JSTL.带标签体的标签，方法和例子

时间：2023-03-09 18:26:46
JSTL.带标签体的标签，方法和例子

1.

实现 forEach 标签: 两个属性: items(集合类型, Collection), var(String 类型)

doTag:
遍历 items 对应的集合
 把正在遍历的对象放入到 pageContext 中, 键: var, 值: 正在遍历的对象.
把标签体的内容直接输出到页面上.

（1）首先建立一个customer类：

package com.lanqiao.javatest;

public class Customer {

    private Integer id;

    private String name;

    private int age;

    public Customer() {

        super();

        // TODO Auto-generated constructor stub

    }

    public Customer(Integer id, String name, int age) {

        super();

        this.id = id;

        this.name = name;

        this.age = age;

    }

    public Integer getId() {

        return id;

    }

    public void setId(Integer id) {

        this.id = id;

    }

    public String getName() {

        return name;

    }

    public void setName(String name) {

        this.name = name;

    }

    public int getAge() {

        return age;

    }

    public void setAge(int age) {

        this.age = age;

    }

    @Override

    public String toString() {

        return "Customer [id=" + id + ", name=" + name + ", age=" + age + "]";

    }

}

（2）ForEachTag类，继承于父类： SimpleTagSupport

package com.atguigu.javaweb;

import java.io.IOException;

import java.util.Collection;

import javax.servlet.jsp.JspException;

import javax.servlet.jsp.tagext.SimpleTagSupport;

public class ForEachTag extends SimpleTagSupport{

    private Collection<?> items;

    public void setItems(Collection<?> items) {

        this.items = items;

    }

    private String var;

    public void setVar(String var) {

        this.var = var;

    }

    @Override

    public void doTag() throws JspException, IOException {

//        * 遍历 items 对应的集合

        if(items != null){

            for(Object obj: items){

        //        * 把正在遍历的对象放入到 pageContext 中, 键: var, 值: 正在遍历的对象.

                getJspContext().setAttribute(var, obj);

                //把标签体的内容直接输出到页面上.

                getJspBody().invoke(null); 

            }

        }

    }

}

（3）在lib下建立一个mytag.tld的xml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>MyTag 1.2 core library</description>

    <display-name>MyTag core</display-name>

    <tlib-version>1.2</tlib-version>

    <short-name>lxn</short-name>

    <uri>http://lxn.com/myTag/core</uri>

    <tag>

        <name>forEach</name>

        <tag-class>com.lanqiao.javatest.ForEachTag</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>true</rtexprvalue>

        </attribute>

    </tag>

</taglib>

（4）建立一个test.jsp页面：

<%@page import="java.util.HashMap"%>

<%@page import="java.util.Map"%>

<%@page import="com.atguigu.javaweb.Customer"%>

<%@page import="java.util.ArrayList"%>

<%@page import="java.util.List"%>

<%@ page language="java" contentType="text/html; charset=UTF-8"

    pageEncoding="UTF-8"%>

<%@ taglib prefix="lxn" uri="http://lxn.com/myTag/core" %>

<!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>Insert title here</title>

</head>

<body>

    <br><br>

    <%

        List<Customer> customers = new ArrayList<Customer>();

        customers.add(new Customer(1, "AAA"));

        customers.add(new Customer(2, "BBB"));

        customers.add(new Customer(3, "CCC"));

        customers.add(new Customer(4, "DDD"));

        customers.add(new Customer(5, "EEE"));

        request.setAttribute("customers", customers);
%>

    <lxn:forEach items="${requestScope.custoemrs }" var="cust">

       -- ${cust.id } -- ${cust.name } <br>

    </lxn:forEach>

    <br><br>

    </body>

</html>

2.使用标签处理器类，其中有一些方法：

package com.atguigu.javaweb;

import java.io.IOException;

import java.io.OutputStreamWriter;

import java.io.PrintWriter;

import java.io.StringWriter;

import javax.servlet.jsp.JspException;

import javax.servlet.jsp.tagext.JspFragment;

import javax.servlet.jsp.tagext.SimpleTagSupport;

//标签处理器类

public class TestJspFragment extends SimpleTagSupport {

    @Override

    public void doTag() throws JspException, IOException {

　　　　//JspFragment，是标签体，getJspBody()是获取标签体

        JspFragment bodyContent = getJspBody();

        //JspFragment.invoke(Witer): Writer 即为标签体内容输出的字符流, 若为 null, 则

        //输出到 getJspContext().getOut(), 即输出到页面上.

        //1. 利用 StringWriter 得到标签体的内容.

        StringWriter sw = new StringWriter();

        bodyContent.invoke(sw);

        //2. 把标签体的内容都变为大写

        String content = sw.toString().toUpperCase();

        //3. 获取 JSP 页面的 out 隐含对象, 输出到页面上

        getJspContext().getOut().print(content);

    }

}

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