JVM(Java virtual machine)

• Classloader：负责loading, linking,initialization 我们编写的代码是.java文件，经过编译之后就是.class字节码文件，classloader就是负责把这些.class文件加载到内存中

  • loading阶段：

    • Bootstrap： 负责加载从RT.jar(RUNTIME MEMORY)加载.class文件，比如System.out，不需要在乎哪里来的System类，
    • Extention: 负责从/jre/lib/ext 文件中加载.class
    • Application：

  • linking阶段

    • verify：Once the class files are loaded to the memory, there is a verify phase where the bytecode class files are verified if they conform the standards
    • prepare: In the preparing phase, memory is allocated for the static variables and default values are assigned.
    • resolve: In the resvolve phase, all the symbolic references are replayed with actual references.

  • initializaton阶段：all the static variables are assigned with the actual values and it also excutes the static initializers at this point of time

• Runtime memory area

  • method area: all the class level data， 比如类静态变量

  • heap area: all the objects and instance variables are stored

  • stack memory: local variable, operand stack, frame data

  • PC register: current executing instructions

  • native method stack: native method information，native code,method也就是非java编写的本地代码，通常是C或C++写的。本地代码指的是在本地（通常是操作系统层面）编写和执行的代码。这些代码可以直接访问操作系统的底层功能和硬件设备，通常用于实现那些Java无法高效完成或无法直接访问的任务。

• Execution engine: 将字节码文件转换成机器码文件并执行指令

  • Interpreter

  • JIT compiler

    • Intermedia code generatoe: Generates intermedia code
    • Code optimizer: optimizes the intermedia code for better performance
    • target code generater: converts intermediate code to native machine code
    • profiler: finding hotpots, methods, which are called repeatedly.

  • Java native method interface: interact with the native libraries and makes it avaliable for the JVM execution engine.

  • Garbage collector : destroy the objects that are no longer used.