JDK 动态代理实现及原理解析

前面介绍了代理模式
，讲了动态代理常见的实现方式，包含了JDK的动态代理和CGLib的动态代理。本文将介绍下JDK 动态代理实现及机制。

首先，要了解的是InvocationHandler接口。

"InvocationHandler is the interface implemented by the invocation handler of a proxy instance.

Each proxy instance has an associated invocation handler. When a method is invoked on a proxy instance, the method invocation is encoded and dispatched to the invoke method of its invocation handler."

InvocationHandler是每个完成方法调用处理的代理实例需要实现的接口。每个代理实例都有关联的一个调用处理者。当一个方法被代理实例执行时，方法调用会被转发到调用处理者执行。

这个接口有一个方法：
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable;
其中proxy是生成代理的对象，method是方法，args是方法参数。来看下具体用法：

/**
* Create by zxb on 2017/4/23
*/
public interface IDBQuery {
String getElement(String id);
}

/**
* Create by zxb on 2017/4/23
*/
public class DBQuery implements IDBQuery {

public String getElement(String id) {
return id + "_JDKProxy";
}
}

/**
* Create by zxb on 2017/4/23
*/
public class DBQueryProxy implements InvocationHandler {

private DBQuery dbQuery;

public DBQueryProxy(DBQuery dbQuery) {
this.dbQuery = dbQuery;
}

public Object invoke(Object o, Method method, Object[] objects) throws Throwable {
return method.invoke(dbQuery, objects);
}
}

/**
* Create by zxb on 2017/4/23
*/
public class TestJDKProxy {

public static void main(String[] args) {
DBQuery dbQuery = new DBQuery();
DBQueryProxy dbQueryProxy = new DBQueryProxy(dbQuery);
IDBQuery query = (IDBQuery) Proxy
.newProxyInstance(dbQuery.getClass().getClassLoader(), dbQuery.getClass().getInterfaces(),
dbQueryProxy);
System.out.println(query.getElement("Hello"));
}
}


执行结果：



其中需要注意的是，实现了InvocationHandler接口的DBQueryProxy，依赖真实对象。开始的时候，对这个不解，但是后来有了了解，才知道是需要这么写的。如果使用method.invoke(o,args)那么就会变成死循环，不断调自己。后面有解释。

所以看完例子，这里最重要的就是

Proxy.newProxyInstance(dbQuery.getClass().getClassLoader(), dbQuery.getClass().getInterfaces(), dbQueryProxy);看下newProxyInstance的实现
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) {
  //检查可访问性
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}

/*
* 查找 or 生成 指定的代理类，这步有用到缓存
*/
Class<?> cl = getProxyClass0(loader, intfs);

/*
* 通过指定的invocation handler来初始化构造函数
*/
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<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//生成实例
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);
}
}通过getProxyClass0方法来获取代理类，
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);
}其中proxyClassCache的定义为
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

这个WeakCache可以根据传入的ClassLoader、Class<?>[] 接口类数组得到对应的key和代理的类，有点类似map。所以重点看ProxyClassFactory。
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// 所有生成的代理类的前缀
private static final String proxyClassNamePrefix = "$Proxy";

// 下一个代理类的唯一标识值
private static final AtomicLong nextUniqueNumber = new AtomicLong();

@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
//校验传入的接口数组，是否由loader加载的？是否是接口？是否重复？
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
for (Class<?> intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
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");
}
/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
*/
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}

String proxyPkg = null; // 定义代理存放的包
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;

/*
* 处理非public类的接口，到同一包中
*/
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) {
// if no non-public proxy interfaces, use com.sun.proxy package
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}

/*
* 设置代理类的名称
*/
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());
}
}
}

OK，可以看到，关键是在
ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);来看ProxyGenerator的generateProxyClass方法传入代理类名称、接口、访问控制标志
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
//这步生成对应代理类的byte数组
final byte[] var4 = var3.generateClassFile();
//saveGeneratedFiles的值，由sun.misc.ProxyGenerator.saveGeneratedFiles变量值决定
//保存生成的文件，否则直接返回byte数组
   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), 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;
}我们来看下，如何生成代理类byte数组

private byte[] generateClassFile() {
添加hascode,equals,toString()方法
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;
//遍历接口，添加方法
  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 {
//添加构造函数
 this.methods.add(this.generateConstructor());
var11 = this.proxyMethods.values().iterator();

while(var11.hasNext()) {
var12 = (List)var11.next();
var15 = var12.iterator();
//添加方法及字段
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());
}
}
//添加静态初始化语句
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);
}
}
}

在前面的Test类中，添加
System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

重新执行，可以在项目的根目录中多了com文件夹，点进去可以在 com\sun\proxy\目录下找到$Proxy0.class文件，这个就是生成的代理类的字节码。反编译出来看下

package com.sun.proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import org.zheng.proxy.jdk.IDBQuery;

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

public $Proxy0(InvocationHandler paramInvocationHandler)
throws
{
super(paramInvocationHandler);
}

public final boolean equals(Object paramObject)
throws
{
try
{
return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
}
throw new UndeclaredThrowableException(localThrowable);
}

public final String getElement(String paramString)
throws
{
try
{
return (String)this.h.invoke(this, m3, new Object[] { paramString });
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
}
throw new UndeclaredThrowableException(localThrowable);
}

public final String toString()
throws
{
try
{
return (String)this.h.invoke(this, m2, null);
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
}
throw new UndeclaredThrowableException(localThrowable);
}

public final int hashCode()
throws
{
try
{
return ((Integer)this.h.invoke(this, m0, null)).intValue();
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
}
throw new UndeclaredThrowableException(localThrowable);
}

static
{
try
{
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
m3 = Class.forName("org.zheng.proxy.jdk.IDBQuery").getMethod("getElement", new Class[] { Class.forName("java.lang.String") });
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
return;
}
catch (NoSuchMethodException localNoSuchMethodException)
{
throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
}
catch (ClassNotFoundException localClassNotFoundException)
{
}
throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
}
}

可以看到：
1）$Proxy0继承了Proxy实现了IDBQuery接口。

2）getElement方法，是this.h.invoke(this, m3, new Object[] { paramString }); 这就是调用我们DBQueryProxy的invoke方法，传入代理类本身的实例，试想，如果还是用method.invoke(o,args)那么就一直是跳入代理类执行了。

3）这个代理类本身是没有真实对象的，所以必须要依赖于外部传入的对象来执行方法。所以在DBQueryProxy需要依赖真实对象，就是这么回事。

好的，理清楚了，下篇写CGLib动态代理的。