Spring技术内幕：Spring AOP的实现原理

Posted 2020-09-27 cxchanpin

7、Advice通知的实现
AopProxy代理对象生成时，其拦截器也一并生成。以下我们来分析下Aop是怎样对目标对象进行增强的。在为AopProxy配置拦截器的实现中，有一个取得拦截器配置过程，这个过程由DefaultAvisorChainFactory实现的，而这个工厂类负责生成拦截器链，在它的getInterceptorsAndDynamicInterceptionA-
dvice方法中，有一个适配器的注冊过程。通过配置Spring预先设计好的拦截器，Spring增加了它对Aop实现的处理。

为具体了解这个过程，先从Defau-
ltAdvisorChainFactory的实现開始。通过以下代码能够看到，在DefaultAdvisorChainFactory实现中。首先构造了一个GlobalAdvisorAdapterRegistry单件，然后对配置的Advisor通知器进行逐个遍历，这些通知链都是配置在interceptorNames中的，从getInterceptorsAndDynamicInterceptionAdvice传递进来的advised參数对象中，能够方便的取得配置的通知器，有了这些通知器，接着就是一个由
GlobalAdvisorAdapterRegistry来完毕的拦截器的适配和注冊。

/**
 * A simple but definitive way of working out an advice chain for a Method,
 * given an {@link Advised} object. Always rebuilds each advice chain;
 * caching can be provided by subclasses.
 * @author Juergen Hoeller
 * @author Rod Johnson
 * @author Adrian Colyer
 * @since 2.0.3
 */
@SuppressWarnings("serial")
public class DefaultAdvisorChainFactory implements AdvisorChainFactory, Serializable {
    public List<Object> getInterceptorsAndDynamicInterceptionAdvice(
            Advised config, Method method, Class targetClass) {
        // This is somewhat tricky... we have to process introductions first,
        // but we need to preserve order in the ultimate list.
        List<Object> interceptorList = new ArrayList<Object>(config.getAdvisors().length);
        boolean hasIntroductions = hasMatchingIntroductions(config, targetClass);
        // 得到注冊器GlobalAdvisorAdapterRegistry，这是一个单件模式的实现
        AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();
        for (Advisor advisor : config.getAdvisors()) {
            if (advisor instanceof PointcutAdvisor) {
                // Add it conditionally.
                PointcutAdvisor pointcutAdvisor = (PointcutAdvisor) advisor;
                if (config.isPreFiltered() || pointcutAdvisor.getPointcut().getClassFilter().matches(targetClass)) {
                    MethodInterceptor[] interceptors = registry.getInterceptors(advisor);
                    MethodMatcher mm = pointcutAdvisor.getPointcut().getMethodMatcher();
                    if (MethodMatchers.matches(mm, method, targetClass, hasIntroductions)) {
                        if (mm.isRuntime()) {
                            // Creating a new object instance in the getInterceptors() method
                            // isn‘t a problem as we normally cache created chains.
                            for (MethodInterceptor interceptor : interceptors) {
                                interceptorList.add(new InterceptorAndDynamicMethodMatcher(interceptor, mm));
                            }
                        }
                        else {
                            interceptorList.addAll(Arrays.asList(interceptors));
                        }
                    }
                }
            }
            else if (advisor instanceof IntroductionAdvisor) {
                IntroductionAdvisor ia = (IntroductionAdvisor) advisor;
                if (config.isPreFiltered() || ia.getClassFilter().matches(targetClass)) {
                    Interceptor[] interceptors = registry.getInterceptors(advisor);
                    interceptorList.addAll(Arrays.asList(interceptors));
                }
            }
            else {
                Interceptor[] interceptors = registry.getInterceptors(advisor);
                interceptorList.addAll(Arrays.asList(interceptors));
            }
        }
        return interceptorList;
    }

GlobalAdvisorAdapterRegistry的getInterceptors方法为AOP的实现做出了非常大的贡献，这种方法封装着advice织入实现的入口。我们先从GlobalAdvisorAdapterRegistry的实现入手，他基本起一个适配器的作用，但同一时候也是单件模式，代码例如以下：

     /**
     * Keep track of a single instance so we can return it to classes that request it.
     */
    // 使用静态变量来保持一个唯一实例
    private static AdvisorAdapterRegistry instance = new DefaultAdvisorAdapterRegistry();
    /**
     * Return the singleton {@link DefaultAdvisorAdapterRegistry} instance.
     */
    public static AdvisorAdapterRegistry getInstance() {
        return instance;
    }

到这里，我们知道在DefaultAdvisorAdapterRegistry中，设置了一系列的adapter适配器，这是这些适配器的实现。为Spring的advice提供了编织能力。以下我们看看DefaultAdvisorAdapterRegistry到底发生了什么？adapter的作用具体分为两个：
1、调用adapter的support方法，通过这种方法来推断取得的advice属于什么类型的advice通知，从而依据不同的advice类型来注冊不同的AdviceInterceptor，也就是前面我们看到的拦截器
2、这些AdviceInterceptor都是Spring AOP框架设计好的，是为实现不同的advice功能提供服务的。

有了这些AdviceInterceptor，能够方便的使用由Spring提供的各种不同的advice来设计AOP应用。也就是说，正是这些AdviceInterceptor终于实现了advice通知在AopProxy对象中的织入功能。

/**
 * Default implementation of the {@link AdvisorAdapterRegistry} interface.
 * Supports {@link org.aopalliance.intercept.MethodInterceptor},
 * {@link org.springframework.aop.MethodBeforeAdvice},
 * {@link org.springframework.aop.AfterReturningAdvice},
 * {@link org.springframework.aop.ThrowsAdvice}.
 *
 * @author Rod Johnson
 * @author Rob Harrop
 * @author Juergen Hoeller
 */
@SuppressWarnings("serial")
public class DefaultAdvisorAdapterRegistry implements AdvisorAdapterRegistry, Serializable {
    private final List<AdvisorAdapter> adapters = new ArrayList<AdvisorAdapter>(3);
    /**
     * Create a new DefaultAdvisorAdapterRegistry, registering well-known adapters.
     */
    public DefaultAdvisorAdapterRegistry() {
        registerAdvisorAdapter(new MethodBeforeAdviceAdapter());
        registerAdvisorAdapter(new AfterReturningAdviceAdapter());
        registerAdvisorAdapter(new ThrowsAdviceAdapter());
    }
    public Advisor wrap(Object adviceObject) throws UnknownAdviceTypeException {
        if (adviceObject instanceof Advisor) {
            return (Advisor) adviceObject;
        }
        if (!(adviceObject instanceof Advice)) {
            throw new UnknownAdviceTypeException(adviceObject);
        }
        Advice advice = (Advice) adviceObject;
        if (advice instanceof MethodInterceptor) {
            // So well-known it doesn‘t even need an adapter.
            return new DefaultPointcutAdvisor(advice);
        }
        for (AdvisorAdapter adapter : this.adapters) {
            // Check that it is supported.
            if (adapter.supportsAdvice(advice)) {
                return new DefaultPointcutAdvisor(advice);
            }
        }
        throw new UnknownAdviceTypeException(advice);
    }
    public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {
        List<MethodInterceptor> interceptors = new ArrayList<MethodInterceptor>(3);
        Advice advice = advisor.getAdvice();
        if (advice instanceof MethodInterceptor) {
            interceptors.add((MethodInterceptor) advice);
        }
        for (AdvisorAdapter adapter : this.adapters) {
            if (adapter.supportsAdvice(advice)) {
                interceptors.add(adapter.getInterceptor(advisor));
            }
        }
        if (interceptors.isEmpty()) {
            throw new UnknownAdviceTypeException(advisor.getAdvice());
        }
        return interceptors.toArray(new MethodInterceptor[interceptors.size()]);
    }
    public void registerAdvisorAdapter(AdvisorAdapter adapter) {
        this.adapters.add(adapter);
    }
}

剥茧抽丝，继续看adapter，在DefaultAdvisorRegistry的getInterceptors调用中。从MethodBeforeAdviceAdapter、AfterReturningAdviceAdapter、ThrowsAdviceAdaper这几个通知适配器的名字上能够看出和Advice一一相应，在这里，他们作为适配器被增加到adapter的List中，他们都是实现AdvisorAdapter接口的同一层次的类。仅仅是各自承担着不同的适配的任务，一对一的服务于不同的advice实现。
以MethodBeforeAdviceAdapter为例。代码例如以下：

class MethodBeforeAdviceAdapter implements AdvisorAdapter, Serializable {

    public boolean supportsAdvice(Advice advice) {
        return (advice instanceof MethodBeforeAdvice);
    }
    // 把advice从通知器中取出
    public MethodInterceptor getInterceptor(Advisor advisor) {
        MethodBeforeAdvice advice = (MethodBeforeAdvice) advisor.getAdvice();
        return new MethodBeforeAdviceInterceptor(advice);
    }
}

到这里就非常清楚了。Spring AOP为了实现advice的织入，设计了特定拦截器对这些功能进行了封装。

尽管应用不会直接用到这些拦截器。但却是advice发挥作用必不可少的准备。还是以MethodBeforeAdviceInterceptor为例，我们看看advice是怎样封装的。

在invoke回调方法中，看到首先触发了advice的before的回调，然后才是MethodInvocation的proceed方法的调用。看到这里。就已经和前面的在ReflectionMethodInvocation的Proceed分析中联系起来。回顾了一下，在AopProxy代理对象触发的ReflectionMethodInvocation的proceed方法中，在取得拦截器以后，启动了对拦截器invoke方法的调用。

依照AOP的规则，ReflectiveMethodInvocation触发的拦截器invoke方法，终于会依据不同的advice类型，触发Spring对不同的advice的拦截器封装，比方对MethodBeforeAdvice,终于会依据不同的advice类型触发Spring对不同的advice的拦截器封装。比方对MethodBeforeAdvice。终于会触发MethodBeforeAdviceInterceptor的invoke方法。在MethodBeforeAdviceInterceptor方法中。会调用advice的before方法。这就是MethodBeforeAdvice所须要的对目标对象的增强效果：在方法调用之前通知增强。

public class MethodBeforeAdviceInterceptor implements MethodInterceptor, Serializable {
    private MethodBeforeAdvice advice;
    /**
     * Create a new MethodBeforeAdviceInterceptor for the given advice.
     * @param advice the MethodBeforeAdvice to wrap
     */
    // 为指定的Advice创建相应的MethodBeforeAdviceInterceptor对象
    public MethodBeforeAdviceInterceptor(MethodBeforeAdvice advice) {
        Assert.notNull(advice, "Advice must not be null");
        this.advice = advice;
    }

    // 这个invoke方法是拦截器的回调方法，会在代理对象的方法被调用时触发回调
    public Object invoke(MethodInvocation mi) throws Throwable {
        this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis() );
        return mi.proceed();
    }
}

完毕MethodBeforeAdviceInterceptor的调用。然后启动advice通知的afterReturning回调，代码例如以下：

/**
     * Create a new AfterReturningAdviceInterceptor for the given advice.
     * @param advice the AfterReturningAdvice to wrap
     */
    public AfterReturningAdviceInterceptor(AfterReturningAdvice advice) {
        Assert.notNull(advice, "Advice must not be null");
        this.advice = advice;
    }
    public Object invoke(MethodInvocation mi) throws Throwable {
        Object retVal = mi.proceed();
        this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
        return retVal;
    }

ThrowsAdvice的实现和上面相似，也是封装在相应的AdviceInterceptor中，ThrowsAdvice的回调方法要复杂一些，他维护了一个exceptionHandlerMap来相应不同的方法调用场景，这个exceptionHandlerMap中的handler的取得时与触发ThrowsAdvice增强的异常相关的。

/**
 * Interceptor to wrap an after-throwing advice.
 *
 * <p>The signatures on handler methods on the {@code ThrowsAdvice}
 * implementation method argument must be of the form:<br>
 *
 * {@code void afterThrowing([Method, args, target], ThrowableSubclass);}
 *
 * <p>Only the last argument is required.
 *
 * <p>Some examples of valid methods would be:
 *
 * <pre class="code">public void afterThrowing(Exception ex)</pre>
 * <pre class="code">public void afterThrowing(RemoteException)</pre>
 * <pre class="code">public void afterThrowing(Method method, Object[] args, Object target, Exception ex)</pre>
 * <pre class="code">public void afterThrowing(Method method, Object[] args, Object target, ServletException ex)</pre>
 *
 * <p>This is a framework class that need not be used directly by Spring users.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 */
public class ThrowsAdviceInterceptor implements MethodInterceptor, AfterAdvice {
    private static final String AFTER_THROWING = "afterThrowing";
    private static final Log logger = LogFactory.getLog(ThrowsAdviceInterceptor.class);
    private final Object throwsAdvice;
    /** Methods on throws advice, keyed by exception class */
    private final Map<Class, Method> exceptionHandlerMap = new HashMap<Class, Method>();
    /**
     * Create a new ThrowsAdviceInterceptor for the given ThrowsAdvice.
     * @param throwsAdvice the advice object that defines the exception
     * handler methods (usually a {@link org.springframework.aop.ThrowsAdvice}
     * implementation)
     */
    public ThrowsAdviceInterceptor(Object throwsAdvice) {
        Assert.notNull(throwsAdvice, "Advice must not be null");
        this.throwsAdvice = throwsAdvice;
        // 配置ThrowsAdvice回调方法
        Method[] methods = throwsAdvice.getClass().getMethods();
        for (Method method : methods) {
            if (method.getName().equals(AFTER_THROWING) &&
                    (method.getParameterTypes().length == 1 || method.getParameterTypes().length == 4) &&
                    Throwable.class.isAssignableFrom(method.getParameterTypes()[method.getParameterTypes().length - 1])
                ) {
                // Have an exception handler
                // 配置异常处理
                this.exceptionHandlerMap.put(method.getParameterTypes()[method.getParameterTypes().length - 1], method);
                if (logger.isDebugEnabled()) {
                    logger.debug("Found exception handler method: " + method);
                }
            }
        }
        if (this.exceptionHandlerMap.isEmpty()) {
            throw new IllegalArgumentException(
                    "At least one handler method must be found in class [" + throwsAdvice.getClass() + "]");
        }
    }
    public int getHandlerMethodCount() {
        return this.exceptionHandlerMap.size();
    }
    /**
     * Determine the exception handle method. Can return null if not found.
     * @param exception the exception thrown
     * @return a handler for the given exception type
     */
    private Method getExceptionHandler(Throwable exception) {
        Class exceptionClass = exception.getClass();
        if (logger.isTraceEnabled()) {
            logger.trace("Trying to find handler for exception of type [" + exceptionClass.getName() + "]");
        }
        Method handler = this.exceptionHandlerMap.get(exceptionClass);
        while (handler == null && !exceptionClass.equals(Throwable.class)) {
            exceptionClass = exceptionClass.getSuperclass();
            handler = this.exceptionHandlerMap.get(exceptionClass);
        }
        if (handler != null && logger.isDebugEnabled()) {
            logger.debug("Found handler for exception of type [" + exceptionClass.getName() + "]: " + handler);
        }
        return handler;
    }
    public Object invoke(MethodInvocation mi) throws Throwable {
        try {
            // 把目标对象方法调用放在try catch中。并在catch中触发。
            // ThrowsAdvice的回调，把异常接着向外抛出，不做过多的处理
            return mi.proceed();
        }
        catch (Throwable ex) {
            Method handlerMethod = getExceptionHandler(ex);
            if (handlerMethod != null) {
                invokeHandlerMethod(mi, ex, handlerMethod);
            }
            throw ex;
        }
    }
    // 通过反射启动对ThrowsAdvice回调方法的调用
    private void invokeHandlerMethod(MethodInvocation mi, Throwable ex, Method method) throws Throwable {
        Object[] handlerArgs;
        if (method.getParameterTypes().length == 1) {
            handlerArgs = new Object[] { ex };
        }
        else {
            handlerArgs = new Object[] {mi.getMethod(), mi.getArguments(), mi.getThis(), ex};
        }
        try {
            method.invoke(this.throwsAdvice, handlerArgs);
        }
        catch (InvocationTargetException targetEx) {
            throw targetEx.getTargetException();
        }
    }
}

未完待续……