Mocking a private constructor

Question

The Site class is provided to me by an external team and has a private constructor.

public class Site
{
   int id;
   String brand;

   private Site(int id, String brand)
   {
      this.id = id;
      this.brand = brand;
   }
}

The SiteUtil class (controlled by team) is

public class SiteUtil
{
   public static Site getSite()
   {
     Site site;
     //Logic
     return site; 
   }
 }

The data the getSite() function applies it logic on requires a network call, therefore it needs to be mocked. It doesn't have a setter currently (maybe to maintain consistency with the data source, not so sure)

I mock it as follows

Site mockSite = new Site(1,"Google");
PowerMockito.when(SiteUtil.getSite(1)).thenReturn(mockSite);

The code above of course dosent compile as I use the public constructor. The solution I read was to mock the private constructor of Site object. I'm however at a loss on how to do that (First time writing unit tests!)

Could you elaborate with some code or references I could look at? — Pranav Kapoor, Sep 26 '16 at 11:37
Ouch. This is a terrible API design. Is the "external team" one within your organization or completely separate? — chrylis -cautiouslyoptimistic-, Sep 26 '16 at 11:43
Field f = Site.class.getDeclaredField("site"); give you a Field object representing the 'private site' filed of the Site class. do a f.setAccessible(true) and you then can get or set it with f.get(instanceOfSite) to get the private field site value of instanceOfSite. See java reflection api. — Tokazio, Sep 26 '16 at 11:43
@chrylis, I cant get the other team to change their code. I'm not their only client. — Pranav Kapoor, Sep 26 '16 at 11:46

Nicolas Filotto · Accepted Answer · 2018-03-26T20:41:13.813

Assuming that your code accesses to the value of id and brand only through getters, you could simply mock your class Site then return this mock when you call the static method SiteUtil.getSite() as next:

// Use the launcher of powermock
@RunWith(PowerMockRunner.class)
public class MyTestClass {

    @Test
    // Prepare the class for which we want to mock a static method
    @PrepareForTest(SiteUtil.class)
    public void myTest() throws Exception{
        // Build the mock of Site
        Site mockSite = PowerMockito.mock(Site.class);
        // Define the return values of the getters of our mock
        PowerMockito.when(mockSite.getId()).thenReturn(1);
        PowerMockito.when(mockSite.getBrand()).thenReturn("Google");
        // We use spy as we only want to mock one specific method otherwise
        // to mock all static methods use mockStatic instead
        PowerMockito.spy(SiteUtil.class);
        // Define the return value of our static method
        PowerMockito.when(SiteUtil.getSite()).thenReturn(mockSite);

        ...
    }
}

It seems to be giving illegal access error for me https://stackoverflow.com/questions/66819621/unable-to-mock-certain-final-classes-with-powermockito-java-lang-illegalaccess — Akshay Hazari, Mar 29 '21 at 05:27

score 1 · Answer 2 · answered Sep 26 '16 at 12:42

As an alternate approach, there's a way to compatibly alter their API if you can get management to back you. Instead of hiding the network lookups in the getSite() method, externalize them into a SiteLookupStrategy:

public class SiteUtil {
    private static SiteLookupStrategy strategy = new DefaultSiteLookupStrategy();

    public static Site getSite(int siteNum) {
        return strategy.lookup(siteNum);
    }

    public static void setLookupStrategy(SiteLookupStrategy strategy) {
        SiteUtil.strategy = strategy;
    }
}

This way, for testing, you could inject your own (mocked) strategy, but existing clients of the code would not need to be changed. (This also has the advantage of making the lookup itself easier to test for that group.)

Mocking a private constructor

2 Answers2

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