评：
一 代理设计模式
代理模式为目标对象提供一种代理以控制对实际对象的访问。在某些情况下，一个客户不想或者不能直接引用另一个对象，而代理对象可以在客户端和目标对象之间起到中介的作用。
为了保持行为的一致性，代理类和实际委托类通常会实现相同的接口，所以在访问者看来两者没有丝毫的区别。
代理模式类图
常见的代理有:
1) 远程代理(Remote proxy)：对一个位于不同的地址空间对象提供一个局域代表对象，如RMI中的stub。
2) 虚拟代理(Virtual proxy)：根据需要将一个资源消耗很大或者比较复杂的对象，延迟加载，在真正需要的时候才创建。
3) 保护代理(Protect or Access Proxy)：控制对一个对象的访问权限。
4) 智能引用(Smart Reference Proxy)：提供比目标对象额外的服务和功能。
通过代理类这一中间层，能够有效控制对实际委托类对象的直接访问，也可以很好地隐藏和保护实际对，实施不同的控制策略，从而在设计上获得了更大的灵活性。
二 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。
动态代理类图
动态代理在代理ProxySubject和RealSubject之间增加了InvocationHandler，这是一种通信间接化， 增加了灵 性性，例如可以把这个中间层实现为一个框架Framework，直接通过xml文件等方式来调用RealSubject。
在普通的设计中，我们一般不会使用动态代理。但是在一些框架结构设计中，动态代理非常重要，如RMI,EJB中都使用动态代理。
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interface Subject
{
public void doSomething();
}
class RealSubject implements Subject
{
public void doSomething()
{
System.out.println( "call doSomething()" );
}
}
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
{
return method.invoke( proxied, args);
}
}
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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();
}
}
}
三 动态代理内部实现
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：
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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());
}
}
}
通过 ProxySubject.java，我们可以看到动态代理的内部是如何实现的，并且我们可以实现自己的一个动态代理生成器。
ProxyGenerator内部是如何生成class二进制数据，可以参考源代码。
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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();标签：JAVA,methods,interfaces,Object,代理,dout,new,动态,class From： https://blog.51cto.com/u_16080829/6412724