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Confusion between using the EL exp tag and JSP expression tags with JSTL [duplicate]

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. What are the alternative JSP 2 lines, and what is this technique called?

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) way back in 2001.
The major disadvantages of scriptlets are:
Reusability: you can't reuse scriptlets.
Replaceability: you can't make scriptlets abstract.
OO-ability: you can't make use of inheritance/composition.
Debuggability: if scriptlet throws an exception halfway, all you get is a blank page.
Testability: scriptlets are not unit-testable.
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 de-emphasizes 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 a 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 overall JSP pages.
If you want to invoke some Java code to process a GET 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 process a POST request, such as gathering data from a submitted HTML form and doing some business stuff with it (conversion, validation, saving in DB, etcetera), 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 ${message} in EL), or just taking to the desired target page in case of success.
If you want to invoke some Java code to control the execution plan and/or the destination of the request and the response, then implement a servlet according to the MVC's Front Controller Pattern. E.g.:
protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
try {
Action action = ActionFactory.getAction(request);
String view = action.execute(request, response);
if (view.equals(request.getPathInfo().substring(1)) {
request.getRequestDispatcher("/WEB-INF/" + view + ".jsp").forward(request, response);
} else {
response.sendRedirect(view);
}
} catch (Exception e) {
throw new ServletException("Executing action failed.", e);
}
}
Or just adopt an MVC framework like JSF, Spring MVC, Wicket, etc so that you end up with just a JSP/Facelets page and a JavaBean class without the need for a custom servlet.
If you want to invoke some Java code to control the flow inside a JSP page, then you need to grab an (existing) flow control taglib like JSTL core. E.g. displaying List<Product> in a table:
<%# taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
...
<table>
<c:forEach items="${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 fit 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 web application 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:
What's the difference between JSP, Servlet and JSF?
How does Servlet, ServletContext, HttpSession and HttpServletRequest/Response work?
Basic MVC example with JSP, Servlet and JDBC
Design patterns in Java web applications
Hidden features of JSP/Servlet

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>

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.

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>

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.

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, and velocity with JSP as a "template engine".
The data provider to those tags depends on frameworks that you are dealing with. Struts 2 and WebWork as an implementation for the MVC pattern uses OGNL "very interesting technique to expose Beans properties to JSP".

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.

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 forwarding, 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>

Learn to customize and write your own tags using JSTL
Note that EL is EviL (runtime exceptions and refactoring).
Wicket may be evil too (performance and toilsome for small applications 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 the server and load the bodyContent.jsp file in the 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>

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.

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.

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 you can write all programming structures in the form of tags.

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 ( <%!, <jsp:declaration, <%=, <jsp:expression, <%, <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

Use JSTL tag libraries in JSP. That will work perfectly.

Just use the JSTL tag and EL expression.

If somebody is really against programming in more languages than one, I suggest GWT. Theoretically, you can avoid all the JavaScript and HTML elements, because Google Toolkit transforms all the client and shared code to JavaScript. You won't have problem with them, so you have a webservice without coding in any other languages. You can even 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 JavaScript 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 it's say worth 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 minimize it.

Using scriptlets in JSPs is not a good practice.
Instead, you can use:
JSTL tags
EL expressions
Custom Tags- you can define your own tags to use.
Please refer to:
http://docs.oracle.com/javaee/1.4/tutorial/doc/JSTL3.html
EL

A neat idea from the Python world is 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 of the above examples 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 internationalisation and localisation 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.

Sure, replace <%! counter++; %> by an 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.

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 is technically all Java code during runtime.
But to answer the question, tag libraries are usually used for applying logic (removing Java code) to JSP pages.

How can I 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.
Source

If we use the following things in a Java web application, Java code can be eliminated from the foreground of the JSP file.
Use the MVC architecture for a web application
Use JSP Tags
a. Standard Tags
b. Custom Tags
Expression Language

Using Scriptlets is a very old way and not recommended. If you want directly output something in your JSP pages, just use Expression Language (EL) along with JSTL.
There are also other options, such as using a templating engine such as Velocity, Freemarker, Thymeleaf, etc. But using plain JSP with EL and JSTL serves my purpose most of the time and it also seems the simplest for a beginner.
Also, take note that it is not a best practice to do business logic in the view layer. You should perform your business logic in the service layer,
and pass the output result to your views through a controller.

Use a Backbone.js or AngularJS-like JavaScript framework for UI design
and fetch the data using a REST API. This will remove the Java dependency from the UI completely.

Nothing of that is used anymore, my friend. My advice is to decouple the view (CSS, HTML, JavaScript, etc.) from the server.
In my case, I do my systems handling the view with Angular and any data needed is brought from the server using REST services.
Believe me, this will change the way you design.

JSP 2.0 has a feature called "Tag Files", and 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 a 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 easily:
/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 (for example: 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 makes 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 tags not only to avoid Java, but also minimize your HTML codes. I myself try to review HTML code and build tags a lot as soon as I see code duplicates start in my pages.
(Even if you end up using Java in your JSP code, which I hope not, you can encapsulate that code in a tag.)

Make your values and parameters inside your servlet classes
Fetch those values and parameters within your JSP using JSTL/Taglib
The good thing about this approach is that your code is also HTML like
code!

A lot of the answers here go the "use a framework" route. There's zero wrong with that. However I don't think it really answers your question, because frameworks may or may not use JSPs, nor are they designed in any way with removing java use in JSPs as a primary goal.
The only good answer to your question "how do I avoid using Java in a JSP" is: you can't. That's what JSPs are for - using Java to render HTML with dynamic data/logic.
The follow up question might be, how much java should I use in my JSPs. Before we answer that question, you should also ponder, "do I need to use JSPs to build web content using Java?" The answer to that last one is, no. There are many alternatives to JSPs for developing web facing applications using Java. Struts for example does not force you to use JSPs - don't get me wrong, you can use them and many implementations do, but you don't absolutely have to. Struts doesn't even force you to use any HTML. A JSP doesn't either, but let's be honest, a JSP producing no HTML is kinda weird. Servlets, famously, allow you to serve any kind of content you like over HTTP dynamically. They are the primary tech behind pretty much everything java web - JSPs are just HTML templates for servlets, really.
So the answer to how much java you should put in a JSP is, "as little as possible". I of course have java in my JSPs, but it consists exclusively of tag library definitions, session and client variables, and beans encapsulating server side objects. The <%%> tags in my HTML are almost exclusively property calls or variable expressions. Rare exceptions include ultra-specific calculations pertaining to a single page and unlikely to ever be reused; bugfixes stemming from page-specific issues only applying to one page; last minute concatenations and arithmetic stemming from unusual requirements limited in scope to a single page; and other similar cases. In a code set of 1.5 million lines, 3000 JSPs and 5000 classes, there are maybe 100 instances of such unique snippets. It would have been quite possible to make these changes in classes or tag library definitions, but it would have been inordinately complex due to the specificity of each case, taken longer to write and debug, and taken more time as a result to get to my users. It's a judgement call. But make no mistake, you cannot write JSPs of any meaning with "no java" nor would you want to. The capability is there for a reason.

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

By using JSTL tags together with EL expressions, 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" %>

Why do we use multiple scriplet tags for a for loop instead of one

I am studying JSP and the Servlets and have question to you guys, because in few videos I saw that the teacher is using scriplets like below, I used an example array without any objectsjust to show better what I mean.
<%
Student[] students = new Student[]{students};
names.setAttribute("students", students)"
%>
But sometimes when he is using a standard for instead of JSTL c:forEach he is writing scriptlets like this one below.
<%
for (String temp : students) { %>
<tr>
<td><%=temp.getFirstName()%></td>
<td><%=temp.getLastname()%></td>
<% } %>
What is the difference between single <% %> for a for loop do we need to use it like above or no?

Either one of these can be used. The only issue with the c:forEach method is that you may have to install and include the JSTL tag library. Depending on your environment and skillset this can be a bit of a pain.
The example you show above uses "pure" Java with JSP tags. Anything between <% and %> is regular Java code. This is built into the JSP language. The <%= is a shortcut for using the out implicit object that is built into JSP (see this page for a list of all variables). This could also be written as:
<%
for (String temp : students) { %>
<tr>
<td><% out.print(temp.getFirstName()) %></td>
<td><% out.print(temp.getLastname()) %></td>
<% } %>
But now a warning. It's fine to learn this and, depending on what you for a living, you very well may run into JSP code. I've done consulting for people who, even today, have JSP code all over the place. But to the point that #GyroGearless makes JSP's are not considered best practice for new projects. Ultimately they mix business logic with display logic. In a big project with UI people and Java people it can make it difficult to separate the work. JSP Tag libraries were created as an attempt to minimize that but it really wasn't the best answer.
New projects today separate out the display logic from the backend logic. You write the front end leveraging the JavaScript library of the week (i.e. AngularJS, ReactJS, Ember, etc.) and your backend using modern technologies like JAX-RS to create an AJAX front end with a REST/JSON interface to the backend.

How can I reuse code in JSP?

For example, a user will be rendered throughout my application as
<div class="user">
<p class="username">${user.name}</p>
<p class="karma">${user.karma}</p>
<img src="/users/${user.id}"/>
</div>
How can I reuse this code block?
Note - my code is running within a tag, so I can't use tags for this (or any JSP) otherwise I get a Scripting elements are disallowed here error.
Edit
I'm trying to use a tag file, but getting PropertyNotFoundException.
This is my tag file called 'user.tag':
<%#tag description="User" pageEncoding="UTF-8" %>
<a href="../user/showUser.do?userId=${user.id}">
<p>${user.name}</p>
<img class='avatar' src='${user.avatarUrl}' alt=""/>
</a>
And usage inside a jsp:
Where job.poster is a java bean with id, name, and avatarUrl properties.
If I add
<%#attribute name="user" %>
to the tag file, then I get an exception
Property 'id' not found on type java.lang.String

Since JSP 2.0, there is yet another kind of tags: Tag files. Tag files are JSP custom tags written as a JSP template itself, which seems to be what you want.
http://fforw.de/post/creating-jsp-layouts-with-page-tags/ shows how to use such a tag file as general layout solution. Using them as component should be even easier.

You should be able to use tag files within tag files; this works for me in a JSP 2.2 container:
<%-- mytag.tag --%>
<%#tag description="demo code" pageEncoding="UTF-8"%>
<%#taglib prefix="cust" tagdir="/WEB-INF/tags" %>
<%#attribute name="message"%>
<cust:mytag2 message="${message}" /><%-- uses mytag2.tag --%>
If that fails, you can use the include directive: <%#include file="/WEB-INF/jspf/fragment.jspf" %>
Note that the spec says about tags:
Directive Available? Interpretation/Restrictions
======================================================================
page no A tag file is not a page. The tag directive must
be used instead. If this directive is used in a
tag file, a translation error must result.
So, fragment.jspf must not have a any elements that are not supported in tags, including a page directive.

For the example you have given it sounds like some templating framework is needed, to display the user badge on each screen. At its simplest level this may just be a jsp:include which always includes your "UserBadge.jsp".
If you are running on a web framework e.g. JSF you may use Facelet templates or write a custom component for this. So the answer depends on what framework you have. Breaking it down to just JSP and JSTL - the included JSP or a javax.servlet.jsp.tagext.Tag would certainly reduce the duplication.
Always be careful to follow the DRY Principle... Don't Repeat Yourself!

I feel sometimes creating a .tag file is overkill (especially if it's only for one page), and I've wanted what you describe for years, so I wrote my own, simple solution. See here:
https://stackoverflow.com/a/25575120/1607642

Why don't you use a custom tag or jsp functions.
Thanks,

Using Servlets and HTML

I have a doubt regarding the use of servlets.
In the application I'm building I use Html pages to ask the user for info.
Imagine a Html page that lets the user request to see on the screen the content of a data base. What I'm doing is:
1.- From the Html page I'm calling a servlet that will open a connection with the database.
2.- The servlet builds the web page that the user will see.
My question is: is there any other way of doing this? Do I have to build the web page in the servlet or is there any way of sending the information contained in the database to an .html file which will build the web page (in my case I need to show on the screen a table containing all the information) ?
Thanks

Servlets are meant to control, preprocess and/or postprocess requests, not to present the data. There the JSP is for as being a view technology providing a template to write HTML/CSS/JS in. You can control the page flow with help of taglibs like JSTL and access any scoped attributes using EL.
First create a SearchServlet and map it on an url-pattern of /search and implement doGet() and doPost() as follows:
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// Preprocess request here and finally send request to JSP for display.
request.getRequestDispatcher("/WEB-INF/search.jsp").forward(request, response);
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// Postprocess request here. Get results from your DAO and store in request scope.
String search = request.getParameter("search");
List<Result> results = searchDAO.find(search);
request.setAttribute("results", results);
request.getRequestDispatcher("/WEB-INF/search.jsp").forward(request, response);
}
Here's how the JSP /WEB-INF/search.jsp would look like, it makes use of JSTL (just drop JAR in /WEB-INF/lib) to control the page flow.
<%# taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%# taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn" %>
...
<form action="search" method="post">
<input type="text" name="search">
<input type="submit" value="search">
</form>
<c:if test="${not empty results}">
<p>There are ${fn:length(results)} results.</p>
<table>
<c:forEach items="${results}" var="result">
<tr>
<td>${result.id}</td>
<td>${result.name}</td>
<td>${result.value}</td>
</tr>
</c:forEach>
</table>
</c:if>
Note that JSP is placed in /WEB-INF to prevent users from direct access by URL. They are forced to use the servlet for that by http://example.com/contextname/search.
To learn more about JSP/Servlets, I can recommend Marty Hall's Coreservlets.com tutorials. To learn more about the logic behind searchDAO, I can recommend this basic DAO tutorial.
To go a step further, you could always consider to make use of a MVC framework which is built on top of the Servlet API, such as Sun JSF, Apache Struts, Spring MVC, etcetera so that you basically end up with only Javabeans and JSP/XHTML files. The average MVC frameworks will take care about gathering request parameters, valitate/convert them, update Javabeans with those values, invoke some Javabean action method to process them, etcetera. This makes the servlet "superfluous" (which is however still used as being the core processor of the framework).

In addition to Servlets, Java has JSP pages, which are a mix of HTML and custom tags (including the Java Standard Tag Library or JSTL) which are eventually compiled into Servlets.
There are other technologies for making web applications, including things like Java Server Faces (JSF), Apache Struts, Spring, etc... Spring in particular is very widely used in modern web application development.
Ultimately, though, it's like Brian Agnew said... you have to communicate back and forth from the browser to the server. These are just various technologies to facilitate that.

Ultimately the browser has to send a request to the server for the database info. You can do this in many ways:
build the whole page in the servlet (perhaps the simplest)
build a page in the servlet containing (say) XML data, and the browser renders it into HTML (via XSL/Javascript etc.). That may be suitable if you want the browser to control the formatting and presentation.
build a page containing AJAX requests to go back to the server and get the data. That may be more suitable for data that updates regularly, or more interactive applications.
There are numerous ways to skin this cat. I suspect you're doing the simplest one, which is fine. I would tend towards this without explicit requirements that mean I need to do something else. Further requirements that complicate matters may include paging the data if there's too much to fit on the screen. All the solutions above can be made to incorporate this in some fashion.

My java is rusty but...
in your Data Access layer, iterate over the result set and build a custom object and insert it into an ArrayList;
class DataAccess {
public ArrayList foo() {
// Connect to DB
// Execute Query
// Populate resultSet
ArrayList result = new ArrayList();
while (resultSet.hasNext()) {
CustomObject o = new CustomObject();
o.setProperty1(resultSet.getInt(1));
o.setProperty2(resultSet.getString(2));
// and so on
result.add(o);
}
return result;
}
}
Call this method from your Servlet. After you have populated the ArrayList, put it on the Request object and forward on to your .jsp page
ArrayList results = DataAccessClass.foo();
Request.setAttribute("Results", results);
In your jsp, build your markup using scriptlets
<% foreach (CustomObject o in Request.getAttribute("Results"))
{%>
<td><%= o.getProperty1()</td>
<td><%= o.getProperty2()</td>
<% } %>
Good luck

Use AJAX instead of TagLib?

I was thinking about the idea of using Ajax instead of TagLib. The most elegant way would be: Using Java Annotation.
The idea is, designers or anybody can make the HTML without any taglib ,just using the "standard" HTML tags with id or name, and call the Javascript. That way any WYSIWYG can be used, developer don't have to care about HTML format or the way it's designed.
In many (at least open-source) WYSIWYG don't show the taglibs in that final result (or have a template of it), so it's hard to "preview". Other reason is, developer should know Java and HTML/TagLibs should not be a must-have, since we got CSS and AJAX.
It should work just like that:
MyClass.java:
import ...
// Use the ResourceBundle resource[.{Locale}].properties
#Jay2JI18n(resourceBundle="org.format.resource",name="MyClassForm")
public class MyClass {
private Integer age;
private String name
private Date dob;
private salary;
#Jay2JLabel(resource="label.name")
#Jay2JMaxLength(value=50,required=true,)
#Jay2JException(resource="exception.message")
public String getName() {
...
}
public void setName(String name) {
if ( name.trim().equal("") ) {
throw new Exception("Name is required");
}
}
/* Getter and setter for age */
...
#Jay2JLabel(message="Salary")
#Jay2JFormat(format="##,###.00",language="en")
#Jay2JFormat(format="##.###,00",language="pt_BR")
// or you could use that to access a property of the ResourceBundle
//#Jay2I18nResource(resource="money.format")
public Date getSalary() {
...
}
/* Setter for salary and getter/setter for the rest */
...
}
Page.html:
<html>
<head>
<SCRIPT>
</SCRIPT>
</head>
<body>
<form onload="Jay2J.formalize(this)">
</form>
</body>
</html>
of it can be a HTML with the fields filled;
PageWithFields.html:
<html>
<head>
<SCRIPT>
</SCRIPT>
</head>
<body>
<form action="myfavoritewaytopostthis" onsubmit="return Jay2J.validate(this)" onload="Jay2J.formalizeExistField(this)">
<label>Name</label><input type="text" name="name" id="name" />
<label>DOB</label><input type="text" name="dateOfBirth" id="dob" />
<label>Salary</label><input type="text" name="salary" id="salary" />
<input type="submit" />
</form>
</body>
</html>
That way the Annotation (no XML, it's like HTML in the way that it's only another file modify and XML is not Java) will define how the HTML will be treated. That way developer can stop developing in HTML and use just JAVA (or JavaScript), do you think that's a valid idea?

When i see your topic title i thought:
You cant use Ajax in stead of a taglib. AJAX is javascript on the client and the taglib is java code on the server.
After reading your post i thought, ah he whats to do what [link text][1] does
But then not entrily the same.
[1]: http://code.google.com/webtoolkit/ GWT

First impression is ... yuck, someone who picks this up will have no idea what they're looking at without learning your (new, different, non-standard) way of doing things. You could do something similar by implementing a tag that takes a bean (value object) and maybe does some minor reflection/annotation inspection to emit the proper html, and you'll save yourself a lot of heartache down the line.
Make your value objects implement a simple interface that your tag will use to extract and format the html, and you can probably get 80-90% of where you're trying to go with 1/2 the work or less.

First impression was, WTF. After reading further, I get a impression that you are trying to address the 'separation of concerns'problem in a different way. Some observations on your approach.
Requires client side scripting to be enabled and hence fails accessibility guide lines.
Reinventing the wheel: Many web frameworks like Tapestry, Wicket try to address these issues and have done a commendable work.
On your comment on binding Java to HTML, the code example doesn't convey the idea very clearly. formalize() seems to create the UI, that implies you have UI (HTML) coded into java (Bad Idea? probably not NakedObjects attempts to you domain models for UI, probably yes if one were to write a page specific code)
validate() is invoked on onSubmit(), Why would I want it to be processed asynchronously!! That aside, using obstrusive java script is way out of fashion (seperation of concerns again)
Your argument on taglibs preventing WYSIWIG, though justifiable, is not entirely valid. Tags cannot be used to compose other tags, each tag is a unique entity that either deals with behaviour or emits some html code. Your argument is valid for the second case. However, if I understand your formalize() correctly, you are doing the same!
Nice to hear some new ideas and Welcome to SO. Also, please use the edit question option until you earn enough reputation to add comments. Adding answers is not the right way!

This idea has some merit, if I understand it correctly.
You could use AOP to modify a servlet that would actually be called for the page. The servlet would then return the html, using the annotations.
This way the programmers don't see the html generation, and if you have a standard javascript library for it, then it may work.
But, just because it works doesn't mean that you should do it.
As was mentioned, there are many frameworks out there that can hide the javascript from programmers, such as JSF, which is basically taglibs and a different navigation scheme.
I remember using the beehive project to do something similar, it was annotation driven so I could basically do everything in java and it generated the javascript, years ago. :)