JavaServer Pages (JSP) is a Java technology that allows
software developers to dynamically generate HTML,
XML or other types of documents in response to a Web client
request. The technology allows Java code and certain pre-defined actions to be embedded into static content.
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. Tag libraries provide a platform independent way of extending the
capabilities of a Web server.
JSPs are compiled into Java Servlets by a JSP compiler. A JSP compiler may generate a servlet in Java code that is then compiled by the
Java compiler, or it may generate byte code for the servlet directly. JSPs can also be
interpreted on-the-fly reducing the time taken to reload changes.
JSP and Servlets
Architecturally, JSP can be viewed as a high-level abstraction of servlets that is
implemented as an extension of the Servlet 2.1 API. Both servlets and JSPs were originally developed at Sun Microsystems, initially created by Anselm Baird-Smith and later
elaborated on as a specification by Satish Dharmaraj. Starting with version 1.2 of the JSP
specification, JavaServer Pages have been developed under the Java Community
Process. JSR 53 defines both the JSP 1.2 and Servlet 2.3 specifications and JSR 152 defines the JSP 2.0 specification. As
of May 2006 the JSP 2.1 specification has been released under JSR 245 as part of
Java EE 5.
JSP syntax
A JavaServer Page may be broken down into the following pieces:
- static data such as HTML
- JSP directives such as the include directive
- JSP scripting elements and variables
- JSP actions
- custom tags with correct library
JSP directives
JSP 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
- There are several options to the page directive.
- 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 page that will 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
- Indicates if the resulting servlet is 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 advisible that you increase it.
- isELIgnored
- Defines whether 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. You won'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 genrated 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=20 %> //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 that a prefix be specified (much
like a namespace in C++) and the URI for the tag library description.
<%@ taglib prefix="myprefix" uri="taglib/mytag.tld" %>
JSP scripting elements and objects
JSP implicit objects
The following JSP implicit objects are exposed by the JSP container and can be referenced by the programmer:
- 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 HTTP response object that can be used to send data back to the client.
- session
- The HTTP session 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 .
Scripting elements
There are three basic kinds of scripting elements that allow java code to be inserted directly into the servlet.
- A declaration tag places a variable definition inside the body of the java servlet class. Static data members may be
defined as well.
<%! 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 the contained statements 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 %>
- Also we can use the following tag to give comments in jsp:
<%-- give your comments here --%>
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 list contains the standard ones:
- jsp:include
- Similar to a subroutine, 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.
- 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: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.
- jsp:fallback
- The content to show if the browser does not support applets.
- jsp:getProperty
- Gets a property from the specified JavaBean.
- jsp:setProperty
- Sets a property in the specified JavaBean.
- jsp:useBean
- Creates or re-uses a JavaBean available to the JSP page.
Examples of tags
jsp:include
<html>
<head></head>
<body>
<jsp:include page="mycommon.jsp" >
<jsp:param name="extraparam" value="myvalue" />
</jsp:include>
name:<%=request.getParameter("extraparam")%>
</body>
</html>
jsp:forward
<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". The request handling does not return to this page.
jsp:plugin
<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>
The plugin example illustrates a <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. 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. However, each
of those jsp:param tags can have a dynamic name and a dynamic value.
jsp:useBean
<jsp:useBean id="myBean" class="com.foo.MyBean" scope="request" />
<jsp:getProperty name="myBean" property="lastChanged" />
<jsp:setProperty name="myBean" property="lastChanged" value="<%= new Date()%>" />
The scope attribute can be request, page, session or application. It has the following meanings:
- request
- the attribute is available for the lifetime of the request. Once the request has been processed by all of the JSPs, the
attribute will be de-referenced.
- page
- the attribute is available for the current page only.
- session
- the attribute is available for the lifetime of the user's session.
- application
- the attribute is available to every instance and is never de-referenced. Same as a global
variable.
The example above will use a Bean Manager to create an instance of the class
com.foo.MyBean and store the instance in the attribute named "myBean". The attribute will be available for the
life-time of the request. It can be shared among all of the JSPs that were included or forwarded-to from the main JSP that first
received the request.
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 and see that the tag 'myaction' is implemented by the java class
'MyActionTag'. 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).
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.
The myaction tag above would have an implementation class that looked like something below:
public class MyActionTag extends TagSupport {
//Releases all instance variables.
public void release() {...}
public MyActionTag() { ... }
//called for the start tag
public int doStartTag() { ... }
//called at the end tag
public int doEndTag(){ ... }
}
Add Body Tag description.
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.
Internationalization
Internationalization in JSP is accomplished the same way as in
a normal Java application, that is by using resource bundles.
JSP 2.0
The new version of the JSP specification includes new features meant to improve programmer productivity. Namely:
Hello, ${param.visitor} <%-- same as: Hello, <%=request.getParameter("param").getVisitor()%> --%>
Model-view-controller paradigm
Sun recommends that the Model-view-controller pattern 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 or Spring framework).
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 <source lang=java>
<%@ page errorPage="myerror.jsp" %>
<%@ page import="com.foo.bar" %>
<html>
<head>
<%! int serverInstanceVariable = 1;%>
...
<% int localStackBasedVariable = 1; %>
...
</source>
Resulting servlet <source lang=java>
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( "\r\n" ); out.print( " \r\n" ); ... } catch ( Exception _exception ) { //clean up and redirect to error page in
<%@ page errorPage="myerror.jsp" %> } } } </source>
See also
Further reading
- Bergsten, Hans (2003). JavaServer Pages, 3rd
Edition, O'Reilly Media. ISBN 978-0-596-00563-4.
- Kathy, Sierra; Bert Bates & Bryan Basham. Head First Servlets &
JSP. O'Reilly Media. ISBN 978-0-596-00540-5.
External links
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