StreamResourceWriter error handling via ErrorHandler interface

Question

I have a FileCreator class that implements StreamResourceWriter interface and MainErrorHandler class that implements ErrorHandler. I'm using the MainErrorHandler class as a centralized Exception handler in my project which mostly logs the exception and shows a notification to the user. The problem is that StreamResourceWriter.accept() method runs in a non UI thread and when an Exception is thrown it is directed to the ErrorHandler which then fails to show a notification due to "IllegalStateException: UI instance is not available". Is there a way to show a notification window to the user from MainErrorHandler when FileCreator throws an error in accept() method?

Below FileCreator snippet.

public class FileCreator implements StreamResourceWriter {
    @Override
    public void accept(OutputStream stream, VaadinSession session) throws IOException {
        // Run in a non ui thread.
        // Writes to OutputStream but an Exception might be thrown during this process
    }
}

Below MainErrorHandler snippet.

/**
 * Centralized error handler
 */
public class MainErrorHandler implements ErrorHandler {
    private static final Logger log = LoggerFactory.getLogger(MainErrorHandler.class);
    @Override
    public void error(ErrorEvent event) {
        log.error("Error occurred", event.getThrowable());
        //Cannot show a notification if ErrorEvent came from FileCreator.
        //Will get an IllegalStateException: UI instance is not available.
        Notification.show("Error occurred");
        //Tried UI.getCurrent but it returns null if ErrorEvent came from FileCreator.
        UI.getCurrent();
    }
}

Using Vaadin 13.0.1.

Edit

One way to solve this issue is to pass UI reference to FileCreator directly. Below an example.

public class FileCreator implements StreamResourceWriter {
    private UI ui;
    //Pass UI reference directly
    public FileCreator(UI ui){
       this.ui = ui;                                                        
    }
    @Override
    public void accept(OutputStream stream, VaadinSession session) throws IOException {
       try{
        // Run in a non ui thread.
        // Writes to OutputStream but an Exception might be thrown during this process
       }catch(Exception e){
           //I don't like this since have to catch all exceptions and have to call ErrorHandeler directly with a UI reference. Also what if somewhere in code ErrorHandler is changed and is not of type MainErrorHandler.
           ((MainErrorHandler)VaadinSession.getCurrent().getErrorHandler()).error(e, ui);
       }
    }
}

As I said in comments I really don't like this approach either since I am forced to catch all Exceptions, have to cast ErrorHandler to MainErrorHandler and calling it directly.

froemijojo · Answer 1 · 2019-03-25T15:03:21.623

0

There is a way, but it's not perfect.

You can get all UI instances via VaadinSession.getCurrent().getUIs().

To filter out the inactive/detached UIs you can check if ui.getSession() returns a VaadinSession (so, not null). The JavaDoc of getSession says:

The method will return null if the UI is not currently attached to a VaadinSession.

Then you can invoke the access method on each of the UIs and create and show the notification inside the UI-context.

for(UI ui : VaadinSession.getCurrent().getUIs()) {
    // Filtering out detached/inactive UIs
    if (ui.getSession() != null) {
        ui.access(() -> {
            // create Notification here
        });
    }

I said it's not perfect because you have to keep in mind that the user can have several UIs opened at the same time(e.g. multiple tabs).

edited Mar 25 '19 at 15:03

answered Mar 22 '19 at 13:46

froemijojo

91
1
4

Hi froemijojo, thank you for your answer. I tried the approach you suggested and got it to work somewhat but there are two limitations that prevents me using it. #1 I get multiple notifications on the same tab somewhere around 4-5 notifications even when I have only one tab active and #2 it seems that I have to manually lock the session before executing the code snippet since otherwise I get IllegalArgumentException "VaadinSession not locked". I'll continue to investigate this approach and other approaches. – Julius Koljonen Mar 25 '19 at 09:43
@JuliusKoljonen i edited my answer, filtering out the detached/inactive UIs should help with locking the session explicitly, because the access method of an attached UI should do that already. Did this also help with the multiple Notification problem? – froemijojo Mar 25 '19 at 15:10
Hi @froemijojo, thanks again for your reply and edit! Your code example works now quite well: no multiple notifications in one tab and no locking issues but as you said if user has multiple tabs open the error notification is shown in all of the open tabs/UIs. I wish for a solution where the error notification is shown only in one UI from where the file download was started. Is there a way to get the UI reference from where occurred exception originated from in your solution? – Julius Koljonen Mar 26 '19 at 08:20
Hmmmm just realised that StreamResourceWriter's accept method is called without session lock and I get "IllegalStateException: Cannot access state in VaadinSession or UI without locking the session" when calling VaadinSession.getCurrent().getUIs(). That's why the interface has VaadinSession session as a parameter so that implementor can lock/unlock the session. Your code snippet works when a StreamResource is instantiated with InputStreamFactory instead of StreamResourceWriter. – Julius Koljonen Mar 26 '19 at 08:40
@JuliusKoljonen you could wrap the Exception into a custom Exception Type which also has a Reference to the UI of the filecreator and check inside your MainErrorHandler if it's that ExceptionType. But as far as i can tell you won't get around storing the UI instance at construction time inside the File Creator, otherwise there's no way you could know in which UI it was created. – froemijojo Mar 26 '19 at 11:18

StreamResourceWriter error handling via ErrorHandler interface

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