JRebel

JRebel is a JVM plugin that allows Java developers to change code and see the effect without redeploying the application or restarting Tomcat. It supports Spring as well (Spring beans additions, removals etc. on the fly).

( from http://zeroturnaround.com/software/jrebel/trial/ )

What Does JRebel Do?

Instant Reload

See all changes to Java code instantly in the browser. Save and refresh!
The same is true for your other project files.

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Ecosystem Support

All major IDEs, application servers, servlet containers and frameworks are
supported out of the box.

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Zero Configuration

Start using inside 5 minutes, no docs needed.