There are overloaded exec methods in which you can include an array of environment variables. For example exec(String command, String[] envp).
Here's an example (with proof) of setting an env variable in a child process you exec:
public static void main(String[] args) throws IOException {
String[] command = { "cmd", "/C", "echo FOO: %FOO%" };
String[] envp = { "FOO=false" };
Process p = Runtime.getRuntime().exec(command, envp);
BufferedReader reader = new BufferedReader(new InputStreamReader(p.getInputStream()));
String s = reader.readLine();
System.err.println(s);
}
However, that sets the variable in the env of the created process, not of your current (Java) process.
Similarly, if you are creating a process from Ant (as you mention in comments to aix) using the exec task, then you can pass environment variables to the child process using nested env elements, e.g.
<exec executable="whatever">
<env key="FOO" value="false"/>
</exec>
