Compiler, Interpreter, JIT Compiler

Java programs go through multiple steps before they run efficiently on your machine.

Compiler

A compiler is a program that translates the entire source code written by a programmer into an intermediate form called bytecode.

• In Java, this happens when you write code in .java files and compile it using the javac command.
• The compiler converts your code into bytecode stored in .class files.
• Bytecode is not machine-specific, meaning it can run on any device that has a Java Virtual Machine (JVM).
• It ensures your code is error-free (syntactically) before converting it to bytecode.
• It makes Java platform-independent because the same bytecode can run on any system with a JVM.
• However, the compiler does not execute the code; it only translates it.

Interpreter

An interpreter is responsible for executing the bytecode produced by the compiler. It reads the bytecode line by line and converts it into machine code (binary) that your computer’s processor understands.

• In Java, the JVM interpreter plays this role.
• The interpreter is useful for debugging since it stops immediately when it encounters an error.
• However, interpreting bytecode line by line can be slower, especially for repetitive tasks or larger programs. This is where the JIT Compiler comes in.

JIT Compiler (Just-In-Time Compiler)

The JIT Compiler is part of the JVM and works at runtime to make Java programs run faster.

• It identifies frequently used code blocks (called “hot spots”) and compiles them into native machine code.
• Once compiled, the native machine code is executed directly by the processor, avoiding the need for repeated interpretation.
• This approach combines the portability of bytecode with the speed of native machine code.
• It improves performance by optimizing code during runtime.
• It reduces the overhead caused by interpreting bytecode line by line.
• The more you run your program, the faster it gets for repetitive tasks because the JIT Compiler keeps optimizing.

How They Work Together in Java

• Compilation: Your .java file is compiled into .class files (bytecode) by the Java compiler.
• Interpretation: The JVM interpreter reads and executes the bytecode.
• JIT Compilation: As the program runs, the JIT Compiler identifies frequently used code and converts it into optimized machine code for faster execution.