JavaServer Pages
JSP or JavaServer Pages is a Java technology that allows developers to dynamically generate HTML, XML or some other type of web page. The technology allows Java code and certain pre-defined actions to be embedded into static content.
The JSP syntax adds additional XML tags, called JSP actions, to be used to invoke built-in functionally. 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 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.
In either case, it is helpful to understand how the JSP compiler transforms the page into a Java servlet.
For an example, see the following input, and its resulting generated java servlet.
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
Static Data
Static data is written out to the HTTP response exactly as it appears in the input file. Thus a valid JSP input would be a normal HTML with no embedded java or actions. In that case, the same data would be sent in the response each and every time by the web server. Of course, the point of JSP is to allow dynamic data to be inserted into the static content.
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 functionally is similar to the one provided by the C preprocessor.
<%@ include file="somefile.ext" %>
- 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. |
| isThreadSafe | indicates if the resulting servlet is thread safe. |
<%@ 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
Note: Only the "import" 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 Variables
Standard Scripting Variables
The following scripting variables are always available:
- 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 HTTP request object.
- response – The HTTP response object.
- session – The Netscape and Microsoft IE 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 plug-in example illustrates a uniform way of embedding applets in a web page. Before the advent of the <OBJECT> tag, there was no common way of embedding applets. This tag is poorly designed and hopefully future specs will allow for dynamic attributes (height="${param.height}", code="${chart}", etc) and dynamic parameters. Currently, the jsp:plugin tag does not allow for dynamically called applets. For example, if you have a charting applet that requires the data points to be passed in as parameters, you can't use jsp:params unless the number of data points are constant. You can't, for example, loop through a ResultSet to create the jsp:param tags. You have to hand code each jsp:param tag. Each of those jsp:param tags however 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 static (i.e. global) variable.
The example above will use a Internationalization in JSP is accomplished the same way as in a normal Java application, that is by using Velocity style templates (among other things).
- A faster/easier way to create new tags.
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 data storage. 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. This JSP should contain only HTML, XML plus either the built-in JSP actions or custom JSP tag extensions. The JSP file may make use of Java beans to get the data.
See also
