${} is not working on my JSP page. How can i get my ${} html tag to work again? [duplicate]

问题

So ${} did work. But the JSTL jar files that I'm using made to where the ${} doesn't work anymore. These are my JSTL jar files. jstl-1.2 (1).jar, jstl-impl-1.2.jar, jstl-standard.jar. I am following Navin tutorial on Servlet & JSP Tutorial | Full Course on youtube. He skipped JSTL jar files. I'm a junior developer trying to understand why my ${} isn't working anymore.

Question: Why did my ${} tag not work anymore?

Please be gentle. :D

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>

    <%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html >
<html>
<head>
<meta charset="ISO-8859-1">
<title>Insert title here</title>
</head>
<body>


    <c:forEach items="${students}" var="s" >
         ${s} <br>

    </c:forEach>



</body>
</html>

package com.Demo;

public class Student {
    int rollno;
    String name;


    @Override
    public String toString() {
        return "Student [rollno=" + rollno + ", name=" + name + "]";
    }


    public int getRollno() {
        return rollno;
    }


    public void setRollno(int rollno) {
        this.rollno = rollno;
    }


    public String getName() {
        return name;
    }


    public void setName(String name) {
        this.name = name;
    }


    public Student(int rollno, String name) {
        super();
        this.rollno = rollno;
        this.name = name;
    }


}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" version="4.0">
  <display-name>JSTLexample</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
</web-app>

package com.Demo;

import java.io.IOException;
import java.util.Arrays;
import java.util.List;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/DemoServlet")
public class DemoServlet extends HttpServlet{

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
    {

        //String name = "Navin";

        List <Student> studs = Arrays.asList(new Student(1, "brandon"), new Student(2, "Micheal"), new Student (3, "Charles"));

        request.setAttribute("students", studs);

        RequestDispatcher rd = request.getRequestDispatcher("display.jsp");
        rd.forward(request, response);

    }
}

回答1:

That "${}" is EL expression language syntax. You require to configure your "web application" web.xml or variant to use the JSTL (Java Standard Tag Library) .jar file in either the servers /commons/lib folder with server.xml (obviously not) or /WEB-INF/lib of your application. Then call in the names of each tag prefix you wish to use declared at the top of your JSP page. Tomcat has a few ways of achieving it.

Also your doctype should be

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

Yout .tld tag library descriptor should be in the WEB-INF folder.

define the taglib location in your web.xml file

    <jsp-config>
<taglib>
    <taglib-uri>http://java.sun.com/jstl/core</taglib-uri>
        <taglib-location>/WEB-INF/c.tld</taglib-location>
        </taglib>
    </jsp-config>

Also, your bean class should be declared in the web.xml and the page declarations to call it in the page at the top too.

Not a bad point to recheck the above if things go wrong.

The servlet you have made is a GET request servlet , they pass parameters on the URL by

?name=**valuOFthisPart&anid=**somethinHere&terminus=

if you use ${Param.student} added as student past the "?" on the url that may be usable to the EL something like this

**?students=name=**valuOFthisPart&next1=somethinHere&next2=somethinHere&terminus=

A POST servlet cannot carry parameters on the URL and is what you are trying to do by the code so is what the request.setAttribute is setting doing is for a POST request (POST requests do carry tokens).

Too, the setAttribute on request object is available by the interface of its class of which it can be done at class level by a wrapper sub class too as next

javax.servlet.ServletRequestWrapper requestWrpp = new javax.servlet.ServletRequestWrapper(request);
requestWrpp.setAttribute("students", studs);

HOWEVER, while more modern versions of web container recognise complex types such as List and Map (but probably not Student class) you may be able to use the code there by what i vaguely think i remember the clause of use of complex objects in JSP processing is and that being it is understood to be convertible to string. Student is unrecognizable to the web app parser rules, however if you wrote Student to extend ( Map int,String ) then the runtime and compiler may be able to use that set up inside as a ( Map K,V ) Actually, this cannot work because you try to do this in the servlet before JSP processing by the response [ ! unless the Student class is only a servlet support class in the classes folder. (not bean syntax class) see next paragraph ].

You are trying to use a class the way a bean operates , and a bean must be declared both in the web.xml and the page and the servlet notified too with a Student object reference from either classes or /classes/beans folder !!! If it is a bean it should be in a bean folder (if it is not a bean and only a support processing class should be in classes with the servlet packaged or not) and called by the response in the JSP parser , but properly loaded, correctly updated current instance of it in the web app user session should be used (something JSF does more easily). You can obtain current session and beans for servlet instance use by acquiring the web app context and initial instance thread to find the bean you want, its current instance and current state (requires to be a session bean unless the values are constant or it only outputs by set get processing instantly) to get its' current values. Bean classes must be declared throughout the app configuration not dissimilar to servlets but are different with rigid rules of declaration for runtime and syntax.

Final note

// the first argument should be a string NOT an int
req.setAttribute("String Object",(java.lang.Object)anObject);

//NOTE: That object must be convertible to a String from a recognizable java language core class !!!

Just a quick note about post requests, actually it can carry a query string but is considered a bizzarre action inside a Java framework.

来源：https://stackoverflow.com/questions/62079182/is-not-working-on-my-jsp-page-how-can-i-get-my-html-tag-to-work-again

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