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How to avoid Java code in JSP files?

Posted by: admin November 2, 2017 Leave a comment

Questions:

I’m new to Java EE and I know that something like the following three lines

<%= x+1 %>
<%= request.getParameter("name") %>
<%! counter++; %>

is an old school way of coding and in JSP version 2 there exists a method to avoid Java code in JSP files. Can someone please tell me the alternative JSP 2 lines, and what this technique is called?

Answers:

The use of scriptlets (those <% %> things) in JSP is indeed highly discouraged since the birth of taglibs (like JSTL) and EL (Expression Language, those ${} things) over a decade ago.

The major disadvantages of scriptlets are:

  1. Reusability: you can’t reuse scriptlets.
  2. Replaceability: you can’t make scriptlets abstract.
  3. OO-ability: you can’t make use of inheritance/composition.
  4. Debuggability: if scriptlet throws an exception halfway, all you get is a blank page.
  5. Testability: scriptlets are not unit-testable.
  6. Maintainability: per saldo more time is needed to maintain mingled/cluttered/duplicated code logic.

Sun Oracle itself also recommends in the JSP coding conventions to avoid use of scriptlets whenever the same functionality is possible by (tag) classes. Here are several cites of relevance:

From JSP 1.2 Specification, it is highly recommended that the JSP Standard Tag Library (JSTL) be used in your web application to help reduce the need for JSP scriptlets in your pages. Pages that use JSTL are, in general, easier to read and maintain.

Where possible, avoid JSP scriptlets whenever tag libraries provide equivalent functionality. This makes pages easier to read and maintain, helps to separate business logic from presentation logic, and will make your pages easier to evolve into JSP 2.0-style pages (JSP 2.0 Specification supports but deemphasizes the use of scriptlets).

In the spirit of adopting the model-view-controller (MVC) design pattern to reduce coupling between the presentation tier from the business logic, JSP scriptlets should not be used for writing business logic. Rather, JSP scriptlets are used if necessary to transform data (also called “value objects”) returned from processing the client’s requests into a proper client-ready format. Even then, this would be better done with a front controller servlet or a custom tag.


How to replace scriptlets entirely depends on the sole purpose of the code/logic. More than often this code is to be placed in a fullworthy Java class:

  • If you want to invoke the same Java code on every request, less-or-more regardless of the requested page, e.g. checking if an user is logged in, then implement a filter and write code accordingly in doFilter() method. E.g.:

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException {
        if (((HttpServletRequest) request).getSession().getAttribute("user") == null) {
            ((HttpServletResponse) response).sendRedirect("login"); // Not logged in, redirect to login page.
        } else {
            chain.doFilter(request, response); // Logged in, just continue request.
        }
    }
    

    When mapped on an appropriate <url-pattern> covering the JSP pages of interest, then you don’t need to copypaste the same piece of code over all JSP pages.


  • If you want to invoke some Java code to preprocess a request, e.g. preloading some list from a database to display in some table, if necessary based on some query parameters, then implement a servlet and write code accordingly in doGet() method. E.g.:

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        try {
            List<Product> products = productService.list(); // Obtain all products.
            request.setAttribute("products", products); // Store products in request scope.
            request.getRequestDispatcher("/WEB-INF/products.jsp").forward(request, response); // Forward to JSP page to display them in a HTML table.
        } catch (SQLException e) {
            throw new ServletException("Retrieving products failed!", e);
        }
    }
    

    This way dealing with exceptions is easier. The DB is not accessed in the midst of JSP rendering, but far before the JSP is been displayed. You still have the possibility to change the response whenever the DB access throws an exception. In the above example, the default error 500 page will be displayed which you can anyway customize by an <error-page> in web.xml.


  • If you want to invoke some Java code to postprocess a request, e.g. processing a form submit, then implement a servlet and write code accordingly in doPost() method. E.g.:

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        User user = userService.find(username, password);
    
        if (user != null) {
            request.getSession().setAttribute("user", user); // Login user.
            response.sendRedirect("home"); // Redirect to home page.
        } else {
            request.setAttribute("message", "Unknown username/password. Please retry."); // Store error message in request scope.
            request.getRequestDispatcher("/WEB-INF/login.jsp").forward(request, response); // Forward to JSP page to redisplay login form with error.
        }
    }
    

    This way dealing with different result page destinations is easier: redisplaying the form with validation errors in case of an error (in this particular example you can redisplay it using ${message} in EL), or just taking to the desired target page in case of success.


  • If you want to invoke some Java code to control the execution plan and/or the destination of the request and the response, then implement a servlet according the MVC’s Front Controller Pattern. E.g.:

    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        try {
            Action action = ActionFactory.getAction(request);
            String view = action.execute(request, response);
    
            if (view.equals(request.getPathInfo().substring(1)) {
                request.getRequestDispatcher("/WEB-INF/" + view + ".jsp").forward(request, response);
            } else {
                response.sendRedirect(view);
            }
        } catch (Exception e) {
            throw new ServletException("Executing action failed.", e);
        }
    }
    

    Or just adopt a MVC framework like JSF, Spring MVC, Wicket, etc so that you end up with just a JSP/Facelets page and a Javabean class without the need for a custom servlet.


  • If you want to invoke some Java code to control the flow inside a JSP page, then you need to grab an (existing) flow control taglib like JSTL core. E.g. displaying List<Product> in a table:

    <%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
    ...
    <table>
        <c:forEach items="${products}" var="product">
            <tr>
                <td>${product.name}</td>
                <td>${product.description}</td>
                <td>${product.price}</td>
            </tr>
        </c:forEach>
    </table>
    

    With XML-style tags which fits nicely among all that HTML, the code is better readable (and thus better maintainable) than a bunch of scriptlets with various opening and closing braces (“Where the heck does this closing brace belong to?”). An easy aid is to configure your web application to throw an exception whenever scriptlets are still been used by adding the following piece to web.xml:

    <jsp-config>
        <jsp-property-group>
            <url-pattern>*.jsp</url-pattern>
            <scripting-invalid>true</scripting-invalid>
        </jsp-property-group>
    </jsp-config>
    

    In Facelets, the successor of JSP, which is part of the Java EE provided MVC framework JSF, it is already not possible to use scriptlets. This way you’re automatically forced to do things “the right way”.


  • If you want to invoke some Java code to access and display “backend” data inside a JSP page, then you need to use EL (Expression Language), those ${} things. E.g. redisplaying submitted input values:

    <input type="text" name="foo" value="${param.foo}" />
    

    The ${param.foo} displays the outcome of request.getParameter("foo").


  • If you want to invoke some utility Java code directly in the JSP page (typically public static methods), then you need to define them as EL functions. There’s a standard functions taglib in JSTL, but you can also easily create functions yourself. Here’s an example how JSTL fn:escapeXml is useful to prevent XSS attacks.

    <%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn" %>
    ...
    <input type="text" name="foo" value="${fn:escapeXml(param.foo)}" />
    

    Note that the XSS sensitivity is in no way specifically related to Java/JSP/JSTL/EL/whatever, this problem needs to be taken into account in every webapplication you develop. The problem of scriptlets is that it provides no way of builtin preventions, at least not using the standard Java API. JSP’s successor Facelets has already implicit HTML escaping, so you don’t need to worry about XSS holes in Facelets.

See also:

Questions:
Answers:

As a Safeguard: Disable Scriptlets For Good

As another question is discussing, you can and always should disable scriptlets in your web.xml web application descriptor.

I would always do that in order to prevent any developer adding scriptlets, especially in bigger companies where you will lose overview sooner or later. The web.xml settings look like this:

<jsp-config>
    <jsp-property-group>
        <url-pattern>*.jsp</url-pattern>
        <scripting-invalid>true</scripting-invalid>
    </jsp-property-group>
</jsp-config>

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Answers:

JSTL offers tags for conditionals, loops, sets, gets, etc. For example:

<c:if test="${someAttribute == 'something'}">
   ...
</c:if>

JSTL works with request attributes – they are most often set in the request by a Servlet, which forwards to the JSP.

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I’m not sure if i get this correct.

You should read something about MVC. Spring MVC & Struts 2 are the two most common solutions.

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Answers:

You can use JSTL tags together with EL expressions to avoid intermixing Java and HTML code:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>
<html>
    <head>
    </head>
    <body>

        <c:out value="${x + 1}" />
        <c:out value="${param.name}" />
    // and so on

    </body>
</html>

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Answers:

There are also component-based frameworks such as Wicket that generate a lot of the HTML for you. The tags that end up in the HTML are extremely basic and there is virtually no logic that gets mixed in. The result is almost empty-like HTML pages with typical HTML elements. The downside is that there are a lot of components in the Wicket API to learn and some things can be difficult to achieve under those constraints.

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Answers:

In the MVC Architectural pattern, JSPs represent the View layer. Embedding java code in JSPs is considered a bad practice.
You can use JSTL, freeMarker, velocity with JSP as “template engine”.
The data provider to those tags depends on frameworks that you are dealing with. Struts 2 and webwork as an implementation for MVC Pattern uses OGNL “very interesting technique to expose Beans Properties to JSP “.

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Experience has shown that JSP’s have some shortcomings, one of them being hard to avoid mixing markup with actual code.

If you can, then consider using a specialized technology for what you need to do. In Java EE 6 there is JSF 2.0, which provides a lot of nice features including gluing Java beans together with JSF pages through the #{bean.method(argument)} approach.

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Wicket is also an alternative which completely separates java from html, so a designer and programmer can work together and on different sets of code with little understanding of each other.

Look at Wicket.

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if you simply want to avoid the drawbacks of Java coding in JSP you can do so even with scriplets. Just follow some discipline to have minimal Java in JSP and almost no calculation and logic in the JSP page.

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%//instantiate a JSP controller
MyController clr = new MyController(request, response);

//process action if any
clr.process(request);

//process page forwaring if necessary

//do all variable assignment here
String showMe = clr.getShowMe();%>

<html>
    <head>
    </head>
    <body>
        <form name="frm1">
            <p><%= showMe %>
            <p><% for(String str : clr.listOfStrings()) { %>
            <p><%= str %><% } %>

            // and so on   
        </form>
    </body>
</html>

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Learn to customize and write your own tags using JSTL

Note that EL is EviL (runtime exceptions, refactoring)
Wicket may be evil too (performance, toilsome for small apps or simple view tier)

Example from java2s,

This must be added to the web application’s web.xml

<taglib>
    <taglib-uri>/java2s</taglib-uri>
    <taglib-location>/WEB-INF/java2s.tld</taglib-location>
</taglib>

create File:java2s.tld in the /WEB-INF/

<!DOCTYPE taglib
  PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN"
   "http://java.sun.com/dtd/web-jsptaglibrary_1_2.dtd">

<!-- a tab library descriptor -->
<taglib xmlns="http://java.sun.com/JSP/TagLibraryDescriptor">
    <tlib-version>1.0</tlib-version>
    <jsp-version>1.2</jsp-version>
    <short-name>Java2s Simple Tags</short-name>

    <!-- this tag manipulates its body content by converting it to upper case
    -->
    <tag>
        <name>bodyContentTag</name>
        <tag-class>com.java2s.BodyContentTag</tag-class>
        <body-content>JSP</body-content>
        <attribute>
          <name>howMany</name>
        </attribute>
    </tag>
</taglib>

compile the following code into WEB-INF\classes\com\java2s

package com.java2s;

import java.io.IOException;
import javax.servlet.jsp.JspWriter;
import javax.servlet.jsp.tagext.BodyContent;
import javax.servlet.jsp.tagext.BodyTagSupport;

public class BodyContentTag extends BodyTagSupport{
    private int iterations, howMany;

    public void setHowMany(int i){
        this.howMany = i;
    }

    public void setBodyContent(BodyContent bc){
        super.setBodyContent(bc);
        System.out.println("BodyContent = '" + bc.getString() + "'");
    }

    public int doAfterBody(){
        try{    
            BodyContent bodyContent = super.getBodyContent();
            String bodyString  = bodyContent.getString();
            JspWriter out = bodyContent.getEnclosingWriter();

            if ( iterations % 2 == 0 ) 
                out.print(bodyString.toLowerCase());
            else
                out.print(bodyString.toUpperCase());

            iterations++;
            bodyContent.clear(); // empty buffer for next evaluation
        }
        catch (IOException e) {
            System.out.println("Error in BodyContentTag.doAfterBody()" + e.getMessage());
            e.printStackTrace();
        } // end of catch

        int retValue = SKIP_BODY;

        if ( iterations < howMany ) 
            retValue = EVAL_BODY_AGAIN;

        return retValue;
    }
}

Start server and load the bodyContent.jsp in browser

<%@ taglib uri="/java2s" prefix="java2s" %>
<html>
    <head>
        <title>A custom tag: body content</title>
    </head>
    <body>
        This page uses a custom tag manipulates its body content.Here is its output:
        <ol>
            <java2s:bodyContentTag howMany="3">
            <li>java2s.com</li>
            </java2s:bodyContentTag>
        </ol>
    </body>
</html>

Questions:
Answers:

You raised a good question and although you got good answers, I would suggest that you get rid of JSP. It is outdated technology which eventually will die. Use a modern approach, like template engines. You will have very clear separation of business and presentation layers, and certainly no Java code in templates, so you can generate templates directly from web presentation editing software, in most cases leveraging WYSIWYG.

And certainly stay away of filters and pre and post processing, otherwise you may deal with support/debugging difficulties since you always do not know where the variable gets the value.

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No matter how much you try to avoid, when you work with other developers, some of them will still prefer scriptlet and then insert the evil code into the project. Therefore, setting up the project at the first sign is very important if you really want to reduce the scriptlet code. There are several techniques to get over this (including several frameworks that other mentioned). However, if you prefer the pure JSP way, then use the JSTL tag file. The nice thing about this is you can also set up master pages for your project, so the other pages can inherit the master pages

Create a master page called base.tag under your WEB-INF/tags with the following content

<%@tag description="Overall Page template" pageEncoding="UTF-8"%>

<%@attribute name="title" fragment="true" %>

<html>
  <head>
    <title>  
       <jsp:invoke fragment="title"></jsp:invoke>
    </title>

  </head>
  <body>
    <div id="page-header">
       ....
    </div>
    <div id="page-body">
      <jsp:doBody/>
    </div>
    <div id="page-footer">
      .....
    </div>
  </body>
</html>

On this mater page, I created a fragment called “title”, so that in the child page, I could insert more codes into this place of the master page. Also, the tag <jsp:doBody/> will be replaced by the content of the child page

Create child page (child.jsp) in your WebContent folder:

<%@ taglib prefix="t" tagdir="/WEB-INF/tags" %>

<t:base>
    <jsp:attribute name="title"> 
        <bean:message key="hello.world" />
    </jsp:attribute>

    <jsp:body>
    [Put your content of the child here]
    </jsp:body>   
</t:base>

<t:base> is used to specify the master page you want to use (which is base.tag at this moment). All the content inside the tag <jsp:body> here will replace the <jsp:doBody/> on your master page. Your child page can also include any tag lib and you can use it normally like the other mentioned. However, if you use any scriptlet code here (<%= request.getParameter("name") %> …) and try to run this page, you will get a JasperException because Scripting elements ( &lt;%!, &lt;jsp:declaration, &lt;%=, &lt;jsp:expression, &lt;%, &lt;jsp:scriptlet ) are disallowed here. Therefore, there is no way other people can include the evil code into the jsp file

Calling this page from your controller:

You can easily call the child.jsp file from your controller. This also works nice with the struts framework

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in order to avoid java code in JSP files java now provides tag libraries like JSTL also java has come up with JSF into which u can write all programming structures in the form of tags

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Use JSTL Tag libraries in JSP, that will work perfect.

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Just use the JSTL tag and EL expression.

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If somebody is really against programming in more languages than one, I suggest GWT, theoretically you can avoid all the JS and HTML elements, because Google Toolkit transforms all the client and shared code to JS, you won’t have problem with them, so you have a webservice without coding in any other languages. Even you can use some default CSS from somewhere as it is given by extensions (smartGWT or Vaadin). You don’t need to learn dozens of annotations.

Of course if you want, you can hack yourself into the depths of the code and inject JS and enrich your HTML page, but really you can avoid it if you want, and the result will be good as it was written in any other frameworks. I say worths a try, and the basic GWT is well-documented.

And of course many fellow programmers hereby described or recommended several other solutions. GWT is for people who really don’t want to deal with the web part or to minimalize it.

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A neat idea from the Python world are Template attribute languages; TAL was introduced by Zope (therefore a.k.a. “Zope Page Templates”, ZPT) and is a standard, with implementations in PHP, XSLT and Java as well (I have used the Python/Zope and PHP incarnations). In this class of templating languages, one above example could look like this:

<table>
    <tr tal:repeat="product products">
        <td tal:content="product/name">Example product</td>
        <td tal:content="product/description">A nice description</td>
        <td tal:content="product/price">1.23</td>
    </tr>
</table>

The code looks like ordinary HTML (or XHTML) plus some special attributes in an XML namespace; it can be viewed with a browser and safely be tweaked by a designer.
There is support for macros and for i18n as well:

<h1 i18n:translate="">Our special offers</h1>
<table>
    <tr tal:repeat="product products">
        <td tal:content="product/name"
            i18n:translate="">Example product</td>
        <td tal:content="product/description"
            i18n:translate="">A nice description</td>
        <td tal:content="product/price">1.23</td>
    </tr>
</table>

If translations of the content are available, they are used.

I don’t know very much about the Java implementation, though.

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Using scriptlets in JSPs is not a good practice.

Instead, you can use:

  1. JSTL tags
  2. EL expressions
  3. Custom Tags- you can define your own tags to use.

Please refer to:

  1. http://docs.oracle.com/javaee/1.4/tutorial/doc/JSTL3.html
  2. EL
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Technically, JSP are all converted to Servlets during runtime. JSP was initially created for the purpose of the decoupling the business logic and the design logic, following the MVC pattern. So JSP are technically all java codes during runtime. But to answer the question, Tag Libraries are usually used for applying logic (removing Java codes) to JSP pages.

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Sure, replace <%! counter++; %> by a event producer-consumer architecture, where the business layer is notified about the need to increment the counter, it reacts accordingly, and notifies the presenters so that they update the views. A number of database transactions are involved, since in future we will need to know the new and old value of the counter, who has incremented it and with what purpose in mind. Obviously serialization is involved, since the layers are entirely decoupled. You will be able to increment your counter over RMI, IIOP, SOAP. But only HTML is required, which you don’t implement, since it is such a mundane case. Your new goal is to reach 250 increments a second on your new shiny E7, 64GB RAM server.

I have more than 20 years in programming, most of the projects fail before the sextet: Reusability Replaceability OO-ability Debuggability Testability Maintainability is even needed. Other projects, run by people who only cared about functionality, were extremely successful. Also, stiff object structure, implemented too early in the project, makes the code unable to be adapted to the drastic changes in the specifications (aka agile).

So I consider as procrastination the activity of defining “layers” or redundant data structures either early in the project or when not specifically required.

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If we use the following things in a java web application, java code can be eliminated from foreground of the JSP.

  1. Use MVC architecture for web application

  2. Use JSP Tags

    a. Standard Tags

    b. Custom Tags

  3. Expression Language

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How to avoid Java code in JSP files?

You can use tab library tags like JSTL in addition to Expression Language (EL). But EL does not work well with JSP. So it’s is probably better to drop JSP completely and use Facelets.

Facelets is the first non JSP page declaration language designed for JSF (Java Server Faces) which provided a simpler and more powerful programming model to JSF developers as compare to JSP. It resolves different issues occurs in JSP for web applications development.

enter image description here

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JSP 2.0 has a feature called “Tag Files”, you can write tags without external java code and tld. You need to create a .tag file and put it in WEB-INF\tags you can even create directory structure to package your tags.

For example:

/WEB-INF/tags/html/label.tag

<%@tag description="Rensders a label with required css class" pageEncoding="UTF-8"%>
<%@attribute name="name" required="true" description="The label"%>

<label class="control-label control-default"  id="${name}Label">${name}</label>

Use it like

<%@ taglib prefix="h" tagdir="/WEB-INF/tags/html"%>
<h:label  name="customer name" />

Also you can read the tag body easly

/WEB-INF/tags/html/bold.tag
<%@tag description="Bold tag" pageEncoding="UTF-8"%>
<b>
  <jsp:doBody/>
</b>

Use it

<%@ taglib prefix="h" tagdir="/WEB-INF/tags/bold"%>
<h:bold>Make me bold</h:bold>

The samples are very simple but you can do lots of complicated tasks here. Please consider you can use other tags (eg: JSTL which has controlling tags like if/forEcah/chosen text manipulation like format/contains/uppercase or even SQL tags select/update), pass all kind parameters, for example Hashmap, access session, request, … in your tag file too.

Tag File are so easy developed as you did not need to restart the server when changing them, like jsp files. This make them easy for development.

Even if you use a framework like struts 2, which have lots of good tags, you may find that having your own tags can reduce your code a lot. You can pass your tag parameters to struts and this way customize your framework tag.

You can use tag not only to avoid java but also minimize your HTML codes. I myself try to review HTML codes and build tags a lot as soon as see code duplicates start in my pages.

(Even if you end up using the java in you jsp code, which I hope not, you can encapsulate that code in a tag)

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As many answers says, use JSTL or create your own custom tags. Here is good explanation about creating custom tags

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By using JSTL tags together with EL expression you can avoid this. Put the following things in your jsp page:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>