Spring – zbudowanie strony do obslugi bledow

Customizing application error pages

I don’t know if you’ve encountered any errors while trying out the reading-list application, but if so you’ve probably seen an error page much like the one in figure 3.1.

Figure 3.1. Spring Boot’s default whitelabel error page.

Spring Boot offers this “whitelabel” error page by default as part of auto-configuration. Even though it’s slightly more attractive than a stack trace, it doesn’t compare with some of the great works of error art available on the internet. In the interest of presenting your application failures as masterpieces, you’ll probably want to create a custom error page for your applications.

The default error handler that’s auto-configured by Spring Boot looks for a view whose name is “error”. If it can’t find one, it uses its default whitelabel error view shown in figure 3.1. Therefore, the easiest way to customize the error page is to create a custom view that will resolve for a view named “error”.

Ultimately this depends on the view resolvers in place when the error view is being resolved. This includes

• Any bean that implements Spring’s View interface and has a bean ID of “error” (resolved by Spring’s BeanNameViewResolver)
• A Thymeleaf template named “error.html” if Thymeleaf is configured
• A FreeMarker template named “error.ftl” if FreeMarker is configured
• A Velocity template named “error.vm” if Velocity is configured
• A JSP template named “error.jsp” if using JSP views

Because we’re using Thymeleaf for the reading-list application, all we must do to customize the error page is create a file named “error.html” and place it in the templates folder along with our other application templates. Listing 3.7 shows a simple, yet effective replacement for the default whitelabel error page.

Listing 3.7. Custom error page for the reading-list application

This custom error template should be named “error.html” and placed in the templates directory for the Thymeleaf template resolver to find. For a typical Maven or Gradle build, that means putting it in src/main/resources/templates so that it’s at the root of the classpath during runtime.

For the most part, this is a simple Thymeleaf template that displays an image and some error text. There are two specific pieces of information that it also renders: the request path of the error and the exception message. These aren’t the only details available to an error page, however. By default, Spring Boot makes the following error attributes available to the error view:

• timestamp—The time that the error occurred
• status—The HTTP status code
• error—The error reason
• exception—The class name of the exception
• message—The exception message (if the error was caused by an exception)
• errors—Any errors from a BindingResult exception (if the error was caused by an exception)
• trace—The exception stack trace (if the error was caused by an exception)
• path—The URL path requested when the error occurred

Some of these attributes, such as path, are useful when communicating the problem to the user. Others, such as trace, should be used sparingly, be hidden, or be used cleverly on the error page to keep the error page as user-friendly as possible.

You’ll also notice that the template references an image named MissingPage.png. The actual content of the image is unimportant, so feel free to flex your graphic design muscles and come up with an image that suits you. But be sure to put it in src/main/resources/static or src/main/resources/public so that it can be served when the application is running.