How to get javax.validation errors messages and check this messages on HTTP 400 error test?

I'm using the javax.validation in my objects and in my annotation I have the errors messages like this:

@NotNull(message = "This name can't be a null parameter")
private String name;

When I use the @Valid on my endpoint the java will check my validations and return a error 400 with a list of errors inside. And in my integration test I check the error like this:

@Test
public void saveUser() throws Exception{
    User user = builder.newUser();

    getMockMvc().perform(post(URL_USER)
        .contentType(TestUtil.APPLICATION_JSON_UTF8)
        .content(TestUtil.convertObjectToJsonBytes(user)))
        .andExpect(status().isBadRequest());
        
}

But this test are incomplete, because I dont check the error message.I need a way to check my message "This name can't be a null parameter". I know... This message come in a array of errors in a parameter called defaultMessage I think. Someone can help me?