Shulou(Shulou.com)06/02 Report--

In this issue, the editor will bring you about what the Java technical system is and how to summarize JVM. The article is rich in content and analyzes and narrates it from a professional point of view. I hope you can get something after reading this article.

1. Brief introduction

Java is not only a programming language, but also a technical system composed of a series of computer software and specifications.

The slogan of Java is "write once, run everywhere". The reason why it can be "cross-platform" is that different virtual machines on each platform shield hardware differences, while Java programs run on virtual machines.

2. Java technology system

In a broad sense, Kotlin, Groovy, JRuby and other programming languages running on the Java virtual machine and their related programs all belong to the Java technology system.

In the traditional sense, the Java technology system officially defined by JCP mainly includes:

Java programming language

Java virtual machines on various hardware platforms

Class file format

Java class library API

Third-party Java class libraries for commercial organizations and open source communities

The three parts of Java programming language, Java virtual machine and Java class library are called JDK (Java Development Kit). JDK is the smallest environment to support Java program development.

The Java SE API subset and Java virtual machine in Java class library API are generally called JRE (Java Runtime Environment). JRE is a standard environment that supports the running of Java programs.

PS: JCP, whose full name is Java Community Process, is the Java community, a community of technology giants in the industry that defines and develops Java's technical specifications.

The technical system of Java includes the following contents:

3. JVM family

"Java virtual machine" is just a concept, its implementation products are numerous, the following is a brief description of its development process.

Sun Classic/Exact VM

Class VM (JDK 1.0)

The world's first commercial Java virtual machine

Execute Java code in pure interpreter mode

Object lookup is based on handle

Exact VM (JDK 1.2)

Accurate memory management: a virtual machine can know the specific type of data at a location in memory

HotSpot VM

Default virtual machines in Sun/OracleJDK and OpenJDK

The most widely used Java virtual machine at present

Because the HotSpot virtual machine is the most widely used and the default virtual machine for OracleJDK and OpenJDK at present, the later content analysis is mainly aimed at this virtual machine.

The terminal executes the java-version command as follows (different machines will be different, here for reference only):

$java-versionjava version "1.8.0mm 191" Java (TM) SE Runtime Environment (build 1.8.0_191-b12) Java HotSpot (TM) 64-Bit Server VM (build 25.191-b12, mixed mode)

The HotSpot (TM) 64-Bit Server VM here refers to the HotSpot virtual machine.

Mobile/Embedded VM

Java ME virtual machine

BEA JRockit/IBM J9 VM

These two virtual machines were once called the "three major commercial Java virtual machines" with HotSpot virtual machines.

JRockit

Focus on server-side applications

No interpreter, all code is compiled and executed by the just-in-time compiler

J9 VM

Separation of responsibilities and modularization are superior to HotSpot VM

BEA Liquid VM/Azul VM

A proprietary virtual machine bound to a specific hardware platform and compatible with hardware and software

Apache Harmony/Google Android Dalvik VM

These two are "virtual machines", not "Java virtual machines".

Harmony VM

Failed to pass TCK certification

Non-large-scale commercial use

Dalvik VM

Does not follow the Java virtual machine specification

Cannot execute Class file of Java directly

Microsoft JVM

Used to be the best performing Java virtual machine on Windows system.

Other

KVM: has been widely used on mobile platform

Java Card VM

There are many other kinds of virtual machines, which are no longer listed one by one. It is worth noting that a relatively new Java virtual machine: Graal VM.

Graal VM

In April 2018, Oracle Labs publicly launched cool techs: Graal VM.

Graal VM, officially known as "Universal VM" and "Polyglot VM", is a cross-language full-stack virtual machine enhanced on the basis of the HotSpot virtual machine. Can be used as a running platform for "any language", including:

Languages based on Java virtual machine, such as Java, Scala, Groovy, Kotlin, etc.

C, C++, Rust and other LLVM-based languages, while supporting JavaScript, Ruby, Python, etc.

As shown in the figure:

And since JDK 10, a new just-in-time compiler has been added to HotSpot: the Graal compiler.

If the HotSpot virtual machine is really replaced one day in the future, Graal VM is likely to be "superior".

4. Summary

This paper mainly introduces the Java technology system and the development process of the Java virtual machine, this part of the content can be understood, and then analyze the main content of the virtual machine.

In order to review and summarize, the above contents are sorted out into a mind map:

The above is what the editor shares with you about the Java technology system and how to summarize JVM. If you happen to have similar doubts, you might as well refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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