MockMvc vs End-to-End Tests

MockMvc is built on Servlet API mock implementations from the spring-test module and does not rely on a running container. Therefore, there are some differences when compared to full end-to-end integration tests with an actual client and a live server running.

The easiest way to think about this is by starting with a blank MockHttpServletRequest. Whatever you add to it is what the request becomes. Things that may catch you by surprise are that there is no context path by default; no jsessionid cookie; no forwarding, error, or async dispatches; and, therefore, no actual JSP rendering. Instead, "forwarded" and "redirected" URLs are saved in the MockHttpServletResponse and can be asserted with expectations.

This means that, if you use JSPs, you can verify the JSP page to which the request was forwarded, but no HTML is rendered. In other words, the JSP is not invoked. Note, however, that all other rendering technologies which do not rely on forwarding, such as Thymeleaf and Freemarker, render HTML to the response body as expected. The same is true for rendering JSON, XML, and other formats through @ResponseBody methods.

Alternatively, you may consider the full end-to-end integration testing support from Spring Boot with @SpringBootTest. See the Spring Boot Reference Guide.

There are pros and cons for each approach. The options provided in Spring MVC Test are different stops on the scale from classic unit testing to full integration testing. To be certain, none of the options in Spring MVC Test fall under the category of classic unit testing, but they are a little closer to it. For example, you can isolate the web layer by injecting mocked services into controllers, in which case you are testing the web layer only through the DispatcherServlet but with actual Spring configuration, as you might test the data access layer in isolation from the layers above it. Also, you can use the standalone setup, focusing on one controller at a time and manually providing the configuration required to make it work.

Another important distinction when using Spring MVC Test is that, conceptually, such tests are server-side tests, so you can check what handler was used, if an exception was handled with a HandlerExceptionResolver, what the content of the model is, what binding errors there were, and other details. That means that it is easier to write expectations, since the server is not an opaque box, as it is when testing it through an actual HTTP client. This is generally an advantage of classic unit testing: it is easier to write, reason about, and debug but does not replace the need for full integration tests. At the same time, it is important not to lose sight of the fact that the response is the most important thing to check. In short, there is room for multiple styles and strategies of testing even within the same project.