jsr-56 - Java Community Process Program

6 DOWNLOADING AND CACHING OF RESOURCES
The JNLP Client can download four different kind of resources from the Web: JAR files, images,
extensions, and JNLP files. This sections describes how they are downloaded, and how they can be cached
on the client system.
JNLP defines three different download protocols, all based on HTTP GET requests:
A basic download protocol which does not require special Web server support. This can be used for
JAR files, images, and JNLP files.
A version-based download protocol that allows greater control over which resources are downloaded
and which supports incremental updating of JAR files. This can be used for JAR files and images.
An extension download protocol which is an addition to the above protocols to include platformspecific
information. This is used for JNLP files containing extension descriptors.
Resources are named uniquely in a JNLP file using the href and version attributes. The following table
summarize the different elements that refer to external resources and which download protocols they
support.
Element href attribute version attribute Supported Protocols
jnlp Optional Optional Basic
icon Required Optional Basic, Version-based
jar Required Optional Basic, Version-based
nativelib Required Optional Basic, Version-based
extension Required Optional Extension
j2se Optional Required Extension
6.1 HTTP FORMAT
The HTTP protocol is used to transfer all information between the Web server and the JNLP Client. The
following describes the common request and response format used by all download protocols.
6.1.1 REQUEST
Each request consists of an HTTP GET request to the URL for the given resource, and potentially a set of
arguments, passed to the Web server in the query string of the URL. The syntax for a request is:
request ::= URL [ "?" arguments ]
arguments ::= key "=" value ( "&" key "=" value ) *
The key and value elements are encoded using the default encoding for parameters passed in URLs. To
convert a key/value, each character is examined in turn:
The ASCII characters ’a’ through ’z’, ’A’ through ’Z’, ’0’ through ’9’, ’.’ , ’*’, ’-’, and ’_’ can remain the
same.
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