jsp – java server page
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JSP – Java Server Page
DBI – Representation and Management of Data on the Internet
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What is JSP
• http://java.sun.com/products/jsp• A way to create dynamic web pages• Based on Java Technology
– Large library base– Platform independence
• Server side processing• Separates the graphical design from the
dynamic content
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Relationships
• In servlets,– HTML code is printed from java code
• In JSP pages– Java code is embadded in HTML code
Java
HTMLJava
HTML
Template text
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In General Lines
• JSP-enabled server – picks up .jsp page
– parses it
– converts it to runnable form
– runs it
• Converts page to a Java servlet (JspPage), with your code inside the _jspService() method– compiles it
– runs servlet protocol
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Separating Graphical Design and Dynamic Content
• Not a new idea (e.g. PHP, mod_perl, shtml, ASP)• Graphical Design and System Design are two
separate and distinct specialities:– Different languages (HTML vs. Java) – Different goals– Different Training
• Should be separated for optimal project management
• JSP does this by allowing special Java tags in HTML
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An Example<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
Transitional//EN">
<HTML><HEAD><TITLE>Hello World Example</TITLE></HEAD>
<BODY><H2>Hello World Example</H2><B><% out.println("Hello World"); %></B></BODY></HTML>
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Translating and Executing JSP Pages• A JSP page is executed in a JSP container,
generally installed in a Web server
– Think of a “JSP container” as a JVM with suitable software installed
• The underlying semantic model is that of a servlet
• A typical JSP container will translate the JSP page to a Java servlet
• By default, translation and compilation of a JSP page is likely to occur the first time the page is accessed
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The Source Code
• In Tomcat 3.2.1, you can find the generated Java and the class files in a subdirectory under tomcat_home/work.
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Translation<H1>A Random Number</H1><%= Math.random() %>
public void _jspService(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setContentType("text/html");HttpSession session = request.getSession(true);JspWriter out = response.getWriter();out.println("<H1>A Random Number</H1>");out.println(Math.random());...
}
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JSP Features
• Standard directives guiding translation of a JSP page to a servlet
• Standard actions in the form of predefined JSP tags
• Script language declarations, scriptlets, and expressions for including Java (or other language) fragments that embed dynamic content
• A portable tag extension mechanism, for building tag libraries—effectively extending the JSP language
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More Details
Writing JSP Pages
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Template HTML
• The HTML code that wraps the code is like a template
• Created as an ordinary HTML
• The dynamic parts are created on runtime and are inserted into the template
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JSP Scripting Elements
• JSP scripting elements let you insert Java code into the servlet that will be generated from the JSP page
• There are three forms: 1. Expressions of the form <%= expression %> that
are evaluated and inserted into the output, 2. Scriptlets of the form <% code %> that are inserted
into the servlet's service method, and 3. Declarations of the form <%! code %> that are
inserted into the body of the servlet class, outside of any existing methods
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JSP Expressions
• A JSP expression is used to insert Java values directly into the output
• It has the following form: <%= Java Expression %>
• The Java expression is – evaluated, – converted to a string, and – 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
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Predefined Variables
• The following predefined variables can be used:– request, the HttpServletRequest– response, the HttpServletResponse – session, the HttpSession associated with the
request (if any)– out, the PrintWriter (a buffered version of
type JspWriter) used to send output to the client
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Examples
• For example, the following shows the date/time that the page was requested:
Current time: <%= new java.util.Date() %>
• For example, the following shows the hostname:
Your hostname:
<%= request.getRemoteHost() %>
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<HTML> <HEAD>
<TITLE>JSP Expressions</TITLE></HEAD><BODY><H2>JSP Expressions</H2><UL><LI>Current time: <%= new java.util.Date() %><LI>Your hostname: <%= request.getRemoteHost() %><LI>Your session ID: <%= session.getId() %><LI>The <CODE>testParam</CODE> form parameter:<%= request.getParameter("testParam") %></UL></BODY></HTML>
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JSP Scriplets
• JSP scriptlets let you insert arbitrary code into the servlet method that will be built to generate the page ( _jspService )
• Scriptlets have the following form: <% Java Code %>
• Scriptlets have access to the same automatically defined variables as expressions
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Producing Code
• Scriplets produce output HTML by printing into the out variable
• Example:
<%
String queryData = request.getQueryString();
out.println("Attached GET data: " + queryData);
%>
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HTML Code in Scriptlets• HTML code before and after the scriplets is
converted to print methods:
<% if (Math.random() < 0.5) { %> You <B>won</B> the game! <% } else { %> You <B>lost</B> the game! <% } %>
if (Math.random() < 0.5) { out.println("You <B>won</B> the game!"); } else { out.println("You <B>lost</B> the game!"); }
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<%= foo() %><% bar(); %>
public void _jspService(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
request.setContentType("text/html");HttpSession session = request.getSession(true);JspWriter out = response.getWriter();out.println(foo());bar();...
}
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Example of Using Scriplets (1)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><HTML><HEAD><TITLE>Color Testing</TITLE></HEAD><%String bgColor = request.getParameter("bgColor");boolean hasExplicitColor;if (bgColor != null) {
hasExplicitColor = true;} else {
hasExplicitColor = false;bgColor = "WHITE";
}%>
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Example of Using Scriplets (2)<BODY BGCOLOR="<%= bgColor %>"><H2 ALIGN="CENTER">Color Testing</H2><%if (hasExplicitColor) {
out.println("You supplied an explicit background color of "+ bgColor + ".");
} else {out.println("Using default background color ” + “of WHITE. Supply the bgColor request ” + “attribute to try a standard color, “ +“an RRGGBB value, or to see if your browser” + “supports X11 color names.");
}%></BODY></HTML>
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JSP Declaration
• A JSP declaration lets you define methods or fields that get inserted into the main body of the servlet class (outside of the service method processing the request)
• It has the following form: <%! Java Code %>
• Declarations do not produce output• They are used, for example, to define variables
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Example
• We want to print out the number of times the current page has been requested since the server booted (or the servlet class was changed and reloaded):
<%! private int accessCount = 0; %>
Accesses to page since server reboot:
<%= ++accessCount %>
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Predefined Variables
• As we have seen before, there are variables that can be used in the code
• There are eight automatically defined variables, sometimes called implicit objects
• The available variables are request, response, out, session, application, config, pageContext, and page
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request
• This is the HttpServletRequest associated with the request
• It lets you – look at the request parameters (via
getParameter), – the request type (GET, POST, HEAD, etc.),
and – the incoming HTTP headers (cookies, etc.)
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response
• This is the HttpServletResponse associated with the response to the client
• The output stream is buffered,
• Thus, it is legal to set HTTP status codes and response headers, even though this is not permitted in regular servlets once any output has been sent to the client
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out
• This is the PrintWriter used to send output to the client
• However, in order to make the response object useful, this is a buffered version of PrintWriter called JspWriter
• Note that you can adjust the buffer size, or even turn buffering off, through use of the buffer attribute of the page directive
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session
• This is the HttpSession object associated with the request
• Sessions are created automatically, so this variable is bound even if there was no incoming session reference (unless session was turned off using the session attribute of the page
directive)
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application
• This is the ServletContext as obtained via getServletConfig().getContext()
• Servlets and JSP pages can hold constant data in the ServletContext object
• Getting and setting attributes is with getAttribute and setAttribute
• The ServletContext is shared by all the servlets in the server
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config
• This is the ServletConfig of the page
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pageContext
• JSP introduced a new class called PageContext
• It encapsulate use of server-specific features like higher performance JspWriters
• The idea is that, if you access the server-specific features through this class rather than directly, your code will still run on "regular" servlet/JSP engines
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page
• This is simply a synonym for this• page is not very useful in Java codes in JSP
pages
• It was created as a placeholder for the time when the scripting language could be something other than Java
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JSP Directives• A JSP directive affects the overall structure of the
servlet class that is created from the JSP page• It usually has the following form:
<%@ directive attribute="value" %> • Multiple attribute settings for a single directive can
be combined: <%@ directive attribute1="value1" attribute2="value2" ... attributeN="valueN" %>
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Directives
• There are three main types of directive: • page, which lets you do things like
– import classes – customize the servlet superclass
• include, which lets you – insert a file into the servlet class at the time the JSP file
is translated into a servlet
• taglib directive– indicates a library of custom tags that the page can
include
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The page Directive
• The page directive lets you define the following attributes:
– import="package.class“<%@ page import="java.util.*" %>
– contentType="MIME-Type" <%@ page contentType="text/plain" %>
(it is the same as
<% response.setContentType("text/plain"); %>)
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More Page Directives
• isThreadSafe=“true|false”– Normal servlet processing or implementing
SingleThreadModel
• session=“true|false”– Allowing/disallowing sessions
• buffer=“sizekb|none”– specifies the buffer size for the JspWriter out
• autoflush=“true|false”– Flush buffer when full or throws an exception when
buffer isfull
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And More Directives
• extends=“package.class”• info=“message”
– A message for the getServletInfo method
• errorPage=“url”– Define a JSP page that handles uncaught
exceptions
• isErrorPage=“true|false”• language=“java”
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The include Directive
• This directive lets you include files at the time the JSP page is translated into a servlet
• The directive looks like this:
<%@ include file="relative url" %>
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Writing JSP in XML
• We can replace the JSP tags with XML tags that represent – Expressions– Scriptles– Declarations– Directives
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<%= Java Expression %><jsp:expression>
Java Expression </jsp:expression>
<% Code %>
<%! declaration %>
<jsp:scriptlet> Code Java
</jsp:scriptlet>
<jsp:declaration> Java Declaration
</jsp:declaration>
<%@ directive %><jsp:directive.typeAttribute = value/>
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Actions
• JSP actions use constructs in XML syntax to control the behavior of the servlet engine
• You can – dynamically insert a file, – reuse JavaBeans components, – forward the user to another page, or – generate HTML for the Java plugin
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Available Actions
• Available actions include: – jsp:include - Include a file at the time the page is
requested– jsp:useBean - Find or instantiate a JavaBean– jsp:setProperty - Set the property of a JavaBean– jsp:getProperty - Insert the property of a JavaBean
into the output– jsp:forward - Forward the requester to a new page – jsp:plugin - Generate browser-specific code that
makes an OBJECT or EMBED tag for the Java plugin
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The jsp:include Action
• This action lets you insert files into the page being generated
• The file inserted when page is requested
• The syntax looks like this: <jsp:include page="relative URL"
flush="true" />
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The jsp:forward Action
• Forwards request to another page• Syntax:
<jsp:forward page="relative URL"/>• It has a single attribute, page, which should consist of a
relative URL• This could be a static value, or could be computed at
request time• Examples: <jsp:forward page="/utils/errorReporter.jsp" /> <jsp:forward page="<%= someJavaExpression %>" />
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Using Beans
• JavaBeans are reusable software components that can be manipulated visually in a builder tool– Introspaction – analyze how the bean works
– Customization – the user can customize the look of the bean
– Events support
– Properties support
– Persistence – allowing saving the bean on a persistent mechanism and loading it
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Some Facts about Beans
• The bean Class should include a constructor with no arguments
• The bean class should not include public variables (fields)
• Access to the attributes of the bean is through methods that look like– getName / setName for non-boolean fields– isName / setName for boolean fields
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The jsp:useBean Action
• This action lets you load in a JavaBean to be used in the JSP page
• The simplest syntax for specifying that a bean should be used is:
<jsp:useBean id="name“
class="package.class" />
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Using a Bean
<jsp:useBean id="name”
class="package.class" />
•Create an object of the given class•Bind the object to a variable with the given name
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Using a Bean
<jsp:useBean id=“db”
class=“dbiClasses.WebDatabase" />
<% dbiClasses.WebDatabase db = new
dbiClasses.WebDatabase(); %>
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Using a Bean
<jsp:useBean id=“handler“
class=“ConnectionHandler" type=“Runnable”/>
<% Runnable handler = new
ConnectionHandler(); %>
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More on Beans
• Creating a Bean with jsp:useBean is like creating an object
• The scope of the bean can be beyond the page where it is declared
• Beans are used for resource collaboration
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Getting the Properties of a Bean
• Use jsp:getProperty to get a property of a bean
• Use the attributes name and property to get the requested property
<jsp:getProperty name=“db”
property=“login” />
<%= db.getLogin() %>
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Setting the Properties of a Bean
• Use jsp:setProperty to set a property of a bean
• Use the attributes name, property and value to set the value to the property
<jsp:setProperty name=“db”
property=“login” value=“snoopy”/>
<%= db.setLogin(“snoopy”); %>
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Examplepackage dbiTests;
public class SimpleBean {
private String message="No message specified";
public String getMessage() { return(message);
}
public void setMessage(String message) { this.message = message;
} }
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<HTML> <HEAD> <TITLE>Reusing JavaBeans in JSP</TITLE> </HEAD>
<BODY> <CENTER> <TABLE BORDER=5> <TR><TH CLASS="TITLE"> Reusing JavaBeans in JSP</TABLE> </CENTER> <P> <jsp:useBean id="test" class=“dbiTest.SimpleBean" /> <jsp:setProperty name="test" property="message" value="Hello WWW" /> <H1>Message: <I><jsp:getProperty name="test" property="message" /> </I></H1> </BODY> </HTML>
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Getting the Values from the Request
<jsp:setProperty name=“db”
property=“login”
value=‘<%=
request.getParameter(“login”)%>’
/>
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Getting the Parameter Automatically
• If we wand that the value of the parameter dblogin of the request will be set to the bean db automatically we can use:
<jsp:setProperty name=“db”
property=“login”
param=“dblogin” />
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All Input Parameters
• What will the following do?
<jsp:setProperty name=“db”
property=“*” />
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Shared Beans
• So far, we assumed that object that were created using jsp:useBean were bound to to the local variables in the _jspService method
• The scope attribute of jsp:useBean gives us more possibilities
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The page Value of scope
• The default value of scope
• The object has a binding to the local variable
• The object is positioned in the PageContext for the current request
• A servlet can get the value by pageCotext.getAttribute
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The application Value of scope
• The object has a binding to the local variable• The object is positioned in the ServletContext• A servlet can get the value by application.getServletCotext().getAttribute
• Allows both servlets and JSP pages to work with the same object
• Allows a servlet to create a bean that will be used in JSP pages
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The session Value of scope
• The object has a binding to the local variable
• The object is positioned in the HttpSession for the current request
• A servlet can get the value by session.getAttribute
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The request Value of scope
• The object has a binding to the local variable
• The object is positioned in the ServletRequest for the current request
• A servlet can get the value by request.getAttribute
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A Conditional Bean
• The jsp:useBean component will create a new bean only if there is no existing bean with the same id and scope
• If an existing bean is found, it will be bound to the variable defined by id
• Since bean objects are not always created you can have a java code that is executed only if a new bean is created
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Example of Conditional
Bean
public class AccessCounterBean {
private String firstPage;
private int accessCount = 1;
public String getFirstPage() {
return(firstPage);
}
public void setFirstPage(String firstPage) {
this.firstPage = firstPage;
}
public int getAccessCount() {
return(accessCount++);
}
}
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Example of Conditional Bean
<jsp:useBean id=“counter“
class=“AccessCounterBean"
scope=“application”/>
<jsp:setProperty name=“counter”
property=“firstPage”
value=“Current Page Name”/>
</jsp:useBean>
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Comments• <%-- comment --%>
– A JSP comment– Ignored by JSP-to-scriptlet translator– Any embedded JSP scripting elements,
directives, or actions are ignored
• <!-- comment -->– An HTML comment– Passed through to resultant HTML – Any embedded JSP scripting elements,
directives, or actions are executed normally
• /* comment */ or // comment– Java comment– Part of the Java code
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Quoting Conventions
• <\% - used in template text (static HTML) and in attributes where you really want “<%”
• %\> - used in scripting elements and in attributes where you really want “%>”
• \‘ \” - for using quotes in attributes• \\ for having \ in an attribute
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Init and Destroy
• JSP pages, like regular servlets, sometimes want to use init and destroy
• Problem: the servlet that gets built from the JSP page might already use init and destroy– Overriding them would cause problems– Thus, it is illegal to use JSP declarations to
declare init or destroy
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Init and Destroy
• Solution: use jspInit and jspDestroy– The auto-generated servlet is guaranteed to call
these methods from init and destroy– The standard versions of jspInit and jspDestroy
are empty (placeholders for you to override)
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JSP vs. Javascript
• Javascript– Client side– Less secure– Browser Dependent– Unstable
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JSP vs. ASP
• Active Server Pages (ASP)– Similarities:
• Server side dynamic web page generators
• Share similar syntax <% %>
• Modular programming (e.g. ActiveX, JavaBeans)
• Focus on database connectivity
– Differences:• ASP is a product while JSP is a specification
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JSP vs. ASP (cont.)
• JSP– Based on Java Technology– Platform independence
• Unix, AS400, VMS, Windows
– More vendors choice
• ASP– Microsoft programming languages– Near Monopoly
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JSP vs. Servlets
• But JSP pages are converted to Servlets? Aren’t they the same?
• Similarities– Provide identical results to the end user– JSP is an additional module to the Servlet
Engine
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JSP versus Servlets (cont.)
• Differences– Servlets: “HTML in Java code”
• HTML code inaccessible to Graphics Designer
• Everything very accessible to Programmer
– JSP: “Java Code Scriptlets in HTML”• HTML code very accessible to Graphics Designer
• Java code very accessible to Programmer
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