JXTA in a Nutshell

Advertisements

We’ve mentioned advertisements in passing a number of times in this chapter; we even created advertisements for pipes and groups using the shell’s mkadv command. Advertisements are one of the basic building blocks of JXTA: they help peers discover any kind of service, including peergroups, other peers, and pipes. In fact, at a basic level, the JXTA infrastructure is simply about sending advertisements for various resources around to interested parties. A JXTA application simply searches for advertisements it is interested in and responds to requests for advertisements that it has published. The basic protocols we’ve mentioned all use advertisements as the mechanism by which they send data. Advertisements provide a platform-independent representation of platform objects that can be exchanged between different platform implementations (Java, C, etc.).

Advertisements are structured XML documents. The JXTA infrastructure defines six such documents:

• Peer advertisement

• Peergroup advertisement

• Pipe advertisement

• Service advertisement

• Content advertisement

• Endpoint advertisement

Developers may subclass these advertisements when they create their own services. For example, a content advertisement could announce that a peer has a particular item (such as a PDF file of this chapter). A JXTA application may create a subclass of the content advertisement to refer only to PDF files of this book. Peers who are interested in generic content can look for standard content ...