【Java技术探索】深入分析JDK动态代理的源码
动态代理步骤
-
创建一个实现接口InvocationHandler的类，它必须实现invoke方法
-
创建被代理的类以及接口
- 通过Proxy的静态方法
通过Proxy的静态方法
ProxyObject proxyObject = new ProxyObject();
InvocationHandler invocationHandler = new DynamicProxy(proxyObject);
ClassLoader classLoader = proxyObject.getClass().getClassLoader();
ProxyObjectInterface proxy = (IRoom) Proxy.newProxyInstance(classLoader,new Class[]
{ProxyObjectInterface.class},invocationHandler);
proxy.execute();
public class DynamicProxy implements InvocationHandler {
private Object object;
public DynamicProxy(Object object){
this.object = object;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
Object result = method.invoke(object,args);
return result;
}
}创建一个代理 newProxyInstance
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
//检验h不为空，h为空抛异常
Objects.requireNonNull(h);
//接口的类对象拷贝一份
final Class<?>[] intfs = interfaces.clone();
//进行一些安全性检查
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
* 查询（在缓存中已经有）或生成指定的代理类的class对象。
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//得到代理类对象的构造函数，这个构造函数的参数由constructorParams指定
//参数constructorParames为常量值：
private static final Class<?>[] constructorParams = { InvocationHandler.class };
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;
}
});
}
//这里生成代理对象，传入的参数new Object[]{h}后面讲
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);
}
}- 先对h进行判空处理。
这段代码核心就是通过getProxyClass0(loader, intfs)得到代理类的Class对象，然后通过Class对象得到构造方法，进而创建代理对象。下一步看getProxyClass0这个方法。从1可知，先接口得到接口类，当接口的数量超过65535，则报异常。
//此方法也是Proxy类下的方法
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
//意思是：如果代理类被指定的类加载器loader定义了，并实现了给定的接口interfaces，
//那么就返回缓存的代理类对象，否则使用ProxyClassFactory创建代理类。
return proxyClassCache.get(loader, interfaces);
}- proxyClassCache 是一个弱引用的缓存
这里看到proxyClassCache，有Cache便知道是缓存的意思，正好呼应了前面Look up or generate the designated proxy class。查询（在缓存中已经有）或生成指定的代理类的class对象这段注释。
在进入get方法之前，我们看下 proxyClassCache是什么？高能预警，前方代码看起来可能有乱，但我们只需要关注重点即可。
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());//K代表key的类型，P代表参数的类型，V代表value的类型。
// WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache 说明proxyClassCache存的值是Class<?>对象，正是我们需要的代理类对象。
final class WeakCache<K, P, V> {
private final ReferenceQueue<K> refQueue
= new ReferenceQueue<>();
// the key type is Object for supporting null key
private final ConcurrentMap<Object, ConcurrentMap<Object, Supplier<V>>> map
= new ConcurrentHashMap<>();
private final ConcurrentMap<Supplier<V>, Boolean> reverseMap
= new ConcurrentHashMap<>();
private final BiFunction<K, P, ?> subKeyFactory;
private final BiFunction<K, P, V> valueFactory;
public WeakCache(BiFunction<K, P, ?> subKeyFactory,
BiFunction<K, P, V> valueFactory) {
this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
this.valueFactory = Objects.requireNonNull(valueFactory);
}其中map变量是实现缓存的核心变量，他是一个双重的Map结构: (key, sub-key) -> value。其中key是传进来的Classloader进行包装后的对象，sub-key是由WeakCache构造函数传人的KeyFactory()生成的。value就是产生代理类的对象，是由WeakCache构造函数传人的ProxyClassFactory()生成的。如下，回顾一下:
proxyClassCache是个WeakCache类的对象，调用proxyClassCache.get(loader, interfaces)； 可以得到缓存的代理类或创建代理类（没有缓存的情况）。
说明WeakCache中有get这个方法。先看下WeakCache类的定义（这里先只给出变量的定义和构造函数），继续看它的get（）；
//K和P就是WeakCache定义中的泛型，key是类加载器，parameter是接口类数组
public V get(K key, P parameter) {
//检查parameter不为空
Objects.requireNonNull(parameter);
//清除无效的缓存
expungeStaleEntries();
// cacheKey就是(key, sub-key) -> value里的一级key，
Object cacheKey = CacheKey.valueOf(key, refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
//根据一级key得到 ConcurrentMap<Object, Supplier<V>>对象。如果之前不存在，则新建一个ConcurrentMap<Object, Supplier<V>>和cacheKey（一级key）一起放到map中。
ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
ConcurrentMap<Object, Supplier<V>> oldValuesMap
= map.putIfAbsent(cacheKey,
valuesMap = new ConcurrentHashMap<>());
if (oldValuesMap != null) {
valuesMap = oldValuesMap;
}
}
// create subKey and retrieve the possible Supplier<V> stored by that
// subKey from valuesMap
//这部分就是调用生成sub-key的代码，上面我们已经看过怎么生成的了
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
//通过sub-key得到supplier
Supplier<V> supplier = valuesMap.get(subKey);
//supplier实际上就是这个factory
Factory factory = null;
while (true) {
//如果缓存里有supplier ，那就直接通过get方法，得到代理类对象，返回，就结束了，一会儿分析get方法。
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> instance
V value = supplier.get();
if (value != null) {
return value;
}
}
// else no supplier in cache
// or a supplier that returned null (could be a cleared CacheValue
// or a Factory that wasn't successful in installing the CacheValue)
// lazily construct a Factory
//下面的所有代码目的就是：如果缓存中没有supplier，则创建一个Factory对象，把factory对象在多线程的环境下安全的赋给supplier。
//因为是在while（true）中，赋值成功后又回到上面去调get方法，返回才结束。
if (factory == null) {
factory = new Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
supplier = valuesMap.putIfAbsent(subKey, factory);
if (supplier == null) {
// successfully installed Factory
supplier = factory;
}
// else retry with winning supplier
} else {
if (valuesMap.replace(subKey, supplier, factory)) {
// successfully replaced
// cleared CacheEntry / unsuccessful Factory
// with our Factory
supplier = factory;
} else {
// retry with current supplier
supplier = valuesMap.get(subKey);
}
}
}
}所以接下来我们看Factory类中的get方法。接下来看supplier的get()
public synchronized V get() { // serialize access
// re-check
Supplier<V> supplier = valuesMap.get(subKey);
//重新检查得到的supplier是不是当前对象
if (supplier != this) {
// something changed while we were waiting:
// might be that we were replaced by a CacheValue
// or were removed because of failure ->
// return null to signal WeakCache.get() to retry
// the loop
return null;
}
// else still us (supplier == this)
// create new value
V value = null;
try {
//代理类就是在这个位置调用valueFactory生成的
//valueFactory就是我们传入的 new ProxyClassFactory()
//一会我们分析ProxyClassFactory()的apply方法
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
} finally {
if (value == null) { // remove us on failure
valuesMap.remove(subKey, this);
}
}
// the only path to reach here is with non-null value
assert value != null;
// wrap value with CacheValue (WeakReference)
//把value包装成弱引用
CacheValue<V> cacheValue = new CacheValue<>(value);
// put into reverseMap
// reverseMap是用来实现缓存的有效性
reverseMap.put(cacheValue, Boolean.TRUE);
// try replacing us with CacheValue (this should always succeed)
if (!valuesMap.replace(subKey, this, cacheValue)) {
throw new AssertionError("Should not reach here");
}
// successfully replaced us with new CacheValue -> return the value
// wrapped by it
return value;
}
}拨云见日，来到ProxyClassFactory的apply方法，代理类就是在这里生成的。
首先看proxyClassCache的定义WeakCache<ClassLoader, Class<?>[], Class<?>>，泛型里面第一个表示加载器K，第二个表示接口类P，第三个则是生成的代理类V。而V的生成则是通过ProxyClassFactory生成的。调用其apply（）；
//这里的BiFunction<T, U, R>是个函数式接口，可以理解为用T，U两种类型做参数，得到R类型的返回值
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// prefix for all proxy class names
//所有代理类名字的前缀
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
//用于生成代理类名字的计数器
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
//验证代理接口，可不看
for (Class<?> intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
Class<?> interfaceClass = null;
try {
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != intf) {
throw new IllegalArgumentException(
intf + " is not visible from class loader");
}
/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
*/
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
//生成的代理类的包名
String proxyPkg = null; // package to define proxy class in
//代理类访问控制符: public ,final
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
/*
* Record the package of a non-public proxy interface so that the
* proxy class will be defined in the same package. Verify that
* all non-public proxy interfaces are in the same package.
*/
//验证所有非公共的接口在同一个包内；公共的就无需处理
//生成包名和类名的逻辑，包名默认是com.sun.proxy，
// 类名默认是$Proxy 加上一个自增的整数值
//如果被代理类是 non-public proxy interface ，则用和被代理类接口一样的包名
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 + ".";
}
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
//代理类的完全限定名，如com.sun.proxy.$Proxy0.calss
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
//核心部分，生成代理类的字节码
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//把代理类加载到JVM中，至此动态代理过程基本结束了
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());
}
}
}然后调用getMethod（）,将equals(),hashcode(),toString()等方法添加进去。然后遍历所有接口的方法，添加到代理类中。最后将这些方法进行排序。
private static List<Method> getMethods(Class<?>[] interfaces) {
List<Method> result = new ArrayList<Method>();
try {
result.add(Object.class.getMethod("equals", Object.class));
result.add(Object.class.getMethod("hashCode", EmptyArray.CLASS));
result.add(Object.class.getMethod("toString", EmptyArray.CLASS));
} catch (NoSuchMethodException e) {
throw new AssertionError();
}
getMethodsRecursive(interfaces, result);
return result;
}
private static void getMethodsRecursive(Class<?>[] interfaces, List<Method> methods) {
for (Class<?> i : interfaces) {
getMethodsRecursive(i.getInterfaces(), methods);
Collections.addAll(methods, i.getDeclaredMethods());
}
}最后输出相关proxy class
package com.zhb.jdk.proxy;
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.Proxy;
import com.zhb.jdk.dynamicProxy.HelloworldImpl;
import sun.misc.ProxyGenerator;
/**
* @author ZHB
* @date 2018年8月31日下午11:35:07
* @todo TODO
*/
public class DynamicProxyTest {
public static void main(String[] args) {
IUserService target = new UserServiceImpl();
MyInvocationHandler handler = new MyInvocationHandler(target);
//第一个参数是指定代理类的类加载器（我们传入当前测试类的类加载器）
//第二个参数是代理类需要实现的接口（我们传入被代理类实现的接口，这样生成的代理类和被代理类就实现了相同的接口）
//第三个参数是invocation handler，用来处理方法的调用。这里传入我们自己实现的handler
IUserService proxyObject = (IUserService) Proxy.newProxyInstance(DynamicProxyTest.class.getClassLoader(),
target.getClass().getInterfaces(), handler);
proxyObject.add("陈粒");
String path = "D:/$Proxy0.class";
byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", HelloworldImpl.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();
}
}
}
}// Decompiled by Jad v1.5.8e2. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://kpdus.tripod.com/jad.html
// Decompiler options: packimports(3) fieldsfirst ansi space
import com.zhb.jdk.proxy.IUserService;
import java.lang.reflect.*;
public final class $Proxy0 extends Proxy
implements IUserService
{
private static Method m1;
private static Method m2;
private static Method m3;
private static Method m0;
//代理类的构造函数，其参数正是是InvocationHandler实例，
//Proxy.newInstance方法就是通过通过这个构造函数来创建代理实例的
public $Proxy0(InvocationHandler invocationhandler)
{
super(invocationhandler);
}
// Object类中的三个方法，equals，toString， hashCode
public final boolean equals(Object obj)
{
try
{
return ((Boolean)super.h.invoke(this, m1, new Object[] {
obj
})).booleanValue();
}
catch (Error ) { }
catch (Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final String toString()
{
try
{
return (String)super.h.invoke(this, m2, null);
}
catch (Error ) { }
catch (Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
//接口代理方法
public final void add(String s)
{
try
{
// invocation handler的 invoke方法在这里被调用
super.h.invoke(this, m3, new Object[] {
s
});
return;
}
catch (Error ) { }
catch (Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final int hashCode()
{
try
{
// 在这里调用了invoke方法。
return ((Integer)super.h.invoke(this, m0, null)).intValue();
}
catch (Error ) { }
catch (Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
// 静态代码块对变量进行一些初始化工作
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.zhb.jdk.proxy.IUserService").getMethod("add", new Class[] {
Class.forName("java.lang.String")
});
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
}
catch (NoSuchMethodException nosuchmethodexception)
{
throw new NoSuchMethodError(nosuchmethodexception.getMessage());
}
catch (ClassNotFoundException classnotfoundexception)
{
throw new NoClassDefFoundError(classnotfoundexception.getMessage());
}
}
}
