Is it possible to set an environment variable at runtime from Java?

Question

Is it possible to set an environment variable at runtime from a Java application? In Java 1.5 java.lang.System class there is the getenv() method, I would only need a setenv

Answer 1

@Narcoleptic Snowman's answer put me in the right track, however it was mysteriously not working with Oracle JDK 1.8.0_231 (he tested with minor version _144). Even though I was able to update the underlying Map (validated by printing out System.getenv() before and after adding a new property to the Map), the changes were not reflected when the new property was retrieved using System.getenv("property").

After some investigation I found out that's because System.getenv() and System.getenv("property") end up using different static attributes of java.lang.ProcessEnvironment, which are initialized in the class' static block. So it doesn't matter if new properties are added to the Map retrieved using System.getenv(); these properties will not be available in the other Map used by System.getenv("property").

So I changed @Narcoleptic Snowman's answer to deal with this scenario and came to the code below. Please note this will only work if you retrieve properties using System.getenv("property"); if you use System.getenv().get("property") then his answer is what you need. The usage is as follows:

@SuppressWarnings("unchecked")
private static Map getModifiableEnvironment() throws Exception
{
    Class pe = Class.forName("java.lang.ProcessEnvironment");
    Method getenv = pe.getDeclaredMethod("getenv", String.class);
    getenv.setAccessible(true);
    Field props = pe.getDeclaredField("theCaseInsensitiveEnvironment");
    props.setAccessible(true);
    return (Map) props.get(null);
}

This method should be used as follows:

getModifiableEnvironment().put("propName", "propValue");
System.getenv("propName"); // this will return "propValue"