How to mock void methods with Mockito

Question

How to mock methods with void return type    I implemented an observer pattern but I can t mock it with Mockito because I don t know how   And I tried to find an example on the Internet but didn t succeed   My class looks like this   public class World        List lt Listener gt  listeners       void addListener Listener item            listeners add item              void doAction Action goal Object obj            setState  i received            goal doAction obj           setState  i finished               private string state        setter getter state     public class WorldTest implements Listener         Test public void word      World  w  mock World class       w addListener this                             interface Listener       void doAction        The system is not triggered with mock   I want to show the above-mentioned system state  And make assertions according to them

User · Answer

First of all  you should always import mockito static  this way the code will be much more readable  and intuitive   import static org mockito Mockito     For partial mocking and still keeping original functionality on the rest mockito offers  quot Spy quot   You can use it as follows  private World world   spy new World      To eliminate a method from being executed you could use something like this  doNothing   when someObject  someMethod anyObject      to give some custom behaviour to a method use  quot when quot  with an  quot thenReturn quot   doReturn  quot something quot   when this world  someMethod anyObject      For more examples please find the excellent mockito samples in the doc

User · Answer

In your example you should mock Listener item and use Mockito verify to check interactions with it

User · Answer

The solution of so-called problem is to use a spy Mockito spy      instead of a mock Mockito mock        Spy enables us to partial mocking  Mockito is  good at this matter   Because you have class which is not complete  in this way you mock some required place in this class

User · Answer

Take a look at the Mockito API docs  As the linked document mentions  Point   12  you can use any of the doThrow   doAnswer   doNothing   doReturn   family of methods from Mockito framework to mock void methods  For example  Mockito doThrow new Exception    when instance  methodName     or if you want to combine it with follow-up behavior  Mockito doThrow new Exception    doNothing   when instance  methodName     Presuming that you are looking at mocking the setter setState String s  in the class World below is the code uses doAnswer method to mock the setState  World mockWorld   mock World class    doAnswer new Answer lt Void gt          public Void answer InvocationOnMock invocation          Object   args   invocation getArguments          System out println  quot called with arguments   quot    Arrays toString args          return null           when mockWorld  setState anyString

User · Answer

How to mock void methods with mockito - there are two options    doAnswer - If we want our mocked void method to do something  mock the behavior despite being void   doThrow - Then there is Mockito doThrow   if you want to throw an exception from the mocked void method    Following is an example of how to use it  not an ideal usecase but just wanted to illustrate the basic usage     Test public void testUpdate          doAnswer new Answer lt Void gt                Override         public Void answer InvocationOnMock invocation  throws Throwable               Object   arguments   invocation getArguments                if  arguments    null  amp  amp  arguments length  gt  1  amp  amp  arguments 0     null  amp  amp  arguments 1     null                     Customer customer    Customer  arguments 0                   String email    String  arguments 1                   customer setEmail email                              return null                   when daoMock  updateEmail any Customer class   any String class            calling the method under test     Customer customer   service changeEmail  old test com    new test com           some asserts     assertThat customer  is notNullValue          assertThat customer getEmail    is equalTo  new test com           Test expected   RuntimeException class  public void testUpdate throwsException          doThrow RuntimeException class  when daoMock  updateEmail any Customer class   any String class            calling the method under test     Customer customer   service changeEmail  old test com    new test com           You could find more details on how to mock and test void methods with Mockito in my post How to mock with Mockito  A comprehensive guide with examples

User · Answer

Adding to what  sateesh said  when you just want to mock a void method in order to prevent the test from calling it  you could use a Spy this way  World world   new World    World spy   Mockito spy world   Mockito doNothing   when spy  methodToMock     When you want to run your test  make sure you call the method in test on the spy object and not on the world object  For example  assertEquals 0  spy methodToTestThatShouldReturnZero

User · Answer

Adding another answer to the bunch  no pun intended      You do need to call the doAnswer method if you can t don t want to use spy s  However  you don t necessarily need to roll your own Answer  There are several default implementations  Notably  CallsRealMethods   In practice  it looks something like this   doAnswer new CallsRealMethods    when mock           voidMethod any SomeParamClass class      Or   doAnswer Answers CALLS REAL METHODS get    when mock           voidMethod any SomeParamClass class

User · Answer

I think I ve found a simpler answer to that question  to call the real method for just one method  even if it has a void return  you can do this   Mockito doCallRealMethod   when  lt objectInstance gt    lt method gt      lt objectInstance gt   lt method gt       Or  you could call the real method for all methods of that class  doing this    lt Object gt   lt objectInstance gt    mock  lt Object gt  class  Mockito CALLS REAL METHODS

User · Answer

In Java 8 this can be made a little cleaner  assuming you have a static import for org mockito Mockito doAnswer   doAnswer i - gt         Do stuff with i getArguments   here   return null     when  mock    method   methodArguments      The return null  is important and without it the compile will fail with some fairly obscure errors as it won t be able to find a suitable override for doAnswer   For example an ExecutorService that just immediately executes any Runnable passed to execute   could be implemented using   doAnswer i - gt        Runnable  i getArguments   0   run      return null     when executor  execute any

User · Answer

I think your problems are due to your test structure  I ve found it difficult to mix mocking with the traditional method of implementing interfaces in the test class  as you ve done here    If you implement the listener as a Mock you can then verify the interaction   Listener listener   mock Listener class   w addListener listener   world doAction      verify listener  doAction      This should satisfy you that the  World  is doing the right thing

[java] How to mock void methods with Mockito

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