public Session readSession(Serializable sessionId) throws UnknownSessionException {
        Session s = doReadSession(sessionId);
        if (s == null) {
            throw new UnknownSessionException("There is no session with id [" + sessionId + "]");
        }
        return s;
    }
Retrieves the Session object from the underlying EIS identified by <tt>sessionId</tt> by delegating to
the {@link #doReadSession(java.io.Serializable)} method.  If {@code null} is returned from that method, an
{@link UnknownSessionException} will be thrown.

@param sessionId the id of the session to retrieve from the EIS.
@return the session identified by <tt>sessionId</tt> in the EIS.
@throws UnknownSessionException if the id specified does not correspond to any session in the EIS.
core/src/main/java/org/apache/shiro/session/mgt/eis/AbstractSessionDAO.java