Spring框架系列(12) - Spring AOP实现原理详解之JDK代理实现

上文我们学习了SpringAOP Cglib动态代理的实现，本文主要是SpringAOP JDK动态代理的案例和实现部分。@pdai

Spring框架系列(12) - Spring AOP实现原理详解之JDK代理实现

引入

上文我们学习了SpringAOP Cglib动态代理的实现，本文主要是SpringAOP JDK动态代理的案例和实现部分。

什么是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代理的案例

这里我们写一个使用jdk代理的简单例子。@pdai

不需要maven依赖

jdk代理不需要任何依赖。

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>tech-pdai-spring-demos</artifactId>
        <groupId>tech.pdai</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>006-spring-framework-demo-aop-proxy-jdk</artifactId>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <!--based on jdk proxy -->
    <dependencies>

    </dependencies>

</project>

定义实体

User

package tech.pdai.springframework.entity;

/**
 * @author pdai
 */
public class User {

    /**
     * user's name.
     */
    private String name;

    /**
     * user's age.
     */
    private int age;

    /**
     * init.
     *
     * @param name name
     * @param age  age
     */
    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

被代理的类和接口

接口如下

package tech.pdai.springframework.service;

import tech.pdai.springframework.entity.User;

import java.util.List;

/**
 * @author pdai
 */
public interface IUserService {

    /**
     * find user list.
     *
     * @return user list
     */
    List<User> findUserList();

    /**
     * add user
     */
    void addUser();
}

实现类如下：

package tech.pdai.springframework.service;

import tech.pdai.springframework.entity.User;

import java.util.Collections;
import java.util.List;

/**
 * @author pdai
 */
public class UserServiceImpl implements IUserService {

    /**
     * find user list.
     *
     * @return user list
     */
    @Override
    public List<User> findUserList() {
        return Collections.singletonList(new User("pdai", 18));
    }

    /**
     * add user
     */
    @Override
    public void addUser() {
        // do something
    }

}

JDK代理类

代理类如下：

package tech.pdai.springframework.proxy;

import tech.pdai.springframework.service.IUserService;
import tech.pdai.springframework.service.UserServiceImpl;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.Arrays;

/**
 * This class is for proxy demo.
 *
 * @author pdai
 */
public class UserLogProxy {

    /**
     * proxy target
     */
    private IUserService target;

    /**
     * init.
     *
     * @param target target
     */
    public UserLogProxy(UserServiceImpl target) {
        super();
        this.target = target;
    }

    /**
     * get proxy.
     *
     * @return proxy target
     */
    public IUserService getLoggingProxy() {
        IUserService proxy;
        ClassLoader loader = target.getClass().getClassLoader();
        Class[] interfaces = new Class[]{IUserService.class};
        InvocationHandler h = new InvocationHandler() {
            /**
             * proxy: 代理对象。 一般不使用该对象 method: 正在被调用的方法 args: 调用方法传入的参数
             */
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                String methodName = method.getName();
                // log - before method
                System.out.println("[before] execute method: " + methodName);

                // call method
                Object result = null;
                try {
                    // 前置通知
                    result = method.invoke(target, args);
                    // 返回通知, 可以访问到方法的返回值
                } catch (NullPointerException e) {
                    e.printStackTrace();
                    // 异常通知, 可以访问到方法出现的异常
                }
                // 后置通知. 因为方法可以能会出异常, 所以访问不到方法的返回值

                // log - after method
                System.out.println("[after] execute method: " + methodName + ", return value: " + result);
                return result;
            }
        };
        /**
         * loader: 代理对象使用的类加载器.
         * interfaces: 指定代理对象的类型. 即代理代理对象中可以有哪些方法.
         * h: 当具体调用代理对象的方法时, 应该如何进行响应, 实际上就是调用 InvocationHandler 的 invoke 方法
         */
        proxy = (IUserService) Proxy.newProxyInstance(loader, interfaces, h);
        return proxy;
    }

}

使用代理

启动类中指定代理目标并执行。

package tech.pdai.springframework;

import tech.pdai.springframework.proxy.UserLogProxy;
import tech.pdai.springframework.service.IUserService;
import tech.pdai.springframework.service.UserServiceImpl;

/**
 * Jdk proxy demo.
 *
 * @author pdai
 */
public class ProxyDemo {

    /**
     * main interface.
     *
     * @param args args
     */
    public static void main(String[] args) {
        // proxy
        IUserService userService = new UserLogProxy(new UserServiceImpl()).getLoggingProxy();

        // call methods
        userService.findUserList();
        userService.addUser();
    }
}

简单测试

我们启动上述类main 函数，执行的结果如下：

[before] execute method: findUserList
[after] execute method: findUserList, return value: [User{name='pdai', age=18}]
[before] execute method: addUser
[after] execute method: addUser, return value: null

JDK代理的流程

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);
        }
    }
}

示例源码

https://github.com/realpdai/tech-pdai-spring-demos

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