该例注意实现对不同集合的迭代

看例子

第一步：编写标签处理器类
public class ForeachTag2 extends SimpleTagSupport {

private Object items;
private String var;
//把所有的集合和数组转换成单列集合
private Collection collection;


public void setVar(String var) {
this.var = var;
}

public void setCollection(Collection collection) {
this.collection = collection;
}

public void setItems(Object items) {
this.items = items;

if(items==null)
collection=null;
//双列集合
if(items instanceof Collection)
{
collection=(Collection) items;
}
//单列集合
if(items instanceof Map){
Map map=(Map)items;
collection=map.entrySet();
}
//数组 不能用instanceof 因为如果是基本数据类型就会报错
if(items.getClass().isArray()){
this.collection=new ArrayList();
int len=Array.getLength(items);
for(int i=0;i<len;i++)
this.collection.add(Array.get(items, i));
}

}

public void doTag() throws JspException, IOException {

if( collection==null) return;
Iterator it=collection.iterator();
while(it.hasNext()){
Object value=it.next();
this.getJspContext().setAttribute(var, value);
this.getJspBody().invoke(null);
}
}
}

第二步：编写标签描述文件


<?xml version="1.0" encoding="UTF-8" ?>

<taglib xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"
version="2.0">
<description>A tag library exercising SimpleTag handlers.</description>
<tlib-version>1.0</tlib-version>
<short-name>SimpleTagLibrary</short-name>
<uri>http://www.liyong.foreach1tag</uri>
<tag>
<tag>
<name>foreach2</name>
<tag-class>com.liyong.foreach.ForeachTag2</tag-class>
<body-content>scriptless</body-content>
<attribute>
<name>items</name>
<required>true</required>
<rtexprvalue>true</rtexprvalue>
</attribute>
<attribute>
<name>var</name>
<required>true</required>
<rtexprvalue>false</rtexprvalue>
</attribute>
</tag>

</taglib>

第三步：编写jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%@taglib uri="http://www.liyong.foreach1tag" prefix="foreach"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">

<title>My JSP 'foreach1.jsp' starting page</title>

<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->

</head>

<body> <br>

<foreach:foreach2 items="${list}" var="str">
${str }<br/>
</foreach:foreach2>

______map_____<br>
<%
Map map=new HashMap();
map.put("aa","111");
map.put("bb","222");
map.put("cc","333");
request.setAttribute("map",map);
%>
<foreach:foreach2 items="${map}" var="entry">
${entry.key } :${entry.value }<br/>
</foreach:foreach2>


______Integer_____<br>
<%
Integer num[]={new Integer(1),new Integer(2),new Integer(3),new Integer(4)};
request.setAttribute("num",num);
%>
<foreach:foreach2 items="${num}" var="i">
${i }<br/>
</foreach:foreach2>

______String_____<br>
<%
String strs[]={"sss","mmm"};
request.setAttribute("strs",strs);
%>
<foreach:foreach2 items="${strs}" var="str">
${str }<br/>
</foreach:foreach2>
______int_____<br>
<%
int num2[]={11,22,33,44};
request.setAttribute("num2",num2);
%>
<foreach:foreach2 items="${num2}" var="i">
${i }
</foreach:foreach2>
</body>
</html>

第四步：测试...