Transcript CIS336 Website design, implementation and management (also

Lecture 9
JavaServer Pages (JSP)
(Based on Møller and Schwartzbach, 2006, Chapter 9)
David Meredith
[email protected]
www.titanmusic.com/teaching/cis336-2006-7.html
CIS336
Website design, implementation and
management
(also Semester 2 of CIS219, CIS221 and
IT226)
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Scripting languages
• Scripting languages, like ASP and PHP,
view Web applications as collections
of active pages
– An active page is an HTML document
with embedded fragments of script code
that are executed on the server
• Scripting languages are OK for simple
applications
– but weaknesses become apparent when
try to use scripting languages to build
large, complex Web applications
• e.g., lack of static type checking (i.e.,
checking the types of parameters and
variables at compile time)
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The JSP framework
• Pages written using the
JavaServer Pages (JSP) scripting
language are translated into
servlets and then compiled using
a Java compiler
– implies full power of Java language
(including, e.g., static type
checking) is preserved
• This is an advantage of JSP over, e.g.,
ASP or PHP
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A simple JSP example
• JSP page above maintains a hit counter and prints
the current server time
• When using a Tomcat server, needs to be saved in
a directory underneath webapps, e.g.,
$CATALINA_HOME/webapps/myjsps/hello.jsp
– JSP pages compiled into servlets automatically
• JSP page is an HTML page with embedded Java code
– variable out declared implicitly to hold current output stream
• JSP should be as simple as HTML but, without tag libraries or
expression language, need a good knowledge of Java
4
Templates
• JSP page written as a template
– Template is a text file (usually an HTML
file) containing snippets of Java code and
JSP-specific directives
– Java code in a JSP page appears as
• expressions: <%= expression %>
– e.g.,
<%= new java.util.Date().toLocaleString() %>
• statements: <% statement %>
– e.g.,
<% synchronized(this) {out.println(++hits); } %>
• declarations: <%! declaration %>
– e.g., <%! int hits = 0; %>
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Implicitly declared variables
JSP page translated into a servlet and following variables are implicitly declared:
•
HttpServletRequest request;
HttpServletResponse response;
HttpSession session;
ServletContext application;
ServletConfig config;
Output stream declared as
•
JspWriter out;
Unlike PrintWriter
–
–
•
•
JspWriter throws a java.io.IOException if print method fails
•
JspWriter may buffer output before printing
Constant parts of a JSP page are also printed to out
PageContext pageContext;
–
adds extra layer of scope and corresponding setAttribute and getAttribute methods
Four successively broader layers of scope for setting and getting attributes:
•
–
page scope: pageContext
–
request scope: request
–
session scope: session
–
application scope: application
•
pageContext.findAttribute()
–
searches for named attribute in successively broader scopes
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Expressions
• Java expressions are embedded in
JSP pages using syntax:
<%= expression %>
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Statements
• Java statements are embedded in JSP
pages using syntax:
<% statement %>
• Statement must have side-effects to
be noticeable, e.g.,
– change state of variables
– print to output stream
8
Declarations
• Java declarations are embedded in a
JSP file using the syntax
<%! declaration %>
• Can be used to define class fields and
methods
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Declaring variables in
declarations and statements
• If line 6 were a statement, then variable hits would be set to
zero every time the page was loaded
– statement would correspond to a line in a method in the servlet
generated from the page
• Using a declaration means that hits is declared as a field in
the servlet generated by the page
– therefore only set to zero once, when the servlet class is
instantiated
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Directives
• JSP directives have the syntax
<%@ directive %>
• Directives provide parameters to the JSP processor, e.g.,
<%@ include file="header.jsp" %>
– includes the file header.jsp at the position where the directive
occurs
• Note how we declare title to be a field so that we can refer to
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it in an expression in header.jsp
The page directive
•
page directive can be used with various attributes to set different
properties of the JSP page
– buffer="size"
• sets size of output stream buffer in bytes
– autoFlush = "true-of-false"
• determines whether output buffer is automatically flushed
– contentType = "mime-type"
• sets mime type for output stream
– default is "text/html"
– pageEncoding = "encoding"
• sets character set for output
– default is ISO-8859-1
– info="string"
• sets descriptive string returned by invoking getServletInfo() on generated
servlet
– errorPage="path"
• sets JSP page that should be invoked if uncaught exception thrown
– isErrorPage="true-or-false"
• determines whether current page is an error page
– if it is, then there is a variable, exception, available that contains the thrown
exception
– import="package"
• specifies package to be imported in generated servlet class
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Using page
directive to
control error
handling
13
Translation into servlets
• Tomcat generates
servlet that
specializes
HttpJspBase (a
subclass of Servlet)
• Expressions wrapped
in out.print(...)
• HTML markup
wrapped in
out.write(...)
• Declarations
translated into fields
and methods
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Translation into servlets
• Translation is lexical
– means that HTML markup and Java code are not
parsed
– means Java code statements don't have to be
individually syntactically well-formed - only
result that has to be well-formed
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XML version of JSP
• JSP processor performs lexical
translation of JSP pages into servlets
– means no syntactic requirements are
enforced
• JSP can also be written using an XML
syntax
– called JSP documents (not pages)
– means JSP documents can be validated
– can be manipulated using XML tools
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Example JSP document
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JSP Expression Language
•
•
•
${expression} occurring anywhere within JSP template text or
attribute values of markup is replaced with string resulting from
evaluating expression
Expression language resembles JavaScript
Supports
– strings, booleans, various numerical types
– arithmetic, logical and comparison operators
•
•
References to named variables resolved using findAttribute
mechanism
Supports operations on certain objects, e.g.,
${gadget.weight} is translated into
pageContext.findAttribute("gadget").getWeight()
•
Some implicit objects defined, e.g.,
– param is a map of the request parameters
– pageContext references the object pageContext
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Tags
• JSP is intended to be used by people
who are not expert Java programmers
• However, hard to use JSP without
knowledge of Java
• Tags designed to allow markup to be
separated cleanly from Java code
– Standard Tag Library can be used by
non-programmers to access JSP through
mark-up
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Tag files
•
•
•
•
•
•
A tag file is a definition of a
new tag (i.e., element) which
can contain arbitrary Java
code to be executed
Means the active content can
be obtained by designer
simply by using appropriate
tags in the markup
Tag file is a JSP page with
the file extension .tag and
the directive
<%@ tag %>
Note use of attribute
directive to declare an
attribute for the new tag
jsp:doBody instruction
indicates where contents of
tag is inserted
taglib directive indicates
where tags are located (cf.
XML namespaces)
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Using jsp:doBody with the var attribute
• Declare a variable
in the var
attribute of the
jsp:doBody
instruction
• Content of new
tag is stored in
the declared
variable
• Each reference to
declared variable
in tag file then
replaced with
content of new
tag when used
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