Java动态代理详解,以及底层JDK源码实现分析(基于Java8)。

java 动态代理模式在框架中大量得到了使用。动态代理也是学习Spring原理的基础。所以我们相当有必要学习好这一设计模式。对于动态代理有两种实现,一种是JDK自带的实现,还有一种是cglib库的实现。这里笔者分析的是JDK自带的实现,以及JDK源码底层是如何做到的。好,说了半天,动态代理到底是个什么东西呢？代理即想找某个人,但是我们无法真正的联系到他，但是可以通过中介的方式联系到。相当于租房子。你只能通过中介联系到房东。并且，你需要付给中介一定的费用。这就是一个现实生活中的代理模式。好,想知道原理我们首先要知道怎么去使用动态代理模式。这里笔者写一个非常简单的demo,相信广大的朋友看了就立刻会明白其中的意思。上代码。

package dynamicProxy;

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

/**
* 契约接口
*/
interface Goal
{
void doSomething();
}

/**
* 被代理的类
*/
class RealGoal implements Goal
{
@Override
public void doSomething()
{
System.out.println( "做某事......" );
}
}

public class DynamicProxyDemo implements InvocationHandler
{
/**
* 被代理的对象
*/
private RealGoal realGoal ;

public DynamicProxyDemo(RealGoal realGoal)
{
this.realGoal = realGoal;
}

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable
{
/**
* 做真正的目的时,我们可以在之前或之后加入一点我们的代理类想要做的事情
*/
System.out.println("做某事之前");
realGoal.doSomething();
System.out.println("做某事之后");

return null;
}

public static void main(String[] args)
{
/**
* new 一个被代理的对象
*/
RealGoal myRealGoal = new RealGoal();

Class<?> clazz = myRealGoal.getClass();

/**
* 得到代理对象(以前笔者都认为拿到的是真实的目标对象,其实不是,这里拿到的是代理对象,而且是java在我们运行时给我们定义好的类相当于 public class  $Proxy0定义了$Proxy0这么一个类 )
* $Proxy0这个类继承了  java.lang.reflect.Proxy,并且实现了被代理类所实现的所有接口.
* 从newProxyInstance()方法第二个参数中我们可以看出来。
*/
Goal goal = (Goal) Proxy.newProxyInstance( clazz.getClassLoader(), clazz.getInterfaces(), new DynamicProxyDemo( myRealGoal ) );

goal.doSomething();

}

}

执行结果是在  做某事的前后加上了一句 做某事之前和做某事之后。前后打印的两句话是额外加上去的，即证明不是直接调用的目标对象里的方法。这里有的朋友可能会问了?为何我没有看到代理类的定义呢？那是因为代理类是在运行的过程中定义的。相当于运行的时候 public class $Proxy0 定义了这么一个对象。产生代理对象的是这行代码。 Goal goal = (Goal) Proxy.newProxyInstance( clazz.getClassLoader(), clazz.getInterfaces(), new DynamicProxyDemo( myRealGoal ) );想要知道底层源码实现，我们可以进去看一看实现过程。源码的讲解我都在注释里面去讲了。package java.lang.reflect;import java.lang.ref.WeakReference;import java.security.AccessController;import java.security.PrivilegedAction;import java.util.Arrays;import java.util.IdentityHashMap;import java.util.Map;import java.util.Objects;import java.util.concurrent.atomic.AtomicLong;import java.util.function.BiFunction;import sun.misc.ProxyGenerator;import sun.misc.VM;import sun.reflect.CallerSensitive;import sun.reflect.Reflection;import sun.reflect.misc.ReflectUtil;import sun.security.util.SecurityConstants;public class Proxy implements java.io.Serializable {private static final long serialVersionUID = -2222568056686623797L;/** parameter types of a proxy class constructor */private static final Class<?>[] constructorParams ={ InvocationHandler.class };/*** a cache of proxy classes*/private static final WeakCache<ClassLoader, Class<?>[], Class<?>>proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());/*** 自己实现的一个InvocationHandler对象。在生成Proxy的子类的时候会被赋值上*/protected InvocationHandler h;/*** Prohibits instantiation.*/private Proxy() {}/*** 在生成Proxy子类的时候,子类的构造方法会有一行这样的代码。super(h);其中h为实现了InvocationHandler接口的对象*/protected Proxy(InvocationHandler h) {Objects.requireNonNull(h);this.h = h;}private static Class<?> getProxyClass0(ClassLoader loader,Class<?>... interfaces) {//判断实现的接口是否超过65535if (interfaces.length > 65535) {throw new IllegalArgumentException("interface limit exceeded");}//从缓存中拿代理类的Class对象,这里实现的过程比较复杂,笔者略过,不做讨论。return proxyClassCache.get(loader, interfaces);}/** a key used for proxy class with 0 implemented interfaces*/private static final Object key0 = new Object();/** Key1 and Key2 are optimized for the common use of dynamic proxies* that implement 1 or 2 interfaces.*//** a key used for proxy class with 1 implemented interface*/private static final class Key1 extends WeakReference<Class<?>> {private final int hash;Key1(Class<?> intf) {super(intf);this.hash = intf.hashCode();}@Overridepublic int hashCode() {return hash;}@Overridepublic boolean equals(Object obj) {Class<?> intf;return this == obj ||obj != null &&obj.getClass() == Key1.class &&(intf = get()) != null &&intf == ((Key1) obj).get();}}/** a key used for proxy class with 2 implemented interfaces*/private static final class Key2 extends WeakReference<Class<?>> {private final int hash;private final WeakReference<Class<?>> ref2;Key2(Class<?> intf1, Class<?> intf2) {super(intf1);hash = 31 * intf1.hashCode() + intf2.hashCode();ref2 = new WeakReference<Class<?>>(intf2);}@Overridepublic int hashCode() {return hash;}@Overridepublic boolean equals(Object obj) {Class<?> intf1, intf2;return this == obj ||obj != null &&obj.getClass() == Key2.class &&(intf1 = get()) != null &&intf1 == ((Key2) obj).get() &&(intf2 = ref2.get()) != null &&intf2 == ((Key2) obj).ref2.get();}}/** a key used for proxy class with any number of implemented interfaces* (used here for 3 or more only)*/private static final class KeyX {private final int hash;private final WeakReference<Class<?>>[] refs;@SuppressWarnings("unchecked")KeyX(Class<?>[] interfaces) {hash = Arrays.hashCode(interfaces);refs = (WeakReference<Class<?>>[])new WeakReference<?>[interfaces.length];for (int i = 0; i < interfaces.length; i++) {refs[i] = new WeakReference<>(interfaces[i]);}}@Overridepublic int hashCode() {return hash;}@Overridepublic boolean equals(Object obj) {return this == obj ||obj != null &&obj.getClass() == KeyX.class &&equals(refs, ((KeyX) obj).refs);}private static boolean equals(WeakReference<Class<?>>[] refs1,WeakReference<Class<?>>[] refs2) {if (refs1.length != refs2.length) {return false;}for (int i = 0; i < refs1.length; i++) {Class<?> intf = refs1[i].get();if (intf == null || intf != refs2[i].get()) {return false;}}return true;}}/*** A function that maps an array of interfaces to an optimal key where* Class objects representing interfaces are weakly referenced.*/private static final class KeyFactoryimplements BiFunction<ClassLoader, Class<?>[], Object>{@Overridepublic Object apply(ClassLoader classLoader, Class<?>[] interfaces) {switch (interfaces.length) {case 1: return new Key1(interfaces[0]); // the most frequentcase 2: return new Key2(interfaces[0], interfaces[1]);case 0: return key0;default: return new KeyX(interfaces);}}}//这是代理类工厂类private static final class ProxyClassFactoryimplements BiFunction<ClassLoader, Class<?>[], Class<?>>{//定义的名字前缀private static final String proxyClassNamePrefix = "$Proxy";// 生成的数,跟在$Proxy这个名字之后,你会发现,动态代理类的名字都是 $ProxyN (N=0,1,2,3,4,5,6,7,8)private static final AtomicLong nextUniqueNumber = new AtomicLong();@Overridepublic Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {Map<Class<?>, 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");}//判断Class对象是否代表的是一个接口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 inint 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 packageproxyPkg = ReflectUtil.PROXY_PACKAGE + ".";}long num = nextUniqueNumber.getAndIncrement();String proxyName = proxyPkg + proxyClassNamePrefix + num;//产生本地字节文件byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);try {//按照产生的字节来产生动态代理类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());}}}@CallerSensitivepublic static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)throws IllegalArgumentException{/*要求h非空,否则抛出空指针异常*/Objects.requireNonNull(h);//克隆一份被代理目标类所实现所有接口的 Class对象数组final Class<?>[] intfs = interfaces.clone();/*中间省略一些代码*///拿到代理类的Class对象Class<?> cl = getProxyClass0(loader, intfs);/** 中间忽略了一些代码*///拿到代理类的Constructor对象，以便用于构造代理对象final Constructor<?> cons = cl.getConstructor(constructorParams);final InvocationHandler ih = h;//判断访问修饰符if (!Modifier.isPublic(cl.getModifiers())) {AccessController.doPrivileged(new PrivilegedAction<Void>() {public Void run() {cons.setAccessible(true);return null;}});}//返回构造的代理类对象,这里留个意,传入的是 实现了InvocationHandler接口的对象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);}}//根据字节数组生成代理类的Class对象private static native Class<?> defineClass0(ClassLoader loader, String name,byte[] b, int off, int len);}有的人问了,那么那么InvocationHandler的invoke()方法是由谁调用的呢?又是什么时候调用的呢?这里笔者让产生的动态代理类保存到了硬盘上已.class 结尾的文件。运行这段代码即可package dynamicProxy;import sun.misc.ProxyGenerator;import java.io.FileOutputStream;import java.io.IOException;public class ProxyGeneratorUtils{/*** 把代理类的字节码写到硬盘上*/public static void writeProxyClassToHardDisk(String path){// 获取代理类的字节码 方法的第二个参数为被代理类所实现的全部接口byte[] classFile = ProxyGenerator.generateProxyClass( "$Proxy11", RealSubject.class.getInterfaces() );FileOutputStream out = null;try{out = new FileOutputStream( path );out.write( classFile );out.flush();}catch(Exception e){e.printStackTrace();}finally{try{out.close();}catch(IOException e){e.printStackTrace();}}}}得到动态代理对象时候,使用反编译工具我们可以查看 $Proxy0这个类的源码。这里笔者反编译的结果是这样的

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

import dynamicProxy.Goal;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $Proxy11 extends Proxy implements Goal {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m0;

/*
这里的构造方法说明了先调用了父类Proxy的构造方法,吧实现了InvocationHandler的对象设置上。

*/
public $Proxy11(InvocationHandler var1) throws  {
super(var1);
}

public final boolean equals(Object var1) throws  {
try {
return ((Boolean)super.h.invoke(this, m1, new Object[]{var1})).booleanValue();
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}

/*从这个方法中我们可以看出InvocationHandler对象的invoke世界上是由Proxy的子类------>运行时动态生成的类来调用的
并且索要的所有参数,都是从这个动态代理的类中传入的。
*/
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)).intValue();
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}

/*

我们可以看到在这个反编译的类中,static代码块拿到了反射API  Method  。 这个Method即为InvocationHandler里的method对象。
再有一个
*/
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[]{Class.forName("java.lang.Object")});
m3 = Class.forName("dynamicProxy.Goal").getMethod("doSomething", new Class[0]);
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}

所有的讲解都写在了代码的注释里面.另外在附上图片一张,对照着里面的代码,你会对动态代理有一个更深更次的认识。