online Java magazine with lots of tips and howtos at www.javaworld.com 
It's actually very good. The series of articles on Double Checked Locking and why Double Checked Locking Is Broken were very informative. They look at all aspects of Java from the very basics (what's the difference between an interface and an abstract class?) to more advanced material on EJBs and the structure of the Java Virtual Machine. --Adewale Oshineye
See also other Java Links.
See original on c2.com