Importing interfaces into JSPs?

[Bob]

Is it possible to import interfaces into JSPs, such as can be done with classes
using the import attribute in the page directive? We have an application
in which we would like our JSPs to be able to use public static final String
constants defined elsewhere, in an interface. This does not seem to discussed
in at least two books which deal extensively with JSPs.

[Larne Pekowsky]

In article <39995e77$1@news.devx.com>, Bob <robert.byrnes@transcore.com> wrote:
>Is it possible to import interfaces into JSPs, such as can be done
>with classes using the import attribute in the page directive?

Yes! Here's a sample interface:


package com.awl.jspbook.ch04;

public interface MyInterface {
public static String TITLE = "Paradox Addendum";
}


and a sample jsp that uses it:

<%@page import="com.awl.jspbook.ch04.MyInterface" %>

The title is <%= MyInterface.TITLE %>


This works as expected.

The import attribute turns into a simple import in the generated
servlet, so there should be no surprises as long as the interface is
somewhere in your classpath.


- Larne

[Fernando Ribeiro]

I think you can even make the generated servlet to implement your interface
but this would require some extra coding.

Is that right?

Fernando Ribeiro
fribeiro@bol.com.br

larnep@canetoad.canetoad.com (Larne Pekowsky) wrote:
>In article <39995e77$1@news.devx.com>, Bob <robert.byrnes@transcore.com>
wrote:
>>Is it possible to import interfaces into JSPs, such as can be done
>>with classes using the import attribute in the page directive?
>
>Yes! Here's a sample interface:
>
>
>package com.awl.jspbook.ch04;
>
>public interface MyInterface {
> public static String TITLE = "Paradox Addendum";
>}
>
>
>and a sample jsp that uses it:
>
><%@page import="com.awl.jspbook.ch04.MyInterface" %>
>
>The title is <%= MyInterface.TITLE %>
>
>
>This works as expected.
>
>The import attribute turns into a simple import in the generated
>servlet, so there should be no surprises as long as the interface is
>somewhere in your classpath.
>
>
> - Larne
>

[Larne Pekowsky]

Fernando Ribeiro <fribeiro@bol.com.br> wrote:
>
>I think you can even make the generated servlet to implement your interface
>but this would require some extra coding.
>
>Is that right?

You're probably thinking of the extends attribute of the page
directive, which forces the generated servlet to extend a class you
specify. In Tomcat by default all JSP-based servlets extend
org.apache.jasper.runtime.HttpJspBase. If you wanted to add some
string constant to all your JSPs you could define a new base class:

public class MyJspBase extends org.apache.jasper.runtime.HttpJspBase {
public final static String TargetURL = "http://www.devx.com";
}


Now if your JSPs extend MyJspBase they'll be able to use TargetURL
without explicitly importing anything:

<%@page extends="MyJspBase">

Click <a href="<%= TargetURL %>">here!</a>


You can also add methods to base classes.

Note that "HttpJspBase" is not part of the JSP specification, and
different JSP engines will use different base classes. Therefore this
feature can't really be used in a platform-independent way, at least
not yet. There are usually safer ways to get the same functionality,
such as importing classes with static fields (which brings this
conversation full circle...)

There is no 'implements' attribute that is to interfaces what
'extends' is to classes. Maybe in a future version.

- Larne