Spring中的AOP

 

Spring中的AOP

Written by Tony Jiang @ 2012-1-18

（转）何为AOP

AOP，面向切面编程。

在不改动代码的前提下，灵活的在现有代码的执行顺序前后，添加进新规机能。

来一个简单的Sample:

目标类：

package com.hyron.tony;

public class CustomerService {
    private String name;
    private String url;

    public void setName(String name) {
        this.name = name;
    }

    public void setUrl(String url) {
        this.url = url;
    }

    public void printName() {
        System.out.println("Customer name : " + this.name);
    }

    public void printURL() {
        System.out.println("Customer website : " + this.url);
    }

    public void printThrowException() {
        throw new IllegalArgumentException();
    }

}

advice：只以Around advice为例

import java.util.Arrays;

import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;

public class HijackAroundMethod implements MethodInterceptor {
    @Override
    public Object invoke(MethodInvocation methodInvocation) throws Throwable {

        System.out.println("Method name : "
                + methodInvocation.getMethod().getName());
        System.out.println("Method arguments : "
                + Arrays.toString(methodInvocation.getArguments()));

        // same with MethodBeforeAdvice
        System.out.println("HijackAroundMethod : Before method hijacked!");

        try {
            // proceed to original method call
            Object result = methodInvocation.proceed();

            // same with AfterReturningAdvice
            System.out.println("HijackAroundMethod : Before after hijacked!");

            return result;

        } catch (IllegalArgumentException e) {
            // same with ThrowsAdvice
            System.out
                    .println("HijackAroundMethod : Throw exception hijacked!");
            throw e;
        }
    }
}

编织切入关系的配置文件：

<bean id="customerService" class="com.mkyong.customer.services.CustomerService">
        <property name="name" value="Yong Mook Kim" />
        <property name="url" value="http://www.mkyong.com" />
    </bean>

    <bean id="hijackAroundMethodBean" class="com.mkyong.aop.HijackAroundMethod" />

    <bean id="customerServiceProxy" class="org.springframework.aop.framework.ProxyFactoryBean">

        <property name="target" ref="customerService" />

        <property name="interceptorNames">
            <list>
                <value>hijackAroundMethodBean</value>
            </list>
        </property>
    </bean>

 

Sample的启动：

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.mkyong.customer.services.CustomerService;

public class App {
    public static void main(String[] args) {
        ApplicationContext appContext = new ClassPathXmlApplicationContext(
                new String[] { "Spring-Customer.xml" });

        CustomerService cust = (CustomerService) appContext
                .getBean("customerServiceProxy");

        System.out.println("*************************");
        cust.printName();
        System.out.println("*************************");
        cust.printURL();
        System.out.println("*************************");
        try {
            cust.printThrowException();
        } catch (Exception e) {

        }

    }
}

 

以上代码，用customerServiceProxy代理CustomerService的执行

在customerServiceProxy的配置中，定义了用hijackAroundMethodBean作为方法拦截器，在hijackAroundMethodBean中利用invoke方法，拦截住所有的方法调用，塞入自己的逻辑业务。

AOP的两种实现

                上面看到的是Spring的Sample。

                其实，Spring的AOP也是调用了其他开源技术实现。

                比较常用的是JDK自己的Proxy，和开源的CGLIB

                两者的区别，Proxy需要Advice必须从接口继承过来。如果切入的目标物是实体类，则无法使用。

                CGLIB则可以用于直接覆盖实体类的方法。

                Spring对以上两种都有支持。

Spring的底层实现

                Spring在配置文件中，通过ProxyFactoryBean编织和实现了切面的构成。

                我们在执行以下这行话的时候

CustomerService cust = (CustomerService) appContext

                                                                .getBean("customerServiceProxy");

 

                其实是将动态对象的生成委托给了ProxyFactoryBean

当配置文件中 <bean>的class属性配置的实现类是FactoryBean时，通过getBean方法返回的不是FactoryBean本身，而是 FactoryBean#getObject()方法所返回的对象，相当于FactoryBean#getObject()代理了getBean()方 法。

 

执行顺序如下：

1.  ProxyFactoryBean中的getObject

/**
     * Return a proxy. Invoked when clients obtain beans from this factory bean.
     * Create an instance of the AOP proxy to be returned by this factory.
     * The instance will be cached for a singleton, and create on each call to
     * <code>getObject()</code> for a proxy.
     * @return a fresh AOP proxy reflecting the current state of this factory
     */
    public Object getObject() throws BeansException {
        initializeAdvisorChain();
        if (isSingleton()) {
            return getSingletonInstance();
        }
        else {
            if (this.targetName == null) {
                logger.warn("Using non-singleton proxies with singleton targets is often undesirable. " +
                        "Enable prototype proxies by setting the 'targetName' property.");
            }
            return newPrototypeInstance();
        }
    }

 

2.  ProxyFactoryBean中的initializeAdvisorChain

从配置文件中的advice list中取得interceptorNames,并将其加入advisorChain

for (String name : this.interceptorNames) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Configuring advisor or advice '" + name + "'");
                }

                if (name.endsWith(GLOBAL_SUFFIX)) {
                    if (!(this.beanFactory instanceof ListableBeanFactory)) {
                        throw new AopConfigException(
                                "Can only use global advisors or interceptors with a ListableBeanFactory");
                    }
                    addGlobalAdvisor((ListableBeanFactory) this.beanFactory,
                            name.substring(0, name.length() - GLOBAL_SUFFIX.length()));
                }

 

3.  被切面改造过的instance的生成

我们以单例举例：

/**
     * Return the singleton instance of this class's proxy object,
     * lazily creating it if it hasn't been created already.
     * @return the shared singleton proxy
     */
    private synchronized Object getSingletonInstance() {
        if (this.singletonInstance == null) {
            this.targetSource = freshTargetSource();
            if (this.autodetectInterfaces && getProxiedInterfaces().length == 0 && !isProxyTargetClass()) {
                // Rely on AOP infrastructure to tell us what interfaces to proxy.
                Class targetClass = getTargetClass();
                if (targetClass == null) {
                    throw new FactoryBeanNotInitializedException("Cannot determine target class for proxy");
                }
                setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));
            }
            // Initialize the shared singleton instance.
            super.setFrozen(this.freezeProxy);
            this.singletonInstance = getProxy(createAopProxy());
        }
        return this.singletonInstance;
    }

 

AopProxy是最终生成instance的地方，但是它是接口，和框架分离开来了

接口的生成在父类的ProxyCreatorSupport中

/**
     * Subclasses should call this to get a new AOP proxy. They should <b>not</b>
     * create an AOP proxy with <code>this</code> as an argument.
     */
    protected final synchronized AopProxy createAopProxy() {
        if (!this.active) {
            activate();
        }
        return getAopProxyFactory().createAopProxy(this);
    }
createAopProxy的实际运行代码：
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
        if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
            Class targetClass = config.getTargetClass();
            if (targetClass == null) {
                throw new AopConfigException("TargetSource cannot determine target class: " +
                        "Either an interface or a target is required for proxy creation.");
            }
            if (targetClass.isInterface()) {
                return new JdkDynamicAopProxy(config);
            }
            if (!cglibAvailable) {
                throw new AopConfigException(
                        "Cannot proxy target class because CGLIB2 is not available. " +
                        "Add CGLIB to the class path or specify proxy interfaces.");
            }
            return CglibProxyFactory.createCglibProxy(config);
        }
        else {
            return new JdkDynamicAopProxy(config);
        }
    }

 

大家注意一下，这里根据targetClass的类型判断，采用JDK Proxy还是CGLIB模式生成动态对象

 

4.  JDK Proxy的生成

如果大家使用过原生的JDK PROXY，下面的代码是在熟悉不过了

JdkDynamicAopProxy中实例化instance的

public Object getProxy(ClassLoader classLoader) {
        if (logger.isDebugEnabled()) {
            logger.debug("Creating JDK dynamic proxy: target source is " + this.advised.getTargetSource());
        }
        Class[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised);
        findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
        return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
    }

 

JdkDynamicAopProxy中的invoke方法

// Get the interception chain for this method.
            List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(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 = AopUtils.invokeJoinpointUsingReflection(target, method, args);
            }
            else {
                // We need to create a method invocation...
                invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
                // Proceed to the joinpoint through the interceptor chain.
                retVal = invocation.proceed();
            }

 

Proceed的真实代码，把对象本身的invoke和拦截器的invoke交织在一起

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);
        }
    }

 

拦截器是如何编织的

我们在JdkDynamicAopProxy中的invoke方法中看到如下拦截器链条的生成

// Get the interception chain for this method.
            List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

 

它对应的代码是DefaultAdvisorChainFactory中的getInterceptorsAndDynamicInterceptionAdvice方法

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);

 

最终advisor的登记由DefaultAdvisorAdapterRegistry完成

我们可以看到DefaultAdvisorAdapterRegistry中首先登记了所有的AdviceAdapter

/**
     * Create a new DefaultAdvisorAdapterRegistry, registering well-known adapters.
     */
    public DefaultAdvisorAdapterRegistry() {
        registerAdvisorAdapter(new MethodBeforeAdviceAdapter());
        registerAdvisorAdapter(new AfterReturningAdviceAdapter());
        registerAdvisorAdapter(new ThrowsAdviceAdapter());
    }

 

在如下代码中按照AdviceAdapter的类型塞入责任练中

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()]);
    }

 

这种做法类似于：

1.  首先为切入Adivsor建立责任链模式

2.       其次将每个Adivsor委托给DefaultAdvisorAdapterRegistry登录

3.       在DefaultAdvisorAdapterRegistry中封装Adivsor到各个专门的Advisor适配器中

比如，AfterReturningAdviceAdapter的代码如下：

/**
 * Adapter to enable {@link org.springframework.aop.AfterReturningAdvice}
 * to be used in the Spring AOP framework.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 */
class AfterReturningAdviceAdapter implements AdvisorAdapter, Serializable {

    public boolean supportsAdvice(Advice advice) {
        return (advice instanceof AfterReturningAdvice);
    }

    public MethodInterceptor getInterceptor(Advisor advisor) {
        AfterReturningAdvice advice = (AfterReturningAdvice) advisor.getAdvice();
        return new AfterReturningAdviceInterceptor(advice);
    }

}

 

4.       在适配器中，将Advisor的方法和目标类方法交织在一起

**
 * Interceptor to wrap am {@link org.springframework.aop.AfterReturningAdvice}.
 * Used internally by the AOP framework; application developers should not need
 * to use this class directly.
 *
 * @author Rod Johnson
 */
public class AfterReturningAdviceInterceptor implements MethodInterceptor, AfterAdvice, Serializable {

    private final AfterReturningAdvice advice;


    /**
     * 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;
    }

}