Spring AOP底层实现分析

Spring AOP代理对象的生成

Spring提供了两种方式来生成代理对象: JdkProxy和Cglib，具体使用哪种方式生成由AopProxyFactory根据AdvisedSupport对象的配置来决定。默认的策略是如果目标类是接口，则使用JDK动态代理技术，否则使用Cglib来生成代理。

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具体逻辑在 org.springframework.aop.framework.DefaultAopProxyFactory类中，源码如下：

/**
 * Spring AOP代理工厂
 */
public class DefaultAopProxyFactory implements AopProxyFactory, Serializable {

    @Override
    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.");
            }
            //如果目标类是接口, 使用JDK动态代理来生成代理类及代理类实例对象
            if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
                return new JdkDynamicAopProxy(config);
            }
            //使用Cglib生成代理类及代理类实例对象
            return new ObjenesisCglibAopProxy(config);
        }
        else {
            return new JdkDynamicAopProxy(config);
        }
    }

    private boolean hasNoUserSuppliedProxyInterfaces(AdvisedSupport config) {
        Class[] ifcs = config.getProxiedInterfaces();
        return (ifcs.length == 0 || (ifcs.length == 1 && SpringProxy.class.isAssignableFrom(ifcs[0])));
    }

}

JDK动态代理技术生成代理类及实例对象

下面我们来研究一下Spring如何使用JDK来生成代理对象，具体的生成代码放在org.springframework.aop.framework.JdkDynamicAopProxy 这个类中，直接上相关代码：

/**
 * JDK动态代理实现类
 */
final class JdkDynamicAopProxy implements AopProxy, InvocationHandler, Serializable {

    /** use serialVersionUID from Spring 1.2 for interoperability */
    private static final long serialVersionUID = 5531744639992436476L;

    /** We use a static Log to avoid serialization issues */
    private static final Log logger = LogFactory.getLog(JdkDynamicAopProxy.class);

    /** Config used to configure this proxy */
    private final AdvisedSupport advised;

    /**
     * Is the {@link #equals} method defined on the proxied interfaces?
     */
    private boolean equalsDefined;

    /**
     * Is the {@link #hashCode} method defined on the proxied interfaces?
     */
    private boolean hashCodeDefined;


    /**
     * Construct a new JdkDynamicAopProxy for the given AOP configuration.
     * @param config the AOP configuration as AdvisedSupport object
     * @throws AopConfigException if the config is invalid. We try to throw an informative
     * exception in this case, rather than let a mysterious failure happen later.
     */
    public JdkDynamicAopProxy(AdvisedSupport config) throws AopConfigException {
        Assert.notNull(config, "AdvisedSupport must not be null");
        if (config.getAdvisors().length == 0 && config.getTargetSource() == AdvisedSupport.EMPTY_TARGET_SOURCE) {
            throw new AopConfigException("No advisors and no TargetSource specified");
        }
        this.advised = config;
    }

    /**
     * 使用JDK动态代理生成代理类
     */
    @Override
    public Object getProxy() {
        return getProxy(ClassUtils.getDefaultClassLoader());
    }

    @Override
    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, true);
        findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
        return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
    }

    /**
     * Finds any {@link #equals} or {@link #hashCode} method that may be defined
     * on the supplied set of interfaces.
     * @param proxiedInterfaces the interfaces to introspect
     */
    private void findDefinedEqualsAndHashCodeMethods(Class[] proxiedInterfaces) {
        for (Class proxiedInterface : proxiedInterfaces) {
            Method[] methods = proxiedInterface.getDeclaredMethods();
            for (Method method : methods) {
                if (AopUtils.isEqualsMethod(method)) {
                    this.equalsDefined = true;
                }
                if (AopUtils.isHashCodeMethod(method)) {
                    this.hashCodeDefined = true;
                }
                if (this.equalsDefined && this.hashCodeDefined) {
                    return;
                }
            }
        }
    }
}

JdkDynamicAopProxy 同时实现了AopProxy和InvocationHandler接口，InvocationHandler是JDK动态代理的核心，生成的代理对象的方法调用都会委托到InvocationHandler.invoke()方法。下面我们就通过分析这个类中实现的invoke()方法来具体看下Spring AOP是如何织入切面的。

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        MethodInvocation invocation;
        Object oldProxy = null;
        boolean setProxyContext = false;

        TargetSource targetSource = this.advised.targetSource;
        Class targetClass = null;
        Object target = null;

        try {
            if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {   //目标类没有实现eqauls()方法
                // The target does not implement the equals(Object) method itself.
                return equals(args[0]);
            }
            else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {  //目标类没有实现hashCode()方法
                // The target does not implement the hashCode() method itself.
                return hashCode();
            }
            else if (method.getDeclaringClass() == DecoratingProxy.class) {
                // There is only getDecoratedClass() declared -> dispatch to proxy config.
                return AopProxyUtils.ultimateTargetClass(this.advised);
            }
            else if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
                    method.getDeclaringClass().isAssignableFrom(Advised.class)) {
                // Service invocations on ProxyConfig with the proxy config...
                return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
            }

            Object retVal;

            if (this.advised.exposeProxy) {
                // Make invocation available if necessary.
                oldProxy = AopContext.setCurrentProxy(proxy);
                setProxyContext = true;
            }

            // 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();
            }

            // Get the interception chain for this method.
            List