How to test Spring's declarative caching support on Spring Data repositories?

If you want to test a technical aspect like caching, don't use a database at all. It's important to understand what you'd like to test here. You want to make sure the method invocation is avoided for the invocation with the very same arguments. The repository fronting a database is a completely orthogonal aspect to this topic.

Here's what I'd recommend:

1. Set up an integration test that configures declarative caching (or imports the necessary bit's and pieces from your production configuration.
2. Configure a mock instance of your repository.
3. Write a test case to set up the expected behavior of the mock, invoke the methods and verify the output accordingly.

Sample

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
public class CachingIntegrationTest {

  // Your repository interface
  interface MyRepo extends Repository<Object, Long> {

    @Cacheable("sample")
    Object findByEmail(String email);
  }

  @Configuration
  @EnableCaching
  static class Config {

    // Simulating your caching configuration
    @Bean
    CacheManager cacheManager() {
      return new ConcurrentMapCacheManager("sample");
    }

    // A repository mock instead of the real proxy
    @Bean
    MyRepo myRepo() {
      return Mockito.mock(MyRepo.class);
    }
  }

  @Autowired CacheManager manager;
  @Autowired MyRepo repo;

  @Test
  public void methodInvocationShouldBeCached() {

    Object first = new Object();
    Object second = new Object();

    // Set up the mock to return *different* objects for the first and second call
    Mockito.when(repo.findByEmail(Mockito.any(String.class))).thenReturn(first, second);

    // First invocation returns object returned by the method
    Object result = repo.findByEmail("foo");
    assertThat(result, is(first));

    // Second invocation should return cached value, *not* second (as set up above)
    result = repo.findByEmail("foo");
    assertThat(result, is(first));

    // Verify repository method was invoked once
    Mockito.verify(repo, Mockito.times(1)).findByEmail("foo");
    assertThat(manager.getCache("sample").get("foo"), is(notNullValue()));

    // Third invocation with different key is triggers the second invocation of the repo method
    result = repo.findByEmail("bar");
    assertThat(result, is(second));
  }
}

As you can see, we do a bit of over-testing here:

1. The most relevant check, I think is that the second call returns the first object. That's what the caching is all about. The first two calls with the same key return the same object, whereas the third call with a different key results in the second actual invocation on the repository.
2. We strengthen the test case by checking that the cache actually has a value for the first key. One could even extend that to check for the actual value. On the other hand, I also think it's fine to avoid doing that as you tend to test more of the internals of the mechanism rather than the application level behavior.

Key take-aways

1. You don't need any infrastructure to be in place to test container behavior.
2. Setting a test case up is easy and straight forward.
3. Well-designed components let you write simple test cases and require less integration leg work for testing.

I tried testing the cache behavior in my app using Oliver's example. In my case my cache is set at the service layer and I want to verify that my repo is being called the right number of times. I'm using spock mocks instead of mockito. I spent some time trying to figure out why my tests are failing, until I realized that tests running first are populating the cache and effecting the other tests. After clearing the cache for every test they started behaving as expected.

Here's what I ended up with:

@ContextConfiguration
class FooBarServiceCacheTest extends Specification {

  @TestConfiguration
  @EnableCaching
  static class Config {

    def mockFactory = new DetachedMockFactory()
    def fooBarRepository = mockFactory.Mock(FooBarRepository)

    @Bean
    CacheManager cacheManager() {
      new ConcurrentMapCacheManager(FOOBARS)
    }

    @Bean
    FooBarRepository fooBarRepository() {
      fooBarRepository
    }

    @Bean
    FooBarService getFooBarService() {
      new FooBarService(fooBarRepository)
    }
  }

  @Autowired
  @Subject
  FooBarService fooBarService

  @Autowired
  FooBarRepository fooBarRepository

  @Autowired
  CacheManager cacheManager

  def "setup"(){
    // we want to start each test with an new cache
    cacheManager.getCache(FOOBARS).clear()
  }

  def "should return cached foobars "() {

    given:
    final foobars = [new FooBar(), new FooBar()]

    when:
    fooBarService.getFooBars()
    fooBarService.getFooBars()
    final fooBars = fooBarService.getFooBars()

    then:
    1 * fooBarRepository.findAll() >> foobars
  }

def "should return new foobars after clearing cache"() {

    given:
    final foobars = [new FooBar(), new FooBar()]

    when:
    fooBarService.getFooBars()
    fooBarService.clearCache()
    final fooBars = fooBarService.getFooBars()

    then:
    2 * fooBarRepository.findAll() >> foobars
  }
}