JVM – Write Once & Run Everywhere!

Most probably you heard “Write Once & Run Everywhere” before, this sentence is because the same java code can run on any machine that has JVM installed.


Java Virtual Machine

  • It’s a virtual machine that is responsible for running the java code over any operating system, as every operating system has its own version of JVM.
  • Platform independent execution environment that converts Java Byte Code into Machine Code and executes it.

Under the hood

  • You write Java code in a .java file.
  • The compiler converts each .java file into a corresponding .class file, which consists of the Byte Code.
  • During the execution, the ClassLoader loads the .class files on the RAM.
  • Then the Execution Engine will convert the Byte Code into a native Machine Code, using JIT (Just In Time) compiling.

JIT Compiler

  • The JIT Compiler runs after the user clicks to start the program, the compiler converts the bytecode to machine code on the fly.

JVM Components

  1. Class Loader: loads the .class files into the RAM.
  2. Byte Code Verifier: checks if there is any access restriction violation in the code.
  3. Execution Engine: converts the bytecode into a native machine code.