自定义标签实现遍历集合

来源：互联网发布：java运行jar包中的类编辑：程序博客网时间：2024/06/05 04:01

比较好的方法是，将所有集合化为Collection类型的集合，然后只需处理Collection集合便可。

1.将Map化为Collection，Map的每一个元素即Entry放进Collection即可。

2.将类数组化为Collection，调用Arrays.asList(objs);方法。

3.若此对象为数组类（即items.isArray()返回true），将每一个数组元素放入Collection，比较巧妙的是使用Array.getLength(items);获得items数组长度，Array.get(items, i); 便可以获得items 数组在 i 处的对象值。然后将每个对象加入Collection中。

注意：int[]a ; 与Integer []a; 是完全不同的,前者归于第三类，后者归于第二类。

最后只需要处理Collection便可

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

这个方法也十分巧妙，将传入的var变量设为索引值，将Collection的对象作为结果，放进pageContext的属性中，便于在jsp用$获取到，每一次遍历，都会覆盖前一次的属性值。

package com.example;import java.io.IOException;import java.lang.reflect.Array;import java.util.ArrayList;import java.util.Arrays;import java.util.Collection;import java.util.Iterator;import java.util.List;import java.util.Map;import javax.servlet.jsp.JspException;import javax.servlet.jsp.tagext.SimpleTagSupport;public class ForEach extends SimpleTagSupport {private Object items;private String var;private Collection collection;public Object getItems() {return items;}public void setItems(Object items) {this.items = items;}public String getVar() {return var;}public void setVar(String var) {this.var = var;}@Overridepublic void doTag() throws JspException, IOException {if(items instanceof Map){Map map=(Map) items;collection=map.entrySet();}else if(items instanceof Collection){collection=(Collection) items;}else if(items instanceof Object[]){Object[]objs=(Object[]) items;collection=Arrays.asList(objs);}else if(items.getClass().isArray()){int length=Array.getLength(items);collection=new ArrayList();for(int i=0;i<length;++i){collection.add(Array.get(items, i));}}Iterator it=collection.iterator();while(it.hasNext()){Object ob=it.next();this.getJspContext().setAttribute(var, ob);getJspBody().invoke(null);}}}

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE taglib        PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN"        "http://java.sun.com/j2ee/dtd/web-jsptaglibrary_1_2.dtd"><taglib>  <tlib-version>1.0</tlib-version>  <jsp-version>1.2</jsp-version>  <short-name>debug</short-name>  <uri>http://www.forEach.com</uri>   <tag>    <name>forEach</name>    <tag-class>com.example.ForEach</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>

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%@taglib uri="http://www.forEach.com"  prefix="wan" %><%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 'ForEach.jsp' starting page</title>    <meta http-equiv="pragma" content="no-cache"><meta http-equiv="cache-control" content="no-cache"><meta http-equiv="expires" content="0">    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"><meta http-equiv="description" content="This is my page"><!--<link rel="stylesheet" type="text/css" href="styles.css">-->  </head>    <body>   <%   Map map=new HashMap();   map.put("aaa", "111"); map.put("bbb", "222");    request.setAttribute("map", map);  Integer numbers[]={1,2,3}; request.setAttribute("numbers", numbers);  List list=new ArrayList(); list.add(11);    list.add(22); request.setAttribute("list", list);  int nums[]={4,5,6}; request.setAttribute("nums", nums);    %>   -----------------map------------------------<br/>   <wan:forEach var="entry" items="${map}">    ${entry.key}:${entry.value}   </wan:forEach>   <br/>   ----------------numbers----------------------<br/>   <wan:forEach var="index" items="${numbers}">    ${index}   </wan:forEach>   <br/>   ----------------list-------------------------------<br/>   <wan:forEach var="index" items="${list}">    ${index}   </wan:forEach>   <br/>   ----------------nums-----------------------------------<br/>   <wan:forEach var="index" items="${nums}">    ${index}   </wan:forEach>  </body></html>

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