Mockito unit testing RestTemplate

Question

I am using RestTemplate postForEntity method to post body to an endpoint. I need help with writing test case for my code using Mockito. The return type is void but it can be changed to Types or code if needed to test. I have referred many other documentation but they are very general, I tried using them but most did not work for me as the request and return type are different. . Any suggestions are appreciated. Thank you

Here is my Java class

    public void postJson(Set<Type> Types){
        try {
            String oneString = String.join(",", Types);
           Map<String, String> requestBody = new HashMap<>();
            requestBody.put("type", oneString);
            JSONObject jsonObject = new JSONObject(requestBody);
            HttpEntity<String> request = new HttpEntity<String>(jsonObject.toString(), null);
ResponseEntity result = restTemplate.exchange(url, HttpMethod.POST,
                    new HttpEntity<>(request, getHttpHeaders()), String.class);

        } 
    }
} 

Answer 1

You are testing the logic inside MyClass class, so you should not mock it. RestTemplate is a dependency inside MyClass, so this is exactly what you need to mock. In general it should look like this inside your test:

This is just a simple example. A good practice would be to check that the arguments passed to your mock equal to the expected ones. One way would be to replace Mockito.eq() with the real expected data. Another is to verify it separately, like this:

public ResponseEntity<String> postJson(Set<Type> Types){
            try {
                String oneString = String.join(",", Types);
               Map<String, String> requestBody = new HashMap<>();
                requestBody.put("type", oneString);
                JSONObject jsonObject = new JSONObject(requestBody);
                HttpEntity<String> request = new HttpEntity<String>(jsonObject.toString(), null);
    ResponseEntity result = restTemplate.exchange(url, HttpMethod.POST,
                        new HttpEntity<>(request, getHttpHeaders()), String.class);
                } 
        }
        return Types;

You can write test for above method as follows

   @Mock
   RestTemplate restTemplate;

   private Poster poster;
   
   HttpEntity<String> request = new HttpEntity<>(jsonObject.toString(), getHttpHeaders());
   
   ResponseEntity<String> result = restTemplate.exchange(uri, HttpMethod.POST, request, String.class);
   
   Mockito.verify(restTemplate, Mockito.times(1)).exchange(
       Mockito.eq(uri),
       Mockito.eq(HttpMethod.POST),
       Mockito.eq(request),
       Mockito.eq(String.class));
   
   Assert.assertEquals(result, poster.postJson(mockData));
   
   HttpHeaders getHttpHeaders() {
       HttpHeaders headers = new HttpHeaders();
       headers.add(// whatever you need to add);
       return headers;
   }