static S _javaCommand() {
  try answer trim(loadTextFile(javaxDataDir("java-command.txt")));
  S javaHome = System.getProperty("java.home");
  if (nempty(javaHome)) {
    File f = new File(javaHome);
    File f = oneOfTheFiles(
      newFile(f, "bin", "java"),
      newFile(f, "bin", java.exe"));
    if (f != null) ret f2s(f);
  }
  if (isOnPATH("java")) ret "java";
  fail("No java command found");
}