Presents your 
JAVA E-NEWSLETTER for June 24, 2002

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LEARN HOW TO MANAGE SERVLET VARIABLES

If managing names within session and request parameters/attributes
causes obscure bugs and is hard to document, use two interfaces to hold the
names.

The Servlet application program interface (API) provides map style
methods that store values in a session or obtain values from a request. 
A string indexes these values, and it's common to use the particular 
string frequently within the code, such as:

    String query = request.getParameter("query");

    Person person = (Person)session.getAttribute("current_person");

This type of inquiry is problematic for two reasons. The first is that
it's not typed, meaning that incorrect name entries or misspellings are
not noticed until the Web page begins to behave oddly. Nothing points out
that the following code is missing an 'e':

    String query = request.getParameter("qury");

or that this code contains invalid capitalization:

    String query = request.getParameter("Query");

The second problem is that there's no easy application documentation of
the request and session variables.

Both of these problems can be solved by the methodical usage of
interfaces to hold the variable names, as well as comments concerning
their use. For example:

package com.generationjava.application.jobspage;

public interface RequestVars {

    // used to get the query field from a search box
    static public String QUERY = "query";

}

and

package com.generationjava.application.jobspage;

public interface SessionVars {

    // the currently logged in person
    static public String CURRENT_PERSON = "current_person";

}

Here's the usage of these typed and documented variables:

    String query = request.getParameter(RequestVars.QUERY);

    Person person = 
(Person)session.getAttribute(SessionVars.CURRENT_PERSON);

By adhering to this coding convention, order is brought to the chaotic
world of request and session variable names.

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