EL & JSTL: JSTL summary

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Catalog ：

（1）set Label use 1

（2）set Labels and EL Expression of

（3）if label

（4）choose label It is equivalent to multiple judgment statements

（5）forEach loop

 （6）forEach Traverse List aggregate

 （7）forEach Traverse Map aggregate

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JSTL: Covered with labels java Tool class ,java Commands become labels , To meet the needs , Not allow java Command falls on jsp On

in order to jsp Can't see in the page java Code , Replace the original in jsp On the page <% java Code %>, A label is equivalent to a java command

Cause you ：

Java Orders cannot fall on jsp The above to , So the java Put a label on the command , Package into a label to use

The other is EL Expression function is too weak

 

 

establish 04Model Import response jar package ：

 

 

（1）set Label use 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" %>
<%-- Need to add jstl The label of --%>
<%@ 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=" Shanghai "></c:set>

 Student number ：${applicationScope.sid}<br>
 Student name ：${sessionScope.sname}<br>
 Age of trainees ：${requestScope.sage}<br>
 Student's native place ：${pageScope.home}

（2）set Labels and EL Expression of

 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" %>
<%-- Need to add jstl The label of --%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<%-- Set the age of the user this year --%>
<c:set scope="request" var="age" value="20"></c:set>

<%-- Set the user's age after two years 
        Execution order , restrictive EL expression , Re execution JSTL,EL The expression starts with request Client reads data +2,
        Then it is not written to the response body , It's about value Properties of the , And then execute set label ,
        hold request scoped 20 Updated to 22
--%>
<c:set scope="request" var="age" value="${requestScope.age+2}"></c:set>

 Two years later ：${age}

（3）if label

  

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" %>
<%-- Need to add jstl The label of --%>
<%@ 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"> welcome </font>
</c:if>

<c:if test="${sessionScope.age lt 18}">
    <font color="red"> Come back in two years </font>
</c:if>

Reported when running the project 500：

 

JSTL Label library Report 500 Error resolution

Cause analysis
Tomcat The lack of jstl-api-1.2.jar and standard-1.1.2.jar And these two jar package .

Solution
take jstl-api-1.2.jar and standard-1.1.2.jar These two bags are put into D:\Program\apache-tomcat-8.0.53\lib Under the table of contents

Normal operation after joining ： 

 

（4）choose label It is equivalent to multiple judgment statements

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" %>
<%-- Need to add jstl The label of --%>
<%@ 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="${salary ge 50000}"> High wages </c:when>
    <c:when test="${salary ge 20000}"> Normal wage </c:when>
    <c:when test="${salary ge 10000}"> Low wages </c:when>
    <c:otherwise> Abnormal wages </c:otherwise>
</c:choose>

 

 （5）forEach loop

 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" %>
<%-- Need to add jstl The label of --%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<select>
    <%
        for (int i=1;i<=5;i++){
    %>
           <option> The first <%=i%> page </option>
    <%
        }
    %>
</select>

<hr>
JSTL Syntax replace the top java sentence <br>
<select>
    <c:forEach var="i" begin="1" end="5" step="1">
        <option> The first ${pageScope.i} page </option>
    </c:forEach>
</select>

 

 （6）forEach Traverse List aggregate

 

 Student class ：

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;
    }
}

establish 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);
    }
}

  stay web.xml: Specify the access method ：

<?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 + "/";
%>
<%-- Need to add jstl The label of --%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<html>
<head>
    <title>Title</title>
    <base href="<%=basePath%>">
</head>
<body>
  java Command to get data <br>
  <table border="2">
      <tr>
          <td> Student number </td>
          <td> Student name </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>
 adopt JSTL Tag to get the data display <br>
    <table border="2">
        <tr>
            <td> Student number </td>
            <td> Student name </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 Traverse Map aggregate

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

        //Map aggregate 
        Map map=new HashMap();
        map.put(" Class one ",stu);
        map.put(" Class two ",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 + "/";
%>
<%-- Need to add jstl The label of --%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<html>
<head>
    <title>Title</title>
    <base href="<%=basePath%>">
</head>
<body>
  java Command to get data <br>
  <table border="2">
      <tr>
          <td> Student number </td>
          <td> Student name </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>
 adopt JSTL Tag to get the data display <br>
    <table border="2">
        <tr>
            <td> Student number </td>
            <td> Student name </td>
        </tr>

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

<hr>
<h1> Traverse map aggregate </h1>
    <table border="2">
        <tr>
            <td> Class name </td>
            <td> Student number </td>
            <td> Student name </td>
        </tr>

        <%-- Traverse Map Set every time from Map What you get from the set is a 【 Key value pair 】
             And then 【 Key value pair 】 Give it to the loop variable 
             Loop variable .key get 【 Key value pair 】 Keyword name in   Class name 
             Loop variable .value  get 【 Key value pair 】 The content in  stu object 
        --%>
        <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>