根据sessionId获取Session对象

Servlet2.1之后不支持SessionContext里面getSession(String id)方法。
但是，我们可以通过HttpSessionListener监听器和全局静态map自己实现一个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添加一个监听器：
<listener> 
<listener-class>listener.MySessionListener</listener-class> 
</listener>
根据sessionId获取Session对象：
String sessionId = request.getParameter("sessionId");
HttpSession session = MySessionContext.getSession(sessionId);