JVM

Python can execute raw code. This means it is a virtual machine that actively translate machine-independent python code into machine-dependent byte code in run time. Python choose to ship machine-independent human readable code, but Java choose to ship machine-independent bytecode for the following reasons: - bytecode is smaller than source code - bytecode is harder to reverse engineer than source code - bytecode is harder to modify than source code - JVM does not need to have a compiler (reduce footprint) - compile once, and run everywhere

Virtual Machine: a virtualization on CPU

Java Virtual Machine (JVM): parse Java's machine-independent byte code.

• it as its own instruction sets in a .class file

• JVM execute bytecode in .class file by translating them to machine code (you don't need JAVA to produce .class file to feed JVM in theory)

• exactly 1 instance of JVM will be paired with a Java program

• Functionality of JVM

  • Loading classes
  • Linking classes
  • Memory allocation

Java made the decision to ship machine-independent is to avoid re-compilation for different platforms. However, it requires the user to at least install machine-dependent JRE.

Java Runtime Environment: just a java virtual machine, used to run .class bytecode

Java Development Kit: superset of JRE, including Java compiler, debugger, and many developing tool for debugging purposes.

Java provides Java Virtual Machine Specification and in theory you can make your own JVM. There are many JVMs in the market.