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
	@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
		}
	}
}