Help troubleshooting Spring MVC unit test failing with a HTTP 400 error code

Note: I’m working in Eclipse with Spring 3 and using jUnit for my tests.

Background

I was writing a unit test for a file upload method on a controller that looks like:

mockMvc.perform(fileUpload("/fileUpload")
    .param("description", "blah")
  )
  .andExpect(status().isOk());

The test was failing with the message:

java.lang.AssertionError: Status expected:<200> but was:<400>

This isn’t super helpful because I wasn’t sure why I was getting the 400. I tried changing the URL that I was testing and the error code changed to a 404 so I knew the request was trying to get to the right controller method. I also put a breakpoint in my controller method but it was never hit so I knew it was the configuration of the test and not the code in the controller method that was causing the issue.

How to get more info

To find out what the problem is, we need to use the debugger to get to a useful message. In my example above, we need to set a breakpoint inside the .andExpect() method call. To do this, and you’ll need a newish version of Eclipse to support this feature, hold control (or command in OSX) and mouse over the .andExpect() method call, then click Open Implementation and it should open something like the following code in your editor:

public ResultActions andExpect(ResultMatcher matcher) throws Exception {
  matcher.match(mvcResult);
  return this;
}

We need to set our breakpoint on the first line; the matcher.match() call. Now run your unit test in debug mode and wait for the breakpoint to trigger. When it triggers, we’ll need the Variables view in Eclipse to inspect stuff. If it’s not in your perspective, add it using Window -> Show View. In the Variables view, you’ll need to navigate to the following path:

this -> val$mvcResult -> mockResponse -> errorMessage

The errorMessage variable is a String and for me, the message was:

Required MultipartFile parameter 'file' is not present

Ta-da! Now we have some useful information. For my example, the way to fix the message is to call the .file() method on the result from the fileUpload() method, so:

MockMultipartFile file = new MockMultipartFile("file", "orig", null, "bar".getBytes());
mockMvc.perform(fileUpload("/questionFileUpload")
  .file(file)
  .param("description", "blah")
  )
  .andExpect(status().isOk());

Unfortunately I can’t tell you how to fix your problem if it’s not what I’ve experienced but hopefully I’ve given you some help in fixing it yourself.