想理解静态代理和动态代理之前得先理解什么是代理：比如说调用A的方法实现业务，使用代理模式之后我们就不再直接调用A的方法了，而是在中间加了一个代理类，由代理类去调用A。相当于从直接调用变成间接调用，好处就是我们可以在代理类中对A的业务进行增强（拓展一些不属于目标方法的功能），在目标方法执行前后添加其他的业务逻辑，比如事务的开启和事务的提交等等。

        代理模式只是一种设计思想，但在具体的实现上又分为静态代理和动态代理。

        本文以JDK动态代理为示例，JDK动态代理是基于接口的方式，代理类和目标类都实现同一个接口。

一：静态代理：

         静态代理的静态指的是代码运行前我们就编写了代理类，并且编译生成了.class文件，程序可以读取到内存并运行这些文件，用一个例子来理解静态代理：

        定义一个接口Action

public interface Action {
    void run();
}

        定义一个执行具体逻辑的实现类ActionImpl,并重写run()方法。

public class ActionImpl implements Action {
    @Override
    public void run() {
        System.out.println("开车");
    }
}

        定义代理类

public class ActionProxy implements Action {

    private Action action;

    public ActionProxy(Action action) {
        this.action = action;
    }

    @Override
    public void run() {

        System.out.println("出发");

        action.run();

        System.out.println("到地");

    }
}

        测试类

    @Test
    public void test01() {
        Action action = new ActionImpl();
        ActionProxy actionProxy = new ActionProxy(action);
        actionProxy.run();
    }

打印

出发
开车
到地

        ActionProxy代理类对ActionImpl目标类进行了代理，执行具体方法时不再调用ActionImpl，同时增加了前置和后置的代码。静态代理类比较好理解，不再深究。

二：动态代理：

        静态代理的缺点十分明显：每一个被代理对象都需要建一个代理类去代理，代码冗杂。而动态代理就解决了这个问题，代理类是在运行过程中产生的，不需要给每一个被代理类创建代理对象。     

        示例以 jdk动态代理为例

        修改代理类，实现InvocationHandler接口，并重写invoke方法，增加getProxyInstance()方法，生成代理实例。

@Data
public class ActionProxy implements InvocationHandler {

    private Object object;

    public ActionProxy(Object object) {
        this.object = object;
    }

    public Object getProxyInstance() {
        return Proxy.newProxyInstance(object.getClass().getClassLoader(), object.getClass().getInterfaces(), this);
    }

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

        System.out.println("出发");

        Object result = method.invoke(object, args);

        System.out.println("到地");

        return result;

    }

}

修改测试类

    @Test
    public void test01() {

        Action action = new ActionImpl();

        ActionProxy actionProxy = new ActionProxy(action);

        Action proxyInstance = (Action) actionProxy.getProxyInstance();

        proxyInstance.run();
    }

打印

出发
开车
到地

        一样的结果

三：源码分析 

        先看getProxyInstance()中的newProxyInstance()方法，字面意思是new一个代理的实例，点进去看源码

    @CallerSensitive
    public static Object newProxyInstance(ClassLoader loader,
                                          Class[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        Objects.requireNonNull(h);

        final Class[] intfs = interfaces.clone();
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        
        Class cl = getProxyClass0(loader, intfs);

        
        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() {
                    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);
        }
    }

        注释翻译：返回指定接口的代理类的实例，将方法调用调度到指定调用的处理程序。

重点看这句

        
        Class cl = getProxyClass0(loader, intfs);

        注释翻译：查找或生成指定代理类，进入内部继续看

    
    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);
    }

        proxyClassCache的定义

private static final WeakCache[], Class>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

        此时一个新类出现在视野：ProxyClassFactory，顾名思义，这是代理类的工厂类，查看定义，发现一个熟悉的字符串$Proxy，我们知道动态代理类都是$Proxy0、$Proxy1这种形式。再往下找

 
    private static final class ProxyClassFactory
        implements BiFunction[], Class>
    {
        // prefix for all proxy class names
        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) {

            Map, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            for (Class intf : interfaces) {
                
                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");
                }
                
                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;

            
            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 + ".";
            }

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

            
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            try {
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {
                
                throw new IllegalArgumentException(e.toString());
            }
        }
    }

重点看这段代码

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

            
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            try {
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {
                
                throw new IllegalArgumentException(e.toString());
            }

        proxyName是$Proxy加上一个long类型的数字组成，接着看下generateProxyClass()

 public static byte[] generateProxyClass(final String var0, Class[] var1, int var2) {
        ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
        final byte[] var4 = var3.generateClassFile();
        if (saveGeneratedFiles) {
            AccessController.doPrivileged(new PrivilegedAction() {
                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));
                            Files.createDirectories(var3);
                            var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
                        } else {
                            var2 = Paths.get(var0 + ".class");
                        }

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

        到这里已经非常清晰了，生成了byte[]的字节码文件，并且有一个boolean类型saveGeneratedFiles参数用来判断是否写进硬盘，true的情况下会生成以刚才拼接的字符串为类名的.class文件。这里我们写段测试代码来看一看生成的class文件（使用Junit设置参数稍复杂一些，这里使用main函数测试）。

    public static void main(String[] args) {
        System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

        Action action = new ActionImpl();

        ActionProxy actionProxy = new ActionProxy(action);

        Action proxyInstance = (Action) actionProxy.getProxyInstance();

        proxyInstance.run();
    }

在com.sun.proxy包下生成了$Proxy的class文件，我们反编译看一下长什么样

package com.sun.proxy;

import com.xxx.test.Action;
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 Action {
    private static Method m1;
    private static Method m3;
    private static Method m2;
    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 run() 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.ruoyi.test.Action").getMethod("run");
            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());
        }
    }
}

        $Proxy0继承Proxy并且实现Action接口，依然存在run()方法，且直接调用的父类中h的invoke()方法，而h正是父类中的InvocationHandler，也就是我们编写的代理类中获取代理实例时newProxyInstance中传参的this，指向的是当前代理类。所以 $Proxy0中的run()方法调用的正是代理类中的invoke方法。所以当执行代理实例中run()方法时，才会执行代理类的invoke()方法，具备了前置和后置处理器的能力。