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AOP - AOP代理

动态代理概述

什么是代理

代理模式(Proxy pattern): 为另一个对象提供一个替身或占位符以控制对这个对象的访问

什么是动态代理？

动态代理就是，在程序运行期，创建目标对象的代理对象，并对目标对象中的方法进行功能性增强的一种技术。

在生成代理对象的过程中，目标对象不变，代理对象中的方法是目标对象方法的增强方法。可以理解为运行期间，对象中方法的动态拦截，在拦截方法的前后执行功能操作。

代理的创建

创建代理的方法是postProcessAfterInitialization：如果bean被子类标识为代理，则使用配置的拦截器创建一个代理

/**
  * Create a proxy with the configured interceptors if the bean is
  * identified as one to proxy by the subclass.
  * @see #getAdvicesAndAdvisorsForBean
  */
@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
  if (bean != null) {
    Object cacheKey = getCacheKey(bean.getClass(), beanName);
    // 如果不是提前暴露的代理
    if (this.earlyProxyReferences.remove(cacheKey) != bean) {
      return wrapIfNecessary(bean, beanName, cacheKey);
    }
  }
  return bean;
}

wrapIfNecessary方法主要用于判断是否需要创建代理，如果Bean能够获取到advisor才需要创建代理

/**
  * Wrap the given bean if necessary, i.e. if it is eligible for being proxied.
  * @param bean the raw bean instance
  * @param beanName the name of the bean
  * @param cacheKey the cache key for metadata access
  * @return a proxy wrapping the bean, or the raw bean instance as-is
  */
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
   // 如果bean是通过TargetSource接口获取
   if (beanName != null && this.targetSourcedBeans.contains(beanName)) {
      return bean;
   }
   // 如果bean是切面类
   if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
      return bean;
   }
   // 如果是aop基础类？是否跳过？
   if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
      this.advisedBeans.put(cacheKey, Boolean.FALSE);
      return bean;
   }

  // 重点：获取所有advisor，如果没有获取到，那说明不要进行增强，也就不需要代理了。
  Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
  if (specificInterceptors != DO_NOT_PROXY) {
    this.advisedBeans.put(cacheKey, Boolean.TRUE);
    // 重点：创建代理
    Object proxy = createProxy(
        bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
    this.proxyTypes.put(cacheKey, proxy.getClass());
    return proxy;
  }

  this.advisedBeans.put(cacheKey, Boolean.FALSE);
  return bean;
}

获取所有的Advisor

我们看下获取所有advisor的方法getAdvicesAndAdvisorsForBean

@Override
@Nullable
protected Object[] getAdvicesAndAdvisorsForBean(
    Class<?> beanClass, String beanName, @Nullable TargetSource targetSource) {

  List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
  if (advisors.isEmpty()) {
    return DO_NOT_PROXY;
  }
  return advisors.toArray();
}

通过findEligibleAdvisors方法获取advisor， 如果获取不到返回DO_NOT_PROXY（不需要创建代理），findEligibleAdvisors方法如下

/**
  * Find all eligible Advisors for auto-proxying this class.
  * @param beanClass the clazz to find advisors for
  * @param beanName the name of the currently proxied bean
  * @return the empty List, not {@code null},
  * if there are no pointcuts or interceptors
  * @see #findCandidateAdvisors
  * @see #sortAdvisors
  * @see #extendAdvisors
  */
protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
  // 和上文一样，获取所有切面类的切面方法生成Advisor
  List<Advisor> candidateAdvisors = findCandidateAdvisors();
  // 找到这些Advisor中能够应用于beanClass的Advisor
  List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
  // 如果需要，交给子类拓展
  extendAdvisors(eligibleAdvisors);
  // 对Advisor排序
  if (!eligibleAdvisors.isEmpty()) {
    eligibleAdvisors = sortAdvisors(eligibleAdvisors);
  }
  return eligibleAdvisors;
}

获取所有切面类的切面方法生成Advisor

/**
  * Find all candidate Advisors to use in auto-proxying.
  * @return the List of candidate Advisors
  */
protected List<Advisor> findCandidateAdvisors() {
  Assert.state(this.advisorRetrievalHelper != null, "No BeanFactoryAdvisorRetrievalHelper available");
  return this.advisorRetrievalHelper.findAdvisorBeans();
}

找到这些Advisor中能够应用于beanClass的Advisor

/**
  * Determine the sublist of the {@code candidateAdvisors} list
  * that is applicable to the given class.
  * @param candidateAdvisors the Advisors to evaluate
  * @param clazz the target class
  * @return sublist of Advisors that can apply to an object of the given class
  * (may be the incoming List as-is)
  */
public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
  if (candidateAdvisors.isEmpty()) {
    return candidateAdvisors;
  }
  List<Advisor> eligibleAdvisors = new ArrayList<>();
  for (Advisor candidate : candidateAdvisors) {
    // 通过Introduction实现的advice
    if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
      eligibleAdvisors.add(candidate);
    }
  }
  boolean hasIntroductions = !eligibleAdvisors.isEmpty();
  for (Advisor candidate : candidateAdvisors) {
    if (candidate instanceof IntroductionAdvisor) {
      // already processed
      continue;
    }
    // 是否能够应用于clazz的Advice
    if (canApply(candidate, clazz, hasIntroductions)) {
      eligibleAdvisors.add(candidate);
    }
  }
  return eligibleAdvisors;
}

创建代理的入口方法

获取所有advisor后，如果有advisor，则说明需要增强，即需要创建代理，创建代理的方法如下：

/**
  * Create an AOP proxy for the given bean.
  * @param beanClass the class of the bean
  * @param beanName the name of the bean
  * @param specificInterceptors the set of interceptors that is
  * specific to this bean (may be empty, but not null)
  * @param targetSource the TargetSource for the proxy,
  * already pre-configured to access the bean
  * @return the AOP proxy for the bean
  * @see #buildAdvisors
  */
protected Object createProxy(Class<?> beanClass, @Nullable String beanName,
    @Nullable Object[] specificInterceptors, TargetSource targetSource) {

  if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
    AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
  }

  ProxyFactory proxyFactory = new ProxyFactory();
  proxyFactory.copyFrom(this);

  if (proxyFactory.isProxyTargetClass()) {
    // Explicit handling of JDK proxy targets (for introduction advice scenarios)
    if (Proxy.isProxyClass(beanClass)) {
      // Must allow for introductions; can't just set interfaces to the proxy's interfaces only.
      for (Class<?> ifc : beanClass.getInterfaces()) {
        proxyFactory.addInterface(ifc);
      }
    }
  }
  else {
    // No proxyTargetClass flag enforced, let's apply our default checks...
    if (shouldProxyTargetClass(beanClass, beanName)) {
      proxyFactory.setProxyTargetClass(true);
    }
    else {
      evaluateProxyInterfaces(beanClass, proxyFactory);
    }
  }

  Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
  proxyFactory.addAdvisors(advisors);
  proxyFactory.setTargetSource(targetSource);
  customizeProxyFactory(proxyFactory);

  proxyFactory.setFrozen(this.freezeProxy);
  if (advisorsPreFiltered()) {
    proxyFactory.setPreFiltered(true);
  }

  // Use original ClassLoader if bean class not locally loaded in overriding class loader
  ClassLoader classLoader = getProxyClassLoader();
  if (classLoader instanceof SmartClassLoader && classLoader != beanClass.getClassLoader()) {
    classLoader = ((SmartClassLoader) classLoader).getOriginalClassLoader();
  }
  return proxyFactory.getProxy(classLoader);
}

proxyFactory.getProxy(classLoader)

/**
  * Create a new proxy according to the settings in this factory.
  * <p>Can be called repeatedly. Effect will vary if we've added
  * or removed interfaces. Can add and remove interceptors.
  * <p>Uses the given class loader (if necessary for proxy creation).
  * @param classLoader the class loader to create the proxy with
  * (or {@code null} for the low-level proxy facility's default)
  * @return the proxy object
  */
public Object getProxy(@Nullable ClassLoader classLoader) {
  return createAopProxy().getProxy(classLoader);
}

依据条件创建代理(jdk或cglib)

DefaultAopProxyFactory.createAopProxy

@Override
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
  if (!NativeDetector.inNativeImage() &&
      (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() || Proxy.isProxyClass(targetClass)) {
      return new JdkDynamicAopProxy(config);
    }
    return new ObjenesisCglibAopProxy(config);
  }
  else {
    return new JdkDynamicAopProxy(config);
  }
}

小结

  • config.isOptimize() 是通过optimize设置，表示配置是自定义的，默认是false；
  • config.isProxyTargetClass()是通过<aop:config proxy-target-class="true" /> 来配置的，表示优先使用cglib代理，默认是false；
  • hasNoUserSuppliedProxyInterfaces(config) 表示是否目标类实现了接口

由此可以知道：

Spring默认在目标类实现接口时是通过JDK代理实现的，只有非接口的是通过Cglib代理实现的。当设置proxy-target-class为true时在目标类不是接口或者代理类时优先使用cglib代理实现。

JDK代理

JDK动态代理是有JDK提供的工具类Proxy实现的，动态代理类是在运行时生成指定接口的代理类，每个代理实例（实现需要代理的接口）都有一个关联的调用处理程序对象，此对象实现了InvocationHandler，最终的业务逻辑是在InvocationHandler实现类的invoke方法上。

JDK代理的流程如下：

JDK代理自动生成的class是由sun.misc.ProxyGenerator来生成的。

ProxyGenerator生成代码

我们看下sun.misc.ProxyGenerator生成代码的逻辑：

/**
    * Generate a proxy class given a name and a list of proxy interfaces.
    *
    * @param name        the class name of the proxy class
    * @param interfaces  proxy interfaces
    * @param accessFlags access flags of the proxy class
*/
public static byte[] generateProxyClass(final String name,
                                        Class<?>[] interfaces,
                                        int accessFlags)
{
    ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
    final byte[] classFile = gen.generateClassFile();
    ...
}

generateClassFile方法如下：

/**
    * Generate a class file for the proxy class.  This method drives the
    * class file generation process.
    */
private byte[] generateClassFile() {

    /* 第一步：将所有方法包装成ProxyMethod对象 */
    
    // 将Object类中hashCode、equals、toString方法包装成ProxyMethod对象
    addProxyMethod(hashCodeMethod, Object.class);
    addProxyMethod(equalsMethod, Object.class);
    addProxyMethod(toStringMethod, Object.class);

    // 将代理类接口方法包装成ProxyMethod对象
    for (Class<?> intf : interfaces) {
        for (Method m : intf.getMethods()) {
            addProxyMethod(m, intf);
        }
    }

    // 校验返回类型
    for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
        checkReturnTypes(sigmethods);
    }

    /* 第二步：为代理类组装字段，构造函数，方法，static初始化块等 */
    try {
        // 添加构造函数，参数是InvocationHandler
        methods.add(generateConstructor());

        // 代理方法
        for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
            for (ProxyMethod pm : sigmethods) {

                // 字段
                fields.add(new FieldInfo(pm.methodFieldName,
                    "Ljava/lang/reflect/Method;",
                        ACC_PRIVATE | ACC_STATIC));

                // 上述ProxyMethod中的方法
                methods.add(pm.generateMethod());
            }
        }

        // static初始化块
        methods.add(generateStaticInitializer());

    } catch (IOException e) {
        throw new InternalError("unexpected I/O Exception", e);
    }

    if (methods.size() > 65535) {
        throw new IllegalArgumentException("method limit exceeded");
    }
    if (fields.size() > 65535) {
        throw new IllegalArgumentException("field limit exceeded");
    }

    /* 第三步：写入class文件 */

    /*
        * Make sure that constant pool indexes are reserved for the
        * following items before starting to write the final class file.
        */
    cp.getClass(dotToSlash(className));
    cp.getClass(superclassName);
    for (Class<?> intf: interfaces) {
        cp.getClass(dotToSlash(intf.getName()));
    }

    /*
        * Disallow new constant pool additions beyond this point, since
        * we are about to write the final constant pool table.
        */
    cp.setReadOnly();

    ByteArrayOutputStream bout = new ByteArrayOutputStream();
    DataOutputStream dout = new DataOutputStream(bout);

    try {
        /*
            * Write all the items of the "ClassFile" structure.
            * See JVMS section 4.1.
            */
                                    // u4 magic;
        dout.writeInt(0xCAFEBABE);
                                    // u2 minor_version;
        dout.writeShort(CLASSFILE_MINOR_VERSION);
                                    // u2 major_version;
        dout.writeShort(CLASSFILE_MAJOR_VERSION);

        cp.write(dout);             // (write constant pool)

                                    // u2 access_flags;
        dout.writeShort(accessFlags);
                                    // u2 this_class;
        dout.writeShort(cp.getClass(dotToSlash(className)));
                                    // u2 super_class;
        dout.writeShort(cp.getClass(superclassName));

                                    // u2 interfaces_count;
        dout.writeShort(interfaces.length);
                                    // u2 interfaces[interfaces_count];
        for (Class<?> intf : interfaces) {
            dout.writeShort(cp.getClass(
                dotToSlash(intf.getName())));
        }

                                    // u2 fields_count;
        dout.writeShort(fields.size());
                                    // field_info fields[fields_count];
        for (FieldInfo f : fields) {
            f.write(dout);
        }

                                    // u2 methods_count;
        dout.writeShort(methods.size());
                                    // method_info methods[methods_count];
        for (MethodInfo m : methods) {
            m.write(dout);
        }

                                        // u2 attributes_count;
        dout.writeShort(0); // (no ClassFile attributes for proxy classes)

    } catch (IOException e) {
        throw new InternalError("unexpected I/O Exception", e);
    }

    return bout.toByteArray();
}

一共三个步骤（把大象装进冰箱分几步？）：

  • 第一步：（把冰箱门打开）准备工作，将所有方法包装成ProxyMethod对象，包括Object类中hashCode、equals、toString方法，以及被代理的接口中的方法
  • 第二步：（把大象装进去）为代理类组装字段，构造函数，方法，static初始化块等
  • 第三步：（把冰箱门带上）写入class文件

从生成的Proxy代码看执行流程

从上述sun.misc.ProxyGenerator类中可以看到，这个类里面有一个配置参数sun.misc.ProxyGenerator.saveGeneratedFiles，可以通过这个参数将生成的Proxy类保存在本地，比如设置为true 执行后，生成的文件如下：

我们看下生成后的代码：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package com.sun.proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import java.util.List;
import tech.pdai.springframework.service.IUserService;

// 所有类和方法都是final类型的
public final class $Proxy0 extends Proxy implements IUserService {
    private static Method m1;
    private static Method m3;
    private static Method m2;
    private static Method m0;
    private static Method m4;

    // 构造函数注入 InvocationHandler
    public $Proxy0(InvocationHandler var1) throws  {
        super(var1);
    }

    public final boolean equals(Object var1) throws  {
        try {
            return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final List findUserList() throws  {
        try {
            return (List)super.h.invoke(this, m3, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final String toString() throws  {
        try {
            return (String)super.h.invoke(this, m2, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final int hashCode() throws  {
        try {
            return (Integer)super.h.invoke(this, m0, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final void addUser() throws  {
        try {
            super.h.invoke(this, m4, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    static {
        try {
            // 初始化 methods, 2个IUserService接口中的方法，3个Object中的接口
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m3 = Class.forName("tech.pdai.springframework.service.IUserService").getMethod("findUserList");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
            m4 = Class.forName("tech.pdai.springframework.service.IUserService").getMethod("addUser");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

上述代码是比较容易理解的，我就不画图了。

主要流程是：

  • ProxyGenerator创建Proxy的具体类$Proxy0
  • 由static初始化块初始化接口方法：2个IUserService接口中的方法，3个Object中的接口方法
  • 由构造函数注入InvocationHandler
  • 执行的时候，通过ProxyGenerator创建的Proxy，调用InvocationHandler的invoke方法，执行我们自定义的invoke方法

SpringAOP中JDK代理的实现

SpringAOP扮演的是JDK代理的创建和调用两个角色，我们通过这两个方向来看下SpringAOP的代码（JdkDynamicAopProxy类）

SpringAOP Jdk代理的创建

代理的创建比较简单，调用getProxy方法，然后直接调用JDK中Proxy.newProxyInstance()方法将classloader和被代理的接口方法传入即可。

@Override
public Object getProxy() {
    return getProxy(ClassUtils.getDefaultClassLoader());
}

@Override
public Object getProxy(@Nullable ClassLoader classLoader) {
    if (logger.isTraceEnabled()) {
        logger.trace("Creating JDK dynamic proxy: " + this.advised.getTargetSource());
    }
    return Proxy.newProxyInstance(classLoader, this.proxiedInterfaces, this);
}

SpringAOP Jdk代理的执行

执行的方法如下：

/**
    * Implementation of {@code InvocationHandler.invoke}.
    * <p>Callers will see exactly the exception thrown by the target,
    * unless a hook method throws an exception.
    */
@Override
@Nullable
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    Object oldProxy = null;
    boolean setProxyContext = false;

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

    try {
        // 执行的是equal方法
        if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
            // The target does not implement the equals(Object) method itself.
            return equals(args[0]);
        }
        // 执行的是hashcode方法
        else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
            // The target does not implement the hashCode() method itself.
            return hashCode();
        }
        // 如果是包装类，则dispatch to proxy config
        else if (method.getDeclaringClass() == DecoratingProxy.class) {
            // There is only getDecoratedClass() declared -> dispatch to proxy config.
            return AopProxyUtils.ultimateTargetClass(this.advised);
        }
        // 用反射方式来执行切点
        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;
        }

        // Get as late as possible to minimize the time we "own" the target,
        // in case it comes from a pool.
        target = targetSource.getTarget();
        Class<?> targetClass = (target != null ? target.getClass() : null);

        // 获取拦截链
        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.
            Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
            retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
        }
        else {
            // We need to create a method invocation...
            MethodInvocation 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.
        Class<?> returnType = method.getReturnType();
        if (retVal != null && retVal == target &&
                returnType != Object.class && returnType.isInstance(proxy) &&
                !RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
            // Special case: it returned "this" and the return type of the method
            // is type-compatible. Note that we can't help if the target sets
            // a reference to itself in another returned object.
            retVal = proxy;
        }
        else if (retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) {
            throw new AopInvocationException(
                    "Null return value from advice does not match primitive return type for: " + method);
        }
        return retVal;
    }
    finally {
        if (target != null && !targetSource.isStatic()) {
            // Must have come from TargetSource.
            targetSource.releaseTarget(target);
        }
        if (setProxyContext) {
            // Restore old proxy.
            AopContext.setCurrentProxy(oldProxy);
        }
    }
}

CGLIB代理

代理的流程

  • 在上图中，我们可以通过在Enhancer中配置更多的参数来控制代理的行为，比如如果只希望增强这个类中的一个方法（而不是所有方法），那就增加callbackFilter来对目标类中方法进行过滤；Enhancer可以有更多的参数类配置其行为，不过我们在学习上述主要的流程就够了。

  • final方法为什么不能被代理？很显然final方法没法被子类覆盖，当然不能代理了。

  • Mockito为什么不能mock静态方法？因为mockito也是基于cglib动态代理来实现的，static方法也不能被子类覆盖，所以显然不能mock。但PowerMock可以mock静态方法，因为它直接在bytecode上工作。

SpringAOP中Cglib代理的实现

SpringAOP封装了cglib，通过其进行动态代理的创建。

我们看下CglibAopProxy的getProxy方法

@Override
public Object getProxy() {
  return getProxy(null);
}

@Override
public Object getProxy(@Nullable ClassLoader classLoader) {
  if (logger.isTraceEnabled()) {
    logger.trace("Creating CGLIB proxy: " + this.advised.getTargetSource());
  }

  try {
    Class<?> rootClass = this.advised.getTargetClass();
    Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");

    // 上面流程图中的目标类
    Class<?> proxySuperClass = rootClass;
    if (rootClass.getName().contains(ClassUtils.CGLIB_CLASS_SEPARATOR)) {
      proxySuperClass = rootClass.getSuperclass();
      Class<?>[] additionalInterfaces = rootClass.getInterfaces();
      for (Class<?> additionalInterface : additionalInterfaces) {
        this.advised.addInterface(additionalInterface);
      }
    }

    // Validate the class, writing log messages as necessary.
    validateClassIfNecessary(proxySuperClass, classLoader);

    // 重点看这里，就是上图的enhancer，设置各种参数来构建
    Enhancer enhancer = createEnhancer();
    if (classLoader != null) {
      enhancer.setClassLoader(classLoader);
      if (classLoader instanceof SmartClassLoader &&
          ((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
        enhancer.setUseCache(false);
      }
    }
    enhancer.setSuperclass(proxySuperClass);
    enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
    enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
    enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(classLoader));

    // 设置callback回调接口，即方法的增强点
    Callback[] callbacks = getCallbacks(rootClass);
    Class<?>[] types = new Class<?>[callbacks.length];
    for (int x = 0; x < types.length; x++) {
      types[x] = callbacks[x].getClass();
    }
    // 上节说到的filter
    enhancer.setCallbackFilter(new ProxyCallbackFilter(
        this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
    enhancer.setCallbackTypes(types);

    // 重点：创建proxy和其实例
    return createProxyClassAndInstance(enhancer, callbacks);
  }
  catch (CodeGenerationException | IllegalArgumentException ex) {
    throw new AopConfigException("Could not generate CGLIB subclass of " + this.advised.getTargetClass() +
        ": Common causes of this problem include using a final class or a non-visible class",
        ex);
  }
  catch (Throwable ex) {
    // TargetSource.getTarget() failed
    throw new AopConfigException("Unexpected AOP exception", ex);
  }
}

获取callback的方法如下，提几个理解的要点吧，具体读者在学习的时候建议把我的例子跑一下，然后打一个断点进行理解。

  • rootClass: 即目标代理类
  • advised: 包含上文中我们获取到的advisor增强器的集合
  • exposeProxy: 在xml配置文件中配置的，背景就是如果在事务A中使用了代理，事务A调用了目标类的的方法a，在方法a中又调用目标类的方法b，方法a，b同时都是要被增强的方法，如果不配置exposeProxy属性，方法b的增强将会失效，如果配置exposeProxy，方法b在方法a的执行中也会被增强了
  • DynamicAdvisedInterceptor: 拦截器将advised(包含上文中我们获取到的advisor增强器)构建配置的AOP的callback（第一个callback)
  • targetInterceptor: xml配置的optimize属性使用的（第二个callback)
  • 最后连同其它5个默认的Interceptor 返回作为cglib的拦截器链，之后通过CallbackFilter的accpet方法返回的索引从这个集合中返回对应的拦截增强器执行增强操作。
private Callback[] getCallbacks(Class<?> rootClass) throws Exception {
  // Parameters used for optimization choices...
  boolean exposeProxy = this.advised.isExposeProxy();
  boolean isFrozen = this.advised.isFrozen();
  boolean isStatic = this.advised.getTargetSource().isStatic();

  // Choose an "aop" interceptor (used for AOP calls).
  Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised);

  // Choose a "straight to target" interceptor. (used for calls that are
  // unadvised but can return this). May be required to expose the proxy.
  Callback targetInterceptor;
  if (exposeProxy) {
    targetInterceptor = (isStatic ?
        new StaticUnadvisedExposedInterceptor(this.advised.getTargetSource().getTarget()) :
        new DynamicUnadvisedExposedInterceptor(this.advised.getTargetSource()));
  }
  else {
    targetInterceptor = (isStatic ?
        new StaticUnadvisedInterceptor(this.advised.getTargetSource().getTarget()) :
        new DynamicUnadvisedInterceptor(this.advised.getTargetSource()));
  }

  // Choose a "direct to target" dispatcher (used for
  // unadvised calls to static targets that cannot return this).
  Callback targetDispatcher = (isStatic ?
      new StaticDispatcher(this.advised.getTargetSource().getTarget()) : new SerializableNoOp());

  Callback[] mainCallbacks = new Callback[] {
      aopInterceptor,  // 
      targetInterceptor,  // invoke target without considering advice, if optimized
      new SerializableNoOp(),  // no override for methods mapped to this
      targetDispatcher, this.advisedDispatcher,
      new EqualsInterceptor(this.advised),
      new HashCodeInterceptor(this.advised)
  };

  Callback[] callbacks;

  // If the target is a static one and the advice chain is frozen,
  // then we can make some optimizations by sending the AOP calls
  // direct to the target using the fixed chain for that method.
  if (isStatic && isFrozen) {
    Method[] methods = rootClass.getMethods();
    Callback[] fixedCallbacks = new Callback[methods.length];
    this.fixedInterceptorMap = CollectionUtils.newHashMap(methods.length);

    // TODO: small memory optimization here (can skip creation for methods with no advice)
    for (int x = 0; x < methods.length; x++) {
      Method method = methods[x];
      List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, rootClass);
      fixedCallbacks[x] = new FixedChainStaticTargetInterceptor(
          chain, this.advised.getTargetSource().getTarget(), this.advised.getTargetClass());
      this.fixedInterceptorMap.put(method, x);
    }

    // Now copy both the callbacks from mainCallbacks
    // and fixedCallbacks into the callbacks array.
    callbacks = new Callback[mainCallbacks.length + fixedCallbacks.length];
    System.arraycopy(mainCallbacks, 0, callbacks, 0, mainCallbacks.length);
    System.arraycopy(fixedCallbacks, 0, callbacks, mainCallbacks.length, fixedCallbacks.length);
    this.fixedInterceptorOffset = mainCallbacks.length;
  }
  else {
    callbacks = mainCallbacks;
  }
  return callbacks;
}

可以结合调试，方便理解

AOP在嵌套方法调用时不生效

在一个实现类中，有2个方法，方法A，方法B，其中方法B上面有个注解切面，当方法B被外部调用的时候，会进入切面方法。
但当方法B是被方法A调用时，并不能从方法B的注解上，进入到切面方法，即我们经常碰到的方法嵌套时，AOP注解不生效的问题。

案例

外部调用AOP方法正常进入

通过外部，调用方法B，可以正常进入切面方法，这个场景的代码如下：

  • 注解类：
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface DemoAnno {

}
  • 切面类
@Aspect
@Order(-1)
@Component
public class DemoAspect {

    @Before("@annotation(da)")
    public void beforDoSomething(JoinPoint point, DemoAnno da) throws Exception {
        System.out.println("before method B, print 'hello,world' " );
    }
}
  • 接口类
public interface DemoService {
    void methodDemoA();

    void methodDemoB();
}
  • 服务实现类
@Service
public class DemoServiceImpl implements DemoService {
    @Override
    public void methodDemoA(){
        System.out.println("this is method A");
    }

    @Override
    @DemoAnno
    public void methodDemoB() {
        System.out.println("this is method B");
    }
}
  • 测试方法
@Autowired
DemoService demoService;
@Test
public void testMethod(){
    demoService.methodDemoA();
    demoService.methodDemoB();
}

输出结果：

this is method A
before method B, print 'hello,world' 
this is method B

方法嵌套调用，AOP不生效

上面的代码，做下修改。在DemoServiceImpl实现类中，通过方法A去调用方法B，然后再单元测试类中，调用方法A。代码修改后如下：

  • 服务实现类：
@Service
public class DemoServiceImpl implements DemoService {
    @Override
    public void methodDemoA(){
        System.out.println("this is method A");
        methodDemoB();
    }

    @Override
    @DemoAnno
    public void methodDemoB() {
        System.out.println("this is method B");
    }
}
  • 输出结果：
this is method A
this is method B

原因分析

场景1中，通过外部调用方法B，是由于spring在启动时，根据切面类及注解，生成了DemoService的代理类，在调用方法B时，实际上是代理类先对目标方法进行了业务增强处理（执行切面类中的业务逻辑），然后再调用方法B本身。所以场景1可以正常进入切面方法；

场景2中，通过外部调用的是方法A，虽然spring也会创建一个cglib的代理类去调用方法A，但当方法A调用方法B的时候，属于类里面的内部调用，使用的是实例对象本身去去调用方法B，非aop的cglib代理对象调用，方法B自然就不会进入到切面方法了。

解决方案

但实际上我们期望的是，方法A在调用方法B的时候，仍然能够进入切面方法，即需要AOP切面生效。这种情况下，在调用方法B的时候，需要使用AopContext.currentProxy()获取当前的代理对象，然后使用代理对象调用方法B。

注：需要开启 exposeProxy=true 的配置，springboot项目中，可以在启动类上面，添加 @EnableAspectJAutoProxy(exposeProxy = true)注解。

@Service
public class DemoServiceImpl implements DemoService {
    @Override
    public void methodDemoA(){
        System.out.println("this is method A");
        DemoService service = (DemoService) AopContext.currentProxy();
        service.methodDemoB();
    }

    @Override
    @DemoAnno
    public void methodDemoB() {
        System.out.println("this is method B");
    }
}
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