/**

     * Returns the currently accessible {@code Subject} available to the calling code depending on

     * runtime environment.

     * <p/>

     * This method is provided as a way of obtaining a {@code Subject} without having to resort to

     * implementation-specific methods.  It also allows the Shiro team to change the underlying implementation of

     * this method in the future depending on requirements/updates without affecting your code that uses it.

     *

     * @return the currently accessible {@code Subject} accessible to the calling code.

     * @throws IllegalStateException if no {@link Subject Subject} instance or

     *                               {@link SecurityManager SecurityManager} instance is available with which to obtain

     *                               a {@code Subject}, which which is considered an invalid application configuration

     *                               - a Subject should <em>always</em> be available to the caller.

     */

    public static Subject getSubject() {

        Subject subject = ThreadContext.getSubject();

        if (subject == null) {

            subject = (new Subject.Builder()).buildSubject();

            ThreadContext.bind(subject);

        }

        return subject;

    }