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JSP Declarations
The JSP you write turns into a class definition. All the scriptlets you
write are placed inside a single method of this class.
You can also add variable and method declarations to this class. You can
then use these variables and methods from your scriptlets and expressions.
To add a declaration, you must use the <%! and %> sequences to
enclose your declarations, as shown below.
<%@ page import="java.util.*" %>
<HTML>
<BODY>
<%!
Date theDate = new Date();
Date getDate()
{
System.out.println( "In getDate() method" );
return theDate;
}
%>
Hello! The time is now <%= getDate() %>
</BODY>
</HTML>
The example has been created a little contrived, to show variable and method
declarations.
Here we are declaring a Date variable theDate, and the method getDate. Both of
these are available now in our scriptlets and expressions.
But this example no longer works! The date will be the same, no matter
how often you reload the page. This is because these are declarations,
and will only be evaluated once when the page is loaded! (Just as if you
were creating a class and had variable initialization declared in it.)
Exercise: Modify the above example to add another function computeDate
which re-initializes theDate. Add a scriptlet that calls computeDate each time.
Note: Now that you know how to do this -- it is in general not a good
idea to use variables as shown here. The JSP usually will run as multiple
threads of one single instance. Different threads would interfere with
variable access, because it will be the same variable for all of them. If you do
have to use variables in JSP, you should use synchronized access, but
that hurts the performance. In general, any data you need should go either in
the session objet or the request objectc (these are introduced a
little later) if passing data between different JSP pages. Variables you declare
inside scriptlets are fine, e.g. <% int i = 45; %> because these
are declared inside the local scope and are not shared. |