2: Chapter 1: understanding JVM specification 1: introduction to JVM;

explain ：

（1）JVM brief introduction , know JVM;

Catalog

1.JVM summary ; 

2.JVM The main function ;

3. Virtual machine is “Java Platform independence ” The protection of ;

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1.JVM summary ; 

（1） Virtual machine can be regarded as a computer system , But it is simulated by software ;

（2） On a real physical machine , Mainly from CPU Execute instruction set ;;; stay JVM in , Simulate instruction set through software ;

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（1） The right part ： We wrote java Source code （.java file ）, And then through java compiler （ Such as javac Wait for the order , Or by IDEA Tools such as ） Compile it into java Bytecode （.class file ）;

          ●  This process is the development process ;

（2） The left part ： When we compile into .class After the document , This Java Platform runtime environment （ namely JVM） Through the class loader （Class Loader）, Put the bytecode file （.class file ） Load into the virtual machine ;

          ●  natural , When loading , Will verify and authenticate bytecode files （ such as , Verify whether it conforms to the virtual machine specification 、 Whether the format is correct ）;

          ●  When loading bytecode files , Also mount java Class library （ It refers to some basic things that must be provided by the virtual organism 、 Necessary support , such as ： Basic data types 、if-else and for Treatment of circulation 、 Basic java Safe handling ,JNI With some hardware processing and so on ）;

（3） The middle part on the left ： When we finish loading , The virtual machine will give these to Java Interpreter ;

          ● Java The interpreter will interpret the bytecode and execute ; It will also involve just in time compilers （JIT）;

          ●  The runtime is much more complex than shown in this figure ;;; The running environment must first allocate memory , And then execute .class Bytecode （ You will use the bytecode execution engine ）, Garbage collection is also needed , Consider concurrent processing , Safety treatment ;

（4） Lower left ： If you need to interact with hardware or operating system （ Such as through JNI Interact with the outside world ）, The virtual machine will come out ; namely java Programs interact with virtual machines 、 Virtual machines interact with operating systems 、 Then interact with the hardware through the operating system ;

2.JVM The main function ;

3. Virtual machine is “Java Platform independence ” The protection of ;

（1） stay Windows On the system 、Linux On different systems, such as the system ,Java Virtual machines are different ;

（2） The same program we developed , Compile it into .class After the document ;

          ●  Throw it into Linux On the system , There is Linux Version of JVM To run ;;; Throw it into Windows On the system , There is Windows Version of JVM To run ;

          ●  Look at it like this , Actually Java It's not really platform independent ; It's just that different virtual machines on different platforms have shielded us ;