--- title: Handling Errors order: 6 layout: page --- [[application.errors]] = Handling Errors [[application.errors.error-indicator]] == Error Indicator and Message All components have a built-in error indicator that is turned on if validating the component fails, and can be set explicitly with [methodname]#setComponentError()#. Usually, the error indicator is placed right of the component caption. The error indicator is part of the component caption, so its placement is usually managed by the layout in which the component is contained, but some components handle it themselves. Hovering the mouse pointer over the field displays the error message. [source, java] ---- textfield.setComponentError(new UserError("Bad value")); button.setComponentError(new UserError("Bad click")); ---- The result is shown in <>. [[figure.application.errors.error-indicator]] .Error Indicator Active image::img/errorindicator-example2.png[scaledwidth=40%] [[application.errors.reconnect]] == Connection Fault If the connection to the server is lost, Vaadin application shows a "lost connection" notification and tries to restore the connection. After several retries, an error message is shown. You can customize the messages, timeouts, and the number of reconnect attempts in the [classname]#ReconnectDialogConfiguration# object, which you can access from your [classname]#UI# with [methodname]#getReconnectDialogConfiguration()#. ifdef::web[] [[application.errors.systemmessages]] == Customizing System Messages System messages are notifications that indicate a major invalid state that usually requires restarting the application. Session timeout is perhaps the most typical such state. System messages are strings managed in the [classname]#SystemMessages# class. sessionExpired:: ((("session", "expiration"))) ((("session", "timeout"))) The Vaadin session expired. A session expires if no server requests are made during the session timeout period. The session timeout can be configured with the [parameter]#session-timeout# parameter in [filename]#web.xml#, as described in <>. communicationError:: An unspecified communication problem between the Vaadin Client-Side Engine and the application server. The server may be unavailable or there is some other problem. authenticationError:: This error occurs if 401 (Unauthorized) response to a request is received from the server. internalError:: A serious internal problem, possibly indicating a bug in Vaadin Client-Side Engine or in some custom client-side code. outOfSync:: The client-side state is invalid with respect to server-side state. cookiesDisabled:: Informs the user that cookies are disabled in the browser and the application does not work without them. Each message has four properties: a short caption, the actual message, a URL to which to redirect after displaying the message, and property indicating whether the notification is enabled. Additional details may be written (in English) to the debug console window described in <>. You can override the default system messages by setting the [interfacename]#SystemMessagesProvider# in the [classname]#VaadinService#. You need to implement the [methodname]#getSystemMessages()# method, which should return a [classname]#SystemMessages# object. The easiest way to customize the messages is to use a [classname]#CustomizedSystemMessages# object. You can set the system message provider in the [methodname]#servletInitialized()# method of a custom servlet class, for example as follows: [source, java] ---- getService().setSystemMessagesProvider( new SystemMessagesProvider() { @Override public SystemMessages getSystemMessages( SystemMessagesInfo systemMessagesInfo) { CustomizedSystemMessages messages = new CustomizedSystemMessages(); messages.setCommunicationErrorCaption("Comm Err"); messages.setCommunicationErrorMessage("This is bad."); messages.setCommunicationErrorNotificationEnabled(true); messages.setCommunicationErrorURL("http://vaadin.com/"); return messages; } }); ---- See <> for information about customizing Vaadin servlets. endif::web[] ifdef::web[] [[application.errors.unchecked-exceptions]] == Handling Uncaught Exceptions Handling events can result in exceptions either in the application logic or in the framework itself, but some of them may not be caught properly by the application. Any such exceptions are eventually caught by the framework. It delegates the exceptions to the [classname]#DefaultErrorHandler#, which displays the error as a component error, that is, with a small red "!" -sign (depending on the theme). If the user hovers the mouse pointer over it, the entire backtrace of the exception is shown in a large tooltip box, as illustrated in <>. [[figure.application.errors.unchecked-exceptions]] .Uncaught Exception in Component Error Indicator image::img/errorindicator-exception.png[] You can customize the default error handling by implementing a custom [interfacename]#ErrorHandler# and enabling it with [methodname]#setErrorHandler()# in any of the components in the component hierarchy, including the [classname]#UI#, or in the [classname]#VaadinSession# object. You can either implement the [interfacename]#ErrorHandler# or extend the [classname]#DefaultErrorHandler#. In the following example, we modify the behavior of the default handler. [source, java] ---- // Here's some code that produces an uncaught exception final VerticalLayout layout = new VerticalLayout(); final Button button = new Button("Click Me!", new Button.ClickListener() { public void buttonClick(ClickEvent event) { ((String)null).length(); // Null-pointer exception } }); layout.addComponent(button); // Configure the error handler for the UI UI.getCurrent().setErrorHandler(new DefaultErrorHandler() { @Override public void error(com.vaadin.server.ErrorEvent event) { // Find the final cause String cause = "The click failed because:
"; for (Throwable t = event.getThrowable(); t != null; t = t.getCause()) if (t.getCause() == null) // We're at final cause cause += t.getClass().getName() + "
"; // Display the error message in a custom fashion layout.addComponent(new Label(cause, ContentMode.HTML)); // Do the default error handling (optional) doDefault(event); } }); ---- The above example also demonstrates how to dig up the final cause from the cause stack. When extending [classname]#DefaultErrorHandler#, you can call [methodname]#doDefault()# as was done above to run the default error handling, such as set the component error for the component where the exception was thrown. See the source code of the implementation for more details. You can call [methodname]#findAbstractComponent(event)# to find the component that caused the error. If the error is not associated with a component, it returns null. endif::web[]