@Aspect结合@Order能建立具有最高优先级的切面？

谁借莪１个温暖的怀抱￠ 2023-06-30 09:18 23阅读 0赞

### 文章目录 ###

*   *   *  疑问
         *  探究原因
         *  总结

### 疑问 ###

一般情况下，比如说@around接口就只有一个参数ProceedingJoinPoint，是不会有问题的（只是侥幸），但是使用多个参数时，就会抛出异常。

如下，around方法有两个参数时，程序抛异常：

@Aspect
    @Order(Ordered.HIGHEST_PRECEDENCE)
    public class TestOrderAspect {
    
        @Pointcut("@annotation(testOrder)")
        public void pointCut(TestOrder testOrder) {
        }
    
        @Around("pointCut(testOrder)")
        public Object around(ProceedingJoinPoint point, TestOrder testOrder) throws Throwable {
            Object result;
            result = point.proceed();
            return result;
        }
    }

### 探究原因 ###

CglibAopProxy的intercept方法中，会先获取一个拦截链，如果该链不为空，则需要创建methodInvocation

@Override
    @Nullable
    public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
       Object oldProxy = null;
       boolean setProxyContext = false;
       Object target = null;
       TargetSource targetSource = this.advised.getTargetSource();
       try {
          ......
          // 获取拦截链    
          List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
          Object retVal;
          // Check whether we only have one InvokerInterceptor: that is,
          // no real advice, but just reflective invocation of the target.
          if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {
             //不创建methodinvocation，直接调用目标方法
             Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
             retVal = methodProxy.invoke(target, argsToUse);
          }
          else {
             // 1.需要创建methodInvocation
             retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
          }
          retVal = processReturnType(proxy, target, method, retVal);
          return retVal;
       }
       finally {
          ......
       }
    }

创建完MethodInvocation后，会调用ReflectiveMethodInvocation.proceed()，进而会调用AspectJExpressionPointcut的matches方法，进行切点匹配

@Override
    	public boolean matches(Method method, Class<?> targetClass, Object... args) {
    	    ......
    		ProxyMethodInvocation pmi = null;
    		Object targetObject = null;
    		Object thisObject = null;
    		try {
                // 尝试获取MethodInvocation，获取不到会抛异常
                // ExposeInvocationInterceptor才会暴露出来MethodInvocation，所以要在链首位
    			MethodInvocation mi = ExposeInvocationInterceptor.currentInvocation();
    			targetObject = mi.getThis();
    			if (!(mi instanceof ProxyMethodInvocation)) {
    				throw new IllegalStateException("MethodInvocation is not a Spring ProxyMethodInvocation: " + mi);
    			}
    			pmi = (ProxyMethodInvocation) mi;
    			thisObject = pmi.getProxy();
    		}
    		catch (IllegalStateException ex) {
    			......
    		}
            try {
    			......
    			if (pmi != null && thisObject != null) {  // there is a current invocation
    				RuntimeTestWalker originalMethodResidueTest = getRuntimeTestWalker(getShadowMatch(method, method));
    				if (!originalMethodResidueTest.testThisInstanceOfResidue(thisObject.getClass())) {
    					return false;
    				}
    				if (joinPointMatch.matches()) {
                          // 绑定参数信息，即绑定除ProceedingJoinPoint外的参数，非常重要！！！
    					bindParameters(pmi, joinPointMatch);
    				}
    			}
                ......
    	}

返回对应的MethodInvocation：但是如果没有将ExposeInvocationInterceptor加至拦截链首部，则无法暴露对应 的MethodInvocation，故而抛异常。

public static MethodInvocation currentInvocation() throws IllegalStateException {
       MethodInvocation mi = invocation.get();
       if (mi == null) {
          throw new IllegalStateException(
                "No MethodInvocation found: Check that an AOP invocation is in progress, and that the " +
                "ExposeInvocationInterceptor is upfront in the interceptor chain. Specifically, note that " +
                "advices with order HIGHEST_PRECEDENCE will execute before ExposeInvocationInterceptor!");
       }
       return mi;
    }

虽然会被matchesf方法捕获掉异常，但是由于pmi和thisObject为null,导致最终没有执行bindParameters绑定参数信息，所以，只要是2个及以上参数，则会报错。

### 总结 ###

在使用@Order来控制切面优先级时，最好不要直接定义该切面优先级为Ordered.HIGHEST\_PRECEDENCE，导致无法暴露methodInvocation，一旦切面中定义的方法参数超过正常情况，就会报错。