セッションIDによるセッションオブジェクトの取得

Servlet2.1 SessionContext getSession(String id) 。
 ， HttpSessionListener map SessionContext。
MySessionContext.java:
public class MySessionContext {
    private static HashMap mymap = new HashMap();
    public static synchronized void AddSession(HttpSession session) {
        if (session != null) {
            mymap.put(session.getId(), session);
        }
    }
    public static synchronized void DelSession(HttpSession session) {
        if (session != null) {
            mymap.remove(session.getId());
        }
    }
    public static synchronized HttpSession getSession(String session_id) {
        if (session_id == null) 
        return null;
        return (HttpSession) mymap.get(session_id);
    }
}
MySessionListener.java:
public class MySessionListener implements HttpSessionListener {
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
    MySessionContext.AddSession(httpSessionEvent.getSession());
    }
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        HttpSession session = httpSessionEvent.getSession();
        MySessionContext.DelSession(session);
    }
}
web.xml ：
<listener> 
<listener-class>listener.MySessionListener</listener-class> 
</listener>
 sessionId Session ：
String sessionId = request.getParameter("sessionId");
HttpSession session = MySessionContext.getSession(sessionId);