Session management in gwt

You could have the server throw an AuthenticationException to the client in case the user has been logged out.
This will be catched in the callbacks onFailure method, which then can redirect the user to the login-page.

Edit:
AuthenticationException is not a standard exception of course, i was just making an example. It might be best to stick with the standard exceptions.

To try if you caught an specific exception you could use the instanceof operator

    public void onFailure(Throwable e) {                  if(e instanceof AuthenticationException) {                        redirecttoLogin();                  }                  else {                    showError(),               }            }

This does not directly apply to those using RPC, but for those of you who are not using RPC, you should send a HTTP 401 from the server. Then you can check that status code in your RequestBuilder callback.

Client: All Callbacks extend a Abstract Callback where you implement the onFailur()

public abstract class AbstrCallback<T> implements AsyncCallback<T> {  @Override  public void onFailure(Throwable caught) {    //SessionData Expired Redirect    if (caught.getMessage().equals("500 " + YourConfig.ERROR_MESSAGE_NOT_LOGGED_IN)) {      Window.Location.assign(ConfigStatic.LOGIN_PAGE);    }    // else{}: Other Error, if you want you could log it on the client  }}

Server: All your ServiceImplementations extend AbstractServicesImpl where you have access to your SessionData. Override onBeforeRequestDeserialized(String serializedRequest) and check the SessionData there. If the SessionData has expire then write a spacific error message to the client. This error message is getting checkt in your AbstrCallback and redirect to the Login Page.

public abstract class AbstractServicesImpl extends RemoteServiceServlet {  protected ServerSessionData sessionData;  @Override  protected void onBeforeRequestDeserialized(String serializedRequest) {    sessionData = getYourSessionDataHere()    if (this.sessionData == null){       // Write error to the client, just copy paste      this.getThreadLocalResponse().reset();      ServletContext servletContext = this.getServletContext();      HttpServletResponse response = this.getThreadLocalResponse();      try {        response.setContentType("text/plain");        response.setStatus(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);        try {          response.getOutputStream().write(            ConfigStatic.ERROR_MESSAGE_NOT_LOGGED_IN.getBytes("UTF-8"));          response.flushBuffer();        } catch (IllegalStateException e) {          // Handle the (unexpected) case where getWriter() was previously used          response.getWriter().write(YourConfig.ERROR_MESSAGE_NOT_LOGGED_IN);          response.flushBuffer();        }      } catch (IOException ex) {        servletContext.log(          "respondWithUnexpectedFailure failed while sending the previous failure to the client",          ex);      }      //Throw Exception to stop the execution of the Servlet      throw new NullPointerException();    }  }}

In Addition you can also Override doUnexpectedFailure(Throwable t) to avoid logging the thrown NullPointerException.

@Overrideprotected void doUnexpectedFailure(Throwable t) {  if (this.sessionData != null) {    super.doUnexpectedFailure(t);  }}