JVM JDK JRE

1. JVM (Java Virtual Machine)

• Definition: JVM is the engine that runs Java bytecode.
• Role: It converts compiled Java bytecode into machine code so your program can run on any device.
• Key Point: JVM is platform-dependent (different JVMs exist for Windows, Linux, Mac).
• Example: You write code once, compile it, and JVM allows it to run anywhere (“Write Once, Run Anywhere”).

2. JRE (Java Runtime Environment)

• Definition: JRE is the environment required to run Java programs.
• Components:
  • JVM
  • Core libraries (like java.lang, java.util)
  • Other runtime components
• Key Point: JRE cannot compile Java code; it only runs it.

3. JDK (Java Development Kit)

• Definition: JDK is the full toolkit for developing Java applications.
• Components:
  • JRE (so it can run Java programs)
  • Java compiler (javac)
  • Tools like javadoc, jar, jdb
• Key Point: If you want to write and compile Java programs, you need the JDK.

4. Java Compiler (javac)

• Definition: The Java compiler converts Java source code (.java files) into bytecode (.class files).
• Key Point: The bytecode is what the JVM executes.

Putting it together:

1. You write Java code (.java).
2. Java compiler (javac) compiles it → .class (bytecode).
3. JVM executes the bytecode.
4. JRE provides the environment needed for JVM to run the program.
5. JDK includes everything you need to develop + run Java programs.

💡 Analogy:

• JVM = engine of a car
• JRE = car (engine + fuel system, etc.)
• JDK = car factory (everything you need to build & run the car)
• Compiler = assembly line that converts raw materials (source code) into parts the car (JVM) can use