Explain Just-In-Time compiler.

Just-In-Time (JIT) compiler is a feature of the runtime interpreter, that instead of interpreting bytecode every time a method is invoked, will compile the bytecode into the machine code instructions of the running machine, and then invoke this object code instead. Here are the steps to explain how it works:

1. Bytecode Interpretation: When a Java program is run, the bytecode of the program is fed into the Java Virtual Machine (JVM) where it is interpreted by the Just-In-Time (JIT) compiler.

2. Compilation: The JIT compiler then compiles the bytecode of the method into native machine code, turning functions that the program uses frequently into a set of machine instructions that the computer's processor can understand. This process is done at runtime (i.e., while the program is running).

3. Execution: Once the bytecode has been compiled into machine code by the JIT compiler, the JVM executes the machine code directly instead of interpreting it. This is faster than interpreting bytecode.

4. Optimization: The JIT compiler also performs optimizations during the compilation process, such as inlining small methods, which can significantly speed up execution time.

5. Caching: The compiled code is cached, so it doesn't need to be recompiled each time the method is called. This means that if the same method is called again, the already compiled code is used, which speeds up the program.

In summary, the Just-In-Time compiler is a part of the JVM that improves the performance of Java applications by compiling bytecodes to native machine code at run time.