gistfile1.java

public class SomeService {
  //これをいれかえたい。
  @Autowired
  SomeRepository someRepository;
  //
  //
  //
}

@Repository
public class SomeRepositoryImpl implements SomeRepository {
  //いろいろな本番処理
}

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
public class SomeServiceTest {
    @Autowired
    SomeService someService;

    @Autowired
    SomeRepository someRepository;

    @Test
    public void testHoge() throws Exception {
        System.out.println(someRepository); // こいつはMock
        System.out.println(someService);
    }
}

SomeServiceTest-context.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean class="org.mockito.Mockito" factory-method="mock">
        <constructor-arg value="com.example.repository.SomeRepository" />
    </bean>

    <bean class="com.example.service.SomeService"/>
</beans>