Must know java Concepts

Annotation  - Annotation is one of the features which (are being and) would be extensively used in various java frameworks and technologies.  Below links provide overview of Annotation -

http://download.oracle.com/javase/tutorial/java/javaOO/annotations.html
http://download.oracle.com/javase/1.5.0/docs/guide/language/annotations.html
http://download.oracle.com/javase/6/docs/technotes/guides/language/annotations.html
http://www.developer.com/java/other/article.php/3556176/An-Introduction-to-Java-Annotations.htm
http://www.programmingforfuture.com/2008/11/self-descriptive-coding-in-java.html
http://www.java2s.com/Code/Java/Language-Basics/JavaAnnotationAnnotationandReflection.htm

Annotation in EJB- http://www.oracle.com/technetwork/articles/javaee/jpa-137156.html
Metadata Annotation - http://download.oracle.com/javaee/6/tutorial/doc/girdd.html
Java ee 5 Annotation - http://download.oracle.com/javaee/5/tutorial/doc/idx-1.html