The JSP container compiles the .java file it has generated. Java compilation creates a binary class -- the JSP page implementation class -- which resides on disk as a .class file.* If there are Java compilation errors, then error messages are generated that identify the error, the JSP line number, and the equivalent .java source file line number where the error took place.
WebSphere Application Server, like most JSP containers, supports the Eclipse Java Development Tools (JDT) compiler, as well as the JDK's javac. WebSphere Application Server started supporting the JDT compiler with Version 6.1. The JDT compiler is the default Java compiler for the JSP container because it operates very efficiently in an Open Services Gateway Initiative (OSGi) environment.