Mockito - mocking legacy class constructor

Question

I am writing JUnit for a class that references a legacy class via constructor. The legacy class is in a third party jar, so I can't refactor it to make life easier....

This is the class being tested...

public MyClass {

    public String methodToTest(String param) {
        LegacyClass legacy = new LegacyClass(param);
        *..... etc ........*
    }
}

This is what I am trying to do in the mockito JUnit.

public MyClassTest {

    @Test
    public void testMethodToTest() throws Exception {
        LegacyClass legacyMock = mock(LegacyClass.class);
        when(*the LegacyClass constructor with param is called*).thenReturn(legacyMock);
        *.... etc.....*
    }
}

Any ideas on how I can do this ?????

A constructor returning something? Are you sure? For normal functions the stubbing you suggested should work. — Dirk, Jun 09 '11 at 07:04
When the "new LegacyClass(param)" code is executed, I want it to return a mock. That way I can test "methodToTest" without actually needing to construct a live LegacyClass. The "when" method may be the wron way to do it. Thats my question, how do I do it ??? — Dave, Jun 09 '11 at 22:52
Hm, this is rather difficult in this case. If you don't have any chance to pass in the legacy class (i.e. `public String methodToTest(String param, LegacyClass lc)`) or create it by using a factory which you can exchange (inside the methodToTest: `lcFactory.create(param)` which then creates an instance, I don't see a way of testing the method except by its outside behaviour. Both those alternatives can also take a mock created by mockito. — Dirk, Jun 10 '11 at 08:15
Does this answer your question? [Mockito junit 5 mock constructor](https://stackoverflow.com/questions/64905956/mockito-junit-5-mock-constructor) — tkruse, Nov 07 '22 at 17:29

score 9 · Answer 1 · answered Jun 13 '11 at 09:17

Make a builder for the LegacyClass:

public class LegacyClassBuilder {

    public LegacyClass build(String param) {
        return new LegacyClass(param);
    }

}

That way your class can be tested so it creates the LegacyClass with correct parameter.

public MyClass {

    private LegacyClassBuilder builder;

    public setBuilder(LegacyClassBuilder builder) {
        this.builder = builder;
    }

    public String methodToTest(String param) {
        LegacyClass legacy = this.builder.build(param);
        ... etc
    }
}

The test will look something like this:

// ARRANGE
LegacyClassBuilder mockBuilder = mock(LegacyClassBuilder.class);
LegacyClass mockLegacy = mock(LegacyClass.class); 
when(mockBuilder.build(anyString()).thenReturn(mockLegacy);

MyClass sut = new MyClass();
sut.setBuilder(mockBuilder);
String expectedParam = "LOLCAT";


// ACT
sut.methodToTest(expectedParam);

// ASSERT
verify(mockBuilder).build(expectedParam);

If LegacyClass happens to be final then you need to create non-final wrapper for LegacyClass that MyClass will use.

score 4 · Answer 2 · edited Sep 25 '12 at 10:44

4

You can use PowerMockito framework:

import static org.powermock.api.mockito.PowerMockito.*;

whenNew(LegacyClass.class)
  .withParameterTypes(String.class)
  .withArguments(Matchers.any(String.class))
  .thenReturn(new MockedLegacyClass(param));

Then write your MockedLegacyClass implementation according to your test needs.

edited Sep 25 '12 at 10:44

oers

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answered Sep 04 '12 at 12:38

user1645629

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See https://stackoverflow.com/questions/64905956 for solution with Mockito but without PowerMockito – tkruse Nov 07 '22 at 17:30

score 0 · Answer 3 · answered Mar 06 '18 at 16:11

I believe it is not possible to mock constructors using Mockito. Instead, I will suggest the following approach:

  public MyClass {

    public String methodToTest(String param) {
     if(legacy== null){
      //when junit runs, you will get mocked object (not null), hence don't initialize 
        LegacyClass legacy = new LegacyClass(param);
      }
        *..... etc ........*
    }
}

public MyClassTest {

@InjectMock
Myclass myclass; // inject mock the real testable class
@Mock
LegacyClass legacy

@Test
public void testMethodToTest() throws Exception {    
    // when(legacy-constructor).thenReturn() ---not Required
    // instead directly mock the required methods using mocked legacy object
    when(legacy.getSomething(Mockito.any(String.class))).thenReturn(null);
        *.... etc.....*
    }
}

Mockito - mocking legacy class constructor

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