XML feeds are the way today to keep a tab on what's happening in the blogosphere as well as to know about site updates and new additions. I had heard about two popular open source feature-rich Java APIs to deal with the feeds, ROME and Informa but could never really savor them.
For one of my own sites where I, sort of hand-built the aggregator as Myjavaserevr does not allow deploying external JARs or Taglibs I did not get any chance to play with these APIs. So when I got some opportunity it was imperative that I tried them.
The following is a very elementary example of using the two news aggregation libraries that imitates a newsreader. The examples, as I said, are pretty basic and they would make a hit to the specified Feed URL every time you call the JSPs. The code snippets are not meant to demonstrate good coding practice.
Both the libraries support almost all versions of RSS, RDF and Atom and features such as dynamic discovery of feed format. Feature wise probably Informa has an upper hand (it supports OPML, recognizes the Enclosure element making it suitable to comprehend Podcast feeds and can be configured to use a persistence mechanism built over Hibernate) but what it lacks is availability of documents. There are no primers at the site and the code is very poorly commented making the Javadocs difficult to come to pace quickly. The two Java.net articles that I could, Google have been outdated, as I used the 0.6.5 version of the library.
ROME, on the other hand, has very nice documentation available at its site, complete with code examples. Many desirable features are unfortunately still on the TODO list. For comprehending the "Enclosure" element ROME needs a separate plugin module (that also supports iTunes extensions). While I have not investigated them, there are a number of sub-projects based on both ROME and Informa, for example: there is a JSP Tag library based on Informa. There is a short review of various libraries here but I guess much stuff on Informa is not relevant now since its latest release.
I am not mincing my words when I say that each API has its own strengths, Informa library is pretty bulky but supports OPML while Rome has a wider support for all kind of XML feeds and has a pluggable architecture. The good thing about these APIs is that they pretty much offer you everything that you may want to do with feeds, reading, generating your own, and creating a digest from multiple feeds and so on.
To run these JSPs, needless to say, you would need to download Informa and ROME libraries. I ran these on jakarta-tomcat-5.0.28 / j2sdk1.4.2_06 and the only dependency I was missing was the JDOM jar that ROME needs.