彻底理解代理模式
代理设计模式
定义：为其他对象提供一种代理以控制对这个对象的访问。
代理模式的结构如下图所示。
动态代理使用
java动态代理机制以巧妙的方式实现了代理模式的设计理念。
代理模式示例代码
public interface Subject
{
public void doSomething();
}
public class RealSubject implements Subject
{
public void doSomething()
{
System.out.println( "call doSomething()" );
}
}
public class ProxyHandler implements InvocationHandler
{
private Object proxied;
public ProxyHandler( Object proxied )
{
this.proxied = proxied;
}
public Object invoke( Object proxy, Method method, Object[] args ) throws Throwable
{
//在转调具体目标对象之前，可以执行一些功能处理
//转调具体目标对象的方法
return method.invoke( proxied, args);
//在转调具体目标对象之后，可以执行一些功能处理
}
} import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import sun.misc.ProxyGenerator;
import java.io.*;
public class DynamicProxy
{
public static void main( String args[] )
{
RealSubject real = new RealSubject();
Subject proxySubject = (Subject)Proxy.newProxyInstance(Subject.class.getClassLoader(),
new Class[]{Subject.class},
new ProxyHandler(real));
proxySubject.doSomething();
//write proxySubject class binary data to file
createProxyClassFile();
}
public static void createProxyClassFile()
{
String name = "ProxySubject";
byte[] data = ProxyGenerator.generateProxyClass( name, new Class[] { Subject.class } );
try
{
FileOutputStream out = new FileOutputStream( name + ".class" );
out.write( data );
out.close();
}
catch( Exception e )
{
e.printStackTrace();
}
}
}
动态代理内部实现
首先来看看类 Proxy 的代码实现 Proxy 的主要静态变量
// 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;Proxy的构造方法
// 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;} Proxy静态方法 newProxyInstance
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());
}
}
类 Proxy 的 getProxyClass 方法调用 ProxyGenerator 的 generateProxyClass 方法产生ProxySubject.class的二进制数据：
public static byte[] generateProxyClass(final String name, Class[] interfaces)我们可以 import sun.misc.ProxyGenerator ，调用 generateProxyClass 方法产生binary data，然后写入文件，最后通过反编译工具来查看内部实现原理。 反编译后的 ProxySubject.java Proxy 静态方法 newProxyInstance
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());
}
}
} ProxyGenerator 内部是如何生成class二进制数据，可以参考源代码。
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 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 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
一个典型的动态代理创建对象过程可分为以下四个步骤：
1、通过实现InvocationHandler接口创建自己的调用处理器 IvocationHandler handler = new InvocationHandlerImpl(…);
2、通过为Proxy类指定ClassLoader对象和一组interface创建动态代理类
Class clazz = Proxy.getProxyClass(classLoader,new Class[]{…});
3、通过反射机制获取动态代理类的构造函数，其参数类型是调用处理器接口类型
Constructor constructor = clazz.getConstructor(new Class[]{InvocationHandler.class});
4、通过构造函数创建代理类实例，此时需将调用处理器对象作为参数被传入
Interface Proxy = (Interface)constructor.newInstance(new Object[] (handler));
为了简化对象创建过程，Proxy类中的newInstance方法封装了2~4，只需两步即可完成代理对象的创建。
生成的ProxySubject继承Proxy类实现Subject接口，实现的Subject的方法实际调用处理器的invoke方法，而invoke方法利用反射调用的是被代理对象的的方法（Object result=method.invoke(proxied,args)）
美中不足
诚然， Proxy 已经设计得非常优美，但是还是有一点点小小的遗憾之处，那就是它始终无法摆脱仅支持 interface 代理的桎梏，因为它的设计注定了这个遗憾。回想一下那些动态生成的代理类的继承关系图，它们已经注定有一个共同的父类叫 Proxy 。Java的继承机制注定了这些动态代理类们无法实现对class的动态代理，原因是多继承在Java中本质上就行不通。有很多条理由，人们可以否定对 class代理的必要性，但是同样有一些理由，相信支持class动态代理会更美好。接口和类的划分，本就不是很明显，只是到了Java中才变得如此的细化。如果只从方法的声明及是否被定义来考量，有一种两者的混合体，它的名字叫抽象类。实现对抽象类的动态代理，相信也有其内在的价值。此外，还有一些历史遗留的类，它们将因为没有实现任何接口而从此与动态代理永世无缘。如此种种，不得不说是一个小小的遗憾。但是，不完美并不等于不伟大，伟大是一种本质，Java动态代理就是佐例。