白话Spring源码（十二）：Spring AOP源码分析-拦截器链的执行过程

怼烎@ 2022-04-24 03:18 234阅读 0赞

上篇博客我们介绍了创建代理对象：[白话Spring源码（十一）：Spring AOP源码分析-创建代理对象][Spring_Spring AOP_-]。现在我们的得到了 bean 的代理对象，且通知也以合适的方式插在了目标方法的前后。接下来要做的事情，就是执行通知逻辑了。通知可能在目标方法前执行，也可能在目标方法后执行。具体的执行时机，取决于用户的配置。当目标方法被多个通知匹配到时，Spring 通过引入拦截器链来保证每个通知的正常执行。在本文中，我们将会通过源码了解到 Spring 的拦截器链的执行过程。

### JDK 动态代理逻辑分析 ###

我来分析一下 JDK 动态代理逻辑。对于 JDK 动态代理，代理逻辑封装在 InvocationHandler 接口实现类的 invoke 方法中。JdkDynamicAopProxy 实现了 InvocationHandler 接口，下面我们就来分析一下 JdkDynamicAopProxy 的 invoke 方法。

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    	
    		MethodInvocation invocation = null;
    		Object oldProxy = null;
    		boolean setProxyContext = false;
    	
    		TargetSource targetSource = advised.targetSource;
    		Class targetClass = null;
    		Object target = null;		
    		
    		try {
    			// Try special rules for equals() method and implementation of the
    			// Advised AOP configuration interface
    			
    			// Short-circuit expensive Method.equals() call, as Object.equals() isn't overloaded
    			if (method.getDeclaringClass() == Object.class && "equals".equals(method.getName())) {
    				// What if equals throws exception!?
    
    				// This class implements the equals() method itself
    				return new Boolean(equals(args[0]));
    			}
    			else if (Advised.class == method.getDeclaringClass()) {
    				// Service invocations on ProxyConfig with the proxy config
    				return AopProxyUtils.invokeJoinpointUsingReflection(this.advised, method, args);
    			}
    			
    			Object retVal = null;
    			
    			// May be null. Get as late as possible to minimize the time we "own" the target,
    			// in case it comes from a pool.
    			target = targetSource.getTarget();
    			if (target != null) {
    				targetClass = target.getClass();
    			}
    			
    			if (this.advised.exposeProxy) {
    				// Make invocation available if necessary
    				oldProxy = AopContext.setCurrentProxy(proxy);
    				setProxyContext = true;
    			}
    		
    			// Get the interception chain for this method
    			List chain = this.advised.advisorChainFactory.getInterceptorsAndDynamicInterceptionAdvice(
    					this.advised, proxy, method, targetClass);
    			
    			// Check whether we have any advice. If we don't, we can fallback on
    			// direct reflective invocation of the target, and avoid creating a MethodInvocation
    			if (chain.isEmpty()) {
    				// We can skip creating a MethodInvocation: just invoke the target directly
    				// Note that the final invoker must be an InvokerInterceptor so we know it does
    				// nothing but a reflective operation on the target, and no hot swapping or fancy proxying
    				retVal = AopProxyUtils.invokeJoinpointUsingReflection(target, method, args);
    			}
    			else {
    				// We need to create a method invocation...
    				//invocation = advised.getMethodInvocationFactory().getMethodInvocation(proxy, method, targetClass, target, args, chain, advised);
    				
    				invocation = new ReflectiveMethodInvocation(proxy, target,
    									method, args, targetClass, chain);
    										
    				// Proceed to the joinpoint through the interceptor chain
    				retVal = invocation.proceed();
    			}
    			
    			// Massage return value if necessary
    			if (retVal != null && retVal == target) {
    				// Special case: it returned "this"
    				// Note that we can't help if the target sets
    				// a reference to itself in another returned object
    				retVal = proxy;
    			}
    			return retVal;
    		}
    		finally {
    			if (target != null && !targetSource.isStatic()) {
    				// Must have come from TargetSource
    				targetSource.releaseTarget(target);
    			}
    			
    			if (setProxyContext) {
    				// Restore old proxy
    				AopContext.setCurrentProxy(oldProxy);
    			}
    			
    			//if (invocation != null) {
    			//	advised.getMethodInvocationFactory().release(invocation);
    			//}
    		}
    	}

如上，上面的代码我做了比较详细的注释。下面我们来总结一下 invoke 方法的执行流程，如下：

1.  检测 expose-proxy 是否为 true，若为 true，则暴露代理对象
2.  获取适合当前方法的拦截器
3.  如果拦截器链为空，则直接通过反射执行目标方法
4.  若拦截器链不为空，则创建方法调用 ReflectiveMethodInvocation 对象
5.  调用 ReflectiveMethodInvocation 对象的 proceed() 方法启动拦截器链
6.  处理返回值，并返回该值

在以上6步中，我们重点关注第2步和第5步中的逻辑。第2步用于获取拦截器链，第5步则是启动拦截器链。下面先来分析获取拦截器链的过程。

### 获取所有的拦截器 ###

所谓的拦截器，顾名思义，是指用于对目标方法的调用进行拦截的一种工具。拦截器的源码比较简单，所以我们直接看源码好了。下面以前置通知拦截器为例，如下：

public class MethodBeforeAdviceInterceptor implements MethodInterceptor, Serializable {
        
        /** 前置通知 */
        private MethodBeforeAdvice advice;
    
        public MethodBeforeAdviceInterceptor(MethodBeforeAdvice advice) {
            Assert.notNull(advice, "Advice must not be null");
            this.advice = advice;
        }
    
        @Override
        public Object invoke(MethodInvocation mi) throws Throwable {
            // 执行前置通知逻辑
            this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
            // 通过 MethodInvocation 调用下一个拦截器，若所有拦截器均执行完，则调用目标方法
            return mi.proceed();
        }
    }

如上，前置通知的逻辑在目标方法执行前被执行。这里先简单向大家介绍一下拦截器是什么，。本节我们先来看看如何如何获取拦截器，如下：

public List getInterceptorsAndDynamicInterceptionAdvice(Advised config, Object proxy, Method method, Class targetClass) {
    		List cached = (List) this.methodCache.get(method);
    		if (cached == null) {
    			// Recalculate
    			cached = AdvisorChainFactoryUtils.calculateInterceptorsAndDynamicInterceptionAdvice(config, proxy, method, targetClass);
    			this.methodCache.put(method, cached);
    		}
    		return cached;
    	}

public static List calculateInterceptorsAndDynamicInterceptionAdvice(Advised config, Object proxy, Method method, Class targetClass) {
    		List interceptors = new ArrayList(config.getAdvisors().length);
    		for (int i = 0; i < config.getAdvisors().length; i++) {
    			Advisor advisor = config.getAdvisors()[i];
    			if (advisor instanceof PointcutAdvisor) {
    				// Add it conditionally
    				PointcutAdvisor pointcutAdvisor = (PointcutAdvisor) advisor;
    				if (pointcutAdvisor.getPointcut().getClassFilter().matches(targetClass)) {
    					MethodInterceptor interceptor = (MethodInterceptor) GlobalAdvisorAdapterRegistry.getInstance().getInterceptor(advisor);
    					MethodMatcher mm = pointcutAdvisor.getPointcut().getMethodMatcher();
    					if (mm.matches(method, targetClass)) {
    						if (mm.isRuntime()) {
    							// Creating a new object instance in the getInterceptor() method
    							// isn't a problem as we normally cache created chains
    							interceptors.add(new InterceptorAndDynamicMethodMatcher(interceptor, mm) );
    						}
    						else {							
    							interceptors.add(interceptor);
    						}
    					}
    				}
    			}
    			else if (advisor instanceof IntroductionAdvisor) {
    				IntroductionAdvisor ia = (IntroductionAdvisor) advisor;
    				if (ia.getClassFilter().matches(targetClass)) {
    					MethodInterceptor interceptor = (MethodInterceptor) GlobalAdvisorAdapterRegistry.getInstance().getInterceptor(advisor);
    					interceptors.add(interceptor);
    				}
    			}
    		}	// for
    		return interceptors;
    	}	// calculateInterceptorsAndDynamicInterceptionAdvice

以上就是获取拦截器的过程，代码有点长，不过好在逻辑不是很复杂。这里简单总结一下以上源码的执行过程，如下：

1.  从缓存中获取当前方法的拦截器链
2.  若缓存未命中，则调用 AdvisorChainFactoryUtils.calculateInterceptorsAndDynamicInterceptionAdvice获取拦截器链
3.  遍历通知器列表
4.  对于 PointcutAdvisor 类型的通知器，这里要调用通知器所持有的切点（Pointcut）对类和方法进行匹配，匹配成功说明应向当前方法织入通知逻辑
5.  调用 getInterceptors 方法对非 MethodInterceptor 类型的通知进行转换
6.  返回拦截器数组，并在随后存入缓存中

这里需要说明一下，部分通知器是没有实现 MethodInterceptor 接口的，这里使用了适配器模式，如下：

public Interceptor getInterceptor(Advisor advisor) throws UnknownAdviceTypeException {
    		Advice advice = advisor.getAdvice();
    		if (advice instanceof Interceptor) {
    			return (Interceptor) advice;
    		}
    		for (int i = 0; i < this.adapters.size(); i++) {
    			AdvisorAdapter adapter = (AdvisorAdapter) this.adapters.get(i);
    			if (adapter.supportsAdvice(advice)) {
    				return adapter.getInterceptor(advisor);
    			}
    		}
    		throw new UnknownAdviceTypeException(advisor.getAdvice());
    	}

适配器接口：

public interface AdvisorAdapter {
    	
    	boolean supportsAdvice(Advice advice);
    	
    	Interceptor getInterceptor(Advisor advisor);
    
    }

适配器实现类：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2hhb3hpbjk2Mw_size_16_color_FFFFFF_t_70][]

我们看一个BeforeAdviceAdapter：

class BeforeAdviceAdapter implements AdvisorAdapter {
    
    	/**
    	 * @see org.springframework.aop.framework.adapter.AdvisorAdapter#supportsAdvice(java.lang.Object)
    	 */
    	public boolean supportsAdvice(Advice advice) {
    		return advice instanceof MethodBeforeAdvice;
    	}
    
    	/**
    	 * @see org.springframework.aop.framework.adapter.AdvisorAdapter#getInterceptor(org.springframework.aop.Advisor)
    	 */
    	public Interceptor getInterceptor(Advisor advisor) {
    		MethodBeforeAdvice advice = (MethodBeforeAdvice) advisor.getAdvice();
    		return new MethodBeforeAdviceInterceptor(advice) ;
    	}
    
    }

现在我们已经获得了拦截器链，那接下来要做的事情就是启动拦截器了。所以接下来，我们一起去看看 Sring 是如何让拦截器链运行起来的。

### 启动拦截器链 ###

我们先来说说 ReflectiveMethodInvocation。ReflectiveMethodInvocation 贯穿于拦截器链执行的始终，可以说是核心。该类的 proceed 方法用于启动启动拦截器链，下面我们去看看这个方法的逻辑。

public class ReflectiveMethodInvocation implements MethodInvocation {
    
    	protected Method method;
    	
    	protected Object[] arguments;
    	
    	protected Object target;
    	
    	protected Object proxy;
    	
    	/** 
    	 * List of Methodnterceptor and InterceptorAndDynamicMethodMatcher that need dynamic checks.
    	 **/
    	protected List interceptorsAndDynamicMethodMatchers;
    	
    	/**
    	 * Index from 0 of the current interceptor we're invoking.
    	 * -1 until we invoke: then the current interceptor
    	 */
    	private int currentInterceptorIndex = -1;
    	
    	private Class targetClass;
    	
    	
    	/**
    	 * Construct a new MethodInvocation with given arguments
    	 * @param interceptorsAndDynamicMethodMatchers interceptors that should be applied,
    	 * along with any InterceptorAndDynamicMethodMatchers that need evaluation at runtime.
    	 * MethodMatchers included in this struct must already have been found to have matched as far
    	 * as was possibly statically. Passing an array might be about 10% faster, but would complicate
    	 * the code. And it would work only for static pointcuts.
    	 */
    	public ReflectiveMethodInvocation(Object proxy, Object target, 
    					Method m, Object[] arguments,
    					Class targetClass, List interceptorsAndDynamicMethodMatchers) {
    		this.proxy = proxy;
    		this.target = target;
    		this.targetClass = targetClass;
    		this.method = m;
    		this.arguments = arguments;
    		this.interceptorsAndDynamicMethodMatchers = interceptorsAndDynamicMethodMatchers;
    	}
    	
    	
    	/**
    	 * Return the method invoked on the proxied interface.
    	 * May or may not correspond with a method invoked on an underlying
    	 * implementation of that interface.
    	 * @return Method
    	 */
    	public final Method getMethod() {
    		return this.method;
    	}
    	
    	public final AccessibleObject getStaticPart() {
    		return this.method;
    	}
    	
    	/**
    	 * Return the proxy that this interception was made through
    	 * @return Object
    	 */
    	public final Object getProxy() {
    		return this.proxy;
    	}
    	
    	/**
    	 * Private optimization method
    	 * @return Object[]
    	 */
    	public final Object[] getArguments() {
    		return this.arguments;
    	}
    	
    	/**
    	 * @see org.aopalliance.intercept.Invocation#proceed
    	 */
    	public Object proceed() throws Throwable {
    		//	We start with an index of -1 and increment early
    		if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
    			return invokeJoinpoint();
    		}
    
    		Object interceptorOrInterceptionAdvice = this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
    		if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
    			// Evaluate dynamic method matcher here: static part will already have
    			// been evaluated and found to match
    			InterceptorAndDynamicMethodMatcher dm = (InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
    			if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {
    				return dm.interceptor.invoke(this);
    			}
    			else {
    				// Dynamic matching failed
    				// Skip this interceptor and invoke the next in the chain
    				return proceed();
    			}
    		}
    		else {
    			// It's an interceptor so we just invoke it: the pointcut will have
    			// been evaluated statically before this object was constructed
    			return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
    		}
    	}
    	
    	/**
    	 * Invoke the joinpoint using reflection. Subclasses can override this to use custom
    	 * invocation.
    	 * @return the return value of the joinpoint
    	 * @throws Throwable if invoking the joinpoint resulted in an exception
    	 */
    	protected Object invokeJoinpoint() throws Throwable {
    		return AopProxyUtils.invokeJoinpointUsingReflection(target, method, arguments);
    	}
    
    
    	/**
    	 * @see org.aopalliance.intercept.Invocation#getThis
    	 */
    	public final Object getThis() {
    		return this.target;
    	}
    	
    	public String toString() {
    		// Don't do toString on target, it may be
    		// proxied
    	
    		// ToString on args may also fail
    		String s =  "Invocation: method=[" + method + "] " +
    				//"args=[" + StringUtils.arrayToDelimitedString(arguments, ",") +
    				"args=" + this.arguments + 
    				"] ";
    	 
    		s += (this.target == null) ? "target is null": 
    				"target is of class " + target.getClass().getName();
    		return s;
    			
    	}
    
    }

这个类我们主要关注proceed方法，这里使用了责任链模式，关于责任链模式请看：[责任链模式(Chain of Responsibility)][Chain of Responsibility]。

好了，到现在Spring AOP源码基本就介绍完了。谢谢大家!

[Spring_Spring AOP_-]: https://blog.csdn.net/haoxin963/article/details/89294956
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2hhb3hpbjk2Mw_size_16_color_FFFFFF_t_70]: /images/20220220/6c3f6e67580f46978f676f6dd5d6728d.png
[Chain of Responsibility]: https://blog.csdn.net/haoxin963/article/details/86715635