轉：https://blog.csdn.net/forezp/article/details/70305336

之前寫過一篇文章《如何在springcloud分布式系統中實現分布式鎖？》（https://blog.csdn.net/forezp/article/details/68957681），由于自己僅僅是閱讀了相關的書籍，和查閱了相關的資料，就認為那樣的是可行的。那篇文章實現的大概思路是用setNx命令和setEx配合使用。 setNx是一個耗時操作，因為它需要查詢這個鍵是否存在，就算redis的百萬的qps，在高并發的場景下，這種操作也是有問題的。關于redis實現分布式鎖，redis官方推薦使用redlock。

一、redlock簡介

在不同進程需要互斥地訪問共享資源時，分布式鎖是一種非常有用的技術手段。實現高效的分布式鎖有三個屬性需要考慮：

1、安全屬性：互斥，不管什么時候，只有一個客戶端持有鎖 2、效率屬性A:不會死鎖 3、效率屬性B：容錯，只要大多數redis節點能夠正常工作，客戶端端都能獲取和釋放鎖。

Redlock是redis官方提出的實現分布式鎖管理器的算法。這個算法會比一般的普通方法更加安全可靠。關于這個算法的討論可以看下官方文檔（https://github.com/antirez/redis-doc/blob/master/topics/distlock.md）。

二、怎么用java使用 redlock

在pom文件引入redis和redisson依賴：

<!-- redis--> <dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-data-redis</artifactId> </dependency> <!-- redisson--> <dependency><groupId>org.redisson</groupId><artifactId>redisson</artifactId><version>3.3.2</version> </dependency>

AquiredLockWorker接口類，，主要是用于獲取鎖后需要處理的邏輯：

/*** Created by fangzhipeng on 2017/4/5.* 獲取鎖后需要處理的邏輯*/ public interface AquiredLockWorker<T> {T invokeAfterLockAquire() throws Exception; }

DistributedLocker 獲取鎖管理類：

/*** Created by fangzhipeng on 2017/4/5.* 獲取鎖管理類*/ public interface DistributedLocker {/*** 獲取鎖* @param resourceName 鎖的名稱* @param worker 獲取鎖后的處理類* @param <T>* @return 處理完具體的業務邏輯要返回的數據* @throws UnableToAquireLockException* @throws Exception*/<T> T lock(String resourceName, AquiredLockWorker<T> worker) throws UnableToAquireLockException, Exception;<T> T lock(String resourceName, AquiredLockWorker<T> worker, int lockTime) throws UnableToAquireLockException, Exception;}

UnableToAquireLockException ，不能獲取鎖的異常類：

/*** Created by fangzhipeng on 2017/4/5.* 異常類*/ public class UnableToAquireLockException extends RuntimeException {public UnableToAquireLockException() {}public UnableToAquireLockException(String message) {super(message);}public UnableToAquireLockException(String message, Throwable cause) {super(message, cause);} }

RedissonConnector 連接類：

/*** Created by fangzhipeng on 2017/4/5.* 獲取RedissonClient連接類*/ @Component public class RedissonConnector {RedissonClient redisson;@PostConstructpublic void init(){redisson = Redisson.create();}public RedissonClient getClient(){return redisson;} }

RedisLocker 類，實現了DistributedLocker：

import org.redisson.api.RLock; import org.redisson.api.RedissonClient; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Component; import java.util.concurrent.TimeUnit;/*** Created by fangzhipeng on 2017/4/5.*/ @Component public class RedisLocker implements DistributedLocker{private final static String LOCKER_PREFIX = "lock:";@AutowiredRedissonConnector redissonConnector;@Overridepublic <T> T lock(String resourceName, AquiredLockWorker<T> worker) throws InterruptedException, UnableToAquireLockException, Exception {return lock(resourceName, worker, 100);}@Overridepublic <T> T lock(String resourceName, AquiredLockWorker<T> worker, int lockTime) throws UnableToAquireLockException, Exception {RedissonClient redisson= redissonConnector.getClient();RLock lock = redisson.getLock(LOCKER_PREFIX + resourceName);// Wait for 100 seconds seconds and automatically unlock it after lockTime secondsboolean success = lock.tryLock(100, lockTime, TimeUnit.SECONDS);if (success) {try {return worker.invokeAfterLockAquire();} finally {lock.unlock();}}throw new UnableToAquireLockException();} }

測試類：

@AutowiredRedisLocker distributedLocker;@RequestMapping(value = "/redlock")public String testRedlock() throws Exception{CountDownLatch startSignal = new CountDownLatch(1);CountDownLatch doneSignal = new CountDownLatch(5);for (int i = 0; i < 5; ++i) { // create and start threadsnew Thread(new Worker(startSignal, doneSignal)).start();}startSignal.countDown(); // let all threads proceeddoneSignal.await();System.out.println("All processors done. Shutdown connection");return "redlock";}class Worker implements Runnable {private final CountDownLatch startSignal;private final CountDownLatch doneSignal;Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {this.startSignal = startSignal;this.doneSignal = doneSignal;}public void run() {try {startSignal.await();distributedLocker.lock("test",new AquiredLockWorker<Object>() {@Overridepublic Object invokeAfterLockAquire() {doTask();return null;}});}catch (Exception e){}}void doTask() {System.out.println(Thread.currentThread().getName() + " start");Random random = new Random();int _int = random.nextInt(200);System.out.println(Thread.currentThread().getName() + " sleep " + _int + "millis");try {Thread.sleep(_int);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName() + " end");doneSignal.countDown();}}

運行測試類：

Thread-48 start Thread-48 sleep 99millis Thread-48 end Thread-49 start Thread-49 sleep 118millis Thread-49 end Thread-52 start Thread-52 sleep 141millis Thread-52 end Thread-50 start Thread-50 sleep 28millis Thread-50 end Thread-51 start Thread-51 sleep 145millis Thread-51 end

從運行結果上看，在異步任務的情況下，確實是獲取鎖之后才能運行線程。不管怎么樣，這是redis官方推薦的一種方案，可靠性比較高。有什么問題歡迎留言。

三、參考資料

https://github.com/redisson/redisson

《Redis官方文檔》用Redis構建分布式鎖 ： http://ifeve.com/redis-lock/

A Look at the Java Distributed In-Memory Data Model (Powered by Redis) ： https://dzone.com/articles/java-distributed-in-memory-data-model-powered-by-r

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