protected void throwImmutableException() throws InvalidSessionException {
        String msg = "This session is immutable and read-only - it cannot be altered.  This is usually because "
                + "the session has been stopped or expired already.";
        throw new InvalidSessionException(msg);
    }
Simply throws an <code>InvalidSessionException</code> indicating that this proxy is immutable.  Used
only in the Session's 'write' methods documented in the top class-level JavaDoc.

@throws InvalidSessionException in all cases - used by the Session 'write' method implementations.
core/src/main/java/org/apache/shiro/session/mgt/ImmutableProxiedSession.java