This technique yields a high degree of portability between different CPUs. That's desirable. But the performance on machines not designed to execute Java code as a native language is potentially poor. Better performance could be achieved if the code executing Java were compiled to the computer's native language and optimized to eliminate slow/unnecessary operations. However, Java is an object oriented language that requires complicated bindings to environment code at run time.
JIT Compile is a technique where the Java program is compiled to native code at run time after the program is invoked and before it is executed. Proponents claim this will significantly improve Java performance on 80x8x class machines. Sun was, at one time projecting a 5x improvement in performance from a package of performance improvements including JIT Compile. In practice, JIT compilers are rather large, and are prone to replace cache contents when run. This introduces substantial unanticipated overhead. Hard data on the degree of improvement (if any) is not available.
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