Jsp intro
- 2. 2 public class HelloServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); String docType = "<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 " + "Transitional//EN">n"; out.println(docType + "<HTML>n" + "<HEAD><TITLE>Hello</TITLE></HEAD>n" + "<BODY BGCOLOR="#FDF5E6">n" + "<H1>Hello World</H1>n" + "</BODY></HTML>"); } } This is mostly Java with a little HTML mixed in
- 3. The purpose of a servlet is to create a Web page in response to a client request Servlets are written in Java, with a little HTML mixed in ◦ The HTML is enclosed in out.println( ) statements JSP (Java Server Pages) is an alternate way of creating servlets ◦ JSP is written as ordinary HTML, with a little Java mixed in ◦ The Java is enclosed in special tags, such as <% ... %> ◦ The HTML is known as the template text JSP files must have the extension .jsp ◦ JSP is translated into a Java servlet, which is then compiled ◦ Servlets are run in the usual way ◦ The browser or other client sees only the resultant HTML, as usual Tomcat knows how to handle servlets and JSP pages 3
- 4. There is more than one type of JSP “tag,” depending on what you want done with the Java <%= expression %> ◦ The expression is evaluated and the result is inserted into the HTML page <% code %> ◦ The code is inserted into the servlet's service method ◦ This construction is called a scriptlet <%! declarations %> ◦ The declarations are inserted into the servlet class, not into a method 4
- 5. <HTML> <BODY> Hello! The time is now <%= new java.util.Date() %> </BODY> </HTML> Notes: ◦ The <%= ... %> tag is used, because we are computing a value and inserting it into the HTML ◦ The fully qualified name (java.util.Date) is used, instead of the short name (Date), because we haven’t yet talked about how to do import declarations 5
- 6. You can declare your own variables, as usual JSP provides several predefined variables ◦ request : The HttpServletRequest parameter ◦ response : The HttpServletResponse parameter ◦ session : The HttpSession associated with the request, or null if there is none ◦ out : A JspWriter (like a PrintWriter) used to send output to the client Example: ◦ Your hostname: <%= request.getRemoteHost() %> 6
- 7. Scriptlets are enclosed in <% ... %> tags ◦ Scriptlets do not produce a value that is inserted directly into the HTML (as is done with <%= ... %>) ◦ Scriptlets are Java code that may write into the HTML ◦ Example: <% String queryData = request.getQueryString(); out.println("Attached GET data: " + queryData); %> Scriptlets are inserted into the servlet exactly as written, and are not compiled until the entire servlet is compiled ◦ Example: <% if (Math.random() < 0.5) { %> Have a <B>nice</B> day! <% } else { %> Have a <B>lousy</B> day! <% } %> 7
- 8. Use <%! ... %> for declarations to be added to your servlet class, not to any particular method ◦ Caution: Servlets are multithreaded, so nonlocal variables must be handled with extreme care ◦ If declared with <% ... %>, variables are local and OK ◦ Data can also safely be put in the request or session objects Example: <%! private int accessCount = 0; %> Accesses to page since server reboot: <%= ++accessCount %> You can use <%! ... %> to declare methods as easily as to declare variables 8
- 9. Directives affect the servlet class itself A directive has the form: <%@ directive attribute="value" %> or <%@ directive attribute1="value1" attribute2="value2" ... attributeN="valueN" %> The most useful directive is page, which lets you import packages ◦ Example: <%@ page import="java.util.*" %> 9
- 10. The include directive inserts another file into the file being parsed ◦ The included file is treated as just more JSP, hence it can include static HTML, scripting elements, actions, and directives Syntax: <%@ include file="URL " %> ◦ The URL is treated as relative to the JSP page ◦ If the URL begins with a slash, it is treated as relative to the home directory of the Web server The include directive is especially useful for inserting things like navigation bars 10
- 11. Actions are XML-syntax tags used to control the servlet engine <jsp:include page="URL " flush="true" /> ◦ Inserts the indicated relative URL at execution time (not at compile time, like the include directive does) ◦ This is great for rapidly changing data <jsp:forward page="URL" /> <jsp:forward page="<%= JavaExpression %>" /> ◦ Jump to the (static) URL or the (dynamically computed) JavaExpression resulting in a URL 11
- 12. JSP can be embedded in XML as well as in HTML Due to XML’s syntax rules, the tags must be different (but they do the same things) HTML: <%= expression %> XML: <jsp:expression>expression</jsp:expression> HTML: <% code %> XML: <jsp:scriptlet>code</jsp:scriptlet> HTML: <%! declarations %> XML: <jsp:declaration>declarations</jsp:declaration> HTML: <%@ include file=URL %> XML: <jsp:directive.include file="URL"/> 12
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