How to write efficient Java code?

As all of you may know, Java code is compiled and interpreted by JVMs. My questions deal with optimization: Is it optimized at run-time by the JVM only or also at compile-time?

In order to write efficient code, where can I find the list of supported optimizations? Or are JVM optimizations powerful enough so that I just have to write code that is readable and easy to maintain regardless of speed performance?