code: public static void initGlobal(SecurityController controller)
      {
          if (controller == null) throw new IllegalArgumentException();
          if (global != null) {
              throw new SecurityException("Cannot overwrite already installed global SecurityController");
          }
          global = controller;
      }
docs: Initialize global controller that will be used for all
  security-related operations. The global controller takes precedence
  over already installed {@link Context}-specific controllers and cause
  any subsequent call to
  {@link Context#setSecurityController(SecurityController)}
  to throw an exception.
  <p>
  The method can only be called once.
  
  @see #hasGlobal()