Get JVM environment variables and operating system environment variables

Get environment variables

The environment variable here refers to the environment variable associated with the JVM, such as the environment variable set in the following way.

 #Set on the command line at runtime: 
java -Ddebug=true YourClass 

 #Set in the program
System.setProperty("debug", "true");  

   #Get in the program
String debug = System.getProperty("debug");

There are also some predefined environment variables, commonly used as follows:
java.version
java.home
os.name
os.version
user.name
user.home
user.dir

Get the environment variables of the operating system

System.getenv()
System.getenv(aString)

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