Jakarta EE Programming/Jakarta Server Pages Syntax

The JavaServer Pages is a technology for inserting dynamic content into an HTML or XML page using a Java servlet container. In other word, instead of sending HTML pages to web clients that are always the same for every one, you can send HTML pages that can be different for each client and each time they receive it (using database data for instance). To explain it, let's have a simple HTML page on a web server. The data flow is simple. To put it simply, the client requests a HTML page by sending the page URL and the server returns the given HTML page:

Client		Server

	URL

		Retrieve the file

Here are the HTML file and the display on the client:

The HTML page	The display
<html> <body> <span style="color: blue;">The current time</span> </body> </html>	The current time

The returned page is always the same. Now we want to display the current time. To do so, we add a servlet container to the web server. The HTML page will no longer exist on the server. When the page will be requested by a client, a Java class will generate the HTML page and the HTML page will be sent to the client. This Java class is called a servlet:

Client		Web server	Servlet container

	URL

		Ask to generate the file
			Execute the Java class

Here are the Java class, the generated HTML file and the display on the client:

The servlet The HTML page The display

package jsp_servlet;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.*;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;

class _myservlet implements javax.servlet.Servlet, javax.servlet.jsp.HttpJspPage {
    public void _jspService(javax.servlet.http.HttpServletRequest request,
    javax.servlet.http.HttpServletResponse response)
    throws javax.servlet.ServletException,
    java.io.IOException {
        javax.servlet.ServletConfig config = …; // Get the servlet config
        Object page = this;
        PageContext pageContext = …;            // Get the page context for this request
        javax.servlet.jsp.JspWriter out = pageContext.getOut();
        HttpSession session = request.getSession(true);
        try {
            out.print("<html>\r\n");
            out.print("<body>\r\n");
            out.print("<span style="color: blue;">");

            DateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
            Calendar cal = Calendar.getInstance();
            out.print(dateFormat.format(cal.getTime()));

            out.print("</span>\r\n");
            out.print( "</body>\r\n" );
            out.print( "</html>\r\n" );
            …
        } catch (Exception _exception) {
            // Clean up and redirect to error page in <%@ page errorPage="myerror.jsp" %>
        }
    }
}

<html>

  <body>
    <span style="color: blue;">2024/10/24 14:47:26</span>
  </body>
</html>

2024/10/24 14:47:26

Now the page is dynamic but the servlet is hard to code and to read. So we will use a JavaServer Page (JSP). A JSP is written like a HTML page. It has the file extension .jsp for HTML page and the file extension .jspx for an XML markup page. In addition to the HTML syntax, it has embedded scriptlets and jsp tags. A scriptlet is a portion of Java code. The servlet container generates a servlet from the JSP and the servlet will be used to generate HTML pages or XML content. The HTML markups will remains as it is. The embedded scriptlets are inserted into the code of the servlet. The jsp tags are transformed into Java code. So let's use a JSP:

Client		Web server	Servlet container

	URL

		Ask to generate the file
			Generate the servlet

			Execute the Java class

Actually, the servlet is generated by the JSP only for the first request call. The same servlet is reused after then. It is generated again when the JSP will change. Now we have a JSP, that generates a servlet, that generates a HTML page, that will be displayed to the client:

The JSP The servlet The HTML page The display

<%@ page errorPage="myerror.jsp" %>
<%@ page import="java.text.DateFormat" %>
<%@ page import="java.text.SimpleDateFormat" %>
<%@ page import="java.util.Calendar" %>

<html>
  <body>
<% DateFormat dateFormat =
              new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
            Calendar cal = Calendar.getInstance();
%>
    <span style="color: blue;">
      <%= dateFormat.format(cal.getTime()) %>
    </span>
  </body>
</html>

package jsp_servlet;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.*;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;

class _myservlet implements javax.servlet.Servlet,
  javax.servlet.jsp.HttpJspPage {
    public void _jspService(
    javax.servlet.http.HttpServletRequest request,
    javax.servlet.http.HttpServletResponse response)
    throws javax.servlet.ServletException,
    java.io.IOException {
        javax.servlet.ServletConfig config = …;
        Object page = this;
        PageContext pageContext = …;
        javax.servlet.jsp.JspWriter out =
          pageContext.getOut();
        HttpSession session = request.getSession(true);
        try {
            DateFormat dateFormat =
              new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
            Calendar cal = Calendar.getInstance();

            out.print("<html>\r\n");
            out.print("<body>\r\n");
            out.print("<span style="color: blue;">\r\n");
            out.print(dateFormat.format(cal.getTime()));
            out.print("\r\n");
            out.print("</span>\r\n");
            out.print( "</body>\r\n" );
            out.print( "</html>\r\n" );
            …
        } catch (Exception _exception) {
        }
    }
}

<html>

  <body>
    <span style="color: blue;">2024/10/24 14:47:26</span>
  </body>
</html>

2024/10/24 14:47:26

Scripting elements

There are three basic kinds of scripting elements that allow java code to be inserted directly into the JSP.

A declaration tag places a variable definition inside the body of the java servlet class. Static data members may be defined as well. Also inner classes can be defined here.

<%! int serverInstanceVariable = 1; %>

Declaration tags also allow methods to be defined.

<%!
   /**
    * Converts the Object into a string or if
    * the Object is null, it returns the empty string.
    */
   public String toStringOrBlank(Object obj) {
       if (obj != null) {
           return obj.toString();
       }
       return "";
   }
%>

A scriptlet tag places all of the statements contained within it, inside the _jspService() method of the java servlet class.

<% int localStackBasedVariable = 1;
out.println(localStackBasedVariable); %>

An expression tag places an expression to be evaluated inside the java servlet class. Expressions should not be terminated with a semi-colon.

<%= "expanded inline data " + 1 %>

A comment tag does nothing. It is ignored. It lets you document the file. It is different from a HTML comment as a HTML comment () will appear in the generated HTML file.

<%-- This is my first JSP. --%>

Directives

JSP directives are added at the top of a JSP page. These directives control how the JSP compiler generates the servlet. The following directives are available:

include: The include directive informs the JSP compiler to include a complete file into the current file. It is as if the contents of the included file were pasted directly into the original file. This functionality is similar to the one provided by the C preprocessor. Included files generally have the extension "jspf" (for JSP Fragment):

<%@ include file="somefile.jspf" %>

page: The page directive has several attributes:

`import`	Results in a Java `import` statement being inserted into the resulting file.
`contentType`	Specifies the content that is generated. This should be used if HTML is not used or if the character set is not the default character set.
`errorPage`	Indicates the address of the page that should be shown if an exception occurs while processing the HTTP request.
`isErrorPage`	If set to true, it indicates that this is the error page. Default value is false.
`isThreadSafe`	True if the resulting servlet must be thread safe.
`autoFlush`	To autoflush the contents. A value of true, the default, indicates that the buffer should be flushed when it is full. A value of false, rarely used, indicates that an exception should be thrown when the buffer overflows. A value of false is illegal when also using buffer="none".
`session`	To maintain session. A value of true (the default) indicates that the predefined variable session (of type `HttpSession`) should be bound to the existing session if one exists, otherwise a new session should be created and bound to it. A value of false indicates that no sessions will be used, and attempts to access the variable session will result in errors at the time the JSP page is translated into a servlet.
`buffer`	To set Buffer Size. The default is 8k and it is advisable that you increase it.
`isELIgnored`	Defines whether Expression Language (EL) expressions are ignored when the JSP is translated.
`language`	Defines the scripting language used in scriptlets, expressions and declarations. Right now, the only possible value is "java".
`extends`	Defines the superclass of the class this JSP will become. Don't use this unless you really know what you're doing — it overrides the class hierarchy provided by the Container.
`info`	Defines a String that gets put into the translated page, just so that you can get it using the generated servlet's inherited `getServletInfo()` method.
`pageEncoding`	Defines the character encoding for the JSP. The default is "ISO-8859-1" (unless the contentType attribute already defines a character encoding, or the page uses XML document syntax).

<%@ page import="java.util.*" %> <%-- example import --%>
<%@ page contentType="text/html" %> <%-- example contentType --%>
<%@ page isErrorPage="false" %> <%-- example for non error page --%>
<%@ page isThreadSafe="true" %> <%-- example for a thread safe JSP --%>
<%@ page session="true" %> <%-- example for using session binding --%>
<%@ page autoFlush="true" %> <%-- example for setting autoFlush --%>
<%@ page buffer="20kb" %> <%-- example for setting Buffer Size --%>

Note: Only the "import" page directive can be used multiple times in the same JSP.
taglib: The taglib directive indicates that a JSP tag library is to be used. The directive requires a prefix (much like a namespace in C++) and the URI for the tag library description.

<%@ taglib prefix="myprefix" uri="taglib/mytag.tld" %>

Example

Regardless of whether the JSP compiler generates Java source code for a servlet or emits the byte code directly, it is helpful to understand how the JSP compiler transforms the page into a Java servlet. For example, consider the following input JSP and its resulting generated Java Servlet.

Input JSP

Resulting servlet

<%@ page errorPage="myerror.jsp" %>
<%@ page import="com.foo.bar" %>
 
<html>
  <head>
  <%! int serverInstanceVariable = 1;%>
 
  <% int localStackBasedVariable = 1; %>
  </head>

  <body>
    <table>
      <%-- This will be ignored. --%>
      <tr><td><%= toStringOrBlank( "expanded inline data " + 1 ) %></td></tr>
    </table>
  </body>
</html>

 package jsp_servlet;
 import java.util.*;
 import java.io.*;
 import javax.servlet.*;
 import javax.servlet.http.*;
 import javax.servlet.jsp.*;
 import javax.servlet.jsp.tagext.*;

 import com.foo.bar; // Imported as a result of <%@ page import="com.foo.bar" %>
 import …

 class _myservlet implements javax.servlet.Servlet, javax.servlet.jsp.HttpJspPage {
    // Inserted as a
    // result of <%! int serverInstanceVariable = 1;%>
    int serverInstanceVariable = 1;
    …

    public void _jspService(javax.servlet.http.HttpServletRequest request,
    javax.servlet.http.HttpServletResponse response)
    throws javax.servlet.ServletException,
    java.io.IOException {
        javax.servlet.ServletConfig config = …; // Get the servlet config
        Object page = this;
        PageContext pageContext = …;            // Get the page context for this request
        javax.servlet.jsp.JspWriter out = pageContext.getOut();
        HttpSession session = request.getSession(true);
        try {
            out.print("<html>\r\n");
            out.print("<head>\r\n");
            …
            // From <% int localStackBasedVariable = 1; %>
            int localStackBasedVariable = 1;
            …
            out.print("</head>\r\n");
            out.print("<body>\r\n");
            out.print("<table>\r\n");
            out.print(" <tr><td>");
            // From <%= toStringOrBlank( "expanded inline data " + 1) %>
            out.print(toStringOrBlank( "expanded inline data " + 1 ));
            out.print(" </td></tr>\r\n");
            out.print("</table>\r\n");
            out.print("</body>\r\n");
            out.print("</html>\r\n");
            …
        } catch (Exception _exception) {
            // Clean up and redirect to error page in <%@ page errorPage="myerror.jsp" %>
        }
    }
 }

Implicit objects

We have seen that we can declare objects in a scriplet that we can use further. There are also already declared objects that can be used by the programmer. They are called implicit object:

`out`	The JspWriter used to write the data to the response stream.
`page`	The servlet itself.
`pageContext`	A PageContext instance that contains data associated with the whole page. A given HTML page may be passed among multiple JSPs.
`request`	The HttpServletRequest object that provides HTTP request information.
`response`	The HttpServletResponse object that can be used to send data back to the client.
`session`	The HttpSession object that can be used to track information about a user from one request to another.
`config`	Provides servlet configuration data.
`application`	Data shared by all JSPs and servlets in the application.
`exception`	Exceptions not caught by application code.

JSP actions

JSP actions are XML tags that invoke built-in web server functionality. They are executed at runtime. Some are standard and some are custom (which are developed by Java developers). The following are the standard ones.

jsp:include

Like the include directive, this tag includes a specified jsp into the returned HTML page but it works differently. The Java servlet temporarily hands the request and response off to the specified JavaServer Page. Control will then return to the current JSP, once the other JSP has finished. Using this, JSP code will be shared between multiple other JSPs, rather than duplicated.

<html>
  <head></head>
  <body>
    <jsp:include page="mycommon.jsp" >
      <jsp:param name="extraparam" value="myvalue" />
    </jsp:include>
    name:<%=request.getParameter("extraparam")%>
  </body>
</html>

jsp:param

Can be used inside a jsp:include, jsp:forward or jsp:params block. Specifies a parameter that will be added to the request's current parameters.

jsp:forward

Used to hand off the request and response to another JSP or servlet. Control will never return to the current JSP.

<jsp:forward page="subpage.jsp" >
  <jsp:param name="forwardedFrom" value="this.jsp" />
</jsp:forward>

In this forwarding example, the request is forwarded to subpage.jsp.

jsp:plugin

Older versions of Netscape Navigator and Internet Explorer used different tags to embed an applet. This action generates the browser specific tag needed to include an applet. The plugin example illustrates an HTML uniform way of embedding applets in a web page. Before the advent of the <OBJECT> tag, there was no common way of embedding applets.

<jsp:plugin type=applet height="100%" width="100%"
   archive="myjarfile.jar, myotherjar.jar"
   codebase="/applets"
   code="com.foo.MyApplet" >
   <jsp:params>
     <jsp:param name="enableDebug" value="true" />
   </jsp:params>
   <jsp:fallback>
     Your browser does not support applets.
   </jsp:fallback>
</jsp:plugin>

Currently, the jsp:plugin tag does not allow for dynamically called applets. For example, jsp:params cannot be used with a charting applet that requires the data points to be passed in as parameters unless the number of data points is constant. You cannot, for example, loop through a ResultSet to create the jsp:param tags. Each jsp:param tag must be hand-coded.

jsp:fallback

The content to show if the browser does not support applets.

jsp:getProperty

Gets a property from the specified JavaBean.

JSP tag libraries

In addition to the pre-defined JSP actions, developers may add their own custom actions using the JSP Tag Extension API. Developers write a Java class that implements one of the Tag interfaces and provide a tag library XML description file that specifies the tags and the java classes that implement the tags.

Consider the following JSP.

<%@ taglib uri="mytaglib.tld" prefix="myprefix" %>
…
<myprefix:myaction> <%-- The start tag --%>
…
</myprefix:myaction> <%-- The end tag --%>
…

The JSP compiler will load the mytaglib.tld XML file:

<taglib>
  <tlib-version>1.0</tlib-version>
  <jsp-version>2.0</jsp-version>
  <short-name>My taglib</short-name>
  <tag>
    <name>myaction</name>
    <tag-class>org.wikibooks.en.MyActionTag</tag-class>
    <body-content>empty</body-content>
  </tag>
</taglib>

The JSP compiler will see that the tag myaction is implemented by the java class MyActionTag:

public class MyActionTag extends TagSupport {
   public MyActionTag() { … }

     // Releases all instance variables.
   public void release() { … }

   // Called for the start tag
   public int doStartTag() { … }

   // Called at the end tag
   public int doEndTag() { … }
}

The first time the tag is used in the file, it will create an instance of MyActionTag. Then (and each additional time that the tag is used), it will invoke the method doStartTag() when it encounters the starting tag. It looks at the result of the start tag, and determines how to process the body of the tag. The body is the text between the start tag and the end tag. The doStartTag() method may return one of the following:

`SKIP_BODY`	The body between the tag is not processed.
`EVAL_BODY_INCLUDE`	Evaluate the body of the tag.
`EVAL_BODY_TAG`	Evaluate the body of the tag and push the result onto stream (stored in the body content property of the tag).

To do:
Add Body Tag description.

Note: If tag extends the BodyTagSupport class, the method doAfterBody() will be called when the body has been processed just prior to calling the doEndTag(). This method is used to implement looping constructs.

When it encounters the end tag, it invokes the doEndTag() method. The method may return one of two values:

`EVAL_PAGE`	This indicates that the rest of the JSP file should be processed.
`SKIP_PAGE`	This indicates that no further processing should be done. Control leaves the JSP page. This is what is used for the forwarding action.

If you want to iterate the body a few times, your java class (tag handler) must implement the IterationTag interface. It returns EVAL_BODY_AGAIN — which means to invoke the body again.

JSP Standard Tag Library (JSTL)

The JavaServer Pages Standard Tag Library (JSTL) is a component of the Java EE Web application development platform. It extends the JSP specification by adding a tag library of JSP tags for common tasks, such as XML data processing, conditional execution, loops and internationalization.

Struts Tag Library

The Struts project includes the Struts Tag Library, the bulk of which is useful independently of the Struts architecture.

Internationalization

Internationalization in JSP is accomplished the same way as in a normal Java application, that is by using resource bundles.

Expression Language

The Expression Language (EL) is a faster/easier way to display parameter values. It is available since JSP 2.0 . For instance, it allows developers to create Velocity-style templates:

Hello, ${param.visitor}

Same as

Hello, <%=request.getParameter("visitor")%>

It also offers a clearer way to navigate nested beans. Consider some beans:

class Person {
   String name;
   // Person nests an organization bean.
   Organization organization;

   public String getName() { return this.name; }
   public Organization getOrganization() { return this.organization; }
}

class Organization {
   String name;
   public String getName() { return this.name; }
}

Then, if an instance of Person was to be placed onto a request attribute under the name "person", the JSP would have:

 Hello, ${person.name}, of company ${person.organization.name}

Same as.

 Hello,
<% Person p = (Person) request.getAttribute("person");
   if (p != null) {
       out.print(p.getName());
   }
%>, of company
<% if ((p != null) && (p.getOrganization() != null)) {
       out.print(p.getOrganization().getName());
   }
%>

JSP Technology in the Java EE 5 Platform

The focus of Java EE 5 has been ease of development by making use of Java language annotations that were introduced by J2SE 5.0. JSP 2.1 supports this goal by defining annotations for dependency injection on JSP tag handlers and context listeners.

Another key concern of the Java EE 5 specification has been the alignment of its webtier technologies, namely JavaServer Pages (JSP), JavaServer Faces (JSF), and JavaServer Pages Standard Tag Library (JSTL).

The outcome of this alignment effort has been the Unified Expression Language (EL), which integrates the expression languages defined by JSP 2.0 and JSF 1.1.

The main key additions to the Unified EL that came out of the alignment work have been: A pluggable API for resolving variable references into Java objects and for resolving the properties applied to these Java objects, Support for deferred expressions, which may be evaluated by a tag handler when needed, unlike their regular expression counterparts, which get evaluated immediately when a page is executed and rendered, and Support for lvalue expression, which appear on the left hand side of an assignment operation. When used as an lvalue, an EL expression represents a reference to a data structure, for example: a JavaBeans property, that is assigned some user input. The Unified EL is defined in its own specification document, which is delivered along with the JSP 2.1 specification.

Thanks to the Unified EL, JSTL tags, such as the JSTL iteration tags, can be used with JSF components in an intuitive way.

JSP 2.1 leverages the Servlet 2.5 specification for its web semantics.

Model-view-controller paradigm

A model-view-controller pattern can be used with the JSP files in order to split the presentation from request processing and computer data storage. Either regular servlets or separate JSP files are used to process the request. After the request processing has finished, control is passed to a JSP used only for creating the output. There are several platforms based on Model-view-controller pattern for web tiers (such as Barracuda, Apache Struts, Stripes, and the Spring MVC framework).

Environment

Both the Java Server (J2EE specification) and the page scripts and/or extended customised programming added operate by (in the runtime context of being loaded programs used) a special pre-installed base program called a Virtual Machine that integrates with the host Operating System, this type being the Java Virtual Machine (JVM).

Because either, both a Compiler-JVM set (called an SDK or JDK) or the lone JVM (called a JRE, Java Runtime Environment) is made for most computer platform OSs and the compiled programs for the JVM are compiled into special Java Byte code files for the JVM the Byte-code files (compiled Java program .class files) can be effectively transferred between platforms with no requirement to be recompiled excepting versioning compatibility, or special circumstance. The source code for these J2EE servlet or J2EE JSP programs is almost always supplied with J2EE JSP material and J2EE Web Applications because the server must call the compiler when loading them. These small extension programs (custom tags, servlets, beans, page scripting) are variable and likely to be updated or changed either shortly before runtime or intermittently but particularly when sending JSP page requests themselves, it requires the JSP server to have access to a Java compiler (SDK or JDK) and the required source code (not simply the JVM JRE and byte code class files) to successfully exploit the method of serving.

JSP syntax has two basic forms, scriptlet and markup though fundamentally the page is either HTML or XML markup. Scriptlet tagging (called Scriptlet Elements) (delimited) blocks of code with the markup are not effectively markup and allows any java server relevant API (e.g. the servers running binaries themselves or database connections API or java mail API) or more specialist JSP API language code to be embedded in an HTML or XML page provided the correct declarations in the JSP file and file extension of the page are used. Scriptlet blocks do not require to be completed in the block itself only the last line of the block itself being completed syntactically correctly as a statement is required, it can be completed in a later block. This system of split inline coding sections is called step over scripting because it can wrap around the static markup by stepping over it. At runtime (during a client request) the code is compiled and evaluated, but compilation of the code generally only occurs when a change to the code of the file occurs. The JSP syntax adds additional XML-like tags, called JSP actions, to be used to invoke built-in functionality. Additionally, the technology allows for the creation of JSP tag libraries that act as extensions to the standard HTML or XML tags. JVM operated Tag libraries provide a platform independent way of extending the capabilities of a Web server. Note that not all company makes of Java servers are J2EE specification compliant.

To do:
Add code and examples.