Shangsilicon Valley JVM Chapter 1 class loading subsystem

1、 Virtual machine introduction

The so-called virtual machine （Virtual Machine）, It's just one Virtual computers . It's a Software , Used to execute a series of virtual computer instructions . Broadly , Virtual machines can be divided into System virtual machine and Program virtual machine .

•   big-name Virtual Box,VMware It belongs to the system virtual machine , They are all simulations of physical computers , It provides a software platform that can run a complete operating system .
• A typical example of a program virtual machine is Java virtual machine , It's designed to execute a single computer program , stay Java The instructions executed in the virtual machine are called Java Bytecode instruction .
• Whether it's a system virtual machine or a program virtual machine , The software running on it is limited to the resources provided by the virtual machine .

2、Java virtual machine

• Java A virtual machine is an execution machine Java Bytecode virtual computer , It has an independent operating mechanism , It runs Java Bytecode may not be created by Java Language compilation .
• JVM Various languages of the platform can be shared Java The cross platform nature of virtual machine 、 Excellent garbage collector , And a reliable just in time compiler .
• Java The core of technology is Java virtual machine （JVM,Java Virtual Machine）, Because of all the Java The program is running in Java Inside the virtual machine .
• Java Virtual machine is the running environment of binary bytecode , Responsible for loading bytecode into its interior , explain / Compile to execute machine instructions on the corresponding platform . Every one of them Java Instructions ,Java There are detailed definitions in the virtual machine specification , How to get the operands , How to deal with operands , Where is the result of the treatment .
   

3、JVM Overall structure

• HotSpot VM It is one of the representative works of high performance virtual machine on the market .
• It uses an interpreter and just in time compiler architecture .
• In today's ,Java The performance of the program has been completely changed , Have reached the point where you can and C/C++ The program is higher and lower .
• The execution engine consists of three parts ： Interpreter , Just in time compiler , Garbage collector

4、JVM Architectural model

Java The flow of instructions input by the compiler is basically a Stack based instruction set architecture , Another instruction set architecture is Register based instruction set architecture . say concretely ： The difference between the two architectures ：

Based on the characteristics of plank structure

• Simpler design and Implementation , For systems with limited resources
• Avoid the problem of register allocation ： Use zero address instruction to assign
• Most of the instructions in the instruction stream are zero address instructions , Its execution depends on Stack operation . The instruction set is smaller , The compiler is easy to implement
• No hardware support required , Better portability , Better Cross platform implementation

Based on the characteristics of register architecture

• The typical application is x86 The binary instruction set of ： Like traditional PC as well as Android Of Davlik virtual machine .
• Instruction set architecture Rely entirely on hardware , High coupling with hardware , Poor portability
• Better performance and more efficient execution
• Spend less instruction to complete an operation
• In most cases , The instruction set based on register architecture usually uses one address instruction 、 Two address instructions and three address instructions are the main , The instruction set based on the stack architecture is mainly zero address instruction
   

 5、 summary

• Because of the cross platform design ,Java The instructions of are designed according to the stack . Different platforms CPU Different architectures , So it can't be designed as register based . The advantage is cross platform , Instruction set is small , The compiler is easy to implement , The disadvantage is that the performance is reduced , More instructions are needed to implement the same function
• today , Although embedded platforms are no longer Java The mainstream running platform of the program （ To be exact, it should be HotSpot VM Our host environment is no longer limited to embedded platforms ）, So why not change the architecture to register based architecture ？
• Because stack based architecture Good cross platform 、 Instruction set is small , Although compared with register based architecture , Stack based architecture compiles more instructions , The execution performance is not as good as register based architecture , But considering its Cross platform and portability , We still choose the stack architecture