrantReadWriteLock](https://mp.weixin.qq.com/s?__biz=MzU4NDQ4MzU5OA==&mid=2247483745&idx=2&sn=6778ee954a19816310df54ef9a3c2f8a&chksm=fd985700caefde16b9970f5e093b0c140d3121fb3a8458b11871e5e9723c5fd1b5a961fd2228&token=1829606453&lang=zh_CN#rd)

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