spring cache注解@Cacheable缓存穿透详解

最近发现线上监控有个SQL调用量很大,但是方法的调用量不是很大,查看接口实现,发现接口是做了缓存操作的,使用Spring cache缓存注解结合tair实现缓存操作。

但是为啥SQL调用量这么大,难道缓存没有生效。测试发现缓存是正常的,分析了代码发现,代码存在缓存穿透的风险。

具体注解是这样的

@Cacheable(value = "storeDeliveryCoverage", key = "#sellerId + '|' + #cityCode", unless = "#result == null")

unless = "#result == null"表明接口返回值不为空的时候才缓存,如果线上有大量不合法的请求参数过来,由于为空的不会缓存起来,每次请求都打到DB上,导致DB的sql调用量巨大,给了黑客可乘之机,风险还是很大的。

找到原因之后就修改,查询结果为空的时候兜底一个null,把这句unless = "#result == null"条件去掉测试了一下,发现为空的话还是不会缓存。于是debug分析了一波源码,终于发现原来是tair的问题。

由于tair自身的特性,无法缓存null。既然无法缓存null,那我们就兜底一个空对象进去,取出来的时候把空对象转化为null。

基于这个思路我把Cache的实现改造了一下

@Override
    public void put(Object key, Object value) {
        if (value == null) {
            // 为空的话,兜底一个空对象,防止缓存穿透(由于tair自身特性不允许缓存null对象的原因,这里缓存一个空对象)
            value = new Nil();
        }
        if (value instanceof Serializable) {
            final String tairKey = String.format("%s:%s", this.name, key);
            final ResultCode resultCode = this.tairManager.put(
                    this.namespace,
                    tairKey,
                    (Serializable) value,
                    0,
                    this.timeout
            );
            if (resultCode != ResultCode.SUCCESS) {
                TairSpringCache.log.error(
                        String.format(
                                "[CachePut]: unable to put %s => %s into tair due to: %s",
                                key,
                                value,
                                resultCode.getMessage()
                        )
                );
            }
        } else {
            throw new RuntimeException(
                    String.format(
                            "[CachePut]: value %s is not Serializable",
                            value
                    )
            );
        }
    }

Nil类默认是一个空对象,这里给了个内部类:

static class Nil implements Serializable {
        private static final long serialVersionUID = -9138993336039047508L;
    }

取缓存的get方法实现

@Override
    public ValueWrapper get(Object key) {
        final String tairKey = String.format("%s:%s", this.name, key);
        final Result result = this.tairManager.get(this.namespace, tairKey);
        if (result.isSuccess() && (result.getRc() == ResultCode.SUCCESS)) {
            final Object obj = result.getValue().getValue();
            // 缓存为空兜底的是Nil对象,这里返回的时候需要转为null
            if (obj instanceof Nil) {
                return null;
            }
            return () -> obj;
        }
        return null;
    }

改好了之后,测试一下,结果发现还是没有生效,缓存没有兜底,请求都打到DB上了。

debug走一遍,看了下Cache的源码,终于发现关键问题所在(具体实现流程参考上一篇:Spring Cache- 缓存拦截器( CacheInterceptor)):

private Object execute(final CacheOperationInvoker invoker, Method method, CacheOperationContexts contexts) {
		// Special handling of synchronized invocation
		if (contexts.isSynchronized()) {
			CacheOperationContext context = contexts.get(CacheableOperation.class).iterator().next();
			if (isConditionPassing(context, CacheOperationExpressionEvaluator.NO_RESULT)) {
				Object key = generateKey(context, CacheOperationExpressionEvaluator.NO_RESULT);
				Cache cache = context.getCaches().iterator().next();
				try {
					return wrapCacheValue(method, cache.get(key, new Callable() {
						@Override
						public Object call() throws Exception {
							return unwrapReturnValue(invokeOperation(invoker));
						}
					}));
				}
				catch (Cache.ValueRetrievalException ex) {
					// The invoker wraps any Throwable in a ThrowableWrapper instance so we
					// can just make sure that one bubbles up the stack.
					throw (CacheOperationInvoker.ThrowableWrapper) ex.getCause();
				}
			}
			else {
				// No caching required, only call the underlying method
				return invokeOperation(invoker);
			}
		}
		// 处理beforeIntercepte=true的缓存删除操作
		processCacheEvicts(contexts.get(CacheEvictOperation.class), true,
				CacheOperationExpressionEvaluator.NO_RESULT);
		// 从缓存中查找,是否有匹配@Cacheable的缓存数据
		Cache.ValueWrapper cacheHit = findCachedItem(contexts.get(CacheableOperation.class));
		// 如果@Cacheable没有被缓存,那么就需要将数据缓存起来,这里将@Cacheable操作收集成CachePutRequest集合,以便后续做@CachePut缓存数据存放。
		List cachePutRequests = new LinkedList();
		if (cacheHit == null) {
			collectPutRequests(contexts.get(CacheableOperation.class),
					CacheOperationExpressionEvaluator.NO_RESULT, cachePutRequests);
		}
		Object cacheValue;
		Object returnValue;
		//如果没有@CachePut操作,就使用@Cacheable获取的结果(可能也没有@Cableable,所以result可能为空)。
		if (cacheHit != null && cachePutRequests.isEmpty() && !hasCachePut(contexts)) {
			//如果没有@CachePut操作,并且cacheHit不为空,说明命中缓存了,直接返回缓存结果
			cacheValue = cacheHit.get();
			returnValue = wrapCacheValue(method, cacheValue);
		}
		else {
			// 否则执行具体方法内容,返回缓存的结果
			returnValue = invokeOperation(invoker);
			cacheValue = unwrapReturnValue(returnValue);
		}
		// Collect any explicit @CachePuts
		collectPutRequests(contexts.get(CachePutOperation.class), cacheValue, cachePutRequests);
		// Process any collected put requests, either from @CachePut or a @Cacheable miss
		for (CachePutRequest cachePutRequest : cachePutRequests) {
			cachePutRequest.apply(cacheValue);
		}
		// Process any late evictions
		processCacheEvicts(contexts.get(CacheEvictOperation.class), false, cacheValue);
		return returnValue;
	}
 
  
 

根据key从缓存中查找,返回的结果是ValueWrapper,它是返回结果的包装器:

private Cache.ValueWrapper findCachedItem(Collection contexts) {
		Object result = CacheOperationExpressionEvaluator.NO_RESULT;
		for (CacheOperationContext context : contexts) {
			if (isConditionPassing(context, result)) {
				Object key = generateKey(context, result);
				Cache.ValueWrapper cached = findInCaches(context, key);
				if (cached != null) {
					return cached;
				}
				else {
					if (logger.isTraceEnabled()) {
						logger.trace("No cache entry for key '" + key + "' in cache(s) " + context.getCacheNames());
					}
				}
			}
		}
		return null;
	}
private Cache.ValueWrapper findInCaches(CacheOperationContext context, Object key) {
		for (Cache cache : context.getCaches()) {
			Cache.ValueWrapper wrapper = doGet(cache, key);
			if (wrapper != null) {
				if (logger.isTraceEnabled()) {
					logger.trace("Cache entry for key '" + key + "' found in cache '" + cache.getName() + "'");
				}
				return wrapper;
			}
		}
		return null;
	}

这里判断缓存是否命中的逻辑是根据cacheHit是否为空,而cacheHit是ValueWrapper类型,查看ValueWrapper是一个接口,它的实现类是SimpleValueWrapper,这是一个包装器,将缓存的结果包装起来了。

而我们前面的get方法取缓存的时候如果为Nil对象,返回的是null,这样缓存判断出来是没有命中,即cacheHit==null,就会去执行具体方法朔源。

所以到这里已经很清晰了,关键问题是get取缓存的结果如果是兜底的Nil对象,应该返回new SimpleValueWrapper(null)。

应该返回包装器,包装的是缓存的对象为null。

测试了一下,发现ok了

具体源码如下:

/**
 * 基于tair的缓存,适配spring缓存框架
 */
public class TairSpringCache implements Cache {
    private static final Logger log = LoggerFactory.getLogger(TairSpringCache.class);
    private TairManager tairManager;
    private final String name;
    private int namespace;
    private int timeout;
    public TairSpringCache(String name, TairManager tairManager, int namespace) {
        this(name, tairManager, namespace, 0);
    }
    public TairSpringCache(String name, TairManager tairManager, int namespace, int timeout) {
        this.name = name;
        this.tairManager = tairManager;
        this.namespace = namespace;
        this.timeout = timeout;
    }
    @Override
    public String getName() {
        return this.name;
    }
    @Override
    public Object getNativeCache() {
        return this.tairManager;
    }
    @Override
    public ValueWrapper get(Object key) {
        final String tairKey = String.format("%s:%s", this.name, key);
        final Result result = this.tairManager.get(this.namespace, tairKey);
        if (result.isSuccess() && (result.getRc() == ResultCode.SUCCESS)) {
            final Object obj = result.getValue().getValue();
            // 缓存为空兜底的是Nil对象,这里返回的时候需要转为null
            if (obj instanceof Nil) {
                return () -> null;
            }
            return () -> obj;
        }
        return null;
    }
    @Override
    public  T get(Object key, Class type) {
        return (T) this.get(key).get();
    }
    public  T get(Object o, Callable callable) {
        return null;
    }
    @Override
    public void put(Object key, Object value) {
        if (value == null) {
            // 为空的话,兜底一个空对象,防止缓存穿透(由于tair自身特性不允许缓存null对象的原因,这里缓存一个空对象)
            value = new Nil();
        }
        if (value instanceof Serializable) {
            final String tairKey = String.format("%s:%s", this.name, key);
            final ResultCode resultCode = this.tairManager.put(
                    this.namespace,
                    tairKey,
                    (Serializable) value,
                    0,
                    this.timeout
            );
            if (resultCode != ResultCode.SUCCESS) {
                TairSpringCache.log.error(
                        String.format(
                                "[CachePut]: unable to put %s => %s into tair due to: %s",
                                key,
                                value,
                                resultCode.getMessage()
                        )
                );
            }
        } else {
            throw new RuntimeException(
                    String.format(
                            "[CachePut]: value %s is not Serializable",
                            value
                    )
            );
        }
    }
    public ValueWrapper putIfAbsent(Object key, Object value) {
        final ValueWrapper vw = this.get(key);
        if (vw.get() == null) {
            this.put(key, value);
        }
        return vw;
    }
    @Override
    public void evict(Object key) {
        final String tairKey = String.format("%s:%s", this.name, key);
        final ResultCode resultCode = this.tairManager.delete(this.namespace, tairKey);
        if ((resultCode == ResultCode.SUCCESS)
                || (resultCode == ResultCode.DATANOTEXSITS)
                || (resultCode == ResultCode.DATAEXPIRED)) {
            return;
        }
        else {
            final String errMsg = String.format(
                    "[CacheDelete]: unable to evict key %s, resultCode: %s",
                    key,
                    resultCode
            );
            TairSpringCache.log.error(errMsg);
            throw new RuntimeException(errMsg);
        }
    }
    @Override
    public void clear() {
        //TODO fgz: implement here later
    }
    public void setTairManager(TairManager tairManager) {
        this.tairManager = tairManager;
    }
    public void setNamespace(int namespace) {
        this.namespace = namespace;
    }
    public void setTimeout(int timeout) {
        this.timeout = timeout;
    }
    static class Nil implements Serializable {
        private static final long serialVersionUID = -9138993336039047508L;
    }
}

测试用例就不贴了。

以上为个人经验,希望能给大家一个参考,也希望大家多多支持脚本之家。

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