Twitter Shifting More Code to JVM, Citing Performance and Encapsulation As Primary Drivers

http://www.infoq.com/articles/twitter-java-use

While it almost certainly remains the largest Ruby on Rails based site in the world, Twitter has gradually been moving more and more of its stack to the JVM. The change is partially motivated by oft-cited advantages of the JVM, such as performance and scalability, but is also driven by a desire for better encapsulation of individual services, and other architectural concerns.