如何在 Java 中获取系统变量值？

Question

How can I get the System Variable value which is present in

MyComputer -> Properties -> Advanced -> Environment Variables -> System Variables

in Java?

Edit

I have used System.getenv() method.

It is printing value if I give

System.out.println(System.getenv("JAVA_HOME"));

and it is showing null value if I try the same for system variable created by me

System.out.println(System.getenv("DBE"));

Answer 1

使用System.getenv(String)方法，传递要读取的变量名称。

Answer 2

To clarify, system variables are the same as environment variables. User environment variables are set per user and are different whenever a different user logs in. System wide environment variables are the same no matter what user logs on.

To access either the current value of a system wide variable or a user variable in Java, see below:

String javaHome = System.getenv("JAVA_HOME");

For more information on environment variables see this wikipedia page .

Also make sure the environment variable you are trying to read is properly set before invoking Java by doing a:

echo %MYENVVAR%

You should see the value of the environment variable. If not, you may need to reopen the shell (DOS) or log off and log back on.

Answer 3

There are a few details of interest when getting system/environment properties.

First, System.getenv(String) was introduced way-back-when, then deprecated. The deprecation (foolishly, IHMO) continued all the way into JSE 1.4 .

It got re-introduced in JSE 5 .

Those are set using the Environment Variables panel in Windows. Changes to the variables may not get picked up until your current VM is shutdown, and the CMD.exe instance is exited.

In contrast to the environment properties, Java also has Java system properties, accessible through System.getProperties() . These variables can be initialized when the VM is started using a series -D name = value command line arguments. For example, the values for the properties maxInMemory and pagingDirectory are set in the command below:

C:\> java.exe -DmaxInMemory=100M -DpagingDirectory=c:\temp -jar myApp.jar

These properties can be modified at runtime, barring security policy restrictions.

Answer 4

Actually the variable can be set or not, so, In Java 8 or superior its nullable value should be wrapped into an Optional object, which allows really good features. In the following example we gonna try to obtain the variable ENV_VAR1 , if it doesnt exist we may throw some custom Exception to alert about it:

String ENV_VAR1 = Optional.ofNullable(System.getenv("ENV_VAR1")).orElseThrow(
  () -> new CustomException("ENV_VAR1 is not set in the environment"));

Answer 5

Google says to check out getenv() :

Returns an unmodifiable string map view of the current system environment.

I'm not sure how system variables differ from environment variables, however, so if you could clarify I could help out more.

Answer 6

As mentioned by sombody above, restarting eclipse worked for me for the user defined environment variable. After I restart eclipse IDE, System.getenv() is picking up my environment variable.

Answer 7

Have you tried rebooting since you set the environment variable?

It appears that Windows keeps it's environment variable in some sort of cache, and rebooting is one method to refresh it. I'm not sure but there may be a different method, but if you are not going to be changing your variable value too often this may be good enough.

Answer 8

Are you on a linux system? If so be sure you are exporting your variable.

myVar=testvalue; export myVar

I get null unless I use export to define the value globally.

Answer 9

This code gives you a map of Environment variables' names and the corresponding values.

public static void main(String[] args) {
            Map<String, String> map = System.getenv();
            System.out.println(map);
        }