Redis分布式锁机制

刺骨的言语ヽ痛彻心扉 2022-03-31 07:18 201阅读 0赞

> ## Redis分布式锁机制 ##

### 【基本机制】： ###

**基于redis实现的Java分布式锁主要依赖redis的SETNX()命令和DEL()命令，SETNX相当于上锁(lock)，DEL相当于释放锁（unlock）。我们只要实现Lock接口重写lock()和unlock()即可。但是这还不够，安全可靠的分布式锁应该满足满足下面三个条件：**

*  l 互斥，不管任何时候，只有一个客户端能持有同一个锁。
 *  l 不会死锁，最终一定会得到锁，即使持有锁的客户端对应的master节点宕掉。
 *  l 容错，只要大多数Redis节点正常工作，客户端应该都能获取和释放锁。

### 【问题】： ###

**什么情况下回不满足上面三个条件呢。多个线程（客户端）同时竞争锁可能会导致多个客户端同时拥有锁。比如，**

（1）线程1在master节点拿到了锁（存入key）

（2）master节点在把线程1创建的key写入slave之前宕机了，此时集群中的节点已经没有锁（key）了，包括master节点的slaver节点

（3）slaver节点升级为master节点

（4）线程2向新的master节点发起锁（存入key）请求，很明显，能请求成功。

可见，线程1和线程2同时获得了锁。如果在更高并发的情况，可能会有更多线程（客户端）获取锁

*  【死锁】：

拥有锁的线程（客户端）长时间的执行或者因为某种原因造成阻塞，就会导致锁无法释放（unlock没有调用），其它线程就不能获取锁而而产生无限期死锁的情况。其它线程在执行lock失败后即使粗暴的执行unlock删除key之后也不能正常释放锁，因为锁就只能由获得锁的线程释放，锁不能正常释放其它线程仍然获取不到锁。

设置锁的有效时间（redis的expire命令），不管是什么原因导致的死锁，有效时间过后，锁将会被自动释放

*  【解决方法】： 
 *   【容错】：（防止阻塞）

只要有Redis节点正常工作，客户端应该都能获取和释放锁，我们必须用相同的key不断循环向Master节点请求锁，当请求时间超过设定的超时时间则放弃请求锁，这个可以防止一个客户端在某个宕掉的master节点上阻塞过长时间，如果一个master节点不可用了，应该尽快尝试下一个master节点。释放锁比较简单，因为只需要在所有节点都释放锁就行，不管之前有没有在该节点获取锁成功

【RedLock算法流程】：

![783994-20160808160818684-1441187825.jpg][]

【Resis分布式锁sping配置依赖】：

<?xml version="1.0" encoding="UTF-8"?>
    
    <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
      <modelVersion>4.0.0</modelVersion>
    
      <groupId>com.caox</groupId>
      <artifactId>spring-demo</artifactId>
      <version>1.0-SNAPSHOT</version>
      <packaging>war</packaging>
    
      <name>spring-demo Maven Webapp</name>
      
      <url>http://www.example.com</url>
    
      <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.7</maven.compiler.source>
        <maven.compiler.target>1.7</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        
        <spring.version>4.1.1.RELEASE</spring.version>
        
        <mybatis.version>3.2.6</mybatis.version>
        
        <slf4j.version>1.7.7</slf4j.version>
        <log4j.version>1.2.17</log4j.version>
        
        <jackson.version>2.5.0</jackson.version>
      </properties>
    
      <dependencies>
        <dependency>
          <groupId>junit</groupId>
          <artifactId>junit</artifactId>
          <version>4.11</version>
          <scope>test</scope>
        </dependency>
    
        
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-test</artifactId>
          <version>${spring.version}</version>
          <scope>test</scope>
        </dependency>
    
        
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-webmvc</artifactId>
          <version>${spring.version}</version>
        </dependency>
    
        
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-core</artifactId>
          <version>${spring.version}</version>
        </dependency>
    
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-beans</artifactId>
          <version>${spring.version}</version>
        </dependency>
    
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-context</artifactId>
          <version>${spring.version}</version>
        </dependency>
    
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-context-support</artifactId>
          <version>${spring.version}</version>
        </dependency>
    
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-aop</artifactId>
          <version>${spring.version}</version>
        </dependency>
    
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-aspects</artifactId>
          <version>4.0.9.RELEASE</version>
          
            
              
              
            
          
        </dependency>
    
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-tx</artifactId>
          <version>${spring.version}</version>
        </dependency>
    
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-web</artifactId>
          <version>${spring.version}</version>
        </dependency>
    
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-jdbc</artifactId>
          <version>${spring.version}</version>
        </dependency>
    
        
        <dependency>
          <groupId>org.aspectj</groupId>
          <artifactId>aspectjrt</artifactId>
          <version>1.7.4</version>
        </dependency>
        <dependency>
          <groupId>org.aspectj</groupId>
          <artifactId>aspectjweaver</artifactId>
          <version>1.7.4</version>
        </dependency>
        <dependency>
          <groupId>cglib</groupId>
          <artifactId>cglib</artifactId>
          <version>3.1</version>
        </dependency>
        
    
        
        <dependency>
          <groupId>mysql</groupId>
          <artifactId>mysql-connector-java</artifactId>
          <version>5.1.34</version>
        </dependency>
        
        <dependency>
          <groupId>log4j</groupId>
          <artifactId>log4j</artifactId>
          <version>${log4j.version}</version>
        </dependency>
    
        <dependency>
          <groupId>org.slf4j</groupId>
          <artifactId>slf4j-api</artifactId>
          <version>${slf4j.version}</version>
        </dependency>
    
        <dependency>
          <groupId>org.slf4j</groupId>
          <artifactId>slf4j-log4j12</artifactId>
          <version>${slf4j.version}</version>
        </dependency>
        
        
        <dependency>
          <groupId>javax.servlet</groupId>
          <artifactId>javax.servlet-api</artifactId>
          <version>3.0.1</version>
          <scope>provided</scope>
        </dependency>
    
        <dependency>
          <groupId>org.apache.commons</groupId>
          <artifactId>commons-dbcp2</artifactId>
          <version>2.0</version>
        </dependency>
    
        
        <dependency>
          <groupId>org.springframework.data</groupId>
          <artifactId>spring-data-redis</artifactId>
          <version>1.4.2.RELEASE</version>
          
            
              
              
            
            
              
              
            
            
              
              
            
            
              
              
            
            
              
              
            
            
              
              
            
          
        </dependency>
    
        <dependency>
          <groupId>redis.clients</groupId>
          <artifactId>jedis</artifactId>
          <version>2.6.2</version>
          
            
              
              
            
          
        </dependency>
    
        <dependency>
          <groupId>org.apache.commons</groupId>
          <artifactId>commons-pool2</artifactId>
          <version>2.4.2</version>
        </dependency>
        
    
        <dependency>
          <groupId>org.projectlombok</groupId>
          <artifactId>lombok</artifactId>
          <version>1.14.4</version>
        </dependency>
      </dependencies>
    
      <build>
        <finalName>spring-demo</finalName>
        <plugins>
          <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
              <source>1.7</source>
              <target>1.7</target>
            </configuration>
          </plugin>
        </plugins>
    
        
        <resources>
          <resource>
            <directory>src/main/java</directory>
            <includes>
              <include>*.properties</include>
              <include>*.xml</include>
            </includes>
            <filtering>true</filtering>
          </resource>
          <resource>
            <directory>src/main/java/resources</directory>
            <includes>
              <include>*.properties</include>
              <include>*.xml</include>
            </includes>
            <filtering>true</filtering>
          </resource>
        </resources>
    
        
      </build>
    </project>

【RedisLock Java 实现】：

package com.caox.redis;
    
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.support.ClassPathXmlApplicationContext;
    import org.springframework.data.redis.core.StringRedisTemplate;
    
    import java.util.Random;
    import java.util.concurrent.TimeUnit;
    import java.util.concurrent.locks.Condition;
    import java.util.concurrent.locks.Lock;
    
    /**
     * @author : nazi
     * @version : 1.0
     * @date : 2018/12/29 14:22
     */
    public class RedisLock implements Lock{
        protected StringRedisTemplate redisStringTemplate;
    
        // 存储到redis中的锁标志
        private static final String LOCKED = "LOCKED";
    
        // 请求锁的超时时间(ms)
        private static final long TIME_OUT = 30000;
    
        // 锁的有效时间(s)
        public static final int EXPIRE = 60;
    
        // 锁标志对应的key;
        private String key;
    
        // state flag
        private volatile boolean isLocked = false;
    
        public RedisLock(String key) {
            this.key = key;
            @SuppressWarnings("resource")
            ApplicationContext ctx =  new ClassPathXmlApplicationContext("classpath*:application-context.xml");
            redisStringTemplate = (StringRedisTemplate)ctx.getBean("redisStringTemplate");
        }
    
        @Override
        public void lock() {
            //系统当前时间，毫秒
            long nowTime = System.nanoTime();
            //请求锁超时时间，毫秒
            long timeout = TIME_OUT*1000000;
            final Random r = new Random();
            try {
                //不断循环向Master节点请求锁，当请求时间(System.nanoTime() - nano)超过设定的超时时间则放弃请求锁
                //这个可以防止一个客户端在某个宕掉的master节点上阻塞过长时间
                //如果一个master节点不可用了，应该尽快尝试下一个master节点
                while ((System.nanoTime() - nowTime) < timeout) {
                    //将锁作为key存储到redis缓存中，存储成功则获得锁
                    if (redisStringTemplate.getConnectionFactory().getConnection().setNX(key.getBytes(),
                            LOCKED.getBytes())) {
                        //设置锁的有效期，也是锁的自动释放时间，也是一个客户端在其他客户端能抢占锁之前可以执行任务的时间
                        //可以防止因异常情况无法释放锁而造成死锁情况的发生
                        redisStringTemplate.expire(key, EXPIRE, TimeUnit.SECONDS);
                        isLocked = true;
                        //上锁成功结束请求
                        break;
                    }
                    //获取锁失败时，应该在随机延时后进行重试，避免不同客户端同时重试导致谁都无法拿到锁的情况出现
                    //睡眠3毫秒后继续请求锁
                    Thread.sleep(3, r.nextInt(500));
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    
        @Override
        public void unlock() {
            //释放锁
            //不管请求锁是否成功，只要已经上锁，客户端都会进行释放锁的操作
            if (isLocked) {
                redisStringTemplate.delete(key);
            }
        }
    
        @Override
        public void lockInterruptibly() throws InterruptedException {
            // TODO Auto-generated method stub
    
        }
    
        @Override
        public boolean tryLock() {
            // TODO Auto-generated method stub
            return false;
        }
    
        @Override
        public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
            // TODO Auto-generated method stub
            return false;
        }
    
        @Override
        public Condition newCondition() {
            // TODO Auto-generated method stub
            return null;
        }
    }

[【参考文献】： 分布式缓存技术redis系列（五）——redis实战（redis与spring整合，分布式锁实现）][redis_redis_redis_spring]

**【springboot + redis分布式 + aop】：**

package com.caox.aop;
    
    import com.caox.annotions.RedisSync;
    import com.caox.service.IRedisService;
    import lombok.extern.slf4j.Slf4j;
    import org.aspectj.lang.ProceedingJoinPoint;
    import org.aspectj.lang.annotation.Around;
    import org.aspectj.lang.annotation.Aspect;
    import org.aspectj.lang.annotation.Pointcut;
    import org.aspectj.lang.reflect.MethodSignature;
    import org.slf4j.Logger;
    import org.slf4j.LoggerFactory;
    import org.springframework.stereotype.Component;
    
    import javax.annotation.Resource;
    import java.lang.reflect.Method;
    import java.util.Objects;
    
    
    /**
     * @author : nazi
     * @version : 1.0
     * @date : 2019/1/3 10:53
     */
    @Slf4j
    @Aspect
    @Component
    public class RedisSyncAop {
        private static final Logger logger = LoggerFactory.getLogger(RedisSyncAop.class);
        @Resource
        private IRedisService iRedisService;
    
        @Pointcut("@annotation(com.caox.annotions.RedisSync)")
        private void anyMethod(){
        }
    
        @Around("anyMethod()")
        public Object around(ProceedingJoinPoint pjp) throws Throwable {
            Object result = null;
            //获得锁
            Method method = ((MethodSignature)pjp.getSignature()).getMethod();
            String key = method.toString();
            RedisSync redisSync = method.getAnnotation(RedisSync.class);
            long waitTime = redisSync.waitTime();
            long currTime = System.currentTimeMillis();
            Boolean state = iRedisService.setNx(key, currTime);
            long saveTime = 0L;
            while (!state) {
                // 之前存在key 并发开始
                Long tempSaveTime = iRedisService.get(key, Long.class);
                // 若锁被释放
                if (tempSaveTime == null) {
                    // 重新加锁
                    state = iRedisService.setNx(key, currTime);
                    continue;
                }
                // 锁被重新获取
                if (!tempSaveTime.equals(saveTime)) {
                    currTime = System.currentTimeMillis();
                    saveTime = tempSaveTime;
                }
                // 判断是否超时
                if (saveTime + redisSync.timeout() < currTime) {
                    // 超时，直接获得锁  获取上一个锁的时间value
                    Object tempTime = iRedisService.getSet(key, currTime);
                    if(tempTime == null){
                        state = iRedisService.setNx(key, currTime);
                        continue;
                    }
                    // 判断锁是否被释放 或 未被抢先获取  saveTime = tempSaveTime; tempTime(获取上一个锁时间value)
                    if (Objects.equals(saveTime, tempTime)) {
                        logger.warn("方法：{}，执行超时，已被强制解锁！", key);
                        break;
                    }
                }
                // 等待
                if(waitTime > 0) {
                    try {
                        Thread.sleep(waitTime);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                state = iRedisService.setNx(key, currTime);
            }
            // 执行方法
            result = pjp.proceed();
            Long currSaveTime = iRedisService.get(key, Long.class);
            // 判断锁未被判定为超时
            if (currSaveTime != null && Objects.equals(currSaveTime, currTime)) {
                // 释放锁
                iRedisService.del(key);
            }
            return result;
        }
    }

[【参考文献】：SpringBoot+Redis 从HelloWorld到分布式锁][SpringBoot_Redis _HelloWorld]

[783994-20160808160818684-1441187825.jpg]: /images/20220331/ddacca2b8fbc4aef84963dbb1d9b5044.png
[redis_redis_redis_spring]: https://www.cnblogs.com/hjwublog/p/5749929.html
[SpringBoot_Redis _HelloWorld]: https://www.liangzl.com/get-article-detail-673.html