title: What are the most important Java libraries?
body: <p>I've read <a href="https://stackoverflow.com/questions/450906/important-frameworks-tools-to-learn">this discussion</a>  on Stackoverflow, but it's been over two years since it was asked. I am a beginning Java programmer and have little knowledge of appreciated and widely-used libraries for this platform.</p>
  
  <p>Of course I do use <em>some</em> libraries (JUnit, parts of Spring), but I'd love to have a list of libraries, that every Java developer should know (i.e. Hibernate, Velocity..?). I am interested in every field of Java applications.</p>
  
  <p>What would you recommend for me (or any other beginner) to check out?</p>