[Java] JDK, JRE, and JVM

JDK, JRE, and JVM

1. JVM

- JVM is an abstraction machine. It is a specification that provides runtime environment in whick java bytecode can be executed

- JVMs are avaliable for many hardware and software platform. 

- JVM, JRE, and JDK  are platform dependent because configuration of each OS differs

- Java is platform independent

1_1. JVM performs following main task

- Loads code

- Verifes code

- Executes code

- Provides runtime enviroment

2. JRE

- JRE is an acronym for Java Runtime Enviroment

- It is used to provide runtime enviroment. 

- It is the implementation of JVM. It physically exists

- It contains set of libraries + other files that JVM uses at runtime

3. JDK

- JDK is an acronym for Java Development Kit

- It contains JRE + development tools

4. At compile time

- At compile time, java file is compiled by java Compiler and convert the java code into bytecode

4_1. At runtime

- classloader : is the subsystem of JVM that is used to load class files

- Bytecode Verifier : checks the code fragments for illegal code that can violate access right to objects

- Interpreter : read bytecode stream then execute the instructions