jvm.md

JVM

• What is JRE?
  • The JRE is the Java Runtime Environment. It is a package of everything necessary to run a compiled Java program, including the Java Virtual Machine (JVM), the Java Class Library, the Java command, and other infrastructure. However, it cannot be used to create new programs.
• What is JDK?
  • The JDK is the Java Development Kit, the full-featured SDK for Java.
  • It has everything the JRE has, the compiler (javac) and tools (like javadoc and jdb).

Three major component

• Class Loader

  • Loading
    • Bootstrap class loader
      • Loads system jar rt.jar class to memory in the JRE/lib folder.
    • Extension class loader
      • Makes precompiled class files available to JVM
    • Application class loader
  • Linking
    • Verify
      • After class files are loaded in memory, there is a verify phase to check conformance to standard.
    • Prepare
      • In the preparation phase, memory for static variables memory is allocated.
    • Resolve
      • Symbolic references are replaced with actual references.
  • Initialization
    • Static variables and init initialized

• Runtime Data Area

  • PC Registers & Stacks per thread

• Execution Engine

  • Interpreter
    • Converts instruction to machine code
  • JIT
    • Hotspot machine code cached
    • Intermediate Code Generetor
    • Code Optimizer
    • Target code generetor
    • Profiler

• The Java standard library needs to call into native code. For this purpose, the JRE contains some .dll (Windows) .dylib (macOS), or .so (Linux) files under bin/ or lib/ with supporting, system-specific native binary code.