The JIT compiler is enabled by default, and is activated when a Java method is called. The JIT compiler compiles the bytecode of that method into native machine code, compiling it "just in time" to run. When a method has been compiled, the JVM calls the compiled code of that method directly instead of interpreting it.

When did Java introduce JIT?

Symantec demoed theirs in March of 1996.

Does Java use JIT compilation?

3.1.

Today, Java installation uses both JIT compilers during the normal program execution. As we mentioned in the previous section, our Java program, compiled by javac, starts its execution in an interpreted mode. The JVM tracks each frequently called method and compiles them.