问题描述

公司项目中自己开发了一个简易的事件服务，使用自定义注解。

实现方式：

实现BeanFactoryPostProcessor,在postProcessBeanFactory中，扫描所有的bean方法，将带有自定义注解EventListener的方法放到map中，然后可以使用服务来fire相关的方法，非常方便。

以前使用没有问题，这次写了一个方法，与以前不同的唯一一点就是可见性由public改为了private，因为这个方法在设计的时候就只打算在类内部使用。可是在使用的时候，连第一行的日志都打印不出来，改为public后调用正常。

调试

打断点进入相关方法，发现事件服务是采用这种方式来调用相关方法的：

通过ApplicationContextHolder在spring启动时保存ApplicationContext,然后使用方法getBean来根据map中的beanName来获取bean

使用反射获取相应的方法

调用方法invoke

可以看到，反射调用是没什么问题的，按照我们平常使用反射的经验来看，也完全不应该出问题。进入到调用的方法，发现类中的实例属性全部为null，第一行的打印log的log实例为空，直接报了NPE(由于项目原因，这个异常的栈也没有打印出来)，所以在日志中连入口日志都没有。

分析

由于以前也读过Spring的源码，猜测应该是Spring使用cglib动态代理的类有问题，对于public和private方法的调用方式可能不同。这个类使用了事务，因此我去Spring中找到动态代理部分：

public Object getProxy(ClassLoader classLoader) {    if (logger.isDebugEnabled()) {    	logger.debug("Creating CGLIB proxy: target source is " + this.advised.getTargetSource());    }    try {    	Class
     rootClass = this.advised.getTargetClass();    	Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");    	Class
     proxySuperClass = rootClass;    	if (ClassUtils.isCglibProxyClass(rootClass)) {    		proxySuperClass = rootClass.getSuperclass();    		Class
    [] additionalInterfaces = rootClass.getInterfaces();    		for (Class
     additionalInterface : additionalInterfaces) {    			this.advised.addInterface(additionalInterface);    		}    	}    	// Validate the class, writing log messages as necessary.    	validateClassIfNecessary(proxySuperClass, classLoader);    	// Configure CGLIB Enhancer...    	Enhancer enhancer = createEnhancer();    	if (classLoader != null) {    		enhancer.setClassLoader(classLoader);    		if (classLoader instanceof SmartClassLoader &&    				((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {    			enhancer.setUseCache(false);    		}    	}    	enhancer.setSuperclass(proxySuperClass);    	enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));    	enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);    	enhancer.setStrategy(new UndeclaredThrowableStrategy(UndeclaredThrowableException.class));                //获取方法调用时的拦截    	Callback[] callbacks = getCallbacks(rootClass);    	Class
    [] types = new Class
    [callbacks.length];    	for (int x = 0; x < types.length; x++) {    		types[x] = callbacks[x].getClass();    	}    	// fixedInterceptorMap only populated at this point, after getCallbacks call above    	enhancer.setCallbackFilter(new ProxyCallbackFilter(    			this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));    	enhancer.setCallbackTypes(types);    	// Generate the proxy class and create a proxy instance.    	return createProxyClassAndInstance(enhancer, callbacks);    }

方法的调用代理到了callback中，我们来看一下callback的生成

private Callback[] getCallbacks(Class
     rootClass) throws Exception {    // Parameters used for optimisation choices...    boolean exposeProxy = this.advised.isExposeProxy();    boolean isFrozen = this.advised.isFrozen();    boolean isStatic = this.advised.getTargetSource().isStatic();    // Choose an "aop" interceptor (used for AOP calls).    Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised);    // Choose a "straight to target" interceptor. (used for calls that are    // unadvised but can return this). May be required to expose the proxy.    Callback targetInterceptor;    if (exposeProxy) {    	targetInterceptor = isStatic ?    			new StaticUnadvisedExposedInterceptor(this.advised.getTargetSource().getTarget()) :    			new DynamicUnadvisedExposedInterceptor(this.advised.getTargetSource());    }    else {    	targetInterceptor = isStatic ?    			new StaticUnadvisedInterceptor(this.advised.getTargetSource().getTarget()) :    			new DynamicUnadvisedInterceptor(this.advised.getTargetSource());    }    // Choose a "direct to target" dispatcher (used for    // unadvised calls to static targets that cannot return this).    Callback targetDispatcher = isStatic ?    		new StaticDispatcher(this.advised.getTargetSource().getTarget()) : new SerializableNoOp();        //callback组成元素    Callback[] mainCallbacks = new Callback[]{    	aopInterceptor, // for normal advice    	targetInterceptor, // invoke target without considering advice, if optimized    	new SerializableNoOp(), // no override for methods mapped to this    	targetDispatcher, this.advisedDispatcher,    	new EqualsInterceptor(this.advised),    	new HashCodeInterceptor(this.advised)    };    Callback[] callbacks;    // If the target is a static one and the advice chain is frozen,    // then we can make some optimisations by sending the AOP calls    // direct to the target using the fixed chain for that method.    if (isStatic && isFrozen) {    	Method[] methods = rootClass.getMethods();    	Callback[] fixedCallbacks = new Callback[methods.length];    	this.fixedInterceptorMap = new HashMap
    
     (methods.length);    	// TODO: small memory optimisation here (can skip creation for methods with no advice)    	for (int x = 0; x < methods.length; x++) {    		List

主要的callback在变量mainCallbacks中，可以看到有aop的拦截，也有Spring定义的equal hashCode方法拦截，也有直接调用回target的拦截器，具体使用哪个，是由ProxyCallbackFilter来决定的，accept方法来决定执行哪个回调

public int accept(Method method) {	if (AopUtils.isFinalizeMethod(method)) {		logger.debug("Found finalize() method - using NO_OVERRIDE");		return NO_OVERRIDE;	}	if (!this.advised.isOpaque() && method.getDeclaringClass().isInterface() &&			method.getDeclaringClass().isAssignableFrom(Advised.class)) {		if (logger.isDebugEnabled()) {			logger.debug("Method is declared on Advised interface: " + method);		}		return DISPATCH_ADVISED;	}	// We must always proxy equals, to direct calls to this.	if (AopUtils.isEqualsMethod(method)) {		logger.debug("Found 'equals' method: " + method);		return INVOKE_EQUALS;	}	// We must always calculate hashCode based on the proxy.	if (AopUtils.isHashCodeMethod(method)) {		logger.debug("Found 'hashCode' method: " + method);		return INVOKE_HASHCODE;	}	Class
     targetClass = this.advised.getTargetClass();	// Proxy is not yet available, but that shouldn't matter.	List
     chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);	boolean haveAdvice = !chain.isEmpty();	boolean exposeProxy = this.advised.isExposeProxy();	boolean isStatic = this.advised.getTargetSource().isStatic();	boolean isFrozen = this.advised.isFrozen();	if (haveAdvice || !isFrozen) {		// If exposing the proxy, then AOP_PROXY must be used.		if (exposeProxy) {			if (logger.isDebugEnabled()) {				logger.debug("Must expose proxy on advised method: " + method);			}			return AOP_PROXY;		}		String key = method.toString();		// Check to see if we have fixed interceptor to serve this method.		// Else use the AOP_PROXY.		if (isStatic && isFrozen && this.fixedInterceptorMap.containsKey(key)) {			if (logger.isDebugEnabled()) {				logger.debug("Method has advice and optimisations are enabled: " + method);			}			// We know that we are optimising so we can use the			// FixedStaticChainInterceptors.			int index = this.fixedInterceptorMap.get(key);			return (index + this.fixedInterceptorOffset);		}		else {			if (logger.isDebugEnabled()) {				logger.debug("Unable to apply any optimisations to advised method: " + method);			}			return AOP_PROXY;		}	}	else {		// See if the return type of the method is outside the class hierarchy		// of the target type. If so we know it never needs to have return type		// massage and can use a dispatcher.		// If the proxy is being exposed, then must use the interceptor the		// correct one is already configured. If the target is not static, then		// cannot use a dispatcher because the target cannot be released.		if (exposeProxy || !isStatic) {			return INVOKE_TARGET;		}		Class
     returnType = method.getReturnType();		if (targetClass == returnType) {			if (logger.isDebugEnabled()) {				logger.debug("Method " + method +						"has return type same as target type (may return this) - using INVOKE_TARGET");			}			return INVOKE_TARGET;		}		else if (returnType.isPrimitive() || !returnType.isAssignableFrom(targetClass)) {			if (logger.isDebugEnabled()) {				logger.debug("Method " + method +						" has return type that ensures this cannot be returned- using DISPATCH_TARGET");			}			return DISPATCH_TARGET;		}		else {			if (logger.isDebugEnabled()) {				logger.debug("Method " + method +						"has return type that is assignable from the target type (may return this) - " +						"using INVOKE_TARGET");			}			return INVOKE_TARGET;		}	}}

可以看到，对于final方法，不作拦截，如果定义了切面，就会使用切面的连接点来织入，其余的也会有对应的回调原则。

到这其实我们还是没有找到为何public方法和private方法调用的行为为什么是不一样的，我把cglib生成的类保存下来，使用反编译来看看生成的类是什么样的。

这里我只展示一下头部和其中的一个方法：

public class VideoAccessServiceImpl$$EnhancerBySpringCGLIB$$4d078710  extends VideoAccessServiceImpl  implements SpringProxy, Advised, Factory{    public final boolean saveVideo(String paramString)    {      try      {        MethodInterceptor tmp4_1 = this.CGLIB$CALLBACK_0;        if (tmp4_1 == null)        {          tmp4_1;          CGLIB$BIND_CALLBACKS(this);        }        MethodInterceptor tmp17_14 = this.CGLIB$CALLBACK_0;        if (tmp17_14 != null)        {          Object tmp41_36 = tmp17_14.intercept(this, CGLIB$saveVideo$0$Method, new     Object[] { paramString }, CGLIB$saveVideo$0$Proxy);          tmp41_36;          return tmp41_36 == null ? false : ((Boolean)tmp41_36).booleanValue();        }        return super.saveVideo(paramString);      }      catch (RuntimeException|Error localRuntimeException)      {        throw localRuntimeException;      }      catch (Throwable localThrowable)      {        throw new UndeclaredThrowableException(localThrowable);      }    }  }

然后搜索了一下私有方法在这里面是不存在的，也就是说cglib对于私有方法并不会代理，而对于公有方法则会调用相应的MethodInterceptor

到这我们已经找到了两者行为不一致的原因：

public 方法被Spring拦截，最终是使用相关的bean来调用的， private 方法则是跟我们平常使用反射一样，得到了原始的方法，里面的实例属性全都没有初始化，都为null。

总结

通过这次问题搜索，也发现了自己在以往读源码，学习新知识的时候挖掘的不够，以前也写过cglib相关的博客，现在来看也是有点草草，没有深入下去，经过这次的问题解决，让自己对cglib又多了一点理解。

在搜索源代码的时候，也发现了自己对Spring代码的陌生，以往看过的也忘得差不多了，以后还需要多多回顾。也会把相关的知识记录下来，以备日后查阅。

转载于:https://my.oschina.net/liufq/blog/1549647

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