JSP自定义标签遍历List、Set、Map、数组 (ct:forEach)

比眉伴天荒 2022-05-09 08:54 487阅读 0赞

问题描述 ：  
jsp 的pageContext域中存在User对象的users列表，想在jsp文件中遍历users. 或者Set、Map、及数组

<%
        class User{
            private String name;
            private String email;
            public User(String name,String email){
                this.name=name;
                this.email=email;
            }
        }
        List<User> users=new ArrayList<>();
        users.add(new User("user1","123@qq.com"));
        users.add(new User("user2","1234@qq.com"));
        users.add(new User("user3","12345@qq.com"));
        users.add(new User("user1","123456@qq.com"));
        users.add(new User("user5","1234567@qq.com"));
        
        pageContext.setAttribute("users",users);
    %>

遍历方式

<ct:forEach items="${users}" , var="user">
    	${user.name}---${user.email}<br/>
    </ct:forEach>

WEB页面输出

user1---123@qq.com
    user2---1234@qq.com
    user3---12345@qq.com
    user4---123456@qq.com
    user5---1234567@qq.com

1、实现类 ForEach.java

package app.tag;
    
    package app.tag;
    
    import javax.servlet.jsp.JspException;
    import javax.servlet.jsp.tagext.SimpleTagSupport;
    import java.io.IOException;
    import java.lang.reflect.Array;
    import java.util.*;
    
    public class ForEachList extends SimpleTagSupport {
        private Object items;
        private String var;
        public void setItems(List items) {
            this.items = items;
        }
        public void setVar(String var) {
            this.var = var;
        }
        public Collection getColl(){
            if(items instanceof List)
                return (List)items;
            else if(items instanceof Set)
                return (Set)items;
            else if(items instanceof Map)
                return ((Map)items).entrySet();
            else if(items.getClass().isArray()){
                List coll=new ArrayList();
                for(int i=0;i< Array.getLength(items);i++){
                    coll.add(Array.get(items,i));
                }
                return coll;
            }
            return null;
        }
        @Override
        public void doTag() throws JspException, IOException{
            for (Object obj:
                 getColl()) {
                this.getJspContext().setAttribute(var,obj); //将obj保存在pageScope中，以便EL表示式读取数据
                this.getJspBody().invoke(null);//将标签体写入输出流
            }
        }
    }

2、注册标签 ct.tld

<?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">
        <tlib-version>1.0</tlib-version>
        <short-name>ct</short-name>
        <uri>http://www.cooooode.com/taglib</uri>
        <tag>
            <name>forEach</name>
            <tag-class>app.tag.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>

3、tablib指令

<%@ taglib uri="http://www.cooooode.com/taglib" prefix="ct"%>