代理模式：爲另一個對象提供一個替身或佔位符以訪問這個對象。代理模式爲另一個對象提供代表，以便控制客戶對對象的訪問，管理訪問的方式有許多種。
遠程代理管理客戶和遠程對象之間的交互。
虛擬代理控制訪問實例化開銷大的對象。
保護代理基於調用者控制對對象方法的訪問。
代理模式有許多變體，例如：緩存代理、同步代理、防火牆代理、寫入時複製代理。
Java內置的代理支持，可以根據需要動態創建代理，並將所有調用分配到所選的調用處理器（InvocationHandler）。
下面 的例子是一種保護代理
類圖：
源代碼如下：
package javaproxy;
/**
* 被代理者接口
* @author Arvon
*
*/
public interface PersonBean {
public String getName();
public String getGender();
public String getInterests();
public int getHotAndNotRating();
public void setHotAndNotRating(int rating);
public void setInterests(String interests);
public void setName(String name);
public void setGender(String gender);
}
package javaproxy;
public class PersonBeanImpl implements PersonBean {
String name;
String gender;
String interests;
int rating, ratingCount =1;
public PersonBeanImpl(String name, String gender, String interests, int rating) {
super();
this.name = name;
this.gender = gender;
this.interests = interests;
this.rating = rating;
}
@Override
public String toString() {
return "PersonBeanImpl [name=" + name + ", gender=" + gender + ", interests=" + interests + ", rating=" + getHotAndNotRating()
+ "]";
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getGender() {
return gender;
}
public void setGender(String gender) {
this.gender = gender;
}
public String getInterests() {
return interests;
}
public void setInterests(String interests) {
this.interests = interests;
}
@Override
public int getHotAndNotRating() {
return ratingCount==0?0:(rating/ratingCount);
}
public void setHotAndNotRating(int rating){
this.rating+=rating;
ratingCount++;
}
}
package javaproxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
/**
* InvocationHandler 負責處理代理類發來的請求（即代理類的調用）
* @author Administrator
*
*/
public class OwnerInvocationHandler implements InvocationHandler {
PersonBean person;
public OwnerInvocationHandler(PersonBean person) {
super();
this.person = person;
}
/**
* Parameters:proxy - the proxy instance that the method was invoked
* onmethod - the Method instance corresponding to the interface method
* invoked on the proxy instance. The declaring class of the Method object
* will be the interface that the method was declared in, which may be a
* superinterface of the proxy interface that the proxy class inherits the
* method through.args - an array of objects containing the values of the
* arguments passed in the method invocation on the proxy instance, or null
* if interface method takes no arguments. Arguments of primitive types are
* wrapped in instances of the appropriate primitive wrapper class, such as
* java.lang.Integer or java.lang.Boolean.Returns:the value to return from
* the method invocation on the proxy instance. If the declared return type
* of the interface method is a primitive type, then the value returned by
* this method must be an instance of the corresponding primitive wrapper
* class; otherwise, it must be a type assignable to the declared return
* type. If the value returned by this method is null and the interface
* method's return type is primitive, then a NullPointerException will be
* thrown by the method invocation on the proxy instance. If the value
* returned by this method is otherwise not compatible with the interface
* method's declared return type as described above, a ClassCastException
* will be thrown by the method invocation on the proxy instance.
*/
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws IllegalAccessException {
try {
if(method.getName().startsWith("get"))
return method.invoke(person, args); //方法的執行者 person
else if(method.getName().equals("setHotAndNotRating"))
throw new IllegalAccessException();
else if(method.getName().startsWith("set"))
return method.invoke(person, args); //方法的執行者 person
} catch (Exception e) {
// e.printStackTrace();
System.out.println("cannot set rating by owner proxy!");
}
return null;//如果調用其它的方法一律不予處理
}
}
package javaproxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class NonOwnerInvocationHandler implements InvocationHandler{
PersonBean person;
public NonOwnerInvocationHandler(PersonBean person) {
super();
this.person = person;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if(method.getName().startsWith("get"))
return method.invoke(person, args); //方法的執行者 person
else if(method.getName().equals("setHotAndNotRating"))
return method.invoke(person, args);
else if(method.getName().startsWith("set"))//他人不能設置自己的信息
throw new IllegalAccessException();
} catch (Exception e) {
System.out.println("cannot set infos from nonowner proxy...");
}
return null;//如果調用其它的方法一律不予處理
}
}
package javaproxy;
import java.lang.reflect.Proxy;
/**
* 動態產生代理類
* @author Administrator
*
*/
public class ProxyHelper {
/**
* Returns an instance of a proxy class for the specified interfaces that
* dispatches method invocations to the specified invocation handler.
* @param person
* @return
*/
public static PersonBean getOwnerProxy(PersonBean person) {
return (PersonBean) Proxy.newProxyInstance(person.getClass().getClassLoader(), person.getClass().getInterfaces(),
new OwnerInvocationHandler(person));
}
public static PersonBean getNonOwnerProxy(PersonBean person) {
return (PersonBean) Proxy.newProxyInstance(person.getClass().getClassLoader(), person.getClass().getInterfaces(),
new NonOwnerInvocationHandler(person));
}
}
package javaproxy;
/**
* 測試類
* @author Administrator
*
*/
public class ProxyTest {
public static void main(String[] args) {
PersonBean person = new PersonBeanImpl("Jay", "F", "Java", 7);
System.out.println(person);
//動態地產生代理類ownerProxy （代理類ownerProxy實現了PersonBean接口）
PersonBean ownerProxy = ProxyHelper.getOwnerProxy(person);
System.out.println("Name is "+ownerProxy.getName());
//代理類的方法setInterests被調用 代理ownerProxy會把這個調用轉發給OwnerInvocationHandler
//ownerProxy 被實例化的時候指定的調用處理器是OwnerInvocationHandler
// 接着 OwnerInvocationHandler 調用它自己的invoke方法
//OwnerInvocationHandler 可能會轉發給被代理者PersonBean person 處理，如 return method.invoke(person, args); //方法的執行者 person
// 也可能做其他的處理 如拋出異常
ownerProxy.setInterests("python");
System.out.println(person);
try {
ownerProxy.setHotAndNotRating(8);
} catch (Exception e) {
}
PersonBean nonOwnerProxy = ProxyHelper.getNonOwnerProxy(person);
System.out.println("Name is "+nonOwnerProxy.getName());
try {
nonOwnerProxy.setHotAndNotRating(9);
nonOwnerProxy.setInterests("python");
} catch (Exception e) {
}
System.out.println(person);
}
}
程序的輸出：
PersonBeanImpl [name=Jay, gender=F, interests=Java, rating=7]
Name is Jay
PersonBeanImpl [name=Jay, gender=F, interests=python, rating=7]
cannot set rating by owner proxy!
Name is Jay
cannot set infos from nonowner proxy...
PersonBeanImpl [name=Jay, gender=F, interests=python, rating=8]