Java, much like any other open source system, suffers from the paradox of choice - too many choices confuse people and makes them like the overall product or system less. There are numerous Java packages out there that do a number of things and it sometimes becomes very difficult for a developer starting on a project to figure out which combination of libraries are needed to meet the requirements.
Here's a list of interesting and useful projects that I have come across so far, ordered by the specific business need. While there are a number of comparison sites for libraries within a particular group, this is more of a subjective view of the libraries that I prefer.
| Business Need | Package |
|---|---|
| Validate US postal information (City, State, Zip) | JPostal |
| Unit testing module | JUnit |
| System or Integration testing with a Wiki-like interface - very helpful for transactional systems | Fitnesse |
| Application framework without the overhead of J2EE | Spring |
| Logging mechanism | Log4J |
| Process Microsoft Office Documents | Apache POI |
| PDF creator / generator | iText |
| Reporting Engine | Jasper Reports |
| Database mapping | Hibernate |
| Search Engine | Lucene |