Table of contents
Preface
The configuration of Java development environment is actually not complicated. It is also a process of downloading and installing-environment variables-terminal test version. The problem is that Java has a lot of concepts that are difficult to distinguish, such as OpenJDK, OracleJDk, JavaSE, JavsEE, JavaME, etc. In fact, they are not complicated to understand.
1.JDK has so many names? What's the difference?
The origin of the matter has to start with the history of Java development. To put it simply, Java used to be managed by Sun. Later, Oracle took over Java and continued to update Java. The Java released by Oracle was called OracleJDK. Later, Oracle open sourced Java and established the OpenJDK project. This The project later became OpenJDK. In other words OracleJDK和OpenJDK是一家公司的, they reach the same goal by different paths , and at least at the code level their differences are minimal. The big difference is in the license:
OpenJDK is released under the license GPL v2.
OracleJDK is licensed under the Oracle Binary Code License.
OpenJDK project home pageOpenJDK
download
pageOracleJDK home page
Later, more companies participated in the maintenance of OpenJDK, and major Internet companies also released JDK with their own commercial standards, as follows:
| Build | LTS | permissive license | TCK test | Unmodified build from upstream | Provide commercial support |
|---|---|---|---|---|---|
| AdoptOpenJDK | Yes | Yes | No | Optional | Optional (IBM) |
| Alibaba Dragonwell | Yes | Yes | Yes | No | No |
| Amazon Correct | Yes | Yes | Yes | No | Optional (on AWS) |
| Zulu Blue | Yes | Yes | Yes | No | Optional |
| BellSoft Liberica JDK | Yes | Yes | Yes | No | Optional |
| Huawei bisheng JDK | Yes | Yes | Yes | No | No |
| IBM Java SDK | Yes | No | Yes | No | Yes |
| Microsoft Build of OpenJDK | Yes | Yes | Yes | No | No (beta) |
| ojdkbuild | Yes | Yes | No | Yes | No |
| OpenLogic OpenJDK | Yes | Yes | Yes | No | Optional |
| Oracle GraalVM Community Edition | No | Yes | Yes | No | No |
| Oracle GraalVM Enterprise Edition | Yes | No | Yes | No | Yes |
| Oracle Java SE | Yes | No | Yes | No | Yes |
| Oracle OpenJDK | No | Yes | Yes | Yes | No |
| Red Hat build of OpenJDK | Yes | Yes | Yes | No | Yes |
| Tencent Kona | Yes | Yes | Yes | No | No |
| SAP SapMachine | Yes | Yes | Yes | No | Optional (for SAP products) |
LTS: Abbreviation for "Long Term Support", that is, long-term support version
TCK: a test provided by the OpenJDK project, which is actually not very important
All in all, no matter what JDK, it can be said to be derived from the OpenJDK project to some extent, but they have slight differences in protocols, interfaces, etc.
2. What are JavaEE, JavaSE and JavaME?
JavaSE: The abbreviation of Java Standard Edition, commonly known as JDK.
JavaEE: The abbreviation of Java Platform Enterprise Edition, which is the Java Platform Enterprise Edition. A set of interfaces and standards are added to JavaSE, which is mostly used for enterprise-level development, including web development. .
JavaME: Java Micro Edition, which is a micro version of Java, is a shortened version of JavaSE. It is mostly used for the development of mobile devices such as mobile phones and set-top boxes.
3. How to choose JDK?
It is not recommended to use the original OpenJDK directly.
It’s not that the more Yes in the above table, the better.
Personally, I recommend those that have installation packages corresponding to the operating system , such as .deb packages for Debian systems, etc. The main reason is that using installation packages is convenient for uninstalling configurations, etc., and some installation packages will set environment variables for you (out of the box!) , Secondly, some Java applications detect the location of Java by detecting the registry. Using the installation package will be extremely convenient.
In the end, the choice is based on personal needs, such as architecture and required JDK version. Alibaba's DragonWell only has JDK8 and JDK11.
I personally recommend BellSoft's JDK (Full version), AdoptiumJDK, and AmazonJDK. The ranking indicates the degree of recommendation. Their common features are: installation packages provided, multiple JDK versions, and fast domestic downloads.
4.JDK installation and configuration
Take BellSoftJDK as an example
Windows
Download the msi installation package from the official website and install it.
Then configure the environment (BellSoft's JDK will directly add environment variables for you. If you don't mind it, you can leave it alone. I choose to delete it directly here)
Create a new one in the system variable JAVA_HOME, all capital letters. Some software obtains the JDK through the JAVA_HOME environment variable by default. If you have multiple JDKs, you can configure them like me, so that you can switch the global JDK with only a simple change.
Remember to declare the following in Path. This Path will not need to be touched in the future.
After directly operating JAVA_HOME, you can still check it with cmd:
Linux
If there is an installation package, install it directly. If not, find a place to unzip it. Anyway, you will get a folder in the end.
If you are installing using an installation package, you can directly skip the step of registering update-alter. If you are installing via zip, you need to manually register the update-alter corresponding to jdk. Reference article
update-alternatives --install /usr/bin/java java /example/jdk-1.8/bin/java 100 ##最后面的数字大小表示优先级
update-alternatives --install /usr/bin/java java /example/jdk-11/bin/java 200 ##倒数第二个参数是JDK目录
update-alternatives --install /usr/bin/java java /example/jdk-17/bin/java 300 ##其它的不用动
update-alternatives --install /usr/bin/javac javac /example/jdk-1.8/bin/javac 100
update-alternatives --install /usr/bin/java javac /example/jdk-11/bin/javac 200
update-alternatives --install /usr/bin/javac javac /example/jdk-17/bin/javac 300
In any case, you can use update-alternatives --config javacand update-alternatives --config javato quickly switch compilers:
you can also add the following environment variables later, which are available in ~/.zshrcand ~/.bashrc. It is not recommended to configure them globally directly.
It should be noted that this environment variable will not affect update-alternatives --config javacthe options. If you choose jdk8, the compiler output will still be jdk8, and it will not change because it is jdk17's java_home.
Note that this must be present/
If you are more skillful, you can also use the alias abbreviation in .zshrc to quickly switch between java and javac versions.
alias changejdk8="echo 2 | sudo update-alternatives --config java && echo 2 | sudo update-alternatives --config javac && java -version && javac -version"
Off topic: VSCode configuration
VSCode's Java plug-in sometimes does not use JDK. In this case, you need to configure it manually:
Linux and Windows are the same
If there are multiple JDKs, you can configure them like this. The one marked default uses the JDK by default.
