Lessons
Start
Start learning
Start learning now
CodeGym/Java Course/Module 3. Java Professional/Introduction to TagLib

Introduction to TagLib

Module 3. Java Professional
Level 13, Lesson 6
Available

7.1 c:if, c:forEach

Everyone liked the code using tags instead of Java code, so they decided to expand the scope. But programming is not limited to creating objects and reading their properties. You need to call methods of objects, work with the database and other services. What to do?

You just need to represent each Java statement as a tag. Was if, will be <if>, was for, will be, <for>and so on. Okay, okay, just kidding, it wasn't like that. Well, it can't be that people actually decide to do that. But no, maybe!

Programmers allowed to add any tags to the code. In principle, there is nothing to worry about - JSP is an extensible standard. But they went further and released the JSP Standard Tag Library - JSTL. The page with it looks like this:

Java university 
<%@ taglib uri = "http://java.sun.com/jsp/jstl/core" prefix = "c" %>

<html>
   <head>
       <title> JSTL Example</title>
   </head>

   <body>
        <c:set var = "salary" scope = "session" value = "${2000*5}"/>
        <c:if test = "${ salary > 2000}">
            <p>My salary is: <c:out value = "${salary}"/><p>
        </c:if>
   </body>
</html>

It is quite possible that you will come across such code in your future projects, there I will give some explanations.

7.2 JSTL functions

JSTL functions fall into 5 categories:

  • Main tags;
  • Formatting tags;
  • SQL tags;
  • XML tags;
  • Calling functions.

I won't list them all, but I'll list the most popular ones. Let's start with the main tags:

1 <c:out> Outputs the specified expression - equivalent to <%= %>
2 <c:set> Writes the result of an expression to a variable
3 <c:remove> Deletes a variable
4 <c:catch> Catches exceptions
5 <c:if> analogue of if
6 <c:choose> analog switch
7 <c:when> Used together with choose
8 <c:otherwise> Used together with choose
9 <c:import> Allows you to include content in the code (equivalent to the import directive)
10 <c:forEach> for each loop
eleven <c:param> Allows you to set options for import
12 <c:redirect> Redirect
13 <c:url> Creates a URL with parameters

I'll give just one example and finish with that. In principle, it is quite possible to read such code after some skill. But I don't recommend writing.

<%@ taglib uri = "http://java.sun.com/jsp/jstl/core" prefix = "c" %>

<html>
   <head>
      <title> Each Tag Example&</title>
   </head>

   <body>
       <c:forEach var = "i" begin = "1" end = "5">
            Item <c:out value = "${i}"/><p>
       </c:forEach>
   </body>
</html>

Think about it, we write Java code in the form of tags, so that the JSP parser will then convert these tags into Java code. Something in this world has gone wrong.

By the way, you can write your own tag libraries. I even once worked in a project where they were. Awesome experience. As soon as any changes are made to the library, the whole jsp immediately breaks.

What do you want? The compiler does not track such changes. They can only be found visually when viewing the generated HTML pages. And if these are some rare scenarios that arise in non-trivial situations ... God bless backend development and static typing!

PreviousPrevious lesson
NextNext lesson
Comments (2)
  • Popular
  • New
  • Old
You must be signed in to leave a comment
Thomas
Level 41 , Bayreuth, Germany
16 March, 12:11
From Servlet version 5.0 on the namespace has been changed from javax to jakarta (that's Tomcat 10.0 and above). So you need to use the new URIs and of course you need to add the dependendy to project configuration file pom.xml pom.xml: 
<dependency>
    <groupId>org.glassfish.web</groupId>
    <artifactId>jakarta.servlet.jsp.jstl</artifactId>
    <version>3.0.1</version>
</dependency>
if you use Glassfish, then it already has the dependency installed and you just need it for your local setup. Just add 
<scope>provided</scope>
to your jakarta.servlet.jsp.jstl dependecy the new URIs are as follows: 
    Functional Area             URI 	                    Prefix

    core                        jakarta.tags.core           c
    XML processing              jakarta.tags.xml            x
    I18N capable formatting     jakarta.tags.fmt            fmt
    relational db access (SQL)  jakarta.tags.sql            sql
    Functions                   jakarta.tags.functions      fn
That'll for example result in 
<%@ taglib prefix="fn" uri="jakarta.tags.functions" %>
for the functions. Besides the core library you should also look into formatting and functions. The first lets you use the default java i18n capabilities (resource bundles) inside jsp pages so that a user can switch language easily, the latter helps you with some nice (mainly String) functions.
Thomas
Level 41 , Bayreuth, Germany
16 March, 16:54
In the end this introduction was to brief. They were just rushing through barely touching the surface. For JavaScript it was even more obvious. Telling you how to do things with jQuery and that it is far easier than doing it in plain JS without telling how it is done in JS. Bummer. Don't get me wrong. It is not necessary to waste a lot of time here but imho it is necessary to understand some underlying techique (dito JS). And you should be able to code a servlet as it is a very basic and 'easy' approach. Later on model 2 (MVC) architecture is recommended like JSF or Spring. And because I'm just complaining, the tasks that make up Codegym are missing. I really miss them!!!