vaadin - 如何集中处理DataProvider方法中抛出的异常

Question

当 DataProvider 获取或计数方法抛出异常时，例如因为用户没有授权，如何集中处理这些异常？我知道有一个HasErrorParameter接口(interface)可以在路由时抛出异常时显示错误 View 。但当 DataProvider 抛出异常时，不会触发这些错误 View 。

示例:

  new AbstractBackEndDataProvider<String, Void>() {
        @Override
        protected Stream<String> fetchFromBackEnd(Query<String, Void> query) {
            ...
        }

        @Override
        protected int sizeInBackEnd(Query<String, Void> query) {
            throw new UnsupportedOperationException("test");
        }
    }

@Route("failed")
public class FailView extends VerticalLayout 
         implements HasErrorParameter<UnsupportedOperationException> {...}

即使我在 DataProvider 方法中执行 try catch，我也不知道如何仅通过使用捕获的异常而不是 View 组件类(此不会触发 setErrorParameter 方法)。

顺便说一句:我错过了 Vaadin Flow 13 文档中的路由器异常处理主题。我想知道他们为什么删除它。

Answer 1

我相信路由时未发生的所有异常都会被传递给发生错误的 VaadinSession 的 ErrorHandler。

设置 ErrorHandler 的最佳方法似乎是重写自定义 SessionInitListener 中的 sessionInit 方法

您可以在自定义 VaadinServlet 的 servletInitialized 方法中添加自定义 SessionInitListener。

class CustomServlet extends VaadinServlet{
    @Override
    protected void servletInitialized() throws ServletException {
        super.servletInitialized();
        getService().addSessionInitListener(new CustomSessionInitListener());
    }
}

并且 SessionInitListener(在此示例中 CustomSessionInitListener)必须设置初始化 session 的 errorHandler。

class CustomSessionInitListener implements SessionInitListener{
    @Override
    public void sessionInit(SessionInitEvent event) throws ServiceException {
        event.getSession().setErrorHandler(new CustomErrorHandler());
    }
}

有关如何创建您自己的 Servlet 的更多信息，请查看 Vaadin's tutorial page (您需要向下滚动到“自定义 Vaadin Servlet”)

编辑: 要显示错误页面，您需要让 Vaadin 重新路由到错误。为了实现这一点，我们可以使用 BeforeEnterEvent，BeforeEnterEvents 有一个 rerouteToError 方法，我们可以使用它让 Vaadin 显示我们的 ErrorView。

但是我们还想传递 Exception 实例，因此我们也必须存储它。我在下面的类(class)中正是这样做的:

@Route("error-view") // Route shown in the user's browser
public class ErrorViewShower extends Div implements BeforeEnterObserver {
    // Class to store the current Exception of each UI in
    private static class UIExceptionContainer extends HashMap<UI, Exception> {

    }

    // Method to call when we want to show an error
    public static void showError(Exception exception) {
        UIExceptionContainer exceptionContainer = VaadinSession.getCurrent().getAttribute(UIExceptionContainer.class);
        // Creating and setting the exceptionContainer in case it hasn't been set yet.
        if (exceptionContainer == null) {
            exceptionContainer = new UIExceptionContainer();
            VaadinSession.getCurrent().setAttribute(UIExceptionContainer.class, exceptionContainer);
        }

        // Storing the exception for the beforeEnter method
        exceptionContainer.put(UI.getCurrent(), exception);

        // Now we navigate to an Instance of this class, to use the BeforeEnterEvent to reroute to the actual error view
        UI.getCurrent().navigate(ErrorViewShower.class);// If this call doesn't work you might want to wrap into UI.access
    }

    @Override
    public void beforeEnter(BeforeEnterEvent event) {
        UIExceptionContainer exceptionContainer = VaadinSession.getCurrent().getAttribute(UIExceptionContainer.class);

        // Retrieving the previously stored exception. You might want to handle if this has been called without setting any Exception.
        Exception exception = exceptionContainer.get(UI.getCurrent());

        //Clearing out the now handled Exception
        exceptionContainer.remove(UI.getCurrent());

        // Using the BeforeEnterEvent to show the error
        event.rerouteToError(exception, "Possible custom message for the ErrorHandler here");
    }

}

它与错误处理程序的结合使用如下所示:

public class CustomErrorHandler implements ErrorHandler {
    @Override
    public void error(ErrorEvent event) {
        // This can easily throw an exception itself, you need to add additional checking before casting.
        // And it's possible that this method is called outside the context of an UI(when a dynamic resource throws an exception for example)
        Exception exception = (Exception) event.getThrowable();
        ErrorViewShower.showError(exception);
    }

}

编辑2: 事实证明，内部方法调用中发生的异常不会由 UI 的 ErrorHandler 或 VaadinSession 的 ErrorHandler 处理，而是由另一个错误处理程序处理，这会导致客户端终止并显示错误通知，

解决方案是捕获 DataProvider 方法内的异常并将其传递给 ErrorViewShower.showError() 并且仍然返回，没有任何异常使堆栈跟踪向上飞行。 (或者不要自己抛出任何异常，而只需将 new 传递给 ErrorViewShower.showError() 方法)。

通过正常返回，Vaadin 甚至不知道出了什么问题。
ErrorViewShower.showError() 调用 ui.navigate，该导航命令似乎在对 DataProvider 的调用后面“排队”，这意味着用户的 View 将在同样的请求。

具有这样实现的数据提供者:

new AbstractBackEndDataProvider<String, Void>() {
    @Override
    protected Stream<String> fetchFromBackEnd(Query<String, Void> query) {
        try{
            //Code that can throw an Exception here
        }catch(Exception e){
            ErrorViewShower.showError(e);
            //We have to make sure that query.getLimit and query.getOffset gets called, otherwise Vaadin throws an Exception with the message "the data provider hasn't ever called getLimit() method on the provided query. It means that the the data provider breaks the contract and the returned stream contains unxpected data."
            query.getLimit();
            query.getOffset();
            return Stream.of(); //Stream of empty Array to return without error
        }
    }

    @Override
    protected int sizeInBackEnd(Query<String, Void> query) {
        //Second way i mentioned, but this will not catch any Exception you didn't create, where as the try...catch has no way to let any Exception reach Vaadin.
        if(badThingsHappened){
            ErrorViewShower.showError(new UnsupportedOperationException("Bad things..."));
            return 0;//Exiting without error
        }
    }
}