可重入锁详解（synchronized ReentrantLock）

可重入锁详解

概述

什么是 “可重入”，可重入就是说某个线程已经获得某个锁，可以再次获取锁而不会出现死锁。例如

package com.test.reen;// 演示可重入锁是什么意思，可重入，就是可以重复获取相同的锁，synchronized和ReentrantLock都是可重入的// 可重入降低了编程复杂性public class WhatReentrant {public static void main(String[] args) {new Thread(new Runnable() {@Overridepublic void run() {synchronized (this) {System.out.println("第1次获取锁，这个锁是：" + this);int index = 1;while (true) {synchronized (this) {System.out.println("第" + (++index) + "次获取锁，这个锁是：" + this);}if (index == 10) {break;}}}}}).start();}}

package com.test.reen;import java.util.Random;import java.util.concurrent.locks.ReentrantLock;// 演示可重入锁是什么意思public class WhatReentrant2 {public static void main(String[] args) {ReentrantLock lock = new ReentrantLock();new Thread(new Runnable() {@Overridepublic void run() {try {lock.lock();System.out.println("第1次获取锁，这个锁是：" + lock);int index = 1;while (true) {try {lock.lock();System.out.println("第" + (++index) + "次获取锁，这个锁是：" + lock);try {Thread.sleep(new Random().nextInt(200));} catch (InterruptedException e) {e.printStackTrace();}if (index == 10) {break;}} finally {lock.unlock();}}} finally {lock.unlock();}}}).start();}}

可以发现没发生死锁，可以多次获取相同的锁

可重入锁有

• synchronized

• ReentrantLock

使用ReentrantLock的注意点

ReentrantLock 和 synchronized 不一样，需要手动释放锁，所以使用 ReentrantLock的时候一定要手动释放锁，并且加锁次数和释放次数要一样

以下代码演示，加锁和释放次数不一样导致的死锁

package com.test.reen;import java.util.Random;import java.util.concurrent.locks.ReentrantLock;public class WhatReentrant3 {public static void main(String[] args) {ReentrantLock lock = new ReentrantLock();new Thread(new Runnable() {@Overridepublic void run() {try {lock.lock();System.out.println("第1次获取锁，这个锁是：" + lock);int index = 1;while (true) {try {lock.lock();System.out.println("第" + (++index) + "次获取锁，这个锁是：" + lock);try {Thread.sleep(new Random().nextInt(200));} catch (InterruptedException e) {e.printStackTrace();}if (index == 10) {break;}} finally {//lock.unlock();// 这里故意注释，实现加锁次数和释放次数不一样}}} finally {lock.unlock();}}}).start();new Thread(new Runnable() {@Overridepublic void run() {try {lock.lock();for (int i = 0; i < 20; i++) {System.out.println("threadName:" + Thread.currentThread().getName());try {Thread.sleep(new Random().nextInt(200));} catch (InterruptedException e) {e.printStackTrace();}}} finally {lock.unlock();}}}).start();}}

由于加锁次数和释放次数不一样，第二个线程始终无法获取到锁，导致一直在等待。