Using redis to realize distributed lock (single redis)

     because redis It's a single threaded program , It can naturally ensure thread safety , As long as our order is a single order , It can ensure the safety of operation , and redis Provided in setnx key value command , Said when redis There is no key The key value of , Will create the value of this key value pair , If there are already , It doesn't work ,java There is valueOperations.setIfAbsent(key,value) And redis Medium setnx It's the same thing . We can use valueOperations.setIfAbsent Realize the operation of distributed locking , You can also set the expiration time for the lock , In this way, even if the client fails, the lock can be released after the expiration time

Boolean isLock=valueOperations.setIfAbsent("k1",value,120,TimeUnit.SECONDS);

    key It's any value ,value Ensure the uniqueness of each user , Easy to release the lock , Because we are releasing the lock, that is del key when , It needs to be ensured that only the program that actually obtains the lock can release the corresponding lock , Not everyone can release the lock , Prevent yourself from getting locked and being released by others , So the step of releasing the lock is ：

1. First judge the value of the lock variable , Whether you got the lock yourself
2. If the lock is obtained by yourself , You can release the lock
3. If you don't get the lock yourself , The lock cannot be released

This is not an order , But we need these commands to be executed atomically , So we use lua Script .Redis Yes lua Script calls are atomic , So some special scenes , Can be placed in lua To realize , Here is a simple example
     First, in the resources Create one in the directory lock.lua file

if redis.call("get",KEYS[1])==ARGV[1] then
    return redis.call("del",KEYS[1])
else
    return 0

end

When you type key[1] stay redis Is equal to the entered argv[1] when , Delete the original key, Release the lock , The lock cannot be released without waiting key[1]： Entered when acquiring the lock key,argv[1]： Entered when acquiring the lock value

@Test
    public void testLock03() {
    
        ValueOperations valueOperations=redisTemplate.opsForValue();
        String value= UUID.randomUUID().toString();
        // If key It's empty , Just set the value 
        Boolean isLock=valueOperations.setIfAbsent("k1",value,120,TimeUnit.SECONDS);
        if(isLock) {
    
            valueOperations.set("name","xxxx");
            String name=(String)valueOperations.get("name");
            System.out.println("name="+name);
            System.out.println(valueOperations.get("k1"));
            Boolean result=(Boolean)redisTemplate.execute(script, Collections.singletonList("k1"),value);
            System.out.println(result);
        }
        else {
    
            System.out.println(" There are threads in use , Please later ");
        }

    }

among singletonList Is an immutable list, And only one element , It's from redis take k1 Corresponding value, Look and method passed in value Same, different , The same thing is direct key（ Release the lock ）.

redisConfig class

// Serialize information , stay redis It looks good in 
@Configuration
public class RedisConfig {
    
    @Bean
    public RedisTemplate<String, Object> redisTemplate(RedisConnectionFactory redisConnectionFactory) {
    
        RedisTemplate<String, Object> redisTemplate = new RedisTemplate<>();
        //key Serialization 
        redisTemplate.setKeySerializer(new StringRedisSerializer());
        //value Serialization 
        redisTemplate.setValueSerializer(new GenericJackson2JsonRedisSerializer());
        //hash type key Serialization 
        redisTemplate.setHashKeySerializer(new StringRedisSerializer());
        //hash type value Serialization 
        redisTemplate.setHashValueSerializer(new GenericJackson2JsonRedisSerializer());
        // Injection connection factory 
        redisTemplate.setConnectionFactory(redisConnectionFactory);
        return redisTemplate;
    }

    @Bean
    public DefaultRedisScript<Boolean>  script() {
    
        DefaultRedisScript<Boolean> redisScript=new DefaultRedisScript<>();
        redisScript.setLocation(new ClassPathResource("lock.lua"));
        redisScript.setResultType(Boolean.class);
        return redisScript;
    }
}