Notes

java virtual machine is an abstract computing machine that serves as runtime environment for executing java bytecode

• translates java bytecode into machine code for execution
• manages memory, security and execution of java application
• provides a consistent environment across different hardware and operating systems.

Architecture

• Class Loader Subsystem -
  • Loading - loads, links and initializes .class files into memory.
  • Linking- ensure bytecode is safe for execution -> allocate memory for static variables and assign default values -> resolve symbolic links to direct references.
  • Initialization - executes static initializers and assigns values to static fields.
• Runtime Data Areas -
  • Method area - stores class level data, class structures, method bytecode, constant pool, static vars. shared accross all threads.
  • Heap - stores all objects and their instance vars.
  • Stack - per-threaded memory for method calls, storing stack frames.
  • PC (program counter) - per-threaded register that stores the address of the current instruction being executed.
  • Native method stack - per-thread memory for native methods.
• Execution Engine -
  • executes bytecode by translating it into machine code.
  • components -
    • interpreter - executes bytecode line by line
    • just in time compiler (JIT) - compiles frequently executed code for faster execution.
    • garbage collector - automatically reclaim memory from unused object in the heap.
• Native method interface - enables java code to interact with native application in c or c++
• Security Manager - enforces security policies, restricted untrusted code from accessing sensitive resources.