Why I received an Error 403 with MockMvc and JUnit?

I know this question is quite old, but this is one of the first results on Google for some queries and I believe this approach is much better and it is described on spring.io blog

1) You can create your mockMvc with Spring Security support easier, so your setUp() gets much shorter:

@Before
public void setUp() throws Exception {
    mockMvc = MockMvcBuilders
            .webAppContextSetup(webApplicationContext)
            .apply(springSecurity())
            .build();
}

2) You can use org.springframework.security.test.web.servlet.request.SecurityMockMvcRequestPostProcessors.csrf() to populate your test request with correct CSRF token like this:

mockMvc.perform(post("/register")
              .with(csrf())
              .param("action", "signup"))
     .andExpect(status().isOk());