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
spring-cache.png
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
@Override
public Collection<CacheOperation> parseCacheAnnotations(Class<?> type) {
DefaultCacheConfig defaultConfig = getDefaultCacheConfig(type);
return parseCacheAnnotations(defaultConfig, type);
}
// 解析方法级别的缓存注解
@Override
public Collection<CacheOperation> parseCacheAnnotations(Method method) {
DefaultCacheConfig defaultConfig = getDefaultCacheConfig(method.getDeclaringClass());
return parseCacheAnnotations(defaultConfig, method);
}
// 解析缓存注解
private Collection<CacheOperation> parseCacheAnnotations(DefaultCacheConfig cachingConfig, AnnotatedElement ae) {
Collection<CacheOperation> ops = null;
// 解析@Cacheable注解
Collection<Cacheable> 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<CacheEvict> 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<CachePut> 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<Caching> cachings = AnnotatedElementUtils.getAllMergedAnnotations(ae, Caching.class);
if (!cachings.isEmpty()) {
ops = lazyInit(ops);
for (Caching caching : cachings) {
Collection<CacheOperation> cachingOps = parseCachingAnnotation(ae, cachingConfig, caching);
if (cachingOps != null) {
ops.addAll(cachingOps);
}
}
}
return ops;
}
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
@Override
protected Collection<CacheOperation> findCacheOperations(final Class<?> clazz) {
return determineCacheOperations(new CacheOperationProvider() {
@Override
public Collection<CacheOperation> getCacheOperations(CacheAnnotationParser parser) {
return parser.parseCacheAnnotations(clazz);
}
});
}
// 查找方法级别的CacheOperation列表
@Override
protected Collection<CacheOperation> findCacheOperations(final Method method) {
return determineCacheOperations(new CacheOperationProvider() {
@Override
public Collection<CacheOperation> getCacheOperations(CacheAnnotationParser parser) {
return parser.parseCacheAnnotations(method);
}
});
}
}
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.
* <p>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<CacheOperation>> attributeCache =
new ConcurrentHashMap<Object, Collection<CacheOperation>>(1024);
// 根据Method、Class反射信息，获取对应的CacheOperation列表
@Override
public Collection<CacheOperation> getCacheOperations(Method method, Class<?> targetClass) {
if (method.getDeclaringClass() == Object.class) {
return null;
}
Object cacheKey = getCacheKey(method, targetClass);
Collection<CacheOperation> 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map
else {
Collection<CacheOperation> 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<CacheOperation> 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<CacheOperation> 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;
}
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
@Override
public 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() {
@Override
public 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();
}
}
}
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：
- 通过CacheOperationSource，获取所有的CacheOperation列表
- 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
- 如果有@Cacheable注解，查询缓存
- 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
- 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
- 如果有@CachePut注解，则新增到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
- 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 AbstractCacheInvoker
implements 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<CacheOperation> 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 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<Object>() {
@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);
}
}
// 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 evictions
processCacheEvicts(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 conditions
Cache.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 found
List<CachePutRequest> cachePutRequests = new LinkedList<CachePutRequest>();
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 hit
cacheValue = 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 @CachePuts
collectPutRequests(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 miss
for (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 evictions
processCacheEvicts(contexts.get(CacheEvictOperation.class), false, cacheValue);
return returnValue;
}
private Cache.ValueWrapper findCachedItem(Collection<CacheOperationContext> 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;
}
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
}
}
}
