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DT211/3 Internet Application Development
JSP: Actions elements and JSTL
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Introduction Previously, identified that JSP provides a variety of techniques to enable dynamic processing: Directive elements So far, have looked at these Scripting elements Action elements and JSTL Java Beans
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JSP Action elements Action elements are an important syntax element in JSP They are represented by tags (as is HTML) They assist JSP developers to develop in tags rather than scriplet programming Instead of <%, they just use the < character (like HTML) <prefix:action_name> body </prefix:action_name >
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Part after the “:” is the Action Name
JSP Action elements JSP tags have a “start tag”, a “tag body” and an “end tag”. The start and end tag have the same name enclosed in < and > The tag names have an embedded colon character “:” in them, the part before the colon (prefix) describes the type of the tag <prefix:action_name> body </prefix:action_name> Part after the “:” is the Action Name Note: Syntax of Action Elementags is based on XML
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<jsp:include page="scripts/login.jsp" />
JSP Action elements Tags have associated attributes (like HTML e.g. <img src = “..”) Full syntax of JSP Action Elements is: <prefix:action_name attr1 = “value” attr2 = “value2” action_body </prefix:action_name> If the element doesn’t have a body, can lose the end tag and use shorthand syntax of: <prefix:action_name attr1 = “value” attr2 = “value2” /> For example: <jsp:include page="scripts/login.jsp" />
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JSP Action elements Two types 1. 2. JSP Pre-defined tags
External tag library (e.g. JSTL) Tags prefix is <jsp: ….> Tags prefix is whatever developer chooses or library recommends Custom and JSTL (Also called Standard Action Elements)
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JSP Action elements: JSP Predefined Tags
Also called JSP Standard Action Elements List of these elements are: <jsp:forward> <jsp:include> <jsp:param> <jsp:plugin> <jsp:useBean> <jsp:getProperty> <jsp:setProperty> See for detailed attributes and values Used for Java Beans
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JSP Predefined Tag Example <jsp:include>
Standard Action Example: <JSP: include> tag Example: <HTML> <BODY> Going to include hello.jsp...<BR> <jsp:include page="hello.jsp"/> </BODY> </HTML> Executes the included JSP page and adds its output into the this page
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JSP Predefined Tag Example <jsp:include>
What’s Difference from Using the ‘include’ directive? e.g. include file = ‘hello.jsp’ %> The include directive includes the contents of another file at compilation time. Good for including common static code e.g. header file, footer file. Good on performance included only once. But, what if including dynamic common code (e.g. a navigation bar where links are read from the dB?).. need to re-run the file each time a request is made - JSP: include JSP: include incorporates the output of the included JSP file at run time
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JSP Predefined Tag Example <jsp:forward>
Standard Action Example: <JSP: forward> tag Stops processing of one page and starts processing the page specified by the page attribute Example: <HTML> <BODY> Error occurred…please wait<BR> <jsp:forward page=“errorpage.jsp"/> </BODY> </HTML>
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JSP Predefined Tag Example <jsp:param>
Standard Action Example: <JSP: param> tag Can be used to pass parameters when using <jsp:include> or <JSP:forward> Example <jsp:include page="login.jsp"> <jsp:param name="username" value="jsmith" /> </jsp:include> Executes a login page jsp:param passes in username to the login page
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JSP Action Elements JSP pre-defined tags (standard actions elements) Done External Tag library: Custom Java Standard tag library
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External tag libraries – custom
JSP Action elements: External tag libraries – custom JSP 1.1. introduced ‘tag libraries’, allowing addition of tags beyond standard action <jsp: > tags Developers can develop their own Custom action elements using custom tag libraries Custom tag libraries are useful where developers are working in teams – reusabletags can be supplied to team via custom tag libraries. The library is identifed in the JSP page using the <taglib> directive
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JSP Action Elements JSP pre-defined tags (standard actions elements) Done External Tag library: Custom Done Java Standard tag library
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External tag libraries – JSTL
JSP Action elements: External tag libraries – JSTL JSP 1.2 introduced supported for a special tag library called the JSP Standard Tag Library (JSTL) Version 1.0 released in June 2002 Version 1.1 released in June 2004 The JSTL saves programmers from having to develop custom tag libraries for a range of common tasks, such as database access, conditional loops etc Enabled developers to produce more maintainable, simpler JSP code Important development for JSP technology
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JSTL The JSP Standard Tag Library groups actions into four libraries as follows**: Library Contents Core Core functions such as conditional processing and looping, important data from external environments etc Formatting Format and parse information SQL read and write relational database data XMl Processing of XML data ** more may be added in later releases…
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JSTL To use any of these libraries in a JSP, need to declare using the taglib directive in the JSP page, specifying the URI and the Prefix Library Prefix URI Core c Formatting fmt SQL sql XMl xml Example of declaring use of core library: taglib prefix = “c” uri = “ %>
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JSTL: Example Example: JSP page using JSTL that outputs 1 to 10 on a webpage using the <c:forEach> and <c:out> tags of the core library taglib uri=" prefix="c" %> <html> <head> <title>Count to 10 Example (using JSTL)</title> </head> <body> <c:forEach var="i" begin="1" end="10" step="1"> <c:out value="${i}" /> <br /> </c:forEach> </body> </html> A taglib directive declare use of core library JSTL tag examples
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JSTL: Example <c:foreach>
Looking more closely at <c:forEach tag> ….. <c:forEach var="i" begin="1" end="10" step="1"> <c:out value="${i}" /> <br /> </c:forEach> </body> </html> The <forEach> tag enables loop logic. In this case, will look through 10 times. Equivalent to java “for” loop Closing tag <c:forEach> placed after the end of body of loop
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JSTL: Example <c:foreach>
All JSTL tags have a set of attributes (similar to HTML tags..) e.g. <c:foreach> tag has 6 attributes: var, items, varStatus, begin, end, step The full details for each attribute is in the JSTL specification document (on distrib). See p 65 for <c:foreach> tag definition Willl need to use this document to verify WHICH tag should be used and HOW is should be used
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JSTL: Example <c:out>
<c:out> .. outputs a value to webpage. Usually uses just one attribute value Examples: <c:out value="${i}" /> <c:out value=“The result of is ${1+2}” /> <c:out value=“param.userName" />
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JSTL: Example <c:if>
<c:if> .. evaluates a condition. Uses an attribute test to hold the condition Example : <%-- Simple if conditions --%> <c:if test='${param.p == "someValue"}'> Generate this template text if p equals someValue </c:if> Example 2 <c:if test='${param.p}'> Generate this template text if p equals "true“
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JSTL: Multiple ‘if’ conditions
An if/else action requires the use of the <c:choose> tag Syntax : <c:choose> body content (<when> and <otherwise> subtags) </c:choose> See page 52 of JSTL specification doc.
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JSTL: Multiple ‘if’ conditions
Uses <c:choose>, <c:when> and <c:otherwise> Example: <c:choose> <c:when test='${param.p == "0"}'> <c:out value = “zero recorded”/> </c:when> <c:when test='${param.p == "1"}'> Generate this <c:out value = “single value”/> <c:otherwise> <c:out value = “Set to ${param.p}”/> </c:otherwise> </c:choose>
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JSTL: Other core <c:..> actions
Other examples: (NOT a complete list!) <c:set> ….sets the value of a variable <c:remove> ….removes a scoped variable <c:catch> ….catches an exception <c:url> ….. encodes a URL <c:import>… imports the content of a resource <c:redirect>.. redirects to another URL <c:param>.. adds a request parameter to other actions
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JSTL: Expression language
Up to now, could only use Java expressions to assign dynamic values syntax errors common JSTL now provides an expression language (EL) to support the tags simpler syntax, less errors The EL is now part of the JSP specification (as of versions JSP2.0) – can be used in JSTL tags or directly in JSP pages.
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JSTL: Expression language
All EL expressions are evaluated at runtime The EL usually handles data type conversion and null values -- easy to use An EL expression always starts with a ${ and ends with a }
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JSTL: Expression language
The expression can include - literals ( “1”, “100” etc) - variables - implicit variables (more later) Examples: <c:out value = “${1+2+3}” /> <c:if test = “${param.Address == “D6”}” /> expression
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JSTL: Expression language - operators
== != < + - * / or div > <= >= Logical operators consist of && (or and), || (or or), and ! (or not). The empty operator is a prefix operator that can used to determine if a value is null or empty. For example: <c:if test=”${empty param.name}”> Please specify your name. </c:if>
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JSTL implicit variables
The JSTL EL allows these objects to be accessed as ‘Implicit Variables’ Implicit variable are just pre-agreed fixed variable names that can be used in JSTL Expressions -- Think of as “variables that are automatically available to your JSP page”..
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JSTL: Expression language – Implicit Objects
Full set of implicit variables on page 211 of JSTL documentation (includes: header, pageScope..) or on page 74 of O’Reilly’s Java Server Pages book Very common implicit object is param param refers to parameter passed in a request message (e.g. information entered into a form by a user). e.g. <c:out value = “${param.userName}”/> Further Examples of using param in next topic
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Comparing JSTL vs Scriptlets
JSTL removes complexity by using tags instead of java code (abstraction) JSP pages using JSTL usually easier to maintain JSTL allows HTML ‘tag’ developers to ‘program’ JSTL often more difficult to debug Note: Using JSTL does not eliminate scriplets entirely.. may still need them sometimes for more complex logic
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Info on JSTL 1. Java Server Pages by Hans Bergsten – full reference on all JSTL tags in the Appendix. 2. Sun tutorial. Contains examples. Chapter 17 is on JSTL at: 3. Sun’s JSTL 1.0 tag specification on distrib. Good for definition of each tag. Poor on examples.
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Setting up JSTL in a web application
JSTL is provided as part of Tomcat 5 Ensure the two JSTL .jar files are copied into the WEB-INF\lib directory of your web application Ensure the taglib directive(s) is in your JSP page
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