Hotswap
From APIDesign
Hotswap is the most important technology that makes developers productive. Why people think that PHP makes them more productive than Java? Because it is enough to change a single line of code in your .php file, save it and reload the webpage. Doing the same with Java requires compilation, packaging, deployment. All of this takes ages and is not flexible enough. At least it was not until Thomas Wuerthinger released his patched Hotspot!