开发一个foreach标签

迈不过友情╰ 2022-08-13 00:45 198阅读 0赞

此标签和sun公司的一样可以迭代Map,List,数组

jsp文件为

<%@page import="java.util.*"%>
 <%@ page language="java" contentType="text/html; charset=UTF-8"
 pageEncoding="UTF-8"%>
 <%@ taglib uri="/simple" prefix="c"%> 
 
 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
 <html>
 <head>
 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
 <title>foreach tag</title>
 </head>
 <body>
 	<%
 		List list = new ArrayList();
 		list.add("aaa");
 		list.add("bbb");
 		list.add("ccc");
 		request.setAttribute("list", list);
 	%>
 	<c:foreach var="str" items="${list}">
 		${str }
 	</c:foreach>
 	 ------------------------------ 
 	<%
 		Map map = new HashMap();
 		map.put("aa", "111");
 		map.put("bb", "222");
 		map.put("aa", "333");
 		request.setAttribute("map", map);
 	%>
 	<c:foreach var="entry" items="${map }">
 		${entry.key } = ${entry.value }
 	</c:foreach>
 	
 	 ------------------------------ 
 	
 	<%
 		Integer num[] = {1,2,3,4}; 
 		request.setAttribute("num", num);
 	%>
 	<c:foreach var="i" items="${num }">
 		${i }
 	</c:foreach>
 	 ------------------------------ 
 	
 	<%
 		int arr[] = {1,2,3,4}; 
 		request.setAttribute("arr", arr);
 	%>
 	<c:foreach var="i" items="${arr }">
 		${i }
 	</c:foreach>
 </body>
 </html>

标签处理器为

package cn.itcast.web.tag;
 
 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;
 
 //开发foreach标签
 public class ForeachTag extends SimpleTagSupport {
 
 	private Object items;
 	private String var;
 	
 	//单列或双列集合存储至此
 	private Collection collection;
 	
 	public void setItems(Object items) {
 		this.items = items;
 		
 		//将map转为单列集合
 		if(items instanceof Map){
 			Map map = (Map) items;
 			collection = map.entrySet();
 		}
 		
 		if(items instanceof Collection){
 			collection = (List) items;
 		}
 		
 		//统一对数组进行处理
 		if(items.getClass().isArray()){
 			this.collection = new ArrayList();
 			int length = Array.getLength(items);
 			for(int i=0; i<length; i++){
 				Object value = Array.get(items, i);
 				collection.add(value);
 			}
 		}
 		
 		
 		/*if(items instanceof Object[]){
 			Object[] obj = (Object[]) items;
 			collection = Arrays.asList(obj); //此方法接受一个对象
 		}*/
 		
 	}
 	
 	public void setVar(String var) {
 		this.var = var;
 	}
 	
 	@Override
 	public void doTag() throws JspException, IOException {
 		
 		Iterator it = collection.iterator();
 		while(it.hasNext()){
 			Object obj = it.next();
 			//对象存在域中
 			this.getJspContext().setAttribute(var, obj);
 			//标签执行
 			this.getJspBody().invoke(null);
 		}
 	}
 	
 	
 }

要记得在tld中对标签描述

<tag>
 <name>foreach</name>
 		<tag-class>cn.itcast.web.tag.ForeachTag</tag-class>
 		<body-content>scriptless</body-content>
 		
 		<attribute>
 			<name>var</name>
 			<required>true</required>
 			<rtexprvalue>true</rtexprvalue>
 		</attribute>
 		
 		<attribute>
 			<name>items</name>
 			<required>true</required>
 			<rtexprvalue>true</rtexprvalue>
 		</attribute>
 </tag>