java 中使用自定义标签模仿jstl的 ForEach（）

最新推荐文章于 2021-02-21 10:11:01 发布

转载最新推荐文章于 2021-02-21 10:11:01 发布 · 511 阅读

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本文介绍了一种在JSP中实现自定义迭代标签的方法，该标签能够遍历集合、映射和数组等多种数据结构，并通过反射机制简化了对数组的操作。文章提供了完整的代码示例，包括标签的实现类、tld文件配置以及JSP页面调用。

1. [代码]使用到了数组反射的一些知识     
//1：EachTag

package cn.itcast.apsliyuan.tag;

import java.lang.reflect.Array;

import java.util.ArrayList;

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.TagSupport;

public class EachTag extends TagSupport {

/**

* 反序列化

*/

private static final long serialVersionUID = 7239081890975877992L;

private String var;

private Iterator<Object> it;

public void setVar(String var) {

this.var = var;

}

public void setItems(Object object) {

if (object instanceof Collection) {

it = ((Collection) object).iterator();

} else if (object instanceof Map) {

it = ((Map) object).entrySet().iterator();

} else if (object.getClass().isArray()) {//数组反射，虽然影响了效率，但是比每种类型相比是简单多了

List<Object> list = new ArrayList<Object>();

int len = Array.getLength(object);

for (int i = 0; i < len; i++) {

list.add(Array.get(object, i));

}

it = list.iterator();

}

}

// 只会执行一次

@Override

public int doStartTag() throws JspException {

if (it.hasNext()) {

pageContext.setAttribute(var, it.next());

return EVAL_BODY_INCLUDE;

} else {

return SKIP_BODY;

}

}

@Override

public int doAfterBody() throws JspException {

// TODO Auto-generated method stub

if (it.hasNext()) {

pageContext.setAttribute(var, it.next());

return EVAL_BODY_AGAIN;// 再次执行body部分

} else {

return SKIP_BODY;

}

}

@Override

public int doEndTag() throws JspException {

pageContext.removeAttribute(var);

return EVAL_PAGE;

}

}
2. [代码][Java]代码
//2：each.tld

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

<taglib xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee

    http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"

    version="2.1">

    <!-- 定义版本 -->

    <tlib-version>1.0</tlib-version>

    <!-- 定义名字 -->

    <short-name>apsliyuan</short-name>

    <!-- 定义uri -->

    <uri>http://my.oschina.net/apsliyuan</uri>

    <tag>

        <name>forEach</name>

        <tag-class>cn.itcast.apsliyuan.tag.EachTag</tag-class>

        <body-content>JSP</body-content>

        <attribute>

            <name>var</name>

            <required>true</required>

            <rtexprvalue>false</rtexprvalue>

            <type>java.lang.String</type>

        </attribute>

        <attribute>

            <name>items</name>

            <required>true</required>

            <rtexprvalue>true</rtexprvalue>

        </attribute>

    </tag>

</taglib>

3. [代码][Java]代码 
//3:jsp代码
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8" import="java.util.*"%>
<%@taglib uri="http://my.oschina.net/aps" prefix="aps"%>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@taglib uri="http://my.oschina.net/apsliyuan" prefix="apsliyuan"%>
<!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>index.jsp</title>
</head>
<body>
 
    <%
        List str = new ArrayList();
        str.add("liyuan");
        str.add("liyuan02");
        str.add("liyuan03");
        str.add("liyuan04");
        pageContext.setAttribute("strs",str);
         
        Map map=new HashMap();
        map.put("01",11);
        map.put("02",12);
        map.put("03",13);
        map.put("04",14);
        pageContext.setAttribute("map",map);
         
        String[] strArray={"12","23","34"};
        pageContext.setAttribute("strArray",strArray);
    %>
 
    <apsliyuan:forEach items="${strs}" var="strss">
        ${strss}<br/>
    </apsliyuan:forEach>
    <hr/>
    <apsliyuan:forEach items="${map}" var="maps">
        ${maps}<br/>
    </apsliyuan:forEach>
    <hr/>
    <apsliyuan:forEach items="${strArray}" var="strArrays">
        ${strArrays} <br/>
    </apsliyuan:forEach>
</body>
</html>