When calling a servlet with more than 2 threads, ocassionaly io#read returns an empty string. After further investigation, we determined that to_io is returning the same IO objects to multiple ruby threads. Therefore once one thread read the content of the IO, the second thread reading the shared IO gets an empty string back. The problem is located @ $jruby-rack_home/src/main/java/org/jruby/rack/DefaultRackApplication.java Headius suggested to define the method "to_io" with getSingletonClass() instead of getMetaClass(). After following headius recommendation, the problem went away.