JAVA的动态代理模式：
A接口，A1子类实现A接口，A2子类实现A接口。那么JAVA的动态代理模式会A1、A2…等所有子类的接口方法提供一样的增强方法。

先来看看怎么使用

A接口

package com.geely.design.pattern.structural.myproxy;

/**
 * @Author: wenbaipei
 * @Date: 2019/11/30 13:01
 * @Version 1.0
 */
public interface HelloInterface {
    void sayHello();
    void sayHelloAgain();
}

A1子类

package com.geely.design.pattern.structural.myproxy;

/**
 * @Author: wenbaipei
 * @Date: 2019/11/30 13:01
 * @Version 1.0
 */
public class Hello implements HelloInterface{
    @Override
    public void sayHello() {
        System.out.println("Hello wenbaipei!");
    }

    @Override
    public void sayHelloAgain() {
        System.out.println("Hey!!Hello wenbaipei!!");
    }
}

代理类：

package com.geely.design.pattern.structural.myproxy.dynamic;

import org.apache.commons.lang3.StringUtils;

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

/**
 * @Author: wenbaipei
 * @Date: 2019/11/30 13:05
 * @Version 1.0
 */
public class ProxyHandler implements InvocationHandler {
    private Object object;
    public ProxyHandler(Object object){
        this.object = object;
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object ret;
        if (StringUtils.equals(method.getName(),"sayHello")){
            System.out.println("Before invoke "  + method.getName());
            ret = method.invoke(object, args);
            System.out.println("After invoke " + method.getName());
        }else {
            System.out.println("Before invoke "  + method.getName());
            ret = method.invoke(object, args);
            System.out.println("After invoke " + method.getName());
        }
        return ret;
    }
}

测试类：

package com.geely.design.pattern.structural.myproxy.dynamic;

import com.geely.design.pattern.structural.myproxy.Hello;
import com.geely.design.pattern.structural.myproxy.HelloInterface;

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

/**
 * @Author: wenbaipei
 * @Date: 2019/11/30 13:05
 * @Version 1.0
 */
public class Test {
    public static void main(String[] args) {
        System.getProperties().setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

        HelloInterface hello = new Hello();

        InvocationHandler handler = new ProxyHandler(hello);

        HelloInterface proxyHello = (HelloInterface) Proxy.newProxyInstance(hello.getClass().getClassLoader(), hello.getClass().getInterfaces(), handler);

        proxyHello.sayHello();
        proxyHello.sayHelloAgain();
    }
}

结果：

分析

Proxy.newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)生成的是一个$Proxy0类的实例对象，那$Proxy0类又是一个怎样的类？

package com.sun.proxy;

import com.geely.design.pattern.structural.myproxy.HelloInterface;
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 HelloInterface {
    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 void sayHello() 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 void sayHelloAgain() throws  {
        try {
            super.h.invoke(this, m4, (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.geely.design.pattern.structural.myproxy.HelloInterface").getMethod("sayHello");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m4 = Class.forName("com.geely.design.pattern.structural.myproxy.HelloInterface").getMethod("sayHelloAgain");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

$Proxy0是一个继承自Proxy，实现了HelloInterface接口的类。
由于继承自Proxy，那$Proxy0类对象也继承InvocationHandler属性h，而且这个属性h指向的是InvocationHandler handler = new ProxyHandler(hello)代码中new出来的ProxyHandler对象（至于为什么，待会再解释）。
所以当$Proxy0类对象实例proxyHello调用方法sayHello()时，super.h.invoke(this, m3, (Object[])null);会调用handler对象的invoke(Object proxy, Method method, Object[] args)方法。
感觉有点绕？那就看图吧！

原理：

怎么获得$Proxy0类的类对象的？

getProxyClass0(loader, intfs);是怎么获得$Proxy0类的类对象的？

private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }

        // If the proxy class defined by the given loader implementing
        // the given interfaces exists, this will simply return the cached copy;
        // otherwise, it will create the proxy class via the ProxyClassFactory
        return proxyClassCache.get(loader, interfaces);
    }

英文已经明确告诉我们：如果代理类的类对象存在，则直接返回。否则由ProxyClassFactory类来创建代理类的类对象。
那我们就去看看ProxyClassFactory类吧。

private static final class ProxyClassFactory
        implements BiFunction<ClassLoader, Class<?>[], Class<?>>{
        // 代理类的名字的前缀统一为“$Proxy”
        private static final String proxyClassNamePrefix = "$Proxy";

        // 每个代理类前缀后面都会跟着一个唯一的编号，如$Proxy0、$Proxy1、$Proxy2
        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) {
                //验证类加载器加载接口得到对象是否与由apply函数参数传入的对象相同
                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");
                }
                //验证这个Class对象是不是接口
                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
                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;

            /*
             * Record the package of a non-public proxy interface so that the
             * proxy class will be defined in the same package.  Verify that
             * all non-public proxy interfaces are in the same package.
             */
            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) {
                // if no non-public proxy interfaces, use com.sun.proxy package
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }

            /*
             * Choose a name for the proxy class to generate.
             */
            long num = nextUniqueNumber.getAndIncrement();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /*
             * 
             * 生成指定代理类的字节码文件
             */
            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());
            }
        }
    }

一系列检查后，调用ProxyGenerator.generateProxyClass来生成字节码文件。
字节码生成后，调用defineClass0来解析字节码，生成了代理类的Class对象。

怎么生成字节码文件的呢？

  public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
        ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
        // 真正用来生成代理类字节码文件的方法在这里
        final byte[] var4 = var3.generateClassFile();
        // 保存代理类的字节码文件
        if(saveGeneratedFiles) {
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    try {
                        int var1 = var0.lastIndexOf(46);
                        Path var2;
                        if(var1 > 0) {
                            Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar), new String[0]);
                            Files.createDirectories(var3, new FileAttribute[0]);
                            var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
                        } else {
                            var2 = Paths.get(var0 + ".class", new String[0]);
                        }

                        Files.write(var2, var4, new OpenOption[0]);
                        return null;
                    } catch (IOException var4x) {
                        throw new InternalError("I/O exception saving generated file: " + var4x);
                    }
                }
            });
        }

        return var4;
    }

生成代理类字节码文件的generateClassFile方法:

     private byte[] generateClassFile() {
        //下面一系列的addProxyMethod方法是将接口中的方法和Object中的方法添加到代理方法中(proxyMethod)
        this.addProxyMethod(hashCodeMethod, Object.class);
        this.addProxyMethod(equalsMethod, Object.class);
        this.addProxyMethod(toStringMethod, Object.class);
        Class[] var1 = this.interfaces;
        int var2 = var1.length;

        int var3;
        Class var4;
        //获得接口中所有方法并添加到代理方法中
        for(var3 = 0; var3 < var2; ++var3) {
            var4 = var1[var3];
            Method[] var5 = var4.getMethods();
            int var6 = var5.length;

            for(int var7 = 0; var7 < var6; ++var7) {
                Method var8 = var5[var7];
                this.addProxyMethod(var8, var4);
            }
        }

        Iterator var11 = this.proxyMethods.values().iterator();

        List var12;
        while(var11.hasNext()) {
            var12 = (List)var11.next();
            checkReturnTypes(var12);
        }

        Iterator var15;
        try {
            //生成代理类的构造函数
            this.methods.add(this.generateConstructor());
            var11 = this.proxyMethods.values().iterator();

            while(var11.hasNext()) {
                var12 = (List)var11.next();
                var15 = var12.iterator();
                    
                while(var15.hasNext()) {
                    ProxyGenerator.ProxyMethod var16 = (ProxyGenerator.ProxyMethod)var15.next();
                    this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));
                    this.methods.add(var16.generateMethod());
                }
            }

            this.methods.add(this.generateStaticInitializer());
        } catch (IOException var10) {
            throw new InternalError("unexpected I/O Exception", var10);
        }

        if(this.methods.size() > '\uffff') {
            throw new IllegalArgumentException("method limit exceeded");
        } else if(this.fields.size() > '\uffff') {
            throw new IllegalArgumentException("field limit exceeded");
        } else {
            this.cp.getClass(dotToSlash(this.className));
            this.cp.getClass("java/lang/reflect/Proxy");
            var1 = this.interfaces;
            var2 = var1.length;

            for(var3 = 0; var3 < var2; ++var3) {
                var4 = var1[var3];
                this.cp.getClass(dotToSlash(var4.getName()));
            }

            this.cp.setReadOnly();
            ByteArrayOutputStream var13 = new ByteArrayOutputStream();
            DataOutputStream var14 = new DataOutputStream(var13);

            try {
                var14.writeInt(-889275714);
                var14.writeShort(0);
                var14.writeShort(49);
                this.cp.write(var14);
                var14.writeShort(this.accessFlags);
                var14.writeShort(this.cp.getClass(dotToSlash(this.className)));
                var14.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
                var14.writeShort(this.interfaces.length);
                Class[] var17 = this.interfaces;
                int var18 = var17.length;

                for(int var19 = 0; var19 < var18; ++var19) {
                    Class var22 = var17[var19];
                    var14.writeShort(this.cp.getClass(dotToSlash(var22.getName())));
                }

                var14.writeShort(this.fields.size());
                var15 = this.fields.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.FieldInfo var20 = (ProxyGenerator.FieldInfo)var15.next();
                    var20.write(var14);
                }

                var14.writeShort(this.methods.size());
                var15 = this.methods.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
                    var21.write(var14);
                }

                var14.writeShort(0);
                return var13.toByteArray();
            } catch (IOException var9) {
                throw new InternalError("unexpected I/O Exception", var9);
            }
        }
    }

动态代理流程图：

参考资料：

https://www.jianshu.com/p/9bcac608c714
https://www.liaoxuefeng.com/wiki/1252599548343744/1264804593397984