1. Introduction to JVM

1. learn JVM What is the purpose ？

Everyone should learn with purpose Rather than say He let me learn, I will learn

that Why did he let you learn ？

Well, no selling

Study JVM It can make us clear in Java Program execution process The method attribute Classes have a clearer understanding And it is also good for learning concurrency If we talk about actual combat That must be JVM Parameter tuning

Now You will no longer be a beginner Gradually move towards the big guy

When you are old , Looking back on life , Will find out ： When to study abroad 、 When to decide on your first career 、 When you choose an object and fall in love 、 When to get married , In fact, it's all a big change of fate . Just standing at a fork in the road , See the wind and clouds and thousands of masts , The day you made your choice , In the diary , Quite dull and ordinary , I thought it was an ordinary day in my life .

For us learn JVM Just an ordinary day But it will lay a foundation for us in the future

2. Brief introduction JVM

Cross platform and cross language

jvm It is a cross language platform Now it supports multiple languages in jvm Up operation

let me put it another way java Platform language and jvm It's a cross language platform

simply our java Source file after javac command Translate it into .class file It is not implemented immediately But through jvm Translate it into machine language Finally implemented It's just that we can't perceive all this

virtual machine

Virtual machines are simply divided into two types System virtual machine and program virtual machine

The former can install an entire operating system Such as windows and linux The latter is designed for a single program The typical representative is jvm

It is necessary to draw a picture here to let you understand jvm Where it is
Be careful jvm It does not interact directly with the hardware It runs on the operating system

3. JVM framework

The overall architecture

The overall structure is as follows Let's make an impression first You need to be able to draw later

Architectural model

Java There are two main types of compiler instruction flow One is stack based One is register based

What's the difference between the two ？

Stack based architecture It's relatively easy to implement And does not need to rely on hardware That is to say, it can be used across platforms

Based on register architecture Depending on the hardware Poor portability

In order not to be limited by hardware , For greater cross platform , therefore java The compiler implementation adopts the stack architecture .

4. JVM Life cycle

JVM The boot class loader creates an initial class to complete the startup of , according to JVM Different implementations of , The definition of this class is different .

When running a java When it comes to programming , One JVM The process is really started , Program end ,JVM And then quit .

There are five situations that can cause JVM sign out , If you ask , Simply answer twoorthree kinds , Answer it all , There are some rolls , The interviewer may not be so clear .

• The program ends normally ,JVM sign out
• An exception causes the program to exit ,JVM And then quit
• Operating system errors cause JVM sign out
• call Runtime.hale() or System.exit() Lead to JVM sign out
• use JNI Invocation API uninstall JVM(C Language call )

5. JVM Historical development

1. The original virtual machine sun classic

This is the oldest virtual machine ,1996 year Sun The first commercial product developed by the company Java virtual machine , It realizes the combination of interpreter and compiler , Explain that it runs fast , High compilation efficiency , Combined use is better .

2. Absolute overlord HotSpot

Occupy absolute market position ,jdk1.3 in the future , Become openJdk and OracleJdk All by default JVM, It is worth mentioning that his hot code detection technology , Find the most valuable code through the counter and cache it , Increase of efficiency .

3. The second overlord JRocket

BEA The company developed the fastest JVM, This may be achieved on their own products , Focus on the server , All the code is compiled and executed by the compiler , Understanding can .

ps： The company was later Oracle Acquisition , stay HotSpot Graft on JRocket Excellent characteristics of .

4. The third overlord J9

IBM Developed by the company , Market position and HotSpot near .

5. Future overlord Grral VM

Really cross language stack JVM, Can be used on any platform , Future period , If one day you want to replace HotSpot, Then it must be the next successor .