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20.5 Hello JSP
We now transition from servlet examples to JSP examples. One of the
shortcomings of any servlet that produces nontrivial output is that
they often contain embedded
HTML tags locked up within Java
classes where web designers cannot get to them. JSP is designed to
improve this situation: instead of embedding HTML within Java code,
it embeds Java code within HTML pages. (And, as
we'll see, JSP 2.0 and the Java Standard Tag
Libraries dramatically reduce the amount of Java code that must be
intermixed with HTML.)Example 20-4 is a simple JSP page that performs a
function similar to the HelloNet servlet of Example 20-1. If invoked with a parameter named
"name", it simply greets the named
user. If there is no parameter, it displays an HTML form to request
the user's name and then greets the user when the
form is submitted.Example 20-4 uses JSP 1.x, and the main thing to note
about the example is that it consists of HTML code intermingled with
Java code which is contained within pseudotags. Remember that JSP
pages are compiled into servlet classes. <% and
%> delimit blocks of Java code, which are
inserted literally into the body of the servlet's
doGet( ) or doPost( ) method.
Note that these "scriptlet" tags
can contain code fragments that are not complete statements. A
fragment may end in the middle of an if block, to
be followed by HTML tags (which are inserted literally into the
servlet's output) and then followed by another
<%...%> scriptlet that closes the
if block.The tag <%=...%> is like a
scriptlet, but contains a single Java expression (instead of
arbitrary statements) whose value is placed in the
servlet's output at that point. Note also that the
example begins with a <%@page...%>
directive, which declares that the JSP page outputs an HTML document.
Table 20-1 summarizes the syntax of JSP 1.x pages.
Example 20-4. hello.jsp
<%@page language='java' contentType='text/html'%>
<% // Begin scriptlet: this is Java code
if (request.getParameter("name") == null) { // If no name, request one
// Note that the scriptlet ends in the middle of a block.
%>
<%-- Now we switch to HTML tags and java expressions --%>
<form>
<i>Please enter your name: </i><input name="name">
<input type="Submit">
</form>
<%
// Back to Java code
} else { // Otherwise, if there was a name parameter, greet the user
%>
Hello <%= request.getParameter("name") %>
<% } // end the Java if/else statement %>
20.5.1 Displaying a JSP Page
If you use the prepackaged je3.war file for the
examples in this chapter, you can try out this example using a URL
like this:
http://localhost:8080/je3/hello.jsp
Servlet containers automatically compile JSP
pages as needed (and recompile them when they change, which is very
useful during development). You can precompile a JSP page to avoid
the compilation overhead on a production web server, but doing that
is not covered in this chapter.
