问题描述
最近我们用Spring Cache + redis来做缓存。在高并发下@Cacheable 注解返回的内容是null。查看了一下源代码，在使用注解获取缓存的时候，RedisCache的get方法会先去判断key是否存在，然后再去获取值。这了就有一个漏铜，当线程1判断了key是存在的，紧接着这个时候这个key过期了，这时线程1再去获取值的时候返回的是null。
RedisCache的get方法源码：
public RedisCacheElement get(final RedisCacheKey cacheKey) {
Assert.notNull(cacheKey, "CacheKey must not be null!");
// 判断Key是否存在
Boolean exists = (Boolean) redisOperations.execute(new RedisCallback() {
@Override
public Boolean doInRedis(RedisConnection connection) throws DataAccessException {
return connection.exists(cacheKey.getKeyBytes());
}
});
if (!exists.booleanValue()) {
return null;
}
// 获取key对应的值
return new RedisCacheElement(cacheKey, fromStoreValue(lookup(cacheKey)));
}
// 获取值
protected Object lookup(Object key) {
RedisCacheKey cacheKey = key instanceof RedisCacheKey ? (RedisCacheKey) key : getRedisCacheKey(key);
byte[] bytes = (byte[]) redisOperations.execute(new AbstractRedisCacheCallback(
new BinaryRedisCacheElement(new RedisCacheElement(cacheKey, null), cacheValueAccessor), cacheMetadata) {
@Override
public byte[] doInRedis(BinaryRedisCacheElement element, RedisConnection connection) throws DataAccessException {
return connection.get(element.getKeyBytes());
}
});
return bytes == null ? null : cacheValueAccessor.deserializeIfNecessary(bytes);
}
解决方案
这个流程有问题，解决方案就是把这个流程倒过来，先去获取值，然后去判断这个key是否存在。不能直接用获取的值根据是否是NULL判断是否有值，因为Reids可能缓存NULL值。
重写RedisCache的get方法:
public RedisCacheElement get(final RedisCacheKey cacheKey) {
Assert.notNull(cacheKey, "CacheKey must not be null!");
RedisCacheElement redisCacheElement = new RedisCacheElement(cacheKey, fromStoreValue(lookup(cacheKey)));
Boolean exists = (Boolean) redisOperations.execute(new RedisCallback() {
@Override
public Boolean doInRedis(RedisConnection connection) throws DataAccessException {
return connection.exists(cacheKey.getKeyBytes());
}
});
if (!exists.booleanValue()) {
return null;
}
return redisCacheElement;
}
完整实现：
重写RedisCache的get方法
package com.xiaolyuh.redis.cache;
import org.springframework.dao.DataAccessException;
import org.springframework.data.redis.cache.RedisCache;
import org.springframework.data.redis.cache.RedisCacheElement;
import org.springframework.data.redis.cache.RedisCacheKey;
import org.springframework.data.redis.connection.RedisConnection;
import org.springframework.data.redis.core.RedisCallback;
import org.springframework.data.redis.core.RedisOperations;
import org.springframework.util.Assert;
/**
* 自定义的redis缓存
*
* @author yuhao.wang
*/
public class CustomizedRedisCache extends RedisCache {
private final RedisOperations redisOperations;
private final byte[] prefix;
public CustomizedRedisCache(String name, byte[] prefix, RedisOperations redisOperations, long expiration) {
super(name, prefix, redisOperations, expiration);
this.redisOperations = redisOperations;
this.prefix = prefix;
}
public CustomizedRedisCache(String name, byte[] prefix, RedisOperations redisOperations, long expiration, boolean allowNullValues) {
super(name, prefix, redisOperations, expiration, allowNullValues);
this.redisOperations = redisOperations;
this.prefix = prefix;
}
/**
* 重写父类的get函数。
* 父类的get方法，是先使用exists判断key是否存在，不存在返回null，存在再到redis缓存中去取值。这样会导致并发问题，
* 假如有一个请求调用了exists函数判断key存在，但是在下一时刻这个缓存过期了，或者被删掉了。
* 这时候再去缓存中获取值的时候返回的就是null了。
* 可以先获取缓存的值，再去判断key是否存在。
*
* @param cacheKey
* @return
*/
@Override
public RedisCacheElement get(final RedisCacheKey cacheKey) {
Assert.notNull(cacheKey, "CacheKey must not be null!");
RedisCacheElement redisCacheElement = new RedisCacheElement(cacheKey, fromStoreValue(lookup(cacheKey)));
Boolean exists = (Boolean) redisOperations.execute(new RedisCallback() {
@Override
public Boolean doInRedis(RedisConnection connection) throws DataAccessException {
return connection.exists(cacheKey.getKeyBytes());
}
});
if (!exists.booleanValue()) {
return null;
}
return redisCacheElement;
}
/**
* 获取RedisCacheKey
*
* @param key
* @return
*/
private RedisCacheKey getRedisCacheKey(Object key) {
return new RedisCacheKey(key).usePrefix(this.prefix)
.withKeySerializer(redisOperations.getKeySerializer());
}
}
重写RedisCacheManager
package com.xiaolyuh.redis.cache;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.cache.Cache;
import org.springframework.data.redis.cache.RedisCacheManager;
import org.springframework.data.redis.core.RedisOperations;
import org.springframework.data.redis.core.StringRedisTemplate;
import java.util.Collection;
/**
* 自定义的redis缓存管理器
* @author yuhao.wang
*/
public class CustomizedRedisCacheManager extends RedisCacheManager {
private static final Logger logger = LoggerFactory.getLogger(CustomizedRedisCacheManager.class);
public CustomizedRedisCacheManager(RedisOperations redisOperations) {
super(redisOperations);
}
public CustomizedRedisCacheManager(RedisOperations redisOperations, Collection cacheNames) {
super(redisOperations, cacheNames);
}
@Override
protected Cache getMissingCache(String name) {
long expiration = computeExpiration(name);
return new CustomizedRedisCache(
name,
(this.isUsePrefix() ? this.getCachePrefix().prefix(name) : null),
this.getRedisOperations(),
expiration);
}
}
配置Redis管理器
@Configuration
public class RedisConfig {
// redis缓存的有效时间单位是秒
@Value("${redis.default.expiration:3600}")
private long redisDefaultExpiration;
@Bean
public RedisCacheManager cacheManager(RedisTemplate redisTemplate) {
RedisCacheManager redisCacheManager = new CustomizedRedisCacheManager(redisTemplate);
redisCacheManager.setUsePrefix(true);
//这里可以设置一个默认的过期时间 单位是秒
redisCacheManager.setDefaultExpiration(redisDefaultExpiration);
return redisCacheManager;
}
/**
* 显示声明缓存key生成器
*
* @return
*/
@Bean
public KeyGenerator keyGenerator() {
return new SimpleKeyGenerator();
}
}
源码：
https://github.com/wyh-spring-ecosystem-student/spring-boot-student/tree/releases
spring-boot-student-cache-redis 工程
为监控而生的多级缓存框架 layering-cache这是我开源的一个多级缓存框架的实现，如果有兴趣可以看一下