JDK的动态代理深入解析（Proxy,InvocationHandler）（转）

一、什么是动态代理

二、jdk的动态代理

2.1、Proxy类

Proxy

"$Proxy"

java.lang.reflect.Proxy

Class

getInterfaces

Class

getMethods

Method

getMethod

Proxy.isProxyClass

Proxy.getProxyClass

Proxy.newProxyInstance

java.security.ProtectionDomain

java.lang.Object

java.security.AllPermission

InvocationHandler

Proxy.newInstance

Proxy.getProxyClass

方法

protected  Proxy(InvocationHandler h)  //使用其调用处理程序的指定值从子类（通常为动态代理类）构建新的 Proxy 实例。

static InvocationHandler getInvocationHandler(Object proxy) //返回指定代理实例的调用处理程序。

static Class<?> getProxyClass(ClassLoader loader, Class<?>... interfaces) //返回代理类的 java.lang.Class 对象，并向其提供类加载器和接口数组。

static boolean isProxyClass(Class<?> cl) //当且仅当指定的类通过 getProxyClass 方法或 newProxyInstance 方法动态生成为代理类时，返回 true。

static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) //返回一个指定接口的代理类实例，该接口可以将方法调用指派到指定的调用处理程序。

2.1、InvocationHandler接口

Object invoke(Object proxy, Method method, Object[] args)

三、示例

public interface Subject
{
public void rent();

public void hello(String str);
}

public class RealSubject implements Subject
{
@Override
public void rent()
{
System.out.println("I want to rent my house");
}

@Override
public void hello(String str)
{
System.out.println("hello: " + str);
}
}

public class DynamicProxy implements InvocationHandler
{
//　这个就是我们要代理的真实对象
private Object subject;

//    构造方法，给我们要代理的真实对象赋初值
public DynamicProxy(Object subject)
{
this.subject = subject;
}

@Override
public Object invoke(Object object, Method method, Object[] args)
throws Throwable
{
//　　在代理真实对象前我们可以添加一些自己的操作
System.out.println("before rent house");

System.out.println("Method:" + method);

//    当代理对象调用真实对象的方法时，其会自动的跳转到代理对象关联的handler对象的invoke方法来进行调用
method.invoke(subject, args);

//　　在代理真实对象后我们也可以添加一些自己的操作
System.out.println("after rent house");

return null;
}

}

public class Client
{
public static void main(String[] args)
{
//    我们要代理的真实对象
Subject realSubject = new RealSubject();

//    我们要代理哪个真实对象，就将该对象传进去，最后是通过该真实对象来调用其方法的
InvocationHandler handler = new DynamicProxy(realSubject);

/*
* 通过Proxy的newProxyInstance方法来创建我们的代理对象，我们来看看其三个参数
* 第一个参数 handler.getClass().getClassLoader() ，我们这里使用handler这个类的ClassLoader对象来加载我们的代理对象
* 第二个参数realSubject.getClass().getInterfaces()，我们这里为代理对象提供的接口是真实对象所实行的接口，表示我要代理的是该真实对象，这样我就能调用这组接口中的方法了
* 第三个参数handler， 我们这里将这个代理对象关联到了上方的 InvocationHandler 这个对象上
*/
Subject subject = (Subject)Proxy.newProxyInstance(handler.getClass().getClassLoader(), realSubject
.getClass().getInterfaces(), handler);

System.out.println(subject.getClass().getName());
subject.rent();
subject.hello("world");
}
}

$Proxy0

before rent house
Method:public abstract void com.xiaoluo.dynamicproxy.Subject.rent()
I want to rent my house
after rent house

before rent house
Method:public abstract void com.xiaoluo.dynamicproxy.Subject.hello(java.lang.String)
hello: world
after rent house

Subject subject = (Subject)Proxy.newProxyInstance(handler.getClass().getClassLoader(), realSubject
.getClass().getInterfaces(), handler);

public Object invoke(Object object, Method method, Object[] args)
throws Throwable
{
//　　在代理真实对象前我们可以添加一些自己的操作
System.out.println("before rent house");

System.out.println("Method:" + method);

//    当代理对象调用真实对象的方法时，其会自动的跳转到代理对象关联的handler对象的invoke方法来进行调用
method.invoke(subject, args);

//　　在代理真实对象后我们也可以添加一些自己的操作
System.out.println("after rent house");

return null;
}

public abstract void com.xiaoluo.dynamicproxy.Subject.rent()

public abstract void com.xiaoluo.dynamicproxy.Subject.hello(java.lang.String)

动态代理内部实现

// 映射表：用于维护类装载器对象到其对应的代理类缓存
private static Map loaderToCache = new WeakHashMap();

// 标记：用于标记一个动态代理类正在被创建中
private static Object pendingGenerationMarker = new Object();

// 同步表：记录已经被创建的动态代理类类型，主要被方法 isProxyClass 进行相关的判断
private static Map proxyClasses = Collections.synchronizedMap(new WeakHashMap());

// 关联的调用处理器引用
protected InvocationHandler h;

// 由于 Proxy 内部从不直接调用构造函数，所以 private 类型意味着禁止任何调用
private Proxy() {}

// 由于 Proxy 内部从不直接调用构造函数，所以 protected 意味着只有子类可以调用
protected Proxy(InvocationHandler h) {this.h = h;}

public static Object newProxyInstance(ClassLoader loader, Class<?>[]interfaces,InvocationHandler h) throws IllegalArgumentException {
// 检查 h 不为空，否则抛异常
if (h == null) {
throw new NullPointerException();
}

// 获得与指定类装载器和一组接口相关的代理类类型对象
Class cl = getProxyClass(loader, interfaces);

// 通过反射获取构造函数对象并生成代理类实例
try {
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());
}
}

public static byte[] generateProxyClass(final String name, Class[] interfaces)

import java.lang.reflect.*;
public final class ProxySubject extends Proxy
implements Subject
{
private static Method m1;
private static Method m0;
private static Method m3;
private static Method m2;
public ProxySubject(InvocationHandler invocationhandler)
{
super(invocationhandler);
}
public final boolean equals(Object obj)
{
try
{
return ((Boolean)super.h.invoke(this, m1, new Object[] {
obj
})).booleanValue();
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final int hashCode()
{
try
{
return ((Integer)super.h.invoke(this, m0, null)).intValue();
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final void doSomething()
{
try
{
super.h.invoke(this, m3, null);
return;
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final String toString()
{
try
{
return (String)super.h.invoke(this, m2, null);
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
static
{
try
{
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] {
Class.forName("java.lang.Object")
});
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
m3 = Class.forName("Subject").getMethod("doSomething", new Class[0]);
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
}
catch(NoSuchMethodException nosuchmethodexception)
{
throw new NoSuchMethodError(nosuchmethodexception.getMessage());
}
catch(ClassNotFoundException classnotfoundexception)
{
throw new NoClassDefFoundError(classnotfoundexception.getMessage());
}
}
}

private byte[] generateClassFile() {
/*
* Record that proxy methods are needed for the hashCode, equals,
* and toString methods of java.lang.Object.  This is done before
* the methods from the proxy interfaces so that the methods from
* java.lang.Object take precedence over duplicate methods in the
* proxy interfaces.
*/
addProxyMethod(hashCodeMethod, Object.class);
addProxyMethod(equalsMethod, Object.class);
addProxyMethod(toStringMethod, Object.class);
/*
* Now record all of the methods from the proxy interfaces, giving
* earlier interfaces precedence over later ones with duplicate
* methods.
*/
for (int i = 0; i < interfaces.length; i++) {
Method[] methods = interfaces[i].getMethods();
for (int j = 0; j < methods.length; j++) {
addProxyMethod(methods[j], interfaces[i]);
}
}
/*
* For each set of proxy methods with the same signature,
* verify that the methods' return types are compatible.
*/
for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
checkReturnTypes(sigmethods);
}
/* ============================================================
* Step 2: Assemble FieldInfo and MethodInfo structs for all of
* fields and methods in the class we are generating.
*/
try {
methods.add(generateConstructor());
for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
for (ProxyMethod pm : sigmethods) {
// add static field for method's Method object
fields.add(new FieldInfo(pm.methodFieldName,
"Ljava/lang/reflect/Method;",
ACC_PRIVATE | ACC_STATIC));
// generate code for proxy method and add it
methods.add(pm.generateMethod());
}
}
methods.add(generateStaticInitializer());
} catch (IOException e) {
throw new InternalError("unexpected I/O Exception");
}
/* ============================================================
* Step 3: Write the final class file.
*/
/*
* Make sure that constant pool indexes are reserved for the
* following items before starting to write the final class file.
*/
cp.getClass(dotToSlash(className));
cp.getClass(superclassName);
for (int i = 0; i < interfaces.length; i++) {
cp.getClass(dotToSlash(interfaces[i].getName()));
}
/*
* Disallow new constant pool additions beyond this point, since
* we are about to write the final constant pool table.
*/
cp.setReadOnly();
ByteArrayOutputStream bout = new ByteArrayOutputStream();
DataOutputStream dout = new DataOutputStream(bout);
try {
/*
* Write all the items of the "ClassFile" structure.
* See JVMS section 4.1.
*/
// u4 magic;
dout.writeInt(0xCAFEBABE);
// u2 minor_version;
dout.writeShort(CLASSFILE_MINOR_VERSION);
// u2 major_version;
dout.writeShort(CLASSFILE_MAJOR_VERSION);
cp.write(dout);   // (write constant pool)
// u2 access_flags;
dout.writeShort(ACC_PUBLIC | ACC_FINAL | ACC_SUPER);
// u2 this_class;
dout.writeShort(cp.getClass(dotToSlash(className)));
// u2 super_class;
dout.writeShort(cp.getClass(superclassName));
// u2 interfaces_count;
dout.writeShort(interfaces.length);
// u2 interfaces[interfaces_count];
for (int i = 0; i < interfaces.length; i++) {
dout.writeShort(cp.getClass(
dotToSlash(interfaces[i].getName())));
}
// u2 fields_count;
dout.writeShort(fields.size());
// field_info fields[fields_count];
for (FieldInfo f : fields) {
f.write(dout);
}
// u2 methods_count;
dout.writeShort(methods.size());
// method_info methods[methods_count];
for (MethodInfo m : methods) {
m.write(dout);
}
// u2 attributes_count;
dout.writeShort(0); // (no ClassFile attributes for proxy classes)
} catch (IOException e) {
throw new InternalError("unexpected I/O Exception");
}
return bout.toByteArray();

总结

美中不足

cglib

Spring AOP

拦截器