How do I set environment variables from Java?

Question

How do I set environment variables from Java? I see that I can do this for subprocesses using ProcessBuilder. I have several subprocesses to start, though, so I'd rather modify the current process's environment and let the subprocesses inherit it.

There's a System.getenv(String) for getting a single environment variable. I can also get a Map of the complete set of environment variables with System.getenv(). But calling put() on that Map throws an UnsupportedOperationException -- apparently they mean for the environment to be read only. And there's no System.setenv().

So, is there any way to set environment variables in the currently running process? If so, how? If not, what's the rationale? (Is it because this is Java and therefore I shouldn't be doing evil nonportable obsolete things like touching my environment?) And if not, any good suggestions for managing the environment variable changes that I'm going to need to be feeding to several subprocesses?

Answer 1

You can pass parameters into your initial java process with -D:

java -cp <classpath> -Dkey1=value -Dkey2=value ...

Answer 2

(Is it because this is Java and therefore I shouldn't be doing evil nonportable obsolete things like touching my environment?)

I think you've hit the nail on the head.

A possible way to ease the burden would be to factor out a method

void setUpEnvironment(ProcessBuilder builder) {
    Map<String, String> env = builder.environment();
    // blah blah
}

and pass any ProcessBuilders through it before starting them.

Also, you probably already know this, but you can start more than one process with the same ProcessBuilder. So if your subprocesses are the same, you don't need to do this setup over and over.

Answer 3

Poking around online, it looks like it might be possible to do this with JNI. You'd then have to make a call to putenv() from C, and you'd (presumably) have to do it in a way that worked on both Windows and UNIX.

If all that can be done, it surely wouldn't be too hard for Java itself to support this instead of putting me in a straight jacket.

A Perl-speaking friend elsewhere suggests that this is because environment variables are process global and Java is striving for good isolation for good design.

Answer 4

public static void set(Map<String, String> newenv) throws Exception {
    Class[] classes = Collections.class.getDeclaredClasses();
    Map<String, String> env = System.getenv();
    for(Class cl : classes) {
        if("java.util.Collections$UnmodifiableMap".equals(cl.getName())) {
            Field field = cl.getDeclaredField("m");
            field.setAccessible(true);
            Object obj = field.get(env);
            Map<String, String> map = (Map<String, String>) obj;
            map.clear();
            map.putAll(newenv);
        }
    }
}