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原文链接： JDK动态代理实现机制 转载请注明出处！
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本文只对JDK动态代理的底层实现进行分析，如需了解代理模式和动态代理的使用请移步：设计模式—代理模式 动态代理的两种实现方式（JDK/Cglib）
在读代码之前先清楚两个概念：
1、Class类存储的是类的所有信息，包括类的所有方法、属性、实现接口等。每个类对应一个Class对象（单例），Class对象是由classLoader加载出来的，使用双亲委派模型来保证class只会被加载一次。
2、classLoader在加载类的时候不管class文件是从哪里来的，无论是从.class文件、网络、数据库类加载器都不关心。他只关心给他的class二进制流是不是能够通过校验。
说明：以下测试代码和 动态代理的两种实现方式（JDK/Cglib）相同
使用JDK动态代理需要实现InvocationHandler接口，同时实现invoke()方法。
package com.zpj.proxy.jdk;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
/**
* Created by Perkins on 2017/4/2.
*/
public class JDKProxy implements InvocationHandler {
private Object person;
public Object getInstance(Object person) {
this.person = person;
return Proxy.newProxyInstance(person.getClass().getClassLoader(), person.getClass().getInterfaces(), this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("doSomething---------start");
method.invoke(person, args);
System.out.println("doSomething---------end");
return null;
}
}
测试代码如下：
package com.zpj.proxy.jdk;
/**
* Created by Perkins on 2017/4/2.
*/
public class Run {
public static void main(String[] args) {
Person person = (Person) new JDKProxy().getInstance(new MrLi());
person.doWork();
}
}
运行的时候在person处打断点可看到person的类名为$Proxy0而不是Person或者MrLi,则说明该返回对象是Person的实现类。
我们添加如下代码把person的class中的方法打印出来
package com.zpj.proxy.jdk;
/**
* Created by Perkins on 2017/4/2.
*/
import java.lang.reflect.Method;
public class Run {
private Method method;
public static void main(String[] args) {
Person person = (Person) new JDKProxy().getInstance(new MrLi());
Method [] methods = person.getClass().getMethods();
for(int i =0; i<methods.length;i++){
System.out.println(methods[i].getName());
}
person.doWork();
}
}
结果如下，很明显红框中的方法不属于Person也不属于Object中的方法。这更进一步说明返回的person并不是Person的实例。
下面就进入底层代码对JDK动态代理进行解析。
这里直接对Proxy.newProxyInstance(person.getClass().getClassLoader(), person.getClass().getInterfaces(), this);进行分析。
public static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces, InvocationHandler h)throws IllegalArgumentException{
if (h == null) {
//验证InvocationHandler不允许为null
throw new NullPointerException();
}
/*
* Look up or generate the designated proxy class.
*/
//调用getProxyClass()获取Class实例，该实例便是返回的代理Person的实例，此方法为重点！！！
Class cl = getProxyClass(loader, interfaces);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
//利用反射机制从Class中取出构造器创建对象
Constructor cons = cl.getConstructor(constructorParams);
return (Object) cons.newInstance(new Object[] { h });
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString());
} catch (IllegalAccessException e) {
throw new InternalError(e.toString());
} catch (InstantiationException e) {
throw new InternalError(e.toString());
} catch (InvocationTargetException e) {
throw new InternalError(e.toString());
}
}
在上面方法中调用public static Class<?> getProxyClass(ClassLoader loader, Class<?>... interfaces)throws IllegalArgumentException获取了Class实例，下面进入该方法进行分析。
public static Class<?> getProxyClass(ClassLoader loader, Class<?>... interfaces) throws IllegalArgumentException {
if (interfaces.length > 65535) {
//验证接口数量不允许超过65535
throw new IllegalArgumentException("interface limit exceeded");
}
//************开始对interface进行循环验证，验证通过则加入interfaceNames中***************************
Class proxyClass = null;
/* collect interface names to use as key for proxy class cache */
String[] interfaceNames = new String[interfaces.length];
Set interfaceSet = new HashSet(); // for detecting duplicates
for (int i = 0; i < interfaces.length; i++) {//循环对所有接口进行操作
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
String interfaceName = interfaces[i].getName();
Class interfaceClass = null;
try {
//根据名称获取接口的Class
interfaceClass = Class.forName(interfaceName, false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != interfaces[i]) {
throw new IllegalArgumentException(interfaces[i] + " 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.contains(interfaceClass)) {
throw new IllegalArgumentException( "repeated interface: " + interfaceClass.getName());
}
interfaceSet.add(interfaceClass);
interfaceNames[i] = interfaceName;
}
//************结束对interface进行循环验证，存储于interfaceNames中***************************
/*
* Using string representations of the proxy interfaces as
* keys in the proxy class cache (instead of their Class
* objects) is sufficient because we require the proxy
* interfaces to be resolvable by name through the supplied
* class loader, and it has the advantage that using a string
* representation of a class makes for an implicit weak
* reference to the class.
*/
Object key = Arrays.asList(interfaceNames);
/*
* Find or create the proxy class cache for the class loader.
*/
Map cache;
synchronized (loaderToCache) {
cache = (Map) loaderToCache.get(loader);
if (cache == null) {
cache = new HashMap();
loaderToCache.put(loader, cache);
}
/*
* This mapping will remain valid for the duration of this
* method, without further synchronization, because the mapping
* will only be removed if the class loader becomes unreachable.
*/
}
/*
* Look up the list of interfaces in the proxy class cache using
* the key. This lookup will result in one of three possible
* kinds of values:
* null, if there is currently no proxy class for the list of
* interfaces in the class loader,
* the pendingGenerationMarker object, if a proxy class for the
* list of interfaces is currently being generated,
* or a weak reference to a Class object, if a proxy class for
* the list of interfaces has already been generated.
*/
synchronized (cache) {
/*
* Note that we need not worry about reaping the cache for
* entries with cleared weak references because if a proxy class
* has been garbage collected, its class loader will have been
* garbage collected as well, so the entire cache will be reaped
* from the loaderToCache map.
*/
do {
Object value = cache.get(key);
if (value instanceof Reference) {
proxyClass = (Class) ((Reference) value).get();
}
if (proxyClass != null) {
// proxy class already generated: return it
return proxyClass;
} else if (value == pendingGenerationMarker) {
// proxy class being generated: wait for it
try {
cache.wait();
} catch (InterruptedException e) {
/*
* The class generation that we are waiting for should
* take a small, bounded time, so we can safely ignore
* thread interrupts here.
*/
}
continue;
} else {
/*
* No proxy class for this list of interfaces has been
* generated or is being generated, so we will go and
* generate it now. Mark it as pending generation.
*/
cache.put(key, pendingGenerationMarker);
break;
}
} while (true);
}
try {
String proxyPkg = null; // package to define proxy class in
/*
* 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.
*/
//寻找到package的包路径，为构建代理类做准备。同时要保证所有的非public代理接口在相同的包中
for (int i = 0; i < interfaces.length; i++) {
int flags = interfaces[i].getModifiers();
if (!Modifier.isPublic(flags)) {
String name = interfaces[i].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,
proxyPkg = ""; // use the unnamed package
}
{
/*
* Choose a name for the proxy class to generate.
*/
long num;
synchronized (nextUniqueNumberLock) {
num = nextUniqueNumber++;
}
//这里构建了Class的全类名：包名+$Proxy+num，这里的proxyClassNamePrefix=“$Proxy”
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Verify that the class loader hasn't already
* defined a class with the chosen name.
*/
/*
* Generate the specified proxy class.
*/
//该方法为核心方法，获取代理类的字节码数据流。也即是proxyName.class文件的数据流
//因为interface的所有class文件都不已经被加载，所以这里只需要根据名称就可以从JVM中读取出所有的二进制数据
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces);
try {
//调用native方法从class字节码文件中创建代理类的Class实例。这里不再进入分析，由JVM负责实现
//获取到代理类的Class实例后，程序就可以根据代理类Class通过反射进行对象创建
proxyClass = 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());
}
}
// add to set of all generated proxy classes, for isProxyClass
proxyClasses.put(proxyClass, null);
} finally {
/*
* We must clean up the "pending generation" state of the proxy
* class cache entry somehow. If a proxy class was successfully
* generated, store it in the cache (with a weak reference);
* otherwise, remove the reserved entry. In all cases, notify
* all waiters on reserved entries in this cache.
*/
synchronized (cache) {
if (proxyClass != null) {
cache.put(key, new WeakReference(proxyClass));
} else {
cache.remove(key);
}
cache.notifyAll();
}
}
//返回代理类的Class实例
return proxyClass;
}
}
在上面的方法中，核心就在于如何构建代理类的class字节码文件。因为该字节码文件是由代理类和目标类组合而成，这也即是在运行期间动态创建class的方法。
即：byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces);该方法的具体实现，看下面代码分析
public static byte[] generateProxyClass(final String proxyName, Class[] interfaces) {
//创建代理生成器
ProxyGenerator proxyGenerator = new ProxyGenerator(proxyName, interfaces);
//调用generateClassFile()生成class的二进制数据流
final byte[] classFile = proxyGenerator.generateClassFile();
//该参数可以在运行时配置，当为true时则程序会生成代理类的class文件保存在磁盘中，即： $Proxy0.class
if(saveGeneratedFiles) {
AccessController.doPrivileged(new PrivilegedAction() {
public Object run() {
try {
FileOutputStream var1 = new FileOutputStream(ProxyGenerator.dotToSlash(proxyName) + ".class");
var1.write(classFile);
var1.close();
return null;
} catch (IOException var2) {
throw new InternalError("I/O exception saving generated file: " + interfaces);
}
}
});
}
return classFile;
}
继续进入proxyGenerator.generateClassFile()分析
private byte[] generateClassFile() {
//注意这三个方法，hashCodeMethod、equalsMethod、toStringMethod
//这三个方法来自于Object，代理类同样需要Object的这三个方法
//把这三个方法名称和与之匹配的Object,class进行缓存
this.addProxyMethod(hashCodeMethod, Object.class);
this.addProxyMethod(equalsMethod, Object.class);
this.addProxyMethod(toStringMethod, Object.class);
int var1;
int var3;
//这里循环对目标类所实现的所有接口中的方法进行缓存
for(var1 = 0; var1 < this.interfaces.length; ++var1) {
//获取接口中的方法
Method[] var2 = this.interfaces[var1].getMethods();
for(var3 = 0; var3 < var2.length; ++var3) {
this.addProxyMethod(var2[var3], this.interfaces[var1]);
}
}
Iterator var7 = this.proxyMethods.values().iterator();
List var8;
//把代理类需要创建的方法缓存在var8中。这里说明一下,因为该源码是从class中读取出来的，所以变量名在进行编译的时候被更改了，这里阅读的时候需要注意一些
while(var7.hasNext()) {
var8 = (List)var7.next();
checkReturnTypes(var8);
}
Iterator var11;
try {
//添加构造器方法，至此代理类中所需要添加的方法添加完成
this.methods.add(this.generateConstructor());
var7 = this.proxyMethods.values().iterator();
//循环把需要的变量和方法添加在fileds和methods中缓存
while(var7.hasNext()) {
var8 = (List)var7.next();
var11 = var8.iterator();
while(var11.hasNext()) {
ProxyGenerator.ProxyMethod var4 = (ProxyGenerator.ProxyMethod)var11.next();
this.fields.add(new ProxyGenerator.FieldInfo(var4.methodFieldName, "Ljava/lang/reflect/Method;", 10));
this.methods.add(var4.generateMethod());
}
}
this.methods.add(this.generateStaticInitializer());
} catch (IOException var6) {
throw new InternalError("unexpected I/O Exception");
}
//对方法和参数进行安全性验证
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");
for(var1 = 0; var1 < this.interfaces.length; ++var1) {
this.cp.getClass(dotToSlash(this.interfaces[var1].getName()));
}
this.cp.setReadOnly();
//构建缓冲流存放动态生成的字节码文件数据
ByteArrayOutputStream var9 = new ByteArrayOutputStream();
DataOutputStream var10 = new DataOutputStream(var9);
try {
//这里就是按照class文件格式进行封装，这里不再详解
var10.writeInt(-889275714);
var10.writeShort(0);
var10.writeShort(49);
this.cp.write(var10);
var10.writeShort(49);
var10.writeShort(this.cp.getClass(dotToSlash(this.className)));
var10.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
var10.writeShort(this.interfaces.length);
for(var3 = 0; var3 < this.interfaces.length; ++var3) {
//添加所有方法的字节码数据
var10.writeShort(this.cp.getClass(dotToSlash(this.interfaces[var3].getName())));
}
var10.writeShort(this.fields.size());
var11 = this.fields.iterator();
while(var11.hasNext()) {
//添加所有变量的字节码数据
ProxyGenerator.FieldInfo var12 = (ProxyGenerator.FieldInfo)var11.next();
var12.write(var10);
}
var10.writeShort(this.methods.size());
var11 = this.methods.iterator();
while(var11.hasNext()) {
ProxyGenerator.MethodInfo var13 = (ProxyGenerator.MethodInfo)var11.next();
var13.write(var10);
}
var10.writeShort(0);
//动态组合class二进制字节码结束，进行返回。这里存储的就是一个完整class文件数据。调用处据此生成Class对象实例
return var9.toByteArray();
} catch (IOException var5) {
throw new InternalError("unexpected I/O Exception");
}
}
}
至此，JDK动态代理的实现结束。由以上可见，其核心就是动态的生成代理类的class字节码数据，然后调用native方法从字节码数据中创建代理对象的Class实例，拿到Class实例后通过Java的反射技术生成代理类。
那么该代理类的结果到底如何？我们可以在main方法中添加 System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");配置，这样在程序运行的时候会把代理类的字节码文件保存在类路径下。通过反编译可以读取到代理类的具体详情。
下面看反编译后的代理类文件
package com.sun.proxy;
import com.zpj.proxy.jdk.Person;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
//注意这里$Proxy0 extends Proxy implements Person,这里也即是目标类必须实现的有接口的原因
/**
* 代理类名称：$Proxy0
* 继承：Proxy
* 实现：Person
*/
public final class $Proxy0 extends Proxy implements Person {
private static Method m1;
private static Method m2;
private static Method m3;
private static Method m0;
public $Proxy0(InvocationHandler paramInvocationHandler) throws {
super(paramInvocationHandler);
}
//继承自Object的方法
public final boolean equals(Object paramObject) throws {
try {
return ((Boolean) this.h.invoke(this, m1, new Object[]{paramObject})).booleanValue();
} catch (Error | RuntimeException localError) {
throw localError;
} catch (Throwable localThrowable) {
throw new UndeclaredThrowableException(localThrowable);
}
}
//继承自Object的方法
public final String toString()throws {
try {
return (String) this.h.invoke(this, m2, null);
} catch (Error | RuntimeException localError) {
throw localError;
} catch (Throwable localThrowable) {
throw new UndeclaredThrowableException(localThrowable);
}
}
//继承自Object的方法
public final int hashCode()throws {
try {
return ((Integer) this.h.invoke(this, m0, null)).intValue();
} catch (Error | RuntimeException localError) {
throw localError;
} catch (Throwable localThrowable) {
throw new UndeclaredThrowableException(localThrowable);
}
}
//实现Person的方法
public final void doWork()throws {
try {
//由此处可以看出目标方法的调用循序
/**
* 代理类先调用实现接口的方法，在该方法中调用InvocationHandler的invoke方法。
* 而在invoke中由通过注入进去的methods通过反射调用目标类的目标方法doWork()
*/
this.h.invoke(this, m3, null);
return;
} catch (Error | RuntimeException localError) {
throw localError;
} catch (Throwable localThrowable) {
throw new UndeclaredThrowableException(localThrowable);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[]{Class.forName("java.lang.Object")});
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
m3 = Class.forName("com.zpj.proxy.jdk.Person").getMethod("doWork", 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());
}
}
}
------------end
来源：https://www.cnblogs.com/PerkinsZhu/p/6658066.html