JVM, JRE, JDK

JAVA BASICS OVERVIEW

• What is JAVA?

  • Platform Independent language
  • Supports OOPS
  • Portability [WORA]
    • Write Once Run Anywhere

• 3 Main Components Of Java

  ┌───────────┐
  │    JDK    │
  │ ┌─────────┐ │
  │ │   JRE   │ │
  │ │ ┌───────┐ │ │
  │ │ │  JVM  │ │ │
  │ │ └───────┘ │ │
  │ └─────────┘ │
  └───────────┘
  

  So JVM, JRE & JDK are 3 main components of Java.

⇒ JVM : Java Virtual Machine

• It's just an abstract machine that doesn't exist physically.

  [Java Program] --Compiler--> [Bytecode] --JVM--> [Machine Code] --> [CPU] --> Output
  

• So JVM is platform dependent.

• So we need to install JVM based on the platform i.e macOS, Linux or Windows. Input for JVM is bytecode & output is machine code.

• Now since bytecode can be run by any JVM, it makes a Java program platform independent.

• JVM has JIT (Just In Time) compiler which takes bytecode & converts it into machine code.

⇒ JRE : Java Runtime Environment

• JRE contains JVM & class libraries i.e the libraries which we've used in the code.

  ┌─────────────────┐
  │       JRE       │
  │  ┌───────────┐  │
  │  │   Class   │  │
  │  │ Libraries │  │
  │  └───────────┘  │
  │  ┌───────────┐  │
  │  │    JVM    │  │
  │  └───────────┘  │
  └─────────────────┘
  

• So if we have JRE, we can run any Java Program but we cannot code the program.

⇒ JDK : Java Development Kit

• It has program language information

• It has compiler (javac)

• It has debugger

• So JDK = JRE + (Program Language + compiler + debugger + other dev. components)

• So JVM, JRE & JDK all three are platform dependent but the compiled bytecode is platform independent.

• Java Editions:

  • JSE: Java Standard Edition
  • JEE: Java Enterprise Edition (Jakarta EE)
  • JME: Java Micro / Mobile Edition

• JSE is the core Java.

• JEE = JSE + Servlets + JSP + Transaction API + Persistence API

• JME = API for mobile applications