jsr-56 - Java Community Process Program

JNLP File
(application
descriptor)
Icon
Figure 1: JNLP File and External Resources
The JNLP file is, in some sense, similar to a traditional executable format. Traditionally, applications are
delivered as binary platform-dependent files. For example, on Windows, an application is delivered as a
MyApp.exe executable. The executable format is designed so the Windows operating system can load
the application and execute it. It also contains information about external dependencies, such as, e.g.,
MyApp.dll. This format is file-centric; all external references are references to files on the local file
system. In contrast, a JNLP file does not contain any binary data itself, but instead contains URLs to
where they can be obtained from. The JNLP file format is Web-centric; the references to external
resources are URLs, instead of file names.
1.2.2 DOWNLOADING RESOURCES
Jar File
Jar File
JNLP File
(extension
descriptor)
The JNLP Client can download 3 different kind of resources: JAR files, images, and JNLP files. All
resources in a JNLP file are uniquely named using either a URL or a URL/version-id pair. A typical
application deployed using JNLP will consist of a set of JAR files and a set of images 3 . JAR files, images,
and JNLP files can be downloaded using standard HTTP GET requests. For example:
http://www.mysite.com/app/MyApp.jar
Jar File
Jar File
This basic download protocol works from a standard unmodified Web server. This leverages existing Web
server technology, which is important to achieve wide-spread use of a new technology on the Internet.
To provide more control and better utilization of bandwidth, a version-based download protocol is also
supported. The version-based protocol is designed to:
Allow several versions of an application to co-exist on a server at a given time. In particular, this
means that an application that is distributed as several JAR files can be safely upgraded. A JNLP
Client that is downloading JAR files right when a Web server is being updated will never download
JAR files that are a mix between two application versions.
Provide a unique URL for an application independent of its version. This allows a JNLP Client to
automatically detect and flush old versions out of the cache.
Make it possible to incrementally update already-downloaded JAR files. This can substantially
minimize the download requirements for upgrading to a new version.
3 The image files described in the JNLP file are icons that can be used by the JNLP Client to integrate the application into the desktop
environment. They are not for use by the application itself. All application resources, such as images, must generally either be included
in one of the JAR files or be explicitly downloaded using, e.g, an HTTP request.
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