Remote clients can access and run
applications on their local machines by using the Java Network
Launching Protocol (JNLP). JNLP applications are like standard Java
applets in that access to the application begins in a Web browser,
and the JNLP can be confined to a secure “sandbox” on the client
machine. Unlike applets, JNLP applications don’t run inside the
client’s browser; instead, the Web browser is used as a jumping-off
or installation vehicle for the application.

You’ll implement JNLP as an application called
Web Start. In order for your clients to access your JNLP
applications, they must first install Java Web Start. (See Sun’s Java
Web Start page for more information on installing and using
Java Web Start.)

One of the biggest advantages JNLP applications
have over standard client applications is that JNLP apps are
self-installing and self-updating. Once the application is
installed (via the Web browser) on the client, the application will
update itself when necessary as long as it has access to the
network. All resources for JNLP applications are delivered via the
network. This spares an application’s administrator from having to
visit each machine to install and update applications.

Web Start applications are regular Java
applications that are written to the JNLP specification; but there
are differences you’ll want to know before you head down the JNLP
path. Here are several things to be mindful of when developing your
Web Start applications:

  • You must store a Web Start application’s
    resources in jar files on the server. These jar files will be
    transferred to the client for access by the application. Since all
    resources must be stored in jar files, the application must use
    ClassLoader’s getResource() to access any files it needs from these
    jar files.
  • A Web Start application starts with a main
    method like most other Java applications. The main class is defined
    in a JNLP file, which is an XML document that stores other
    application configuration information such as description,
    application icon, code base, permissions, etc.
  • You must configure your Web server to deliver
    a specific MIME type for your JNLP documents. This configuration is
    usually simple, but if it’s not done, your applications may not
    deploy properly.
  • Like Java applets, your application will be
    constrained to a sandbox on the client unless you used signed jars
    and configure permissions correctly in your application’s JNLP
    file.

Even with the above caveats, using JNLP and
Java Web Start for your next distributed application may be an
option. To get detailed information on what it’s like and to test
out a Java Web Start application, visit Sun’s
Java Web Start home page.

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