JavaServer Pages (JSP): Overview and Implementation Guidelines

1. Lecture 9 JavaServer Pages (JSP) (Based on Møller and Schwartzbach, 2006, Chapter 9) CIS336Website design, implementation and management(also Semester 2 of CIS219, CIS221 and IT226) David Meredith d.meredith@gold.ac.uk www.titanmusic.com/teaching/cis336-2006-7.html

2. 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)

3. 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

4. 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

5. 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; %>

6. 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

7. Expressions • Java expressions are embedded in JSP pages using syntax: <%= expression %>

8. 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

9. Declarations • Java declarations are embedded in a JSP file using the syntax <%! declaration %> • Can be used to define class fields and methods

10. 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

11. 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 it in an expression in header.jsp

12. 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

13. Using page directive to control error handling

14. 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

15. 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

16. 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

17. Example JSP document

18. 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 intopageContext.findAttribute("gadget").getWeight() • Some implicit objects defined, e.g., • param is a map of the request parameters • pageContext references the object pageContext

19. 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

20. 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)

21. 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