Re: Have you seen anything like this?

["Richard Dettinger" <cujo@xxxxxxxxxx>]

FWIW:  I believe you don't actually need to use a static initializer for a
servlet.  The init() method has the single threaded, first touch concept as
part of its contract.  I think code in a servlet's init() method and code
in a static initializer, while being different in when they are executed,
are functionally equivalent for application programs.  A common technique
that builds on this is to then define one servlet for a web application
that gets loaded when the server starts up (allowing the rest to get loaded
on an as needed basis.  For example, I'm working on a project where we have
"SetupServlet" which looks like this:

public class SetupServlet
extends HttpServlet
{
    public void init(ServletConfig config)
    throws ServletException
    {

        super.init(config);

        // Do application level setup here.
    }

    public void destroy() {
        // Do application level cleanup here.
    }

}

Then you define the file in your web.xml file to load on startup:
  <servlet>
    <servlet-name>setup</servlet-name>
    <servlet-class>SetupServlet</servlet-class>
    <load-on-startup>0</load-on-startup>
  </servlet>

That's it.  You don't every have to call or reference that servlet at all
and it can serve as a mechanism for you to make sure that
you have things setup correctly that the rest of your application relies
on.

One possible advantage of this approach is that, when there are problems in
initialization, it will be easier to handle them gracefully.
For example, you can throw a ServletException from init(), but exceptions
thrown from static initializers are pretty spotty in what will
happen to the application and container.

============================================

One place where static initializers are very popular (and actually now
required by the specification) is in JDBC drivers.  When a
JDBC driver loads, it is supposed to register itself with the
DriverManager.  So when a user application types:

Class.forName("com.ibm.db2.jdbc.app.DB2Driver");

It cause the native JDBC driver to load.  There is a static initializer in
the DB2Driver class that does this:

java.sql.DriverManager.registerDriver(new DB2Driver());


Richard D. Dettinger
iSeries Java Data Access Team

Democracy's enemies have always underestimated the courage of the American
people.
It was true at Concord Bridge.  It was true at Pearl Harbor.  And it was
true today.

         Rochester Post-Bulletin
         Tuesday September 11, 2001


|---------+---------------------------->
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  |       Subject:  Re: Have you seen anything like this?                       
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>-----------------------------------------------------------------------------------------------------------------------------|




As has been said, these are usually called "static initializers"

>In my java class yesterday, my instructor went over a new constructor
>type thing. He called it a class constructor. Here is an example:

>public class Test {
>    static {
>       // set of instructions here
>    }
>
>    Test() {
>      // set of instructions here
>}
> }

In servlets, I have found these static (class) initializers extremely
useful.  You have guaranteed, single-threaded execution at class load time.
This is a great opportunity to do one-time things like get connections,
prepare SQL statements, initialize Vectors and Hashtables and so on.
Especially good for "immutable" type objects that don't have to by
otherwise synchronized because you can't or won't change them after you
build them in the static initializer.


Larry W. Loen  -   Senior Linux, Java, and iSeries Performance Analyst
                             Dept HP4, Rochester MN


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