Java動態(tài)代理原理和源碼分析

導讀

代理模式就是自己做不了或不想做的事情找別人做迹蛤，比如我們買不到票，找黃牛買襟士，這就是代理模式盗飒。

代理模式分為調用方、代理陋桂、目標三部分逆趣。

我們常用的Java代理模式主要有兩種:

靜態(tài)代理
動態(tài)代理

靜態(tài)代理是設計模式中的一種，也就是硬編碼嗜历，一旦需要代理的類或方法多了宣渗，操作使用很不方便。今天主要講動態(tài)代理梨州。

基本用法

場景痕囱，普通粉絲通過黃牛購買演唱會的門票，
先看一下代碼實現(xiàn)

接口類 TicketCenter.java

public interface TicketCenter {
    void buyTicket(Integer amount);
    void refundTicket();
    void sellTicket();
}

接口實現(xiàn)類 RealTicketCenter.java

public class RealTicketCenter implements TicketCenter {

    @Override
    public void buyTicket(Integer amount) {
        System.out.println("buyTicket,need ￥" + amount);
    }

    @Override
    public void refundTicket() {
        System.out.println("refundTicket");
    }

    @Override
    public void sellTicket() {
        System.out.println("sellTicket");
    }
}

代理類 ProxyTicketCenter.java

public class ProxyTicketCenter implements InvocationHandler {
    RealTicketCenter realTicketCenter;

    public ProxyTicketCenter(RealTicketCenter realTicketCenter) {
        this.realTicketCenter = realTicketCenter;
    }

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

        System.out.println("before");
        Object object = null;
        object=method.invoke(realTicketCenter,args);
        System.out.println("after");
        return object;
    }
}

使用 Main.java

public class Main {
    public static void main(String args[]){
        //保留生成的字節(jié)碼文件
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        TicketCenter ticketCenter = (TicketCenter) Proxy.newProxyInstance(Main.class.getClassLoader(),new Class[]{TicketCenter.class},new ProxyTicketCenter(new RealTicketCenter()));
        ticketCenter.buyTicket(10);
    }
}

執(zhí)行結果如下

before
buyTicket,need ￥10
after

源碼分析

我們可以看到在執(zhí)行buyTicket方法前后執(zhí)行了我們代理需要做的事情暴匠。
我們進入到newProxyInstance方法


        /*
         * Look up or generate the designated proxy class.
         */
        Class<?> cl = getProxyClass0(loader, intfs);

這里是找到或生成指定的代理類鞍恢，
進入到getProxyClass0方法

   /**
     * Generate a proxy class.  Must call the checkProxyAccess method
     * to perform permission checks before calling this.
     */
    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);
    }

從緩存中獲取，如果沒有則生成每窖，我們看一下proxyClassCache這個對象帮掉，存的是代理類緩存


    /**
     * a cache of proxy classes
     */
    private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

這里重點要看下ProxyClassFactory，代理類工廠岛请，是如何生成的

            /*
             * Generate the specified proxy class.
             */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);

看到這段代碼旭寿，就豁然開朗了，生成代理類的字節(jié)碼崇败，那么一個代理類就生成了盅称，有興趣的同學可以進入generateProxyClass方法看他是怎么生成的肩祥。
那生成的代碼是如何的呢？
我們在Main類有一段代碼就起作用了

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

這段代碼會將生成的字節(jié)碼保存到本地
一起看下生成的關鍵代碼


public final class $Proxy0 extends Proxy implements TicketCenter {
    private static Method m1;
    private static Method m5;
    private static Method m2;
    private static Method m3;
    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 refundTicket() throws  {
        try {
            super.h.invoke(this, m5, (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 buyTicket(Integer var1) throws  {
        try {
            super.h.invoke(this, m3, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final void sellTicket() 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"));
            m5 = Class.forName("TicketCenter").getMethod("refundTicket");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("TicketCenter").getMethod("buyTicket", Class.forName("java.lang.Integer"));
            m4 = Class.forName("TicketCenter").getMethod("sellTicket");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

我們在Main方法調用的 ticketCenter.buyTicket(10);
其實就是調用了這個類里面的buyTicket方法缩膝，在buyTicket方法里調用了InvocationHandler 的invoke方法混狠。這樣一來，我們就明白了是動態(tài)代理是如何實現(xiàn)的了疾层。
源碼見lean-proxy模塊 https://github.com/HuangPugang/Java-lean