Here’s a nice article on java.sun.com about what’s new in Java Standard Edition 6 Mustang.

Version 6 of the Java Platform, Standard Edition (Java SE), code-named Mustang, is currently in its second beta release – which is right on track for moving along to its Fall general release. So here are the top 10 things you need to know about Mustang, if you’re still hovering hesitantly over that Beta 2 Download Page link. Subsequent articles will provide more in-depth information.

Really aimed at people who create tools for Java development and for frameworks like JavaServer Pages (JSP) or Personal Home Page construction kit (PHP) engines that need to generate a bunch of classes on demand, the compiler API opens up programmatic access to javac for in-process compilation of dynamically generated Java code. The compiler API is not directly intended for the everyday developer, but for those of you deafened by your screaming inner geek, roll up your sleeves and give it a try. And the rest of us will happily benefit from the tools and the improved Java frameworks that use this.