jsp – java server pages part 1 representation and management of data on the internet
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JSP – Java Server PagesJSP – Java Server PagesPart 1Part 1
Representation and Management of Data on the Internet
IntroductionIntroduction
What is JSP Good For?What is JSP Good For?
• Servlets allow us to write dynamic Web pages
- Easy access to request, session and context data
- Easy manipulation of the response (cookies, etc.)
- And lots more...
• It is not convenient to write and maintain long
static HTML using Servlets
out.println("<h1>Bla Bla</h1>" + "bla bla bla bla"
+ "lots more here...")
JSP IdeaJSP Idea
• Use HTML for most of the page
• Write Servlet code directly in the HTML page, marked with special tags
• The server automatically translates a JSP page to a Servlet class and the latter is actually invoked- In Tomcat 5.0, you can find the generated Servlet
code under $CATALINA_BASE/work/
RelationshipsRelationships
• Servlets: HTML code is printed in Java code
• JSP: Java code is embedded in HTML code
Java
HTMLJava
HTML
ExampleExample
<html>
<head>
<title>Hello World</title>
</head>
<body>
<h2><%= new java.util.Date() %></h2>
<h1>Hello World</h1>
</body>
</html>
Tomcat 5.0 Generated Servlet
Generated Servlet HierarchyGenerated Servlet Hierarchy(Tomcat 5.0 Implementation)(Tomcat 5.0 Implementation)
Apache
Implementation
Generated
Servlet
Sun
Specifications
GenericServlet
Servlet
JspPage
HttpJspPageHttpServlet
HttpJspBase
mypage_jsp
JSP Limitations and AdvantagesJSP Limitations and Advantages
• JSP can only do what a Servlet can do
• Easier to write and maintain HTML
• Easier to separate HTML from code
• Can use a "reverse engineering technique": create static HTML and then replace static data with Java code
JSP Life CycleJSP Life Cycle
JSP Life CycleJSP Life Cycle
The following table describes the life cycle of JSP generated Servlet in details:
JSP Life CycleJSP Life Cycle
Page first w
ritten
Request#1
Request#2
Server restarted
Request#3
Request#4
Page m
odified
Request#5
Request#6
JSP page translated into servlet
Yes No No No Yes No
Servlet compiled Yes No No No Yes No
Servlet instantiated and loaded into server's memory
Yes No Yes No Yes No
init (or equivalent) called
Yes No Yes No Yes No
doGet (or equivalent) called
Yes Yes Yes Yes Yes Yes
Written by Marty Hall. Core Servlets & JSP book: www.coreservlets.com
JSP TranslationJSP Translation
• When the JSP file is modified, JSP is translated into a Servlet - Application need not be reloaded when JSP file is modified
• Server does not generate the Servlet class after startup, if the latter already exists- Generated Servlet acts just like any other Servlet
init()init() and and destroy()destroy()
• init() of the generated Servlet is called every time the Servlet class is loaded into memory and instantiated
• destroy() of the generated Servlet is called every time the generated Servlet is removed
• The latter two happen even if the reason is modification of the JSP file
Thread SynchronizationThread Synchronization
• After the Servlet is generated, one instance of it serves requests in different threads, just like any other Servlet
• In particular, the service method (_jspService) may be executed by several concurrent threads
• Thus, like Servlets, JSP programming requires concurrency management
Basic JSP ElementsBasic JSP Elements
Basic Elements in a JSP fileBasic Elements in a JSP file
• HTML code: <html-tag>content</html-tag>
• JSP Comments: <%-- comment --%>
• Expressions: <%= expression %>
• Scriptlets: <% code %>
• Declarations: <%! code %>
• Directives: <%@ directive attribute="value" %>
• Actions: <jsp:forward.../>, <jsp:include.../>
• EL Expressions: ${expression} Covered Later...
JSP ExpressionsJSP Expressions
• A JSP expression is used to insert Java values directly into the output
• It has the form: <%= expression %> , where expression can be a Java object, a numerical expression, a method call that returns a value, etc...
• For example:
<%= new java.util.Date() %>
<%= "Hello"+" World" %>
<%= (int)(100*Math.random()) %>
JSP ExpressionsJSP Expressions
• A JSP Expression is evaluated
• The result is converted to a string
• The string is inserted into the page
• This evaluation is performed at runtime (when the page is requested), and thus has full access to information about the request, the session, etc...
Expression TranslationExpression Translation
<h1>A Random Number</h1><%= Math.random() %>
public void _jspService(HttpServletRequest request, HttpServletResponse response)
throws java.io.IOException, ServletException { ... response.setContentType("text/html"); ... out.write("<h1>A Random Number</h1>\r\n"); out.print( Math.random() );
out.write("\r\n"); ...
}
Predefined Variables (Implicit Objects)Predefined Variables (Implicit Objects)
• The following predefined variables can be used:
- request: the HttpServletRequest
- response: the HttpServletResponse
- session: the HttpSession associated with the request
- out: the PrintWriter (a buffered version of type JspWriter) used to fill the response content
- application: The ServletContext
• These variables and more will be discussed in details
<html>
<head>
<title>JSP Expressions</title>
</head>
<body>
<h2>JSP Expressions</h2>
<ul>
<li>Current time: <%= new java.util.Date() %></li>
<li>Your hostname:<%= request.getRemoteHost() %></li>
<li>Your session ID: <%= session.getId() %></li>
<li>The <code>testParam</code> form parameter:
<%= request.getParameter("testParam") %></li>
</ul>
</body>
</html>
JSP ScripletsJSP Scriplets
• JSP scriptlets let you insert arbitrary code into the Servlet service method ( _jspService )
• Scriptlets have the form: <% Java Code %> • The code is inserted verbatim into the service
method, according to the location of the scriplet
• Scriptlets have access to the same automatically defined variables as expressions
<%= foo() %> <% bar(); %>
public void _jspService(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
...response.setContentType("text/html");...out.print(foo());bar();...
}
Scriptlet TranslationScriptlet Translation
An Interesting ExampleAn Interesting Example
Scriptlets don't have to be complete code blocks:
<% if (Math.random() < 0.5) { %> You <b>won</b> the game! <% } else { %> You <b>lost</b> the game! <% } %>
if (Math.random() < 0.5) { out.write("You <b>won</b> the game!"); } else { out.write("You <b>lost</b> the game!"); }
JSP DeclarationsJSP Declarations
• A JSP declaration lets you define methods or members that get inserted into the Servlet class (outside of all methods)
• It has the following form: <%! Java Code %>
• For example: <%! private int someField = 5; %>
<%! private void someMethod(...) {...} %>
• It is usually of better design to define methods in a separate Java class...
Declaration ExampleDeclaration Example
• Print the number of times the current page has been requested since the Servlet initialization:
<%! private int accessCount = 0; %>
<%! private synchronized int incAccess() {
return ++accessCount;
} %>
<h1>Accesses to page since Servlet init:
<%= incAccess() %> </h1>
public class serviceCount_jsp extends... implements...
throws... {
private int accessCount = 0;
private synchronized int incAccess() {
return ++accessCount;
}
public void _jspService(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
...
...
out.write("<h1>Accesses to page since Servlet init: ");
out.print(incAccess());
... } ... }
jspInit jspInit andand jspDestroy jspDestroy
• In JSP pages, like regular Servlets, we sometimes want to implement init and destroy
• It is illegal to use JSP declarations to override init or destroy, since they are (usually) already implemented by the generated Servlet
• Instead, override the methods jspInit and jspDestroy- The generated servlet is guaranteed to call these methods from
init and destroy, respectively
- The standard versions of jspInit and jspDestroy are empty (placeholders for you to override)
JSP DirectivesJSP Directives
• A JSP directive affects the structure of the
Servlet class that is generated from the JSP page
• It usually has the following form:
<%@ directive attribute1="value1" ...
attributeN="valueN" %>
• Three directives: page, include and taglib
• include and taglib will be discussed later
pagepage-Directive Attributes-Directive Attributes
• import attribute: A comma separated list of
classes/packages to import
<%@ page import="java.util.*, java.io.*" %>
• contentType attribute: Sets the MIME-Type of the
resulting document (default is text/html)
<%@ page contentType="text/plain" %>
• What is the difference between setting the contentType
attribute, and writing <
%response.setContentType("...");%> ?
More More pagepage-Directive Attributes-Directive Attributes
• session="true|false" - use a session?
• buffer="sizekb|none"
- Specifies the content-buffer size (out)
• autoFlush="true|false"
- Specifies whether the buffer should be flushed when
it fills, or throw an exception otherwise
• isELIgnored ="true|false"
- Specifies whether JSP expression language is used
- EL is discussed later
Variables in JSP Variables in JSP
Implicit ObjectsImplicit Objects
• As seen before, some useful variables, like request and session are predefined
• These variables are called implicit objects
• Implicit objects are defined in the scope of the service method- Can these be used in JSP declarations?
• Implicit objects are part of the JSP specifications
The objectsThe objects request request and and responseresponse
• request and response are the HttpServletRequest and HttpServletResponse arguments of the service method
• Using these objects, you can:
• Read request parameters
• Set response headers
• etc. (everything we learned in Servlet lectures)
The objectThe object out out
• This is the Writer used to add write output into the response body
• This object implements the interface JspWriter that combines the functionality of PrintWriter and BufferedWriter
• Recall that you can adjust the buffer size, or turn buffering off, through use of the buffer attribute of the page directive
The objectThe object pageContext pageContext
• pageContext is a new object introduced by JSP
• This object stores all important elements used by
the generated Servlet, like the application
context, the session, the output writer, etc.
• It enable vendors to elegantly extend their JSP
implementation
• This object is also used to store page-scoped
attributes (e.g., Java Beans - discussed later)
The objectThe object session session
• This is the HttpSession object associated with the request
• If the session attribute in the page directive is turned off (<%@ page session="false" %>) then this object is not available
• Recall that a session is created by default
The objectThe object config config
• This is the ServletConfig of the page, as received in the init() method
• Remember: contains Servlet specific initialization parameters
• Later, we will study how initialization parameters are passed to JSP pages
• Recall that you can also obtain the ServletContext from config
The objectThe object application application
• This is the ServletContext as obtained via
getServletConfig().getContext()
• Remember:
- The ServletContext is shared by all Web-application
Servlets (including ones generated from JSP)
- Getting and setting attributes is with getAttribute and
setAttribute of ServletContext
- You can use this object to get application-wide
initialization parameters
Page ScopePage Scope
• service() local variables
• pageContext attributes
client 1
client 2
a.jsp
b.jsp
Request ScopeRequest Scope
• request attributes
client 1
client 2
a.jsp
b.jsp
client 1
client 2
a.jsp
b.jsp
Session ScopeSession Scope
• session attributes
client 1
client 2
a.jsp
b.jsp
Servlet ScopeServlet Scope
• Servlet members
client 1
client 2
a.jsp
b.jsp
Application ScopeApplication Scope
• application attributes
client 1
client 2
a.jsp
b.jsp
Servlet Package and Helper ClassesServlet Package and Helper Classes
• The generated Servlet has a named package
• In Tomcat, this package is: org.apache.jsp
• In Java, you cannot use classes from the default package (i.e. with no package declaration) from a named package!
• Therefore, helper classes used by JSP pages must have a named package
JSP and XMLJSP and XML
Simple XML ProductionSimple XML Production
<?xml version="1.0"?>
<!DOCTYPE colors SYSTEM "colors.dtd">
<?xml-stylesheet type="text/xsl" href="colors.xsl"?>
<%! static String[] colors = {"red","blue","green"}; %>
<%@ page contentType="text/xml" %>
<colors>
<% for(int i=0; i<3; ++i) { %>
<color id="<%=i%>"><%= colors[i] %></color>
<% } %>
</colors>
Generated XMLGenerated XML
<?xml version="1.0"?>
<!DOCTYPE colors SYSTEM "colors.dtd">
<?xml-stylesheet type="text/xsl" href="colors.xsl"?>
<colors>
<color id="0">red</color>
<color id="1">blue</color>
<color id="2">green</color>
</colors>
• JSPX files are JSP files that have the extension jspx and have XML syntax
• Non-XML symbols <%, <%@, etc. are replaced with special JSP tags
• Default content type of JSPX is text/xml (and not text/html)
• Thus, JSPX files generate XML and can be edited using XML tools
JSPX FilesJSPX Files
<%= Expression %><jsp:expression> Expression
</jsp:expression>
<% Code %><jsp:scriptlet>
Code
</jsp:scriptlet>
<%! Declaration %><jsp:declaration> Declaration
</jsp:declaration>
<%@ Directive %><jsp:directive.typeAttribute="value"/>
Problems with JSPXProblems with JSPX
• The XML declaration (<?xml version="1.0"?>) and the DOCTYPE definition are now those of the JSPX file. How do we include those in the result XML?- Solution: use the <jsp:output> tag to explicitly require
DOCTYPE and XML declarations
• How do we generate dynamic attribute values and still keep the document well formed?- Solution 1: use <jsp:element> for explicit element construction
- Solution 2: use an EL expression (discussed later)
<colors xmlns:jsp="http://java.sun.com/JSP/Page">
<jsp:output doctype-root-element="colors"
doctype-system="colors.dtd" />
<jsp:output omit-xml-declaration="false"/>
<jsp:declaration>static String[] colors = {"red","blue","green"};
</jsp:declaration><jsp:scriptlet><![CDATA[ for(int i=0; i<3; ++i) { ]]></jsp:scriptlet>
<jsp:element name="color"><jsp:attribute name="id">
<jsp:expression>i</jsp:expression></jsp:attribute>
<jsp:expression>colors[i]</jsp:expression></jsp:element><jsp:scriptlet>}</jsp:scriptlet>
</colors>
JSP as JSPXJSP as JSPX
• You can tell the container that a JSP file acts as a JSPX file
• Use the <jsp:root> element as the document root
<jsp:root version="2.0" xmlns:jsp="http://java.sun.com/JSP/Page"> <jsp:output doctype-root-element="colors" doctype-system="colors.dtd" />
<jsp:output omit-xml-declaration="false"/>
<![CDATA[<?xml-stylesheet type="text/xsl" href="colors.xsl"?>]]>
<colors >
<jsp:declaration>static String[] colors = {"red","blue","green"};
</jsp:declaration>
<jsp:scriptlet><![CDATA[ for(int i=0; i<3; ++i) { ]]></jsp:scriptlet>
<jsp:element name="color">
<jsp:attribute name="id">
<jsp:expression>i</jsp:expression></jsp:attribute>
<jsp:expression>colors[i]</jsp:expression>
</jsp:element>
<jsp:scriptlet>}</jsp:scriptlet>
</colors>
</jsp:root>
Using External ParametersUsing External Parameters
JSP Initial ParametersJSP Initial Parameters
• Like Servlets, initialization parameters can be passed to JSP files using the <servlet> element of the application configuration file web.xml
• Use the sub-element <jsp-file> instead of the sub-element <servlet-class>
• A <servlet-mapping> is also needed
- Use the real JSP URL as the <url-pattern>
<web-app>
<context-param>
<param-name>dbLogin</param-name>
<param-value>snoopy</param-value>
</context-param>
<context-param>
<param-name>dbPassword</param-name>
<param-value>snoopass</param-value>
</context-param>
An ExampleAn Example
web.xml
<servlet>
<servlet-name>ParamPage</servlet-name>
<jsp-file>/paramPage.jsp</jsp-file>
<init-param>
<param-name>tableName</param-name>
<param-value>users</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>ParamPage</servlet-name>
<url-pattern>/paramPage.jsp</url-pattern>
</servlet-mapping>
</web-app>
web.xml
<html>
<head><title>JSP initial parameters</title></head>
<body>
<h1>Hello</h1>
<h2>I should use the table
<i><%= config.getInitParameter("tableName") %></i>
</h2>
<h2>To access the Database, I should use the login
<i><%= application.getInitParameter("dbLogin") %></i>
and the password
<i><%= application.getInitParameter("dbPassword") %></i>.
</h2>
</body>
</html>
paramPage.jsp