tomcat共享多个web应用会话的实现方法

 public HttpSession getSession(boolean create) {

    if (crossContext) {

      // There cannot be a session if no context has been assigned yet
      if (context == null)
        return (null);

      // Return the current session if it exists and is valid
      if (session != null && session.isValid()) {
        return (session.getSession());
      }

      HttpSession other = super.getSession(false);
      if (create && (other == null)) {
        // First create a session in the first context: the problem is
        // that the top level request is the only one which can 
        // create the cookie safely
        other = super.getSession(true);
      }
      if (other != null) {
        Session localSession = null;
        try {
          localSession =
            context.getManager().findSession(other.getId());
          if (localSession != null && !localSession.isValid()) {
            localSession = null;
          }
        } catch (IOException e) {
          // Ignore
        }
        if (localSession == null && create) {
          localSession = 
            context.getManager().createSession(other.getId());
        }
        if (localSession != null) {
          localSession.access();
          session = localSession;
          return session.getSession();
        }
      }
      return null;

    } else {
      return super.getSession(create);
    }

  }

request.getSession().getServletContext().getContext("/app2").getAttribute("att2"); 

public ServletContext getContext(String uri) {

    // Validate the format of the specified argument
    if (uri == null || !uri.startsWith("/")) {
      return null;
    }

    Context child = null;
    try {
      // Look for an exact match
      Container host = context.getParent();
      child = (Context) host.findChild(uri);

      // Non-running contexts should be ignored.
      if (child != null && !child.getState().isAvailable()) {
        child = null;
      }

      // Remove any version information and use the mapper
      if (child == null) {
        int i = uri.indexOf("##");
        if (i > -1) {
          uri = uri.substring(0, i);
        }
        // Note: This could be more efficient with a dedicated Mapper
        //    method but such an implementation would require some
        //    refactoring of the Mapper to avoid copy/paste of
        //    existing code.
        MessageBytes hostMB = MessageBytes.newInstance();
        hostMB.setString(host.getName());

        MessageBytes pathMB = MessageBytes.newInstance();
        pathMB.setString(uri);

        MappingData mappingData = new MappingData();
        ((Engine) host.getParent()).getService().findConnectors()[0].getMapper().map(
            hostMB, pathMB, null, mappingData);
        child = (Context) mappingData.context;
      }
    } catch (Throwable t) {
      ExceptionUtils.handleThrowable(t);
      return null;
    }

    if (child == null) {
      return null;
    }

    if (context.getCrossContext()) {
      // If crossContext is enabled, can always return the context
      return child.getServletContext();
    } else if (child == context) {
      // Can still return the current context
      return context.getServletContext();
    } else {
      // Nothing to return
      return null;
    }
  }