Spring cache源码分析

Spring cache是一个缓存API层，封装了对多种缓存的通用操作，可以借助注解方便地为程序添加缓存功能。 常见的注解有@Cacheable、@CachePut、@CacheEvict，有没有想过背后的原理是什么？楼主带着疑问，阅读完Spring cache的源码后，做一个简要总结。 先说结论，核心逻辑在CacheAspectSupport类 ，封装了所有的缓存操作的主体逻辑，下面详细介绍。

题外话：如何阅读开源代码？

有2种方法，可以结合起来使用：

• 静态代码阅读 ：查找关键类、方法的usage之处，熟练使用find usages功能，找到所有相关的类、方法，静态分析核心逻辑的执行过程，一步步追根问底，直至建立全貌
• 运行时debug ：在关键方法上加上断点，并且写一个单元测试调用类库/框架，熟练使用step into/step over/resume来动态分析代码的执行过程

核心类图

如图所示，可以分成以下几类class：

• Cache、CacheManager：Cache抽象了缓存的通用操作，如get、put，而CacheManager是Cache的集合，之所以需要多个Cache对象，是因为需要多种缓存失效时间、缓存条目上限等
• CacheInterceptor、CacheAspectSupport、AbstractCacheInvoker：CacheInterceptor是一个AOP方法拦截器，在方法前后做额外的逻辑，也即查询缓存、写入缓存等，它继承了CacheAspectSupport（缓存操作的主体逻辑）、AbstractCacheInvoker（封装了对Cache的读写）
• CacheOperation、AnnotationCacheOperationSource、SpringCacheAnnotationParser：CacheOperation定义了缓存操作的缓存名字、缓存key、缓存条件condition、CacheManager等，AnnotationCacheOperationSource是一个获取缓存注解对应CacheOperation的类，而SpringCacheAnnotationParser是真正解析注解的类，解析后会封装成CacheOperation集合供AnnotationCacheOperationSource查找

源码分析（带注释解释）

下面对Spring cache源码做分析，带注释解释，只摘录核心代码片段。

1、解析注解

首先看看注解是如何解析的。注解只是一个标记，要让它真正工作起来，需要对注解做解析操作，并且还要有对应的实际逻辑。

SpringCacheAnnotationParser：负责解析注解，返回CacheOperation集合

public class SpringCacheAnnotationParser implements CacheAnnotationParser, Serializable { // 解析类级别的缓存注解@Overridepublic Collection parseCacheAnnotations(Class type) {DefaultCacheConfig defaultConfig = getDefaultCacheConfig(type);return parseCacheAnnotations(defaultConfig, type);} // 解析方法级别的缓存注解@Overridepublic Collection parseCacheAnnotations(Method method) {DefaultCacheConfig defaultConfig = getDefaultCacheConfig(method.getDeclaringClass());return parseCacheAnnotations(defaultConfig, method);} // 解析缓存注解private Collection parseCacheAnnotations(DefaultCacheConfig cachingConfig, AnnotatedElement ae) {Collection ops = null;  // 解析@Cacheable注解Collection cacheables = AnnotatedElementUtils.getAllMergedAnnotations(ae, Cacheable.class);if (!cacheables.isEmpty()) {ops = lazyInit(ops);for (Cacheable cacheable : cacheables) {ops.add(parseCacheableAnnotation(ae, cachingConfig, cacheable));}}  // 解析@CacheEvict注解Collection evicts = AnnotatedElementUtils.getAllMergedAnnotations(ae, CacheEvict.class);if (!evicts.isEmpty()) {ops = lazyInit(ops);for (CacheEvict evict : evicts) {ops.add(parseEvictAnnotation(ae, cachingConfig, evict));}}  // 解析@CachePut注解Collection puts = AnnotatedElementUtils.getAllMergedAnnotations(ae, CachePut.class);if (!puts.isEmpty()) {ops = lazyInit(ops);for (CachePut put : puts) {ops.add(parsePutAnnotation(ae, cachingConfig, put));}}  // 解析@Caching注解Collection cachings = AnnotatedElementUtils.getAllMergedAnnotations(ae, Caching.class);if (!cachings.isEmpty()) {ops = lazyInit(ops);for (Caching caching : cachings) {Collection cachingOps = parseCachingAnnotation(ae, cachingConfig, caching);if (cachingOps != null) {ops.addAll(cachingOps);}}}return ops;}复制代码

AnnotationCacheOperationSource：调用SpringCacheAnnotationParser获取注解对应CacheOperation

public class AnnotationCacheOperationSource extends AbstractFallbackCacheOperationSource implements Serializable { // 查找类级别的CacheOperation列表@Overrideprotected Collection findCacheOperations(final Class clazz) {return determineCacheOperations(new CacheOperationProvider() {@Overridepublic Collection getCacheOperations(CacheAnnotationParser parser) {return parser.parseCacheAnnotations(clazz);}});} // 查找方法级别的CacheOperation列表@Overrideprotected Collection findCacheOperations(final Method method) {return determineCacheOperations(new CacheOperationProvider() {@Overridepublic Collection getCacheOperations(CacheAnnotationParser parser) {return parser.parseCacheAnnotations(method);}});}}复制代码

AbstractFallbackCacheOperationSource：AnnotationCacheOperationSource的父类，实现了获取CacheOperation的通用逻辑

public abstract class AbstractFallbackCacheOperationSource implements CacheOperationSource {/** * Cache of CacheOperations, keyed by method on a specific target class. * 
As this base class is not marked Serializable, the cache will be recreated * after serialization - provided that the concrete subclass is Serializable. */private final Map<Object, Collection> attributeCache =new ConcurrentHashMap<Object, Collection>(1024);// 根据Method、Class反射信息，获取对应的CacheOperation列表@Overridepublic Collection getCacheOperations(Method method, Class targetClass) {if (method.getDeclaringClass() == Object.class) {return null;}Object cacheKey = getCacheKey(method, targetClass);Collection cached = this.attributeCache.get(cacheKey);  // 因解析反射信息较耗时，所以用map缓存，避免重复计算  // 如在map里已记录，直接返回if (cached != null) {return (cached != NULL_CACHING_ATTRIBUTE ? cached : null);}  // 否则做一次计算，然后写入mapelse {Collection cacheOps = computeCacheOperations(method, targetClass);if (cacheOps != null) {if (logger.isDebugEnabled()) {logger.debug("Adding cacheable method '" + method.getName() + "' with attribute: " + cacheOps);}this.attributeCache.put(cacheKey, cacheOps);}else {this.attributeCache.put(cacheKey, NULL_CACHING_ATTRIBUTE);}return cacheOps;}} // 计算缓存操作列表，优先用target代理类的方法上的注解，如果不存在则其次用target代理类，再次用原始类的方法，最后用原始类private Collection computeCacheOperations(Method method, Class targetClass) {// Don't allow no-public methods as required.if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {return null;}// The method may be on an interface, but we need attributes from the target class.// If the target class is null, the method will be unchanged.Method specificMethod = ClassUtils.getMostSpecificMethod(method, targetClass);// If we are dealing with method with generic parameters, find the original method.specificMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);  // 调用findCacheOperations（由子类AnnotationCacheOperationSource实现），最终通过SpringCacheAnnotationParser来解析// First try is the method in the target class.Collection opDef = findCacheOperations(specificMethod);if (opDef != null) {return opDef;}// Second try is the caching operation on the target class.opDef = findCacheOperations(specificMethod.getDeclaringClass());if (opDef != null && ClassUtils.isUserLevelMethod(method)) {return opDef;}if (specificMethod != method) {// Fallback is to look at the original method.opDef = findCacheOperations(method);if (opDef != null) {return opDef;}// Last fallback is the class of the original method.opDef = findCacheOperations(method.getDeclaringClass());if (opDef != null && ClassUtils.isUserLevelMethod(method)) {return opDef;}}return null;}复制代码

2、逻辑执行

以@Cacheable背后的逻辑为例。预期是先查缓存，如果缓存命中了就直接使用缓存值，否则执行业务逻辑，并把结果写入缓存。

ProxyCachingConfiguration：是一个配置类，用于生成CacheInterceptor类和CacheOperationSource类的Spring bean

CacheInterceptor：是一个AOP方法拦截器，它通过CacheOperationSource获取第1步解析注解的CacheOperation结果（如缓存名字、缓存key、condition条件），本质上是拦截原始方法的执行，在之前、之后增加逻辑

// 核心类，缓存拦截器public class CacheInterceptor extends CacheAspectSupport implements MethodInterceptor, Serializable { // 拦截原始方法的执行，在之前、之后增加逻辑@Overridepublic Object invoke(final MethodInvocation invocation) throws Throwable {Method method = invocation.getMethod();  // 封装原始方法的执行到一个回调接口，便于后续调用CacheOperationInvoker aopAllianceInvoker = new CacheOperationInvoker() {@Overridepublic Object invoke() {try {     // 原始方法的执行return invocation.proceed();}catch (Throwable ex) {throw new ThrowableWrapper(ex);}}};try {   // 调用父类CacheAspectSupport的方法return execute(aopAllianceInvoker, invocation.getThis(), method, invocation.getArguments());}catch (CacheOperationInvoker.ThrowableWrapper th) {throw th.getOriginal();}}}复制代码

CacheAspectSupport：缓存切面支持类，是CacheInterceptor的父类，封装了所有的缓存操作的主体逻辑

主要流程如下：

1. 通过CacheOperationSource，获取所有的CacheOperation列表
2. 如果有@CacheEvict注解、并且标记为在调用前执行，则做删除/清空缓存的操作
3. 如果有@Cacheable注解，查询缓存
4. 如果缓存未命中（查询结果为null），则新增到cachePutRequests，后续执行原始方法后会写入缓存
5. 缓存命中时，使用缓存值作为结果；缓存未命中、或有@CachePut注解时，需要调用原始方法，使用原始方法的返回值作为结果
6. 如果有@CachePut注解，则新增到cachePutRequests
7. 如果缓存未命中，则把查询结果值写入缓存；如果有@CachePut注解，也把方法执行结果写入缓存
8. 如果有@CacheEvict注解、并且标记为在调用后执行，则做删除/清空缓存的操作

// 核心类，缓存切面支持类，封装了所有的缓存操作的主体逻辑public abstract class CacheAspectSupport extends AbstractCacheInvokerimplements BeanFactoryAware, InitializingBean, SmartInitializingSingleton { // CacheInterceptor调父类的该方法protected Object execute(CacheOperationInvoker invoker, Object target, Method method, Object[] args) {// Check whether aspect is enabled (to cope with cases where the AJ is pulled in automatically)if (this.initialized) {Class targetClass = getTargetClass(target);   // 通过CacheOperationSource，获取所有的CacheOperation列表Collection operations = getCacheOperationSource().getCacheOperations(method, targetClass);if (!CollectionUtils.isEmpty(operations)) {    // 继续调一个private的execute方法执行return execute(invoker, method, new CacheOperationContexts(operations, method, args, target, targetClass));}}  // 如果spring bean未初始化完成，则直接调用原始方法。相当于原始方法没有缓存功能。return invoker.invoke();} private的execute方法private Object execute(final CacheOperationInvoker invoker, Method method, CacheOperationContexts contexts) {// Special handling of synchronized invocationif (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() {@Overridepublic 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 methodreturn invokeOperation(invoker);}}  // 如果有@CacheEvict注解、并且标记为在调用前执行，则做删除/清空缓存的操作// Process any early evictionsprocessCacheEvicts(contexts.get(CacheEvictOperation.class), true,CacheOperationExpressionEvaluator.NO_RESULT);  // 如果有@Cacheable注解，查询缓存// Check if we have a cached item matching the conditionsCache.ValueWrapper cacheHit = findCachedItem(contexts.get(CacheableOperation.class));  // 如果缓存未命中（查询结果为null），则新增到cachePutRequests，后续执行原始方法后会写入缓存// Collect puts from any @Cacheable miss, if no cached item is foundList cachePutRequests = new LinkedList();if (cacheHit == null) {collectPutRequests(contexts.get(CacheableOperation.class),CacheOperationExpressionEvaluator.NO_RESULT, cachePutRequests);}Object cacheValue;Object returnValue;if (cacheHit != null && cachePutRequests.isEmpty() && !hasCachePut(contexts)) {   // 缓存命中的情况，使用缓存值作为结果// If there are no put requests, just use the cache hitcacheValue = cacheHit.get();returnValue = wrapCacheValue(method, cacheValue);}else {   // 缓存未命中、或有@CachePut注解的情况，需要调用原始方法// Invoke the method if we don't have a cache hit   // 调用原始方法，得到结果值returnValue = invokeOperation(invoker);cacheValue = unwrapReturnValue(returnValue);}  // 如果有@CachePut注解，则新增到cachePutRequests// Collect any explicit @CachePutscollectPutRequests(contexts.get(CachePutOperation.class), cacheValue, cachePutRequests);  // 如果缓存未命中，则把查询结果值写入缓存；如果有@CachePut注解，也把方法执行结果写入缓存// Process any collected put requests, either from @CachePut or a @Cacheable missfor (CachePutRequest cachePutRequest : cachePutRequests) {cachePutRequest.apply(cacheValue);}  // 如果有@CacheEvict注解、并且标记为在调用后执行，则做删除/清空缓存的操作// Process any late evictionsprocessCacheEvicts(contexts.get(CacheEvictOperation.class), false, cacheValue);return returnValue;}private Cache.ValueWrapper findCachedItem(Collection contexts) {Object result = CacheOperationExpressionEvaluator.NO_RESULT;for (CacheOperationContext context : contexts) {   // 如果满足condition条件，才查询缓存if (isConditionPassing(context, result)) {    // 生成缓存key，如果注解中指定了key，则按照Spring表达式解析，否则使用KeyGenerator类生成Object key = generateKey(context, result);    // 根据缓存key，查询缓存值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()) {   // 调用父类AbstractCacheInvoker的doGet方法，查询缓存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;}复制代码

AbstractCacheInvoker：CacheAspectSupport的父类，封装了最终查询Cache接口的逻辑

public abstract class AbstractCacheInvoker { // 最终查询缓存的方法protected Cache.ValueWrapper doGet(Cache cache, Object key) {try {   // 调用Spring Cache接口的查询方法return cache.get(key);}catch (RuntimeException ex) {getErrorHandler().handleCacheGetError(ex, cache, key);return null;  // If the exception is handled, return a cache miss}}}复制代码