Spring AOP：jdk动态代理

Posted 2021-02-19 Chinda

JDK动态代理

JDK动态代理是代理模式的一种实现方式。只能代理接口类，因为Java语言为单继承语言，所以只能以实现接口的方式构建代理对象。

简单示例

package com.chinda.aop;

/**
 * 被代理接口
 *
 * @author Wang Chinda
 * @date 2020/5/16
 * @see
 * @since 1.0
 */
public interface Subject {
    void doSomething();
}

package com.chinda.aop;

/**
 * 真正的对象
 *
 * @author Wang Chinda
 * @date 2020/5/16
 * @see
 * @since 1.0
 */
public class RealSubject implements Subject {
    @Override
    public void doSomething() {
        System.out.println("realSubject to do something!");
    }
}

import com.chinda.aop.RealSubject;
import com.chinda.aop.Subject;
import org.junit.Test;

import java.lang.reflect.Proxy;

/**
 * @author Wang Chinda
 * @date 2020/5/16
 * @see
 * @since 1.0
 */
public class JdkDynamicProxyTest {

    @Test
    public void test() {
        // 保存生成的代理类的字节码文件, 方便查看(可省略)
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

        RealSubject realSubject = new RealSubject();
        Subject subjectProxy = (Subject) Proxy.newProxyInstance(realSubject.getClass().getClassLoader(), realSubject.getClass().getInterfaces(), (proxy, method, args) -> {
            System.out.println("before to do something!");
            Object result = method.invoke(realSubject, args);
            System.out.println("after to do something!");
            return result;
        });
        subjectProxy.doSomething();
    }
}

控制台输出

before to do something!
realSubject to do something!
after to do something!

注意: 保存代理类字节码文件需要配置jvm参数：-Dsun.misc.ProxyGenerator.saveGeneratedFiles=true

代理类字节码文件反编译

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

package com.sun.proxy;

import com.chinda.aop.Subject;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $Proxy4 extends Proxy implements Subject {
    private static Method m1;
    private static Method m3;
    private static Method m2;
    private static Method m0;

    public $Proxy4(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 void doSomething() throws  {
        try {
            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);
        }
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m3 = Class.forName("com.chinda.aop.Subject").getMethod("doSomething");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

1. 代理类继承了Proxy类并且实现了要代理的接口，由于java不支持多继承，所以JDK动态代理不能代理类。

2. 重写了equals、hashCode、toString。

3. 有一个静态代码块，通过反射或者代理类的所有方法。

4. 通过invoke执行代理类中的目标方法doSomething。

源码分析

jdk版本

$ java -version
java version "1.8.0_162"
Java(TM) SE Runtime Environment (build 1.8.0_162-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.162-b12, mixed mode)

粗略流程

1. 生成代理类字节文件，继承Proxy并实现被代理接口。
2. 用加载被代理接口的ClassLoader将代理类字节码文件加载到虚拟机中，创建代理对象。
3. 执行目标对象的目标方法。

查看Proxy.newProxyInstance

@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
                                      Class<?>[] interfaces,
                                      InvocationHandler h)
    throws IllegalArgumentException
{
    // 检查调用控制器是否为null, 若是null抛异常
    Objects.requireNonNull(h);

    // 克隆接口Class[]
    final Class<?>[] intfs = interfaces.clone();
    final SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
        // 检查代理是否可以创建
        checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
    }

    /*
     * Look up or generate the designated proxy class.
     * 生成接口代理类的字节码文件
     */
    Class<?> cl = getProxyClass0(loader, intfs);

    /*
     * 使用自定义的InvocationHandler构造代理对象实例
     */
    try {
        if (sm != null) {
            checkNewProxyPermission(Reflection.getCallerClass(), cl);
        }

        final Constructor<?> cons = cl.getConstructor(constructorParams);
        final InvocationHandler ih = h;
        if (!Modifier.isPublic(cl.getModifiers())) {
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    cons.setAccessible(true);
                    return null;
                }
            });
        }
        return cons.newInstance(new Object[]{h});
    } catch (IllegalAccessException|InstantiationException e) {
        throw new InternalError(e.toString(), e);
    } catch (InvocationTargetException e) {
        Throwable t = e.getCause();
        if (t instanceof RuntimeException) {
            throw (RuntimeException) t;
        } else {
            throw new InternalError(t.toString(), t);
        }
    } catch (NoSuchMethodException e) {
        throw new InternalError(e.toString(), e);
    }
}

查看创建代理类字节码文件getProxyClass0

private static Class<?> getProxyClass0(ClassLoader loader,
                                       Class<?>... interfaces) {
    // 判断接口数是否超过65535
    if (interfaces.length > 65535) {
        throw new IllegalArgumentException("interface limit exceeded");
    }

    // 如果缓存中已经存在相应接口的代理类，直接返回；否则，使用ProxyClassFactory创建代理类
    return proxyClassCache.get(loader, interfaces);
}

查看proxyClassCache.get(loader, interfaces)

// 创建代理类
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));

查看ProxyClassFactory

private static final class ProxyClassFactory
    implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
    // 代理类名称前缀
    private static final String proxyClassNamePrefix = "$Proxy";

    // 下一个代理类名称后缀:比如$Proxy0.class、$Proxy1.class
    private static final AtomicLong nextUniqueNumber = new AtomicLong();

    @Override
    public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {

        Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
        for (Class<?> intf : interfaces) {
            // 校验类加载器是否能通过接口名称加载该类
            Class<?> interfaceClass = null;
            try {
                interfaceClass = Class.forName(intf.getName(), false, loader);
            } catch (ClassNotFoundException e) {
            }
            if (interfaceClass != intf) {
                throw new IllegalArgumentException(
                    intf + " is not visible from class loader");
            }
		// 校验代理类是否是接口
            if (!interfaceClass.isInterface()) {
                throw new IllegalArgumentException(
                    interfaceClass.getName() + " is not an interface");
            }
            /*
             * 校验接口是否重复
             */
            if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                throw new IllegalArgumentException(
                    "repeated interface: " + interfaceClass.getName());
            }
        }

        String proxyPkg = null;     // package to define proxy class in
        int accessFlags = Modifier.PUBLIC | Modifier.FINAL;

        for (Class<?> intf : interfaces) {
            int flags = intf.getModifiers();
            if (!Modifier.isPublic(flags)) {
                accessFlags = Modifier.FINAL;
                String name = intf.getName();
                int n = name.lastIndexOf(‘.‘);
                String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
                if (proxyPkg == null) {
                    proxyPkg = pkg;
                } else if (!pkg.equals(proxyPkg)) {
                    throw new IllegalArgumentException(
                        "non-public interfaces from different packages");
                }
            }
        }

        if (proxyPkg == null) {
            // 构建代理类包名
            proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
        }

        long num = nextUniqueNumber.getAndIncrement();
        String proxyName = proxyPkg + proxyClassNamePrefix + num;

        /*
         * 生成指定的代理类字节流
         */
        byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
            proxyName, interfaces, accessFlags);
        try {
            // 加载字节流文件
            return defineClass0(loader, proxyName,
                                proxyClassFile, 0, proxyClassFile.length);
        } catch (ClassFormatError e) {
            throw new IllegalArgumentException(e.toString());
        }
    }
}

封装上述示例

接口，实现类保持不变。

引入依赖包

<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.18.12</version>
</dependency>

创建动态代理功能类

package com.chinda.aop;

import lombok.AllArgsConstructor;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

/**
 * @author Wang Chinda
 * @date 2020/5/16
 * @see
 * @since 1.0
 */
@AllArgsConstructor
public class JdkDynamicProxy implements InvocationHandler {
    /**
     * 被代理类
     */
    private Object target;

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object result = null;
        try {
            System.out.println("before to do something!");
            result = method.invoke(target, args);
            System.out.println("return to do something! result: " + result);
        } catch (Exception e) {
            e.printStackTrace();
            System.out.println("exception to do something");
        } finally {
            System.out.println("after to do something!");
        }
        return result;
    }
}

创建测试方法

import com.chinda.aop.JdkDynamicProxy;
import com.chinda.aop.RealSubject;
import com.chinda.aop.Subject;
import org.junit.Test;

import java.lang.reflect.Proxy;

/**
 * @author Wang Chinda
 * @date 2020/5/16
 * @see
 * @since 1.0
 */
public class JdkDynamicProxyTest {

    @Test
    public void test2() {
        RealSubject realSubject = new RealSubject();
        Subject subjectProxy = (Subject) Proxy.newProxyInstance(realSubject.getClass().getClassLoader(), realSubject.getClass().getInterfaces(), new JdkDynamicProxy(realSubject));
        subjectProxy.doSomething();
    }
}

继续封装

创建获取代理接口

package com.chinda.aop;

/**
 * @author Wang Chinda
 * @date 2020/5/16
 * @see
 * @since 1.0
 */
public interface AopProxy {

    /**
     * 获取代理类
     * @return
     */
    Object getProxy();
}

修改动态代理功能类

package com.chinda.aop;

import lombok.AllArgsConstructor;

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

/**
 * @author Wang Chinda
 * @date 2020/5/16
 * @see
 * @since 1.0
 */
@AllArgsConstructor
public class JdkDynamicProxy implements AopProxy, InvocationHandler {
    /**
     * 被代理类
     */
    private Object target;

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object result = null;
        try {
            System.out.println("before to do something!");
            result = method.invoke(target, args);
            System.out.println("return to do something! result: " + result);
        } catch (Exception e) {
            e.printStackTrace();
            System.out.println("exception to do something");
        } finally {
            System.out.println("after to do something!");
        }
        return result;
    }

    @Override
    public Object getProxy() {
        return Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), this);
    }
}

创建测试方法

import com.chinda.aop.JdkDynamicProxy;
import com.chinda.aop.RealSubject;
import com.chinda.aop.Subject;
import org.junit.Test;

import java.lang.reflect.Proxy;

/**
 * @author Wang Chinda
 * @date 2020/5/16
 * @see
 * @since 1.0
 */
public class JdkDynamicProxyTest {

    @Test
    public void test3() {
        Subject subjectProxy = (Subject) new JdkDynamicProxy(new RealSubject()).getProxy();
        subjectProxy.doSomething();
    }
}