Jvm-jvm overview

Basic introduction

JVM： Full name Java Virtual Machine, namely Java virtual machine , A specification , Itself is a virtual computer , Interact directly with the operating system , No direct interaction with hardware , And the operating system can help us complete the work of interacting with hardware

characteristic ：

• Java Virtual machines are based on Binary bytecode perform , By a set of bytecode instructions 、 A set of registers 、 A stack 、 A garbage collection heap 、 A method area, etc
• JVM Information related to the operating system platform is masked , So as to make Java The program only needs to be generated in JVM Bytecode file running on , Achieved through this mechanism Cross-platform sex

Java Code execution process ：Java Program --（ compile ）--> Bytecode file --（ Explain to perform ）--> operating system （Win,Linux）

JVM structure ：

JVM、JRE、JDK contrast ：

• JDK(Java SE Development Kit)：Java Standard development kit , Provides compilation 、 function Java The various tools and resources required for a program
• JRE( Java Runtime Environment)：Java Running environment , Used to interpret execution Java Bytecode file

Reference books ：https://book.douban.com/subject/34907497/

Reference video ：https://www.bilibili.com/video/BV1PJ411n7xZ

Reference video ：https://www.bilibili.com/video/BV1yE411Z7AP

Architectural model

Java The instruction stream input by the compiler is a stack based instruction set architecture . Because of the cross platform design ,Java The instructions of are designed according to the stack , Different platforms CPU Different architectures , Therefore, it cannot be designed based on register architecture

• Based on the characteristics of plank structure ：
  • Simple design and implementation , For systems with limited resources
  • Use zero address instruction to assign , The execution process depends on the operation stack , The instruction set is smaller , The compiler is easy to implement
    • Zero address instruction ： A kind of machine instruction , It is an instruction without address field in the instruction system , There is only operation code but no address code . There are two cases of this kind of instruction ： One is that no operands are needed , The other is that the operands are default （ The implicit ）, The default is that the operand is in the register （ACC） in , Instructions have direct access to registers
    • An address instruction ： An operation code corresponds to an address code , Find the operand through the address code
  • No hardware support required , Better portability , Better cross platform
• Based on the characteristics of register architecture ：
  • Need hardware support , Poor portability
  • Better performance , More efficient execution , Registers are faster than memory
  • Command with an address 、 2. Address instruction 、 3. Address instruction is the main

Life cycle

JVM The life cycle of is divided into three stages , Respectively ： start-up 、 function 、 Death

• start-up ： When a Java The program , By booting the class loader （bootstrap class loader） Create an initial class （initial class）, For possession main The class of the function is JVM Starting point of instance running

• function ：

  • main() Method is the initial starting point of a program , Any thread can be started here by

  • stay JVM There are two thread types inside , Respectively ： User threads and daemons ,JVM The daemon thread is used ,main() And other threads use user threads , The daemon thread will end with the end of the user thread

  • Execute one Java The program , What is really being implemented is a Java Process of virtual machine

  • JVM There are two modes of operation Server And Client, The difference between the two models is ：Client Mode starts faster ,Server Mode starts slowly ; But after a long period of stable operation Server Mode is faster than Client Much faster

    Server Mode enabled JVM It's a heavyweight virtual machine , More optimizations of the program ;Client Mode enabled JVM It's a lightweight virtual machine

• Death ：

  • When all user threads in the program are terminated ,JVM You're going to quit
  • The normal execution of the program ends 、 The program terminates abnormally due to an exception or error 、 An operating system error caused the to terminate
  • Thread calls Runtime class halt Method or System class exit Method , also Java The security manager allows this exit or halt operation