由于迭代子模式应用广泛，文章在此不再赘述模式的具体内容了，我使用具体的例子说明如何使用AspectJ来完成模式所述的功能。此例子参考了<<Java与模式>>书中关于迭代子模式的示例。

图1 ——例子系统UML图

例子系统首先定义一个抽象的购物筐类Purchase，以便给出所有的具体购物筐的行为和需要实现的方法。购物筐PurchaseOfCopA和PurchaseOfCopB继承自Purchase，它们分别创建一个前向迭代ForwardIterator和一个后向迭代子BackawrdIterator用于元素的遍历。系统包含一个抽象方面IteratorAspect,它使用Inter-type声明了Purchase类的方法createIterator用来创建迭代子以及适配器类PurchaseIterator，它实现接口java.util.Iterator，同时还定义了一个抽象的pointcut iterator供子方面使用。子方面ForwardIteratorAspect和BackwardIteratorAspect分别定义各自的iterator pointcut捕捉对应的Purchase子类的createIterator方法调用并提供Advice around实现createIterator方法返回对应类别的迭代子。最重要的是这两个子方面中的内部类ForwardIterator和BackwardIterator,它们实现了前向迭代和后向迭代的逻辑。

抽象类Purchase.java

import java.util.*;

public abstract class Purchase {

private Collection elements=new ArrayList();

public void append(Object obj){//添家元素

elements.add(obj);

}

public void remove(Object obj){//删除元素

elements.remove(obj);

}

public Object currentItem(int index){//当前元素

return ((ArrayList)elements).get(index);

}

public int count(){//元素总数

return elements.size();

}

}

具体类PurchaseOfCopA和PurchaseOfCopB

public class PurchaseOfCopA {}

public class PurchaseOfCopB{}

适配器类PurchaseIterator.java

public class PurchaseIterator {}

抽象方面IteratorAspect.java

import java.util.Iterator;

public abstract aspect IteratorAspect {

public Iterator Purchase.createIterator(){ return null; }

abstract pointcut iterator(Purchase purchase);

//PurchaseIterator实现Iterator,作为适配器类，供子方面使用

declare parents : PurchaseIterator implements Iterator;

int PurchaseIterator.current=0;

Purchase PurchaseIterator.purchase;

//Iterator接口方法的缺省实现

public boolean PurchaseIterator.hasNext(){ return false;}

public Object PurchaseIterator.next(){ return null;}

public void PurchaseIterator.remove(){}

public void PurchaseIterator.append(Object obj){}

}

具体方面ForwardIteratorAspect.java

import java.util.Iterator;

public aspect ForwardIteratorAspect extends IteratorAspect{

declare parents : PurchaseOfCopA extends Purchase;

pointcut iterator(Purchase purchase) : target(purchase)

&& call(Iterator createIterator())

&& if(purchase instanceof PurchaseOfCopA); //捕捉前向迭代子的创建方法调用

Iterator around(Purchase purchase) : iterator(purchase){//创建方法执行逻辑，构造前向迭代器

return new ForwardIterator(purchase);

}

private static class ForwardIterator extends PurchaseIterator{//前向迭代子内部类

public ForwardIterator(Purchase purchase){

super();

this.purchase=purchase;

current=0; //当前游标为第一个元素

}

public boolean hasNext(){

return (current<=purchase.count()-1);

}

public Object next(){

if(!hasNext()){

throw new ArrayIndexOutOfBoundsException("Iterator out of Bounds");

}

return purchase.currentItem(current++);

}

}

}

具体方面BackwardIteratorAspect.java

import java.util.Iterator;

public aspect BackwardIteratorAspect extends IteratorAspect{

declare parents : PurchaseOfCopB extends Purchase;

pointcut iterator(Purchase purchase) : target(purchase)

&& call(Iterator createIterator())

&& if(purchase instanceof PurchaseOfCopB);//捕捉后向迭代子的创建方法调用

Iterator around(Purchase purchase) : iterator(purchase){//创建方法执行逻辑，构造后向迭代器

return new BackwardIterator(purchase);

}

private static class BackardIterator extends PurchaseIterator{//后向迭代子内部类

public BackwardIterator(Purchase purchase){

super();

this.purchase=purchase;

current=purchase.count()-1;//当前游标为最后元素

}

public boolean hasNext(){

return (current>=0);

}

public Object next(){

if (!hasNext()) {

throw new ArrayIndexOutOfBoundsException("Iterator out of Bounds");

} else {

return purchase.currentItem(current--);

}

}

}

}

示例代码Demo.java

public class Demo {

public static void main(String [] args)throws Exception{

Purchase purchaseA=new PurchaseOfCopA();

Purchase purchaseB=new PurchaseOfCopB();

purchaseA.append("Dish Washer");

purchaseA.append("Hair Dresser");

purchaseA.append("Microwave");

System.out.println(“Creating forward iterator for purchase A”);

printItems(purchaseA);

purchaseB.append("Hair Dresser");

purchaseB.append("Diskman");

purchaseB.append("Digital Camera");

purchaseB.append("PC");

purchaseB.append("Dish Washer");

System.out.println(“Creating backward iterator for purchase B”);

printItems(purchaseB);

}

private static void printItems(Purchase purchase){//格式化输出元素

int index=1;

if(isBackforward(purchase)){

index=purchase.count();

}

Iterator it=purchase.createIterator();

while(it.hasNext()){

System.out.println("Item :No "+index+":"+it.next().toString());

if(isBackforward(purchase)){

index--;

}

else{

index++;

}

}

}

private static boolean isBackforward(Purchase purchase){//是后向迭代吗？

if(purchase instanceof PurchaseOfCopB){

return true;

}

return false;

}

}

输出结果如下

Creating forward iterator for purchase A

Item No.1 : Dish Washer

Item No.2 : Hair Dresser

Item No.3 : Microwave

Creating backward iterator for purchase B

Item No.5 : Dish Washer

Item No.4 : PC

Item No.3 : Digital Camera

Item No.2 : Discman

Item No.1 : Hair Dresser

至此，我已经使用AspectJ实现了一个使用内禀迭代子的例子，有兴趣的读者可以尝试实现使用外禀迭代子的相同例子。另因为下个月本人要参加重要的考试，所以本系列的其余文章可能会推出的慢一点，还请各位读者见谅。同时由于许多读者不知道AspectJ语法，所以我将会在另一系列中详细介绍AspectJ的语法、配置和使用范例，希望能提高大家对AspectJ以及AOP面向方面编程的兴趣。