• JDK is intend to developer as it contain development tools such as compiler, javadoc, java debugger etc.
  • Virtual Machine is software implementation of machine.
  • Java is designed to run on virtual machine.


  • Self Contained simulation of physical machine.
  • Single program execution VM or System VM for execution of OS.
  • Process : Instruction Fetch, Decoding, execution and storing result.

Features of JVM :

  • Stack Based : Because push and pop operation. (Content Link)
    • Operands addressed implicitly,Stack Pointer,Push/Pop Operation
    • Register Based
      • Instruction need to have address of operands
      • Advantage : Fast Execution & Optimization (result future use)
      • Disadvantage: Average Instruction size is more than Stack
  • Symbolic Reference:
    • All Types referred through run time constant pool
    • All primitive type referred through explicitly memory address based.
  • Garbage Collection: Class instance created by used, destroyed by automatic GB.
  • Guarantee Platform Independence (int size in c/C++ platform to platform)
  • Network byte order

JVM Structure:

  • Java Source File converted to Byte Code by java compiler.
  • Class Loader: It loads the byte code to run time data access and then provide to execution environment for execution
  • Dynamic Load Feature: Load and link every class first time at run time but not at compile time. JVM class loader execute this.

Class Loader Feature:

  • Hierarchical Structure (Bootstrap Class Loader is Parent of all)
  • Delegation Mode: Checked Parent for loading, if not then requesting loader will load
  • Visibility Limit: Only child can see class loaded by parent.
  • Unload is not allowed: Only new creation is allowed (if want create new loader)

FQCN (Fully Classified Class Name) Store Identifier for every class of classloader.


Process of loading & linking a class by class loader :


Execution Engine:

  • Byte code assigned to run time data areas via loaded executed by this engine.
  • Execute one by one instruction.
  • Byte code is human readable so it will be converted to machine specific code.
  • Conversion to machine code done in following ways.
    • Interpreter : Read, Interpret and execute one by one. But slowly execute interpreted language.
    • Just In Time Compiler (JIT): This run interpreter first then at appropriate time JIT compile entire byte code to native code.

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