Spring cache源码分析

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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:

  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 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
        }
    }
}
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