介绍

Spring动态代理可以选择使用jdk动态代理, 或者cglib动态代理, jdk动态代理位于 java.lang.reflect 包下.

使用时涉及
接口: java.lang.reflect.InvocationHandler
动态代理类: java.lang.reflect.Proxy

注意: JDK 动态代理是基于接口的代理, 只能对实现接口的类生成代理, 不能对类进行代理

使用步骤

1.创建InvocationHandler接口的实现类, 并编写invoke方法的实现
2.创建被代理类的接口及实现类
3.使用动态代理类Proxy的静态方法生成代理类实例
4.使用实例调用方法

代码演示

按照步骤编写简易逻辑代码.

创建InvocationHandler接口的实现类

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/**
 * JDK 动态代理
 * 基于接口的代理, 只能对实现接口的类生成代理, 不能对类进行代理
 *
 * @author liuzhihang
 * @date 2018/5/17 10:36
 */
public class MyInvocationHandler implements InvocationHandler {

    /**
     * 目标对象
     */
    private Object target;

    public MyInvocationHandler(Object target) {
        this.target = target;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

        System.out.println("jdk 动态代理 before . . . ");

        System.out.println("当前代理方法为:" + method);
        Object invoke = method.invoke(target, args);

        System.out.println("jdk 动态代理 after . . . ");
        return invoke;
    }
}

创建被代理类的接口及实现类

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/**
 * 被代理类的接口
 * @author liuzhihang
 * @date 2018/5/17 10:47
 */
public interface Subject {

    /**
     * 获取名字
     * @return
     */
    String getName();

    /**
     * 获取年龄
     * @param name
     * @return
     */
    String getAge(String name);
}
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/**
 * 被代理类
 *
 * @author liuzhihang
 * @date 2018/5/17 10:48
 */
public class SubjectImpl implements Subject {

    @Override
    public String getName() {
        System.out.println("SubjectImpl的获取名字方法 . . .");
        return "liuzhihang";
    }

    @Override
    public String getAge(String name) {

        System.out.println(name + "开始获取年龄 . . .");
        return "25";
    }
}

使用动态代理类Proxy的静态方法生成代理类实例

获取代理类实例有以下两种方式, 一种是通过Proxy.newProxyInstance(..)获取, 一种是通过 Proxy.getProxyClass(..) 方式获取
1.Proxy.newProxyInstance(..)

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/**
 * 当代理类实例调用方法时, 会自动跳转到代理类关联的 handler 对象, 通过 method.invoke(target, args) 进行调用
 *
 *
 * @author liuzhihang
 * @date 2018/5/17 10:49
 */
public class ProxyMainTest {

    public static void main(String[] args) {

        Subject subject = new SubjectImpl();

        ClassLoader classLoader = subject.getClass().getClassLoader();
        Class<?>[] interfaces = subject.getClass().getInterfaces();

        MyInvocationHandler handler = new MyInvocationHandler(subject);
        // 生成代理类实例
        Subject proxyInstance = (Subject) Proxy.newProxyInstance(classLoader, interfaces, handler);

        String name = proxyInstance.getName();
        String instanceAge = proxyInstance.getAge("liuzhihang");
        System.err.println(name + " " + instanceAge);

    }
}

2.Proxy.getProxyClass(..)

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/**
 * 当代理类实例调用方法时, 会自动跳转到代理类关联的 handler 对象, 通过 method.invoke(target, args) 进行调用
 * 此方式有异常抛出
 *
 * @author liuzhihang
 * @date 2018/5/17 10:49
 */
public class ProxyMainTest {

    public static void main(String[] args) {
        try {

        Subject subject = new SubjectImpl();

        ClassLoader classLoader = subject.getClass().getClassLoader();
        Class<?>[] interfaces = subject.getClass().getInterfaces();

        MyInvocationHandler handler = new MyInvocationHandler(subject);

            Class<?> proxyClass = Proxy.getProxyClass(classLoader, interfaces);
            Constructor<?> constructor = proxyClass.getConstructor(InvocationHandler.class);

            Subject subject1 = (Subject) constructor.newInstance(handler);
            String name1 = subject1.getName();
            String instanceAge1 = subject1.getAge("liuzhihang");
            System.err.println(name1 + " " + instanceAge1);

        } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException | InstantiationException e) {
            e.printStackTrace();
        }

    }

}

执行结果

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D:\jdk1.8\bin\java.exe . . .
liuzhihang 25
jdk 动态代理 before . . .
当前代理方法为:public abstract java.lang.String com.liuzhihang.tool.proxy.jdk.Subject.getName()
SubjectImpl的获取名字方法 . . .
jdk 动态代理 after . . .
jdk 动态代理 before . . .
当前代理方法为:public abstract java.lang.String com.liuzhihang.tool.proxy.jdk.Subject.getAge(java.lang.String)
liuzhihang开始获取年龄 . . .
jdk 动态代理 after . . .

Process finished with exit code 0

结论: 代理实例在每次调用方法是都会通过代理类进行调用

相关源码解析

完整注释可自己查看相关源码, 源码过程应当DeBug多走走.
1.调用 Proxy.newProxyInstance 方法

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 /**
 * 返回指定接口的代理类实例，该接口将方法调用分派给指定的调用处理程序
 */
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException {

    // 非空校验
    Objects.requireNonNull(h);

    final Class<?>[] intfs = interfaces.clone();

    // 获取系统安全接口
    final SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
        // 校验权限
        checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
    }

    /*
     * 从缓存中获取代理类 或者 生成新的代理类
     */
    Class<?> cl = getProxyClass0(loader, intfs);

    /*
     * 通过反射获取构造函数对象并生成代理类实例
     */
    try {
        if (sm != null) {
            checkNewProxyPermission(Reflection.getCallerClass(), cl);
        }
        // 获取构造
        final Constructor<?> cons = cl.getConstructor(constructorParams);
        final InvocationHandler ih = h;
        // 验证代理类的修饰符
        if (!Modifier.isPublic(cl.getModifiers())) {
            // 修改访问权限
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    // 将此对象的可访问标志设置为指定的布尔值, true表示反射对象在使用时应禁止Java语言访问检查, false表示反射对象应强制执行Java语言访问检查
                    cons.setAccessible(true);
                    return null;
                }
            });
        }
        //生成实例, 并将参数传入构造
        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);
    }
}

可以看出获取代理类是在 Class<?> cl = getProxyClass0(loader, intfs); 处, 继续相关逻辑
2.获取代理类相关逻辑

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/**
 * 生成代理类, 之前必须进行权限检查
 */
private static Class<?> getProxyClass0(ClassLoader loader,
                                       Class<?>... interfaces) {
    if (interfaces.length > 65535) {
        throw new IllegalArgumentException("interface limit exceeded");
    }

    //如果由实现给定接口的给定加载器定义的代理类存在，则它将简单地返回缓存副本; 否则，它将通过Proxy Class Factory创建代理类
    return proxyClassCache.get(loader, interfaces);
}

3.proxyClassCache.get(loader, interfaces);
java.lang.reflect.WeakCache#get(..) 介绍

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 /**
 * 通过缓存查找值, 如果缓存中没有给定的（key，sub Key）对的条目或条目已被清除，则它总是评估{Key sub Key Factory}函数并可选择评估{Factory value}函数
 */
public V get(K key, P parameter) {
    // 非空校验
    Objects.requireNonNull(parameter);
    // 判断移除队列
    expungeStaleEntries();
    // 缓存key
    Object cacheKey = CacheKey.valueOf(key, refQueue);

    // 延迟加载使用二级map
    ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
    if (valuesMap == null) {
        ConcurrentMap<Object, Supplier<V>> oldValuesMap
                = map.putIfAbsent(cacheKey,
                valuesMap = new ConcurrentHashMap<>());
        if (oldValuesMap != null) {
            valuesMap = oldValuesMap;
        }
    }

    // 创建子key 并根据key 检索supplier
    Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
    // 根据key获取supplier
    Supplier<V> supplier = valuesMap.get(subKey);
    Factory factory = null;

    while (true) {
        if (supplier != null) {
            // supplier 可能为 Factory 或者 CacheValue<V> 的实例, 从缓存中获取到则直接返回
            V value = supplier.get();
            if (value != null) {
                return value;
            }
        }

        // factory不存在则创建
        if (factory == null) {
            factory = new Factory(key, parameter, subKey, valuesMap);
        }

        // supplier 为null
        if (supplier == null) {
            // 从valuesMap获取supplier
            supplier = valuesMap.putIfAbsent(subKey, factory);
            if (supplier == null) {
                // successfully installed Factory
                supplier = factory;
            }
            // else retry with winning supplier
        } else {
            if (valuesMap.replace(subKey, supplier, factory)) {
                // successfully replaced
                // cleared CacheEntry / unsuccessful Factory
                // with our Factory
                supplier = factory;
            } else {
                // retry with current supplier
                supplier = valuesMap.get(subKey);
            }
        }
    }
}

可以发现重点在 Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter)); 获取 subKey 的过程中.
4.subKeyFactory.apply(key, parameter)
Debug发现在此处调用的是 java.lang.reflect.Proxy.ProxyClassFactory 静态内部类,
此处根据接口的数量生成二级缓存

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 /**
 * 一个工厂函数, 用于生成, 定义并返回给定ClassLoader和接口数组的代理类
 */
private static final class ProxyClassFactory
        implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
    // 所有代理类的前缀
    private static final String proxyClassNamePrefix = "$Proxy";

    // next number to use for generation of unique proxy class names
    private static final AtomicLong nextUniqueNumber = new AtomicLong();

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

        // 在IdentityHashMap中, 当且仅当两个key严格相等（key1==key2）时，IdentityHashMap才认为两个key相等
        Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
        // 循环接口数组
        for (Class<?> intf : interfaces) {
            /*
             * 验证类加载器是否将此接口的名称解析为同一个Class对象
             */
            Class<?> interfaceClass = null;
            try {
                // 获取接口的 class
                interfaceClass = Class.forName(intf.getName(), false, loader);
            } catch (ClassNotFoundException e) {
            }
            if (interfaceClass != intf) {
                throw new IllegalArgumentException(
                        intf + " is not visible from class loader");
            }
            /*
             * 验证interfaceClass是否为接口
             */
            if (!interfaceClass.isInterface()) {
                throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
            }
            /*
             * 验证接口是否重复
             */
            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;

        /*
         * 验证所有非公开代理接口是否在同一个包中
         */
        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) {
            // 如果没有非公开的代理接口，使用 com.sun.proxy package
            proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
        }

        /*
         * 为要生成的代理类选择一个名称
         */
        long num = nextUniqueNumber.getAndIncrement();
        String proxyName = proxyPkg + proxyClassNamePrefix + num;

        /*
         * 生成代理类
         */
        byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);
        try {
            return defineClass0(loader, proxyName,
                    proxyClassFile, 0, proxyClassFile.length);
        } catch (ClassFormatError e) {
            /*
             * A ClassFormatError here means that (barring bugs in the
             * proxy class generation code) there was some other
             * invalid aspect of the arguments supplied to the proxy
             * class creation (such as virtual machine limitations
             * exceeded).
             */
            throw new IllegalArgumentException(e.toString());
        }
    }
}

5.生辰给代理类
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);
可以在测试类中添加以下内容打印出代理类:

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System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

代理类内容如下:

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

package com.sun.proxy;

import com.liuzhihang.tool.proxy.jdk.Subject;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $Proxy0 extends Proxy implements Subject {
    private static Method m1;
    private static Method m3;
    private static Method m2;
    private static Method m4;
    private static Method m0;

    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 String getName() throws  {
        try {
            return (String)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 String getAge(String var1) throws  {
        try {
            return (String)super.h.invoke(this, m4, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    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.liuzhihang.tool.proxy.jdk.Subject").getMethod("getName");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m4 = Class.forName("com.liuzhihang.tool.proxy.jdk.Subject").getMethod("getAge", Class.forName("java.lang.String"));
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

可以看出生成的$Proxy0类继承Proxy动态代理类并实现了Subject被代理接口, 实现所有方法
通过 super.h.invoke(this, m1, new Object[]{var1}) 内部调用了 InvocationHandler.invoke(…)方法, 通过反射调用代理实例的方法