使用 mockito 模拟使用通配符返回泛型的方法

原创:嗯，这更多是泛型的问题，而不是 Mockito 的问题.对于泛型，您应该阅读 Angelika Langer 在它们上写的内容.对于当前主题，即通配符，请阅读此部分.p>

但简而言之，您可以使用 Mockito 的其他语法来帮助您解决当前的情况:

doReturn(interfaces).when(classAMock).getMyInterfaces();

或者使用 BDD 别名:

willReturn(interfaces).given(classAMock).getMyInterfaces();

尽管如此，您可以编写更通用友好的包装器.这将有助于未来的开发人员使用相同的第 3 方 API.

附带说明:您不应该模拟您不拥有的类型，这可能会导致许多错误和问题.相反，你应该有一些包装.例如 DAO 和存储库就代表了这样的想法，人们将模拟 DAO 或存储库接口，而不是 JDBC/JPA/hibernate 的东西.有很多关于此的博客文章:

• http://davesquared.net/2011/04/dont-mock-types-you-dont-own.html
• http://blog.8thlight.com/eric-smith/2011/10/27/thats-not-yours.html
• https://web.archive.org/web/20140923101818/http://freshbrewedcode.com/derekgreer/2012/04/01/tdd-best-practices-dont-mock-others/
• ...

I'm using mockito 1.9.5. I have the following code:

public class ClassA  {

public List<? extends MyInterface> getMyInterfaces() {
    return null;
}

public static void testMock() {
    List<MyInterface> interfaces = new ArrayList<>();
    ClassA classAMock = mock(ClassA.class);
    when(classAMock.getMyInterfaces()).thenReturn(interfaces);      
}

I get a compilation error for the thenReturn(interfaces) saying:

"The method thenReturn(List<capture#1-of ? extends MyInterface>) in the type 
 OngoingStubbing<List<capture#1-of ? extends MyInterface>> is not applicable for the arguments 
 (List<MyInterface>)"

However, when I use the thenAnswer method of mockito, I don't get the error. Can anyone tell me what's going on? Why do I get the error when I use the thenReturn method? Is there any other way to solve this problem when ClassA is provided by a 3rd party and cannot be modified?

EDIT : Starting from Mockito 1.10.x, generics types that are embedded in the class are now used by Mockito for deep stubs. ie.

public interface A<T extends Observer & Comparable<? super T>>  {
  List<? extends B> bList();
  T observer();
}

B b = deep_stubbed.bList().iterator().next(); // returns a mock of B ; mockito remebers that A returns a List of B
Observer o = deep_stubbed.observer(); // mockito can find that T super type is Observer
Comparable<? super T> c = deep_stubbed.observer(); // or that T implements Comparable

Mockito tries its best to get type information that the compiler embeds, but when erasure applies, mockito cannot do anything but return a mock of Object.

Original : Well that's more of an issue with generics than with Mockito. For generics, you should read what Angelika Langer wrote on them. And for the current topic, i.e. wildcards, read this section.

But for short, what you could use is the other syntax of Mockito to help with your current situation :

doReturn(interfaces).when(classAMock).getMyInterfaces();

Or with the BDD aliases :

willReturn(interfaces).given(classAMock).getMyInterfaces();

Nevertheless, you could write wrappers that are more generic friendly. That will help future developers working with same 3rd party API.

As a side note: you shouldn't mocks type you don't own, it can lead to many errors and issues. Instead you should have some wrapper. DAO and repositories for example represent such idea, one will mock the DAO or repository interface, but not the JDBC / JPA / hibernate stuff. There are many blog posts about that:

• http://davesquared.net/2011/04/dont-mock-types-you-dont-own.html
• http://blog.8thlight.com/eric-smith/2011/10/27/thats-not-yours.html
• https://web.archive.org/web/20140923101818/http://freshbrewedcode.com/derekgreer/2012/04/01/tdd-best-practices-dont-mock-others/
• ...

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