单机 Redis 的分布式锁

àì夳堔傛蜴生んèń 2023-07-17 12:40 83阅读 0赞

### 文章目录 ###

*  定义锁的抽象类
 *  错误版：
 *  改进一
 *  改进二
 *  改进三 （lua原子操作+刷新过期时间）
 *  最终代码
 *  总结

# 定义锁的抽象类 #

抽象类RedisLock实现java.util.concurrent包下的Lock接口，然后对一些方法提供默认实现，子类只需实现lock方法和unlock方法即可。代码如下：

public abstract class RedisLock implements Lock { 
    
        protected Jedis jedis;
        protected String lockKey;
    
        public RedisLock(Jedis jedis,String lockKey) { 
            this(jedis, lockKey);
        }
    
        public void sleepBySencond(int sencond){ 
            try { 
                Thread.sleep(sencond*1000);
            } catch (InterruptedException e) { 
                e.printStackTrace();
            }
        }
    
        @Override
        public void lockInterruptibly(){ }
    
        @Override
        public Condition newCondition() { 
            return null;
        }
    
        @Override
        public boolean tryLock() { 
            return false;
        }
    
        @Override
        public boolean tryLock(long time, TimeUnit unit){ 
            return false;
        }
    
    }

# 错误版： #

public class LockCase1 extends RedisLock { 
    
          public LockCase1(Jedis jedis, String name) { 
              super(jedis, name);
    	  }
    	  
          @Override
          public void lock() { 
              while(true){ 
                  String result = jedis.set(lockKey, "value", "NX");
                  if(OK.equals(result)){ 
    			 	 System.out.println(Thread.currentThread().getId()+"加锁成功!");
    				 break; 
    			  }
    	  	  } 
    	  }
    	  
          @Override
          public void unlock() { 
              jedis.del(lockKey);
          }
    }

假设有两个客户端A和B，A获取到分布式的锁。A执行了一会，突然A所在的服务器断电了（或者其他什么的），也就是客户端A挂了。这时出现一个问题，这个锁一直存在，且不会释放，其他客户端永远获取不到锁。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI0MDk1MDU1_size_16_color_FFFFFF_t_70]

# 改进一 #

public void lock() { 
    	while(true){ 
    		String result = jedis.set(lockKey, "value", NOT_EXIST,SECONDS,30);
    		if(OK.equals(result)){ 
    			System.out.println(Thread.currentThread().getId()+"加锁成功!");
    			break; 
    		}
    	} 
    }

这时又出现一个问题，问题出现的步骤如下：

*  1.客户端A获取锁成功，过期时间30秒。
 *  2.客户端A在某个操作上阻塞了50秒。
 *  3.30秒时间到了，锁自动释放了。
 *  4.客户端B获取到了对应同一资源的锁。
 *  5.客户端A从阻塞中恢复出来，释放掉了客户端B持有的锁。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI0MDk1MDU1_size_16_color_FFFFFF_t_70 1]

# 改进二 #

抽象类RedisLock增加lockValue字段，lockValue字段的默认值为UUID随机值假设当前线程ID。

实现类里lock和unlock方法

public void lock() { 
    	while(true){ 
    		String result = jedis.set(lockKey, lockValue, NOT_EXIST,SECONDS, 30);
    		if(OK.equals(result)){ 
    			System.out.println(Thread.currentThread().getId()+"加锁成功!");
    			break; 
    		}
    	} 
    }
    
    ...
    
    public void unlock() { 
    	String lockValue = jedis.get(lockKey);
    	if (lockValue.equals(lockValue)){ 
    	    jedis.del(lockKey);
    	}
    }

问题在于如果调用jedis.del()方法的时候，这把锁已经不属于当前客户端的时候会解除他人加的锁。那么是否真的有这种场景？答案是肯定的，比如客户端A加锁，一段时间之后客户端A解锁，在执行jedis.del()之前，锁突然过期了，此时客户端B尝试加锁成功，然后客户端A再执行del()方法，则将客户端B的锁给解除了。

> 根本原因是if判断和del操作不是原子性的。

# 改进三 （lua原子操作+刷新过期时间） #

public void unlock() { 
    	// 使用lua脚本进行原子删除操作
    	String checkAndDelScript = "if redis.call('get', KEYS[1]) == ARGV[1] then" +
    	                           "return redis.call('del', KEYS[1]) " +
    	                           "else " +
    	                           "return 0 " +
    	                           "end";
    	jedis.eval(checkAndDelScript, 1, lockKey, lockValue);
    }

这时还有最后一个问题：过期时间如何保证大于业务执行时间。  
抽象类中RedisLock增加一个boolean类型的属性isOpenExpirationRenewal，用来标识是否开启定时刷新过期时间。

在增加一个scheduleExpirationRenewal方法用于开启刷新过期时间的线程。

package com.zhuawa.course.biz.redis.distributedlock;
    
    
    import redis.clients.jedis.Jedis;
    
    import java.util.UUID;
    import java.util.concurrent.TimeUnit;
    import java.util.concurrent.locks.Condition;
    import java.util.concurrent.locks.Lock;
    
    public abstract class RedisLock implements Lock { 
    
        protected Jedis jedis;
        protected String lockKey;
        protected String lockValue;
        protected volatile boolean isOpenExpirationRenewal = true;
    
        public RedisLock(Jedis jedis,String lockKey) { 
            this(jedis, lockKey, UUID.randomUUID().toString()+Thread.currentThread().getId());
        }
    
        public RedisLock(Jedis jedis, String lockKey, String lockValue) { 
            this.jedis = jedis;
            this.lockKey = lockKey;
            this.lockValue = lockValue;
        }
    
        public void sleepBySencond(int sencond){ 
            try { 
                Thread.sleep(sencond*1000);
            } catch (InterruptedException e) { 
                e.printStackTrace();
            }
        }
    
        /** * 开启定时刷新 */
        protected void scheduleExpirationRenewal(){ 
            Thread renewalThread = new Thread(new ExpirationRenewal());
            renewalThread.start();
        }
        /** * 刷新key的过期时间 */
        private class ExpirationRenewal implements Runnable{ 
            @Override
            public void run() { 
                while (isOpenExpirationRenewal){ 
                    System.out.println("执行延迟失效时间中...");
    
                    String checkAndExpireScript = "if redis.call('get', KEYS[1]) == ARGV[1] then " +
                            "return redis.call('expire',KEYS[1],ARGV[2]) " +
                            "else " +
                            "return 0 end";
                    jedis.eval(checkAndExpireScript, 1, lockKey, lockValue, "30");
    
                    //休眠10秒
                    sleepBySencond(10);
                }
            }
        }
    
        @Override
        public void lockInterruptibly(){ }
    
        @Override
        public Condition newCondition() { 
            return null;
        }
    
        @Override
        public boolean tryLock() { 
            return false;
        }
    
        @Override
        public boolean tryLock(long time, TimeUnit unit){ 
            return false;
        }
    
    }

加锁代码在获取锁成功后将isOpenExpirationRenewal置为true，并且调用scheduleExpirationRenewal方法，开启刷新过期时间的线程。

public void lock() { 
        while (true) { 
            String result = jedis.set(lockKey, lockValue, NOT_EXIST, SECONDS, 10);
            if (OK.equals(result)) { 
                System.out.println("线程id:"+Thread.currentThread().getName() + "加锁成功!时间:"+LocalTime.now());
    
                //开启定时刷新过期时间
                isOpenExpirationRenewal = true;
                scheduleExpirationRenewal();
                break;
            }
            System.out.println("线程id:"+Thread.currentThread().getName() + "获取锁失败，休眠10秒!时间:"+LocalTime.now());
            //休眠10秒
            sleepBySencond(10);
        }
    }

解锁代码增加一行代码，将isOpenExpirationRenewal属性置为false，停止刷新过期时间的线程轮询。

public void unlock() { 
        System.out.println("线程id:"+Thread.currentThread().getId() + "解锁!时间:"+LocalTime.now());
    
        String checkAndDelScript = "if redis.call('get', KEYS[1]) == ARGV[1] then " +
                                    "return redis.call('del', KEYS[1]) " +
                                    "else " +
                                    "return 0 " +
                                    "end";
        jedis.eval(checkAndDelScript, 1, lockKey, lockValue);
        isOpenExpirationRenewal = false;
    
    }

# 最终代码 #

常量类：

public class LockConstants { 
        public static final String OK = "OK";
    
        /** NX|XX, NX -- Only set the key if it does not already exist. XX -- Only set the key if it already exist. **/
        public static final String NOT_EXIST = "NX";
        public static final String EXIST = "XX";
    
        /** expx EX|PX, expire time units: EX = seconds; PX = milliseconds **/
        public static final String SECONDS = "EX";
        public static final String MILLISECONDS = "PX";
    
        private LockConstants() { }
    }

抽象类：

import redis.clients.jedis.Jedis;
    
    import java.util.UUID;
    import java.util.concurrent.TimeUnit;
    import java.util.concurrent.locks.Condition;
    import java.util.concurrent.locks.Lock;
    
    public abstract class RedisLock implements Lock { 
    
        protected Jedis jedis;
        protected String lockKey;
        protected String lockValue;
        protected volatile boolean isOpenExpirationRenewal = true;
    
        public RedisLock(Jedis jedis,String lockKey) { 
            this(jedis, lockKey, UUID.randomUUID().toString()+Thread.currentThread().getId());
        }
    
        public RedisLock(Jedis jedis, String lockKey, String lockValue) { 
            this.jedis = jedis;
            this.lockKey = lockKey;
            this.lockValue = lockValue;
        }
    
        public void sleepBySencond(int sencond){ 
            try { 
                Thread.sleep(sencond*1000);
            } catch (InterruptedException e) { 
                e.printStackTrace();
            }
        }
    
        /** * 开启定时刷新 */
        protected void scheduleExpirationRenewal(){ 
            Thread renewalThread = new Thread(new ExpirationRenewal());
            renewalThread.start();
        }
        /** * 刷新key的过期时间 */
        private class ExpirationRenewal implements Runnable{ 
            @Override
            public void run() { 
                while (isOpenExpirationRenewal){ 
                    System.out.println("执行延迟失效时间中...");
    
                    String checkAndExpireScript = "if redis.call('get', KEYS[1]) == ARGV[1] then " +
                            "return redis.call('expire',KEYS[1],ARGV[2]) " +
                            "else " +
                            "return 0 end";
                    jedis.eval(checkAndExpireScript, 1, lockKey, lockValue, "30");
    
                    //休眠10秒
                    sleepBySencond(10);
                }
            }
        }
    
        @Override
        public void lockInterruptibly(){ }
    
        @Override
        public Condition newCondition() { 
            return null;
        }
    
        @Override
        public boolean tryLock() { 
            return false;
        }
    
        @Override
        public boolean tryLock(long time, TimeUnit unit){ 
            return false;
        }
    
    }

实现类：

import redis.clients.jedis.Jedis;
    
    import java.time.LocalTime;
    
    import static com.zhuawa.course.biz.redis.distributedlock.LockConstants.NOT_EXIST;
    import static com.zhuawa.course.biz.redis.distributedlock.LockConstants.OK;
    import static com.zhuawa.course.biz.redis.distributedlock.LockConstants.SECONDS;
    
    
    public class RedisDistributedLock extends RedisLock { 
    
        public RedisDistributedLock(Jedis jedis, String lockKey) { 
            super(jedis, lockKey);
    
        }
    
        @Override
        public void lock() { 
            while (true) { 
                String result = jedis.set(lockKey, lockValue, NOT_EXIST, SECONDS, 10);
                if (OK.equals(result)) { 
                    System.out.println("线程id:"+Thread.currentThread().getName() + "加锁成功!时间:"+LocalTime.now());
    
                    //开启定时刷新过期时间
                    isOpenExpirationRenewal = true;
                    scheduleExpirationRenewal();
                    break;
                }
                System.out.println("线程id:"+Thread.currentThread().getName() + "获取锁失败，休眠10秒!时间:"+LocalTime.now());
                //休眠10秒
                sleepBySencond(10);
            }
        }
    
        @Override
        public void unlock() { 
            System.out.println("线程id:"+Thread.currentThread().getId() + "解锁!时间:"+LocalTime.now());
    
            String checkAndDelScript = "if redis.call('get', KEYS[1]) == ARGV[1] then " +
                                        "return redis.call('del', KEYS[1]) " +
                                        "else " +
                                        "return 0 " +
                                        "end";
            jedis.eval(checkAndDelScript, 1, lockKey, lockValue);
            isOpenExpirationRenewal = false;
    
        }
    }

# 总结 #

setnx+lua  
4大要点：

*  set命令要用`set key value px milliseconds nx`
 *  value要具有唯一性
 *  释放锁时要验证value的值，不能误解锁
 *  定时刷新过期时间保障过期时间大于业务执行时间

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI0MDk1MDU1_size_16_color_FFFFFF_t_70]: /images/20230528/d85b45e5df3d4b7e8ad9dca75985aded.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI0MDk1MDU1_size_16_color_FFFFFF_t_70 1]: /images/20230528/a3c3dcfeb2a34f34a431af94bda1f28f.png