EL&JSTL：JSTL总结

目录：

（1）set标签使用1

（2）set标签和EL的表达式联合使用

（3）if标签

（4）choose标签相当于多个判断语句

（5）forEach循环

（6）forEach遍历List集合

（7）forEach遍历Map集合

JSTL:披着标签的java工具类，java命令变成标签的形式，来满足需求，不让java命令落在jsp上

为了jsp页面中看不见java代码，替换原来在jsp页面中的<% java代码%>，一个标签相当于一个java命令

产生你原因：

Java命令不能落在jsp上面去，所以把java命令披上一个标签，封装成一个标签使用

另一个是EL表达式功能过于弱小

创建04Model导入响应的jar包：

（1）set标签使用1

index.jsp:

<%--
  Created by IntelliJ IDEA.
  User: DELL
  Date: 2022/7/20
  Time: 7:11
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%--需要加jstl的标签--%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<c:set scope="application" var="sid" value="10"></c:set>
<c:set scope="session" var="sname" value="mike"></c:set>
<c:set scope="request" var="sage" value="26"></c:set>
<c:set scope="page" var="home" value="上海"></c:set>

学员编号：${applicationScope.sid}<br>
学员姓名：${sessionScope.sname}<br>
学员年龄：${requestScope.sage}<br>
学员籍贯：${pageScope.home}

（2）set标签和EL的表达式联合使用

index_2.jsp:

<%--
  Created by IntelliJ IDEA.
  User: DELL
  Date: 2022/7/20
  Time: 8:19
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%--需要加jstl的标签--%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<%--设置用户今年的年龄--%>
<c:set scope="request" var="age" value="20"></c:set>

<%--设置用户两年之后的年龄
       执行顺序，限制性EL表达式，再执行JSTL，EL表达式先从request客户端读取数据+2，
       然后并没有写到响应体中，而是写到value属性中，然后再执行set标签，
       把request作用域中的20更新为22
--%>
<c:set scope="request" var="age" value="${requestScope.age+2}"></c:set>

两年之后的年龄：${age}

（3）if标签

index_3.jsp:

<%--
  Created by IntelliJ IDEA.
  User: DELL
  Date: 2022/7/20
  Time: 8:19
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%--需要加jstl的标签--%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<c:set scope="request" var="age" value="23"></c:set>


<c:if test="${sessionScope.age ge 18}">
    <font color="red">欢迎光临</font>
</c:if>

<c:if test="${sessionScope.age lt 18}">
    <font color="red">过两年再来</font>
</c:if>

在运行项目的时候报了500：

JSTL标签库报500错误解决方案

原因分析
Tomcat缺少jstl-api-1.2.jar和standard-1.1.2.jar和这两个jar包。

解决方案
将jstl-api-1.2.jar和standard-1.1.2.jar这两个包放入到D:\Program\apache-tomcat-8.0.53\lib目录下

加入之后正常运行：

（4）choose标签相当于多个判断语句

index_4.jsp:

<%--
  Created by IntelliJ IDEA.
  User: DELL
  Date: 2022/7/20
  Time: 9:18
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%--需要加jstl的标签--%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<c:set scope="page" var="salary" value="15000"></c:set>
<c:choose>
    <c:when test="${salary ge 50000}">高工资</c:when>
    <c:when test="${salary ge 20000}">正常工资</c:when>
    <c:when test="${salary ge 10000}">低工资</c:when>
    <c:otherwise>不正常工资</c:otherwise>
</c:choose>

（5）forEach循环

index_5.jsp:

<%--
  Created by IntelliJ IDEA.
  User: DELL
  Date: 2022/7/20
  Time: 9:42
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%--需要加jstl的标签--%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<select>
    <%
        for (int i=1;i<=5;i++){
    %>
           <option>第<%=i%>页</option>
    <%
        }
    %>
</select>

<hr>
JSTL语法替换上方的java语句<br>
<select>
    <c:forEach var="i" begin="1" end="5" step="1">
        <option>第${pageScope.i}页</option>
    </c:forEach>
</select>

（6）forEach遍历List集合

Student类：

package com.bjpowernode.model;

public class Student {
    private Integer sid;
    private String sname;

    public Student(Integer sid, String sname) {
        this.sid = sid;
        this.sname = sname;
    }

    public Integer getSid() {
        return sid;
    }

    public void setSid(Integer sid) {
        this.sid = sid;
    }

    public String getSname() {
        return sname;
    }

    public void setSname(String sname) {
        this.sname = sname;
    }
}

创建Servlet：SixServlet：

package com.bjpowernode.controller;

import com.bjpowernode.model.Student;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

public class SixServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Student stu=new Student(10,"mike");
        Student stu2=new Student(20,"allen");

        List list=new ArrayList();
        list.add(stu);
        list.add(stu2);

        request.setAttribute("key",list);
        request.getRequestDispatcher("/index_6.jsp").forward(request,response);
    }
}

在web.xml:中指定访问方式：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    
    <servlet>
        <servlet-name>SixServlet</servlet-name>
        <servlet-class>com.bjpowernode.controller.SixServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>SixServlet</servlet-name>
        <url-pattern>/six.do</url-pattern>
    </servlet-mapping>
    
</web-app>

index_6.jsp:

<%@ page import="java.util.List" %>
<%@ page import="com.bjpowernode.model.Student" %><%--
  Created by IntelliJ IDEA.
  User: DELL
  Date: 2022/7/20
  Time: 10:11
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%
    String path = request.getContextPath();
    String basePath = request.getScheme() + "://" + request.getServerName() + ":" + request.getServerPort() + path + "/";
%>
<%--需要加jstl的标签--%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<html>
<head>
    <title>Title</title>
    <base href="<%=basePath%>">
</head>
<body>
  java命令获取数据<br>
  <table border="2">
      <tr>
          <td>学员编号</td>
          <td>学员姓名</td>
      </tr>
      <%
          List<Student> stuList=(List)request.getAttribute("key");
          for(Student stu:stuList){
      %>
           <tr>
               <td><%=stu.getSid()%></td>
               <td><%=stu.getSname()%></td>
           </tr>
      <%
          }
      %>
  </table>
<hr>
通过JSTL标签来获取数据显示<br>
    <table border="2">
        <tr>
            <td>学员编号</td>
            <td>学员姓名</td>
        </tr>

        <c:forEach items="${requestScope.key}" var="stu">
            <tr>
                <td>${stu.sid}</td>
                <td>${stu.sname}</td>
            </tr>
        </c:forEach>
    </table>
</body>
</html>

（7）forEach遍历Map集合

控制器：SixServlet:

package com.bjpowernode.controller;

import com.bjpowernode.model.Student;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class SixServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Student stu=new Student(10,"mike");
        Student stu2=new Student(20,"allen");

        //List集合
        List list=new ArrayList();
        list.add(stu);
        list.add(stu2);

        //Map集合
        Map map=new HashMap();
        map.put("一班",stu);
        map.put("二班",stu2);
        request.setAttribute("mapKey",map);

        request.setAttribute("key",list);
        request.getRequestDispatcher("/index_6.jsp").forward(request,response);
    }
}

index_6.jsp:

<%@ page import="java.util.List" %>
<%@ page import="com.bjpowernode.model.Student" %><%--
  Created by IntelliJ IDEA.
  User: DELL
  Date: 2022/7/20
  Time: 10:11
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%
    String path = request.getContextPath();
    String basePath = request.getScheme() + "://" + request.getServerName() + ":" + request.getServerPort() + path + "/";
%>
<%--需要加jstl的标签--%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<html>
<head>
    <title>Title</title>
    <base href="<%=basePath%>">
</head>
<body>
  java命令获取数据<br>
  <table border="2">
      <tr>
          <td>学员编号</td>
          <td>学员姓名</td>
      </tr>
      <%
          List<Student> stuList=(List)request.getAttribute("key");
          for(Student stu:stuList){
      %>
           <tr>
               <td><%=stu.getSid()%></td>
               <td><%=stu.getSname()%></td>
           </tr>
      <%
          }
      %>
  </table>
<hr>
通过JSTL标签来获取数据显示<br>
    <table border="2">
        <tr>
            <td>学员编号</td>
            <td>学员姓名</td>
        </tr>

        <c:forEach items="${requestScope.key}" var="stu">
            <tr>
                <td>${stu.sid}</td>
                <td>${stu.sname}</td>
            </tr>
        </c:forEach>
    </table>

<hr>
<h1>遍历map集合</h1>
    <table border="2">
        <tr>
            <td>班级名称</td>
            <td>学员编号</td>
            <td>学员名称</td>
        </tr>

        <%--遍历Map集合每次从Map集合中得到的是一个【键值对】
            再把【键值对】交给循环变量
            循环变量.key获得【键值对】中关键字名字 班级名称
            循环变量.value 获得【键值对】中的内容 stu对象
        --%>
        <c:forEach items="${mapKey}" var="key_Value">
            <tr>
                <td>${key_Value.key}</td>
                <td>${key_Value.value.sid}</td>
                <td>${key_Value.value.sname}</td>
            </tr>
        </c:forEach>
    </table>
</body>
</html>

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