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The JIT compiler

Although not dictated anywhere, all popular JVM implementations are not just simple interpreters; they feature complex JIT compilers along with their interpreters.

When you launch a Java application, JVM is launched and initialized first. Once this is done, it immediately starts interpreting and running the Java bytecode. If the interpreter believes it makes sense, it will compile sections of the programs and load libraries to native executable code in memory and start executing that version of the code instead of the interpreted Java bytecode version. This often results in code that could be executed much faster.

Whether the code is compiled or interpreted depends on many things. If a routine is called often, it becomes a probable candidate for the JIT compiler to compile it to the native code.

The advantage of the JIT approach is that the distributed files can be cross-platform and the user does not have to wait for native compiling of the whole application. Applications start executing immediately after JVM is initialized, and the optimization is done under the hood.