原文：http://www.cnblogs.com/flyoung2008/archive/2013/08/11/3251148.html
代理設計模式
定義：爲其他對象提供一種代理以控制對這個對象的訪問。
代理模式的結構如下圖所示。
動態代理使用
java動態代理機制以巧妙的方式實現了代理模式的設計理念。
代理模式示例代碼
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
{
//在轉調具體目標對象之前，可以執行一些功能處理
//轉調具體目標對象的方法
return method.invoke( proxied, args);
//在轉調具體目標對象之後，可以執行一些功能處理
}
}
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();
}
}
}
動態代理內部實現
首先來看看類Proxy的代碼實現 Proxy的主要靜態變量
// 映射表：用於維護類裝載器對象到其對應的代理類緩存 private static Map loaderToCache = new WeakHashMap(); // 標記：用於標記一個動態代理類正在被創建中 private static Object pendingGenerationMarker = new Object(); // 同步表：記錄已經被創建的動態代理類類型，主要被方法 isProxyClass 進行相關的判斷 private static Map proxyClasses = Collections.synchronizedMap(new WeakHashMap()); // 關聯的調用處理器引用 protected InvocationHandler h;
Proxy的構造方法
// 由於 Proxy 內部從不直接調用構造函數，所以 private 類型意味着禁止任何調用
private Proxy() {}
// 由於 Proxy 內部從不直接調用構造函數，所以 protected 意味着只有子類可以調用
protected Proxy(InvocationHandler h) {this.h = h;}
Proxy靜態方法newProxyInstance
public static Object newProxyInstance(ClassLoader loader, Class<?>[]interfaces,InvocationHandler h) throws IllegalArgumentException {
// 檢查 h 不爲空，否則拋異常
if (h == null) {
throw new NullPointerException();
}
// 獲得與指定類裝載器和一組接口相關的代理類類型對象
Class cl = getProxyClass(loader, interfaces);
// 通過反射獲取構造函數對象並生成代理類實例
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());
}
}
類Proxy的getProxyClass方法調用ProxyGenerator的 generateProxyClass方法產生ProxySubject.class的二進制數據：
public static byte[] generateProxyClass(final String name, Class[] interfaces)
我們可以import sun.misc.ProxyGenerator，調用 generateProxyClass方法產生binary data，然後寫入文件，最後通過反編譯工具來查看內部實現原理。 反編譯後的ProxySubject.java Proxy靜態方法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內部是如何生成class二進制數據，可以參考源代碼。
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();
總結
一個典型的動態代理創建對象過程可分爲以下四個步驟：
1、通過實現InvocationHandler接口創建自己的調用處理器 IvocationHandler handler = new InvocationHandlerImpl(...);
2、通過爲Proxy類指定ClassLoader對象和一組interface創建動態代理類
Class clazz = Proxy.getProxyClass(classLoader,new Class[]{...});
3、通過反射機制獲取動態代理類的構造函數，其參數類型是調用處理器接口類型
Constructor constructor = clazz.getConstructor(new Class[]{InvocationHandler.class});
4、通過構造函數創建代理類實例，此時需將調用處理器對象作爲參數被傳入
Interface Proxy = (Interface)constructor.newInstance(new Object[] (handler));
爲了簡化對象創建過程，Proxy類中的newInstance方法封裝了2~4，只需兩步即可完成代理對象的創建。
生成的ProxySubject繼承Proxy類實現Subject接口，實現的Subject的方法實際調用處理器的invoke方法，而invoke方法利用反射調用的是被代理對象的的方法（Object result=method.invoke(proxied,args)）
美中不足
誠然，Proxy已經設計得非常優美，但是還是有一點點小小的遺憾之處，那就是它始終無法擺脫僅支持interface代理的桎梏，因爲它的設計註定了這個遺憾。回想一下那些動態生成的代理類的繼承關係圖，它們已經註定有一個共同的父類叫Proxy。Java的繼承機制註定了這些動態代理類們無法實現對class的動態代理，原因是多繼承在Java中本質上就行不通。有很多條理由，人們可以否定對 class代理的必要性，但是同樣有一些理由，相信支持class動態代理會更美好。接口和類的劃分，本就不是很明顯，只是到了Java中才變得如此的細化。如果只從方法的聲明及是否被定義來考量，有一種兩者的混合體，它的名字叫抽象類。實現對抽象類的動態代理，相信也有其內在的價值。此外，還有一些歷史遺留的類，它們將因爲沒有實現任何接口而從此與動態代理永世無緣。如此種種，不得不說是一個小小的遺憾。但是，不完美並不等於不偉大，偉大是一種本質，Java動態代理就是佐例。