鸡尾酒
Mockito是Java的单元测试Mock框架。它的logo是一杯古巴最著名的鸡尾酒Mojito，Ｍojito鸡尾酒，源自古巴的哈瓦那，带有浓厚的加勒比海风情。并不浓烈，但是喝一杯下去，脸上会泛起红晕，象少女的羞涩。味道很清新，有一点青涩、有点甜蜜。
Stub & Mock
Stub和Mock是Test Double类型中的2种。Test Double一共有5种类型，Dummy，Stub，Spy，Mock，Fake。
Test Double是测试复制品，用来统称模拟真实对象的假对象。因使用场景有略微不同，而有这5种类型。
- Dummy，通常只用来填充参数列表。有可能是null对象引用，或Object类实例等。
- Fake，是简化版的实现，比如基于内存实现的数据库，不会真的去做数据库操作，用简单的HashMap来存放数据。
- Stub，Stub用来替代SUT(System Under Test)依赖的组件，但是只模拟一个外部依赖，不做断言。
- Spy，介于Stub和Mock之间。如果真实对象没有被打桩，当调用Spy对象时，真实对象也会被调用。
- Mock，可以理解为Stub+Verification，既模拟外部依赖，也会定义预期结果。
看一个实例，发送邮件，
public interface MailService {public void send(Message msg);
}先写个Stub，
public class MailServiceStub implements MailService {private List<Message> messages = new ArrayList<Message>();public void send(Message msg) {messages.add(msg);}public int numberSent() {return messages.size();}
}实现Stub的状态验证，
class OrderStateTester...
public void testOrderSendsMailIfUnfilled() {Order order = new Order(TALISKER, 51);MailServiceStub mailer = new MailServiceStub();order.setMailer(mailer);order.fill(warehouse);assertEquals(1, mailer.numberSent());
}只做了简单的测试，断言发出了1封邮件。没有测试是否发送给了对的收件人，或者邮件正文是否正确。不过不影响跟Mock比较。
如果用Mock，会怎么写呢？
class OrderInteractionTester...
public void testOrderSendsMailIfUnfilled() {Order order = new Order(TALISKER, 51);Mock warehouse = mock(Warehouse.class);Mock mailer = mock(MailService.class);order.setMailer((MailService) mailer.proxy());mailer.expects(once()).method("send");warehouse.expects(once()).method("hasInventory").withAnyArguments().will(returnValue(false));order.fill((Warehouse) warehouse.proxy());
}结合一张图，就一下全明白了，
怎么喝Mockito？
添加Maven dependency，
<dependency><groupId>org.mockito</groupId><artifactId>mockito-core</artifactId><version>3.3.3</version><scope>test</scope>
</dependency>现在可以开始Mock了，先Mock一个List Interface试试，(示例只是玩语法，实际应使用instance)
//Let's import Mockito statically so that the code looks clearer
import static org.mockito.Mockito.*;// mock creation
List mockedList = mock(List.class);// using mock object
mockedList.add("one");
mockedList.clear();// verification
verify(mockedList).add("one");
verify(mockedList).clear();接着用Mockito来做Stub：
// You can mock concrete classes, not just interfaces
LinkedList mockedList = mock(LinkedList.class);// stubbing
when(mockedList.get(0)).thenReturn("first");
when(mockedList.get(1)).thenThrow(new RuntimeException());// following prints "first"
System.out.println(mockedList.get(0));// following throws runtime exception
System.out.println(mockedList.get(1));// following prints "null" because get(999) was not stubbed
System.out.println(mockedList.get(999));// Although it is possible to verify a stubbed invocation, usually it's just redundant
// If your code cares what get(0) returns, then something else breaks
// (often even before verify() gets executed).
// If your code doesn't care what get(0) returns, then it should not be stubbed.
verify(mockedList).get(0);mock默认会返回null，或原始值，或空集合。如int/Integer返回0，boolean/Boolean返回false。
小结
本文首先介绍了Mockito是Java的单元测试Mock框架，然后比较了单元测试中Stub和Mock的区别，最后演示了Mockito如何使用。
参考资料：https://javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/Mockito.html
https://martinfowler.com/articles/mocksArentStubs.html
http://xunitpatterns.com/Test Double.html