What is the easiest way to hide HTML code from users eye?

What is the easiest way to hide HTML code from users eye? - java

I am currently working with JSP pages and would like to know what would be the easiest way to comment HTML/Java code from being able to see it when you open source on the web. As for now, I was used to use
<%-- html/Java code --%>
However, later on it screw up all highlighting of JSP file. Maybe there is another way of commenting multiple lines without possibility to see them in HTML source on web.

Look:
<%-- Comment --%> - commenting out html
<% /*java code*/ %> - commenting out java code
<!-- comment --> - html comment, ignored by the browser

I would like to quote Oracle on this one: Code Convention
Please note the following (excerpt from the link):
JSP comments (also called server-side comments) are visible only on the server side (that is, not propagated to the client side). Pure JSP comments are preferred over JSP comments with scripting language comments, as the former is less dependent on the underlying scripting language, and will be easier to evolve into JSP 2.0-style pages.
Examples:
<% /** ... */ %>
<% /* ... */ %>
<% //... %>
<% //... %>
Client-side comments can be used to annotate the responses sent to the client with additional information about the responses. They should not contain information about the behavior and internal structure of the server application or the code to generate the responses.
Let's note that these comments:
<!-- ... -->
ignore all elements except #import statements.

Related

when to use jsp include and page directive include [duplicate]

It seems that there are two methods for templating with JSP. Including files with one of these statements
<%# include file="foo.html" %>
<jsp:include page="foo.html" />
or using JSP tag files
// Save this as mytag.tag
<%# tag description="Description" pageEncoding="UTF-8"%>
<html>
<head>
</head>
<body>
<jsp:doBody/>
</body>
</html>
And in another JSP page call it with
<%# taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:mytag>
<h1>Hello World</h1>
</t:mytag>
So which method should I use? Is one now considered deprecated or are they both valid and cover different use cases?
Edit
Isn't using this tag file the same as using an include?
// Save this as product.tag
<%# tag description="Product templage" pageEncoding="UTF-8"%>
<%# tag import="com.myapp.Product" %>
<%# attribute name="product" required="true" type="com.myapp.Product"%>
Product name: ${product.name} <br/>
Quantity: ${product.quantity} <br/>
And call it on another JSP with
<%# taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:product>
<c:forEach items="${cart.products}" var="product">
<t:product product="${product}"/>
</c:forEach>
</t:product>
That seems to me to be the very same as using an include and passing parameters to it. So are Tag Files the same as includes?

Overview of JSP Syntax Elements
First, to make things more clear, here is a short overview of JSP syntax elements:
Directives: These convey information regarding the JSP page as a
whole.
Scripting elements: These are Java coding elements such as
declarations, expressions, scriptlets, and comments.
Objects and scopes: JSP objects can be created either explicitly or
implicitly and are accessible within a given scope, such as from
anywhere in the JSP page or the session.
Actions: These create objects or affect the output stream in the JSP
response (or both).
How content is included in JSP
There are several mechanisms for reusing content in a JSP file.
The following 4 mechanisms to include content in JSP can be categorized as direct reuse:
(for the first 3 mechanisms quoting from "Head First Servlets and JSP")
1) The include directive:
<%# include file="header.html" %>
Static: adds the content from the value of the file attribute to the current page at translation time. The directive was
originally intended for static layout templates, like HTML headers.
2) The <jsp:include> standard action:
<jsp:include page="header.jsp" />
Dynamic: adds the content from the value of the page attribute to the current page at request time. Was intended more for dynamic
content coming from JSPs.
3) The <c:import> JSTL tag:
<c:import url=”http://www.example.com/foo/bar.html” />
Dynamic: adds the content from the value of the URL attribute to the current page, at request time. It works a lot like
<jsp:include>, but it’s more powerful and flexible: unlike the
other two includes, the <c:import> URL can be from outside the
web Container!
4) Preludes and codas:
Static: preludes and codas can be applied only to the beginnings and ends of pages.
You can implicitly include preludes (also called headers) and codas
(also called footers) for a group of JSP pages by adding
<include-prelude> and <include-coda> elements respectively within
a <jsp-property-group> element in the Web application web.xml deployment descriptor.
Read more here:
• Configuring Implicit Includes at the Beginning and End of JSPs
• Defining implicit includes
Tag File is an indirect method of content reuse, the way of encapsulating reusable content.
A Tag File is a source file that contains a fragment of JSP code that is reusable as a custom tag.
The PURPOSE of includes and Tag Files is different.
Tag file (a concept introduced with JSP 2.0) is one of the options for creating custom tags. It's a faster and easier way to build custom tags.
Custom tags, also known as tag extensions, are JSP elements that allow custom logic and output provided by other Java components to be inserted into JSP pages. The logic provided through a custom tag is implemented by a Java object known as a tag handler.
Some examples of tasks that can be performed by custom tags include operating on implicit objects, processing forms, accessing databases and other enterprise services such as email and directories, and implementing flow control.
Regarding your Edit
Maybe in your example (in your "Edit" paragraph), there is no difference between using direct include and a Tag File. But custom tags have a rich set of features. They can
Be customized by means of attributes passed from the calling page.
Pass variables back to the calling page.
Access all the objects available to JSP pages.
Communicate with each other. You can create and initialize a JavaBeans component, create a public EL variable that refers to that bean in one tag, and then use the bean in another tag.
Be nested within one another and communicate by means of private variables.
Also read this from "Pro JSP 2": Understanding JSP Custom Tags.
Useful reading.
Difference between include directive and include action in
JSP
JSP tricks to make templating
easier
Very informative and easy to understand tutorial from coreservlet.com with beautiful
explanations that include <jsp:include> VS. <%# include %>
comparison table:
Including Files and Applets in JSP
Pages
Another nice tutorial from coreservlets.com related to tag libraries and
tag files:
Creating Custom JSP Tag Libraries: The
Basics
The official Java EE 5 Tutorial with examples:
Encapsulating Reusable Content
Using Tag
Files.
This page from the official Java EE 5 tutorial should give you even
more understanding:
Reusing Content in JSP
Pages.
This excerpt from the book "Pro JSP 2" also discuses why do you need
a Tag File instead of using static include:
Reusing Content with Tag
Files
Very useful guide right from the Oracle documentation:
Static Includes Versus Dynamic Includes
Conclusion
Use the right tools for each task.
Use Tag Files as a quick and easy way of creating custom tags that can help you encapsulate reusable content.
As for the including content in JSP (quote from here):
Use the include directive if the file changes rarely. It’s the fastest mechanism. If your container doesn’t automatically detect changes, you can force the changes to take effect by deleting the main page class file.
Use the include action only for content that changes often, and if which page to include cannot be decided until the main page is requested.

Possible Duplicate Question
<#include> - The directive tag instructs the JSP compiler to merge contents of the included file into the JSP before creating the generated servlet code. It is the equivalent to cutting and pasting the text from your include page right into your JSP.
Only one servlet is executed at run time.
Scriptlet variables declared in the parent page can be accessed in the included page (remember, they are the same page).
The included page does not need to able to be compiled as a standalone JSP. It can be a code fragment or plain text. The included page will never be compiled as a standalone. The included page can also have any extension, though .jspf has become a conventionally used extension.
One drawback on older containers is that changes to the include pages may not take effect until the parent page is updated. Recent versions of Tomcat will check the include pages for updates and force a recompile of the parent if they're updated.
A further drawback is that since the code is inlined directly into the service method of the generated servlet, the method can grow very large. If it exceeds 64 KB, your JSP compilation will likely fail.
<jsp:include> - The JSP Action tag on the other hand instructs the container to pause the execution of this page, go run the included page, and merge the output from that page into the output from this page.
Each included page is executed as a separate servlet at run time.
Pages can conditionally be included at run time. This is often useful for templating frameworks that build pages out of includes. The parent page can determine which page, if any, to include according to some run-time condition.
The values of scriptlet variables need to be explicitly passed to the include page.
The included page must be able to be run on its own.
You are less likely to run into compilation errors due to the maximum method size being exceeded in the generated servlet class.
Depending on your needs, you may either use <#include> or
<jsp:include>

Main advantage of <jsp:include /> over <%# include > is:
<jsp:include /> allows to pass parameters
<jsp:include page="inclusion.jsp">
<jsp:param name="menu" value="objectValue"/>
</jsp:include>
which is not possible in <%#include file="somefile.jsp" %>

All three template options - <%#include>, <jsp:include> and <%#tag> are valid, and all three cover different use cases.
With <#include>, the JSP parser in-lines the content of the included file into the JSP before compilation (similar to a C #include). You'd use this option with simple, static content: for example, if you wanted to include header, footer, or navigation elements into every page in your web-app. The included content becomes part of the compiled JSP and there's no extra cost at runtime.
<jsp:include> (and JSTL's <c:import>, which is similar and even more powerful) are best suited to dynamic content. Use these when you need to include content from another URL, local or remote; when the resource you're including is itself dynamic; or when the included content uses variables or bean definitions that conflict with the including page. <c:import> also allows you to store the included text in a variable, which you can further manipulate or reuse. Both these incur an additional runtime cost for the dispatch: this is minimal, but you need to be aware that the dynamic include is not "free".
Use tag files when you want to create reusable user interface components. If you have a List of Widgets, say, and you want to iterate over the Widgets and display properties of each (in a table, or in a form), you'd create a tag. Tags can take arguments, using <%#tag attribute> and these arguments can be either mandatory or optional - somewhat like method parameters.
Tag files are a simpler, JSP-based mechanism of writing tag libraries, which (pre JSP 2.0) you had to write using Java code. It's a lot cleaner to write JSP tag files when there's a lot of rendering to do in the tag: you don't need to mix Java and HTML code as you'd have to do if you wrote your tags in Java.

According to:
Java Revisited
Resources included by include directive are loaded during jsp translation time, while resources included by include action are loaded during request time.
Any change on included resources will not be visible in case of include directive until jsp file compiles again. While in case of include action, any change in included resource will be visible in the next request.
Include directive is static import, while include action is dynamic import.
Include directive uses file attribute to specify resources to be included while include action uses page attribute for the same purpose.

jsp:include vs %#include for %#taglib functionality [duplicate]

It seems that there are two methods for templating with JSP. Including files with one of these statements
<%# include file="foo.html" %>
<jsp:include page="foo.html" />
or using JSP tag files
// Save this as mytag.tag
<%# tag description="Description" pageEncoding="UTF-8"%>
<html>
<head>
</head>
<body>
<jsp:doBody/>
</body>
</html>
And in another JSP page call it with
<%# taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:mytag>
<h1>Hello World</h1>
</t:mytag>
So which method should I use? Is one now considered deprecated or are they both valid and cover different use cases?
Edit
Isn't using this tag file the same as using an include?
// Save this as product.tag
<%# tag description="Product templage" pageEncoding="UTF-8"%>
<%# tag import="com.myapp.Product" %>
<%# attribute name="product" required="true" type="com.myapp.Product"%>
Product name: ${product.name} <br/>
Quantity: ${product.quantity} <br/>
And call it on another JSP with
<%# taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:product>
<c:forEach items="${cart.products}" var="product">
<t:product product="${product}"/>
</c:forEach>
</t:product>
That seems to me to be the very same as using an include and passing parameters to it. So are Tag Files the same as includes?

Overview of JSP Syntax Elements
First, to make things more clear, here is a short overview of JSP syntax elements:
Directives: These convey information regarding the JSP page as a
whole.
Scripting elements: These are Java coding elements such as
declarations, expressions, scriptlets, and comments.
Objects and scopes: JSP objects can be created either explicitly or
implicitly and are accessible within a given scope, such as from
anywhere in the JSP page or the session.
Actions: These create objects or affect the output stream in the JSP
response (or both).
How content is included in JSP
There are several mechanisms for reusing content in a JSP file.
The following 4 mechanisms to include content in JSP can be categorized as direct reuse:
(for the first 3 mechanisms quoting from "Head First Servlets and JSP")
1) The include directive:
<%# include file="header.html" %>
Static: adds the content from the value of the file attribute to the current page at translation time. The directive was
originally intended for static layout templates, like HTML headers.
2) The <jsp:include> standard action:
<jsp:include page="header.jsp" />
Dynamic: adds the content from the value of the page attribute to the current page at request time. Was intended more for dynamic
content coming from JSPs.
3) The <c:import> JSTL tag:
<c:import url=”http://www.example.com/foo/bar.html” />
Dynamic: adds the content from the value of the URL attribute to the current page, at request time. It works a lot like
<jsp:include>, but it’s more powerful and flexible: unlike the
other two includes, the <c:import> URL can be from outside the
web Container!
4) Preludes and codas:
Static: preludes and codas can be applied only to the beginnings and ends of pages.
You can implicitly include preludes (also called headers) and codas
(also called footers) for a group of JSP pages by adding
<include-prelude> and <include-coda> elements respectively within
a <jsp-property-group> element in the Web application web.xml deployment descriptor.
Read more here:
• Configuring Implicit Includes at the Beginning and End of JSPs
• Defining implicit includes
Tag File is an indirect method of content reuse, the way of encapsulating reusable content.
A Tag File is a source file that contains a fragment of JSP code that is reusable as a custom tag.
The PURPOSE of includes and Tag Files is different.
Tag file (a concept introduced with JSP 2.0) is one of the options for creating custom tags. It's a faster and easier way to build custom tags.
Custom tags, also known as tag extensions, are JSP elements that allow custom logic and output provided by other Java components to be inserted into JSP pages. The logic provided through a custom tag is implemented by a Java object known as a tag handler.
Some examples of tasks that can be performed by custom tags include operating on implicit objects, processing forms, accessing databases and other enterprise services such as email and directories, and implementing flow control.
Regarding your Edit
Maybe in your example (in your "Edit" paragraph), there is no difference between using direct include and a Tag File. But custom tags have a rich set of features. They can
Be customized by means of attributes passed from the calling page.
Pass variables back to the calling page.
Access all the objects available to JSP pages.
Communicate with each other. You can create and initialize a JavaBeans component, create a public EL variable that refers to that bean in one tag, and then use the bean in another tag.
Be nested within one another and communicate by means of private variables.
Also read this from "Pro JSP 2": Understanding JSP Custom Tags.
Useful reading.
Difference between include directive and include action in
JSP
JSP tricks to make templating
easier
Very informative and easy to understand tutorial from coreservlet.com with beautiful
explanations that include <jsp:include> VS. <%# include %>
comparison table:
Including Files and Applets in JSP
Pages
Another nice tutorial from coreservlets.com related to tag libraries and
tag files:
Creating Custom JSP Tag Libraries: The
Basics
The official Java EE 5 Tutorial with examples:
Encapsulating Reusable Content
Using Tag
Files.
This page from the official Java EE 5 tutorial should give you even
more understanding:
Reusing Content in JSP
Pages.
This excerpt from the book "Pro JSP 2" also discuses why do you need
a Tag File instead of using static include:
Reusing Content with Tag
Files
Very useful guide right from the Oracle documentation:
Static Includes Versus Dynamic Includes
Conclusion
Use the right tools for each task.
Use Tag Files as a quick and easy way of creating custom tags that can help you encapsulate reusable content.
As for the including content in JSP (quote from here):
Use the include directive if the file changes rarely. It’s the fastest mechanism. If your container doesn’t automatically detect changes, you can force the changes to take effect by deleting the main page class file.
Use the include action only for content that changes often, and if which page to include cannot be decided until the main page is requested.

Main advantage of <jsp:include /> over <%# include > is:
<jsp:include /> allows to pass parameters
<jsp:include page="inclusion.jsp">
<jsp:param name="menu" value="objectValue"/>
</jsp:include>
which is not possible in <%#include file="somefile.jsp" %>

According to:
Java Revisited
Resources included by include directive are loaded during jsp translation time, while resources included by include action are loaded during request time.
Any change on included resources will not be visible in case of include directive until jsp file compiles again. While in case of include action, any change in included resource will be visible in the next request.
Include directive is static import, while include action is dynamic import.
Include directive uses file attribute to specify resources to be included while include action uses page attribute for the same purpose.

JSP - when use directives and when jsp tags?

For example, consider you want forward user to another page.
Which solution is better?
<%# page forward = "anotherPage.jsp" %>
or
<jsp:forward page = "anotherPage.jsp" %>
In this case, I really can't find any differences between them.

First of all, forwarding to another page shouldn't be done from a JSP. That's a responsibility of the controller, which should be written in Java (as a servlet, or action of your preferred MVC controller).
Now to answer your question: there is no forward directive. So you can't use it. The only existing directives are page, include and taglib. See http://careerride.com/JSP-directives.aspx (for example) for additional details.

What's the difference between including files with JSP include directive, JSP include action and using JSP Tag Files?

It seems that there are two methods for templating with JSP. Including files with one of these statements
<%# include file="foo.html" %>
<jsp:include page="foo.html" />
or using JSP tag files
// Save this as mytag.tag
<%# tag description="Description" pageEncoding="UTF-8"%>
<html>
<head>
</head>
<body>
<jsp:doBody/>
</body>
</html>
And in another JSP page call it with
<%# taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:mytag>
<h1>Hello World</h1>
</t:mytag>
So which method should I use? Is one now considered deprecated or are they both valid and cover different use cases?
Edit
Isn't using this tag file the same as using an include?
// Save this as product.tag
<%# tag description="Product templage" pageEncoding="UTF-8"%>
<%# tag import="com.myapp.Product" %>
<%# attribute name="product" required="true" type="com.myapp.Product"%>
Product name: ${product.name} <br/>
Quantity: ${product.quantity} <br/>
And call it on another JSP with
<%# taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:product>
<c:forEach items="${cart.products}" var="product">
<t:product product="${product}"/>
</c:forEach>
</t:product>
That seems to me to be the very same as using an include and passing parameters to it. So are Tag Files the same as includes?

Overview of JSP Syntax Elements
First, to make things more clear, here is a short overview of JSP syntax elements:
Directives: These convey information regarding the JSP page as a
whole.
Scripting elements: These are Java coding elements such as
declarations, expressions, scriptlets, and comments.
Objects and scopes: JSP objects can be created either explicitly or
implicitly and are accessible within a given scope, such as from
anywhere in the JSP page or the session.
Actions: These create objects or affect the output stream in the JSP
response (or both).
How content is included in JSP
There are several mechanisms for reusing content in a JSP file.
The following 4 mechanisms to include content in JSP can be categorized as direct reuse:
(for the first 3 mechanisms quoting from "Head First Servlets and JSP")
1) The include directive:
<%# include file="header.html" %>
Static: adds the content from the value of the file attribute to the current page at translation time. The directive was
originally intended for static layout templates, like HTML headers.
2) The <jsp:include> standard action:
<jsp:include page="header.jsp" />
Dynamic: adds the content from the value of the page attribute to the current page at request time. Was intended more for dynamic
content coming from JSPs.
3) The <c:import> JSTL tag:
<c:import url=”http://www.example.com/foo/bar.html” />
Dynamic: adds the content from the value of the URL attribute to the current page, at request time. It works a lot like
<jsp:include>, but it’s more powerful and flexible: unlike the
other two includes, the <c:import> URL can be from outside the
web Container!
4) Preludes and codas:
Static: preludes and codas can be applied only to the beginnings and ends of pages.
You can implicitly include preludes (also called headers) and codas
(also called footers) for a group of JSP pages by adding
<include-prelude> and <include-coda> elements respectively within
a <jsp-property-group> element in the Web application web.xml deployment descriptor.
Read more here:
• Configuring Implicit Includes at the Beginning and End of JSPs
• Defining implicit includes
Tag File is an indirect method of content reuse, the way of encapsulating reusable content.
A Tag File is a source file that contains a fragment of JSP code that is reusable as a custom tag.
The PURPOSE of includes and Tag Files is different.
Tag file (a concept introduced with JSP 2.0) is one of the options for creating custom tags. It's a faster and easier way to build custom tags.
Custom tags, also known as tag extensions, are JSP elements that allow custom logic and output provided by other Java components to be inserted into JSP pages. The logic provided through a custom tag is implemented by a Java object known as a tag handler.
Some examples of tasks that can be performed by custom tags include operating on implicit objects, processing forms, accessing databases and other enterprise services such as email and directories, and implementing flow control.
Regarding your Edit
Maybe in your example (in your "Edit" paragraph), there is no difference between using direct include and a Tag File. But custom tags have a rich set of features. They can
Be customized by means of attributes passed from the calling page.
Pass variables back to the calling page.
Access all the objects available to JSP pages.
Communicate with each other. You can create and initialize a JavaBeans component, create a public EL variable that refers to that bean in one tag, and then use the bean in another tag.
Be nested within one another and communicate by means of private variables.
Also read this from "Pro JSP 2": Understanding JSP Custom Tags.
Useful reading.
Difference between include directive and include action in
JSP
JSP tricks to make templating
easier
Very informative and easy to understand tutorial from coreservlet.com with beautiful
explanations that include <jsp:include> VS. <%# include %>
comparison table:
Including Files and Applets in JSP
Pages
Another nice tutorial from coreservlets.com related to tag libraries and
tag files:
Creating Custom JSP Tag Libraries: The
Basics
The official Java EE 5 Tutorial with examples:
Encapsulating Reusable Content
Using Tag
Files.
This page from the official Java EE 5 tutorial should give you even
more understanding:
Reusing Content in JSP
Pages.
This excerpt from the book "Pro JSP 2" also discuses why do you need
a Tag File instead of using static include:
Reusing Content with Tag
Files
Very useful guide right from the Oracle documentation:
Static Includes Versus Dynamic Includes
Conclusion
Use the right tools for each task.
Use Tag Files as a quick and easy way of creating custom tags that can help you encapsulate reusable content.
As for the including content in JSP (quote from here):
Use the include directive if the file changes rarely. It’s the fastest mechanism. If your container doesn’t automatically detect changes, you can force the changes to take effect by deleting the main page class file.
Use the include action only for content that changes often, and if which page to include cannot be decided until the main page is requested.

Main advantage of <jsp:include /> over <%# include > is:
<jsp:include /> allows to pass parameters
<jsp:include page="inclusion.jsp">
<jsp:param name="menu" value="objectValue"/>
</jsp:include>
which is not possible in <%#include file="somefile.jsp" %>

According to:
Java Revisited
Resources included by include directive are loaded during jsp translation time, while resources included by include action are loaded during request time.
Any change on included resources will not be visible in case of include directive until jsp file compiles again. While in case of include action, any change in included resource will be visible in the next request.
Include directive is static import, while include action is dynamic import.
Include directive uses file attribute to specify resources to be included while include action uses page attribute for the same purpose.

How can I avoid Java code in JSP files, using JSP 2?

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" %>

Develop Reference

Java is a programming language and computing platform first released by Sun Microsystems in 1995.