Not able to catch errors in websocket onMessage

Question

I'm using the library https://github.com/MrStahlfelge/gdx-websockets with LibGDX and Java.

I'm having trouble dealing with errors occurring in the onMessage method.

It seems to silently suppress all errors even if I set setSendGracefully(false).

private WebSocket socket;

socket = WebSockets.newSocket(WebSockets.toSecureWebSocketUrl("myDomain.com", 8080));
socket.setSendGracefully(false);
socket.addListener(new WebSocketListener() {
    @Override
    public boolean onOpen(WebSocket webSocket) {
        Gdx.app.log(TAG, "onOpen"); // line reached
        
        return false;
    }

    @Override
    public boolean onClose(WebSocket webSocket, int closeCode, String reason) {
        Gdx.app.log(TAG, "onClose"); // line reached
        
        return false;
    }

    public boolean onMessage(WebSocket webSocket, String packet) {
        Gdx.app.log(TAG, "onMessage: " + packet); // line reached
        
        Vector2 vtest = null;
        try {
            vtest.x = 1;

        }
        catch (Exception ex) {
             // line not reached
            Gdx.app.log(TAG, "error onMessage: " + ex.getCause().getMessage() + "; " + ex.getMessage());
        } catch (Throwable throwable) { // Catch both exceptions and errors
             // line not reached
            Gdx.app.error(TAG, "error onMessage:", throwable);
        }
    
        return false;
    }
    
    @Override
    public boolean onError(WebSocket webSocket, Throwable error) {
        // it never reaches this method
        Gdx.app.log(TAG, "onError: " + error.getCause().getMessage() + "; " + error.getMessage());
        
        return false;
    }
});
socket.connect();

All others Gdx.app.log lines print out the log.

Do you have any suggestions? Am I doing something wrong/leaving something out?

Thanks

Answer 1

It is because the NullPointerException that is being throw by vtest.x = 1; does not have an underlying cause, so getCause() returns null, causing another NullPointerException to be throw when you try to log.

 // ex.getCause() will return null here, causing the log statement to throw and not log anything
 Gdx.app.log(TAG, "error onMessage: " + ex.getCause().getMessage() + "; " + ex.getMessage());

Consider using

Gdx.app.error(TAG", "some_message", ex);

instead of Gdx.app.log to log Exceptions.