TDD（测试驱动开发）中Mockito使用浅谈

Mockito简介

• 什么是mock？
  在软件开发的世界之外, "mock"一词是指模仿或者效仿。 因此可以将“mock”理解为一个替身，替代者. 在软件开发中提及"mock"，通常理解为模拟对象或者Fake。

• 为什么需要Mock?
  Mock是为了解决units之间由于耦合而难于被测试的问题。所以mock object是unit test的一部分。

Mock的好处是什么?

• 提前创建测试，TDD（测试驱动开发）

  这是个最大的好处吧。如果你创建了一个Mock那么你就可以在service接口创建之前写Service Tests了，这样你就能在开发过程中把测试添加到你的自动化测试环境中了。换句话说，模拟使你能够使用测试驱动开发。

• 团队可以并行工作

  这类似于上面的那点；为不存在的代码创建测试。但前面讲的是开发人员编写测试程序，这里说的是测试团队来创建。当还没有任何东西要测的时候测试团队如何来创建测试呢？模拟并针对模拟测试！这意味着当service借口需要测试时，实际上QA团队已经有了一套完整的测试组件；没有出现一个团队等待另一个团队完成的情况。这使得模拟的效益型尤为突出了。

• 你可以创建一个验证或者演示程序。

  由于Mocks非常高效，Mocks可以用来创建一个概念证明，作为一个示意图，或者作为一个你正考虑构建项目的演示程序。这为你决定项目接下来是否要进行提供了有力的基础，但最重要的还是提供了实际的设计决策。

• 为无法访问的资源编写测试

  这个好处不属于实际效益的一种，而是作为一个必要时的“救生圈”。有没有遇到这样的情况？当你想要测试一个service接口，但service需要经过防火墙访问，防火墙不能为你打开或者你需要认证才能访问。遇到这样情况时，你可以在你能访问的地方使用MockService替代，这就是一个“救生圈”功能。

• Mock 可以交给用户

  在有些情况下，某种原因你需要允许一些外部来源访问你的测试系统，像合作伙伴或者客户。这些原因导致别人也可以访问你的敏感信息，而你或许只是想允许访问部分测试环境。在这种情况下，如何向合作伙伴或者客户提供一个测试系统来开发或者做测试呢？最简单的就是提供一个mock，无论是来自于你的网络或者客户的网络。soapUI mock非常容易配置，他可以运行在soapUI或者作为一个war包发布到你的java服务器里面。

• 隔离系统

  有时，你希望在没有系统其他部分的影响下测试系统单独的一部分。由于其他系统部分会给测试数据造成干扰，影响根据数据收集得到的测试结论。使用mock你可以移除掉除了需要测试部分的系统依赖的模拟。当隔离这些mocks后，mocks就变得非常简单可靠，快速可预见。这为你提供了一个移除了随机行为，有重复模式并且可以监控特殊系统的测试环境。

Mockito使用示例

模拟对象

// 模拟LinkedList 的一个对象  
LinkedList mockedList = mock(LinkedList.class);
// 此时调用get方法，会返回null，因为还没有对方法调用的返回值做模拟
System.out.println(mockedList.get(0));

模拟方法调用的返回值

// 模拟获取第一个元素时，返回字符串first。  给特定的方法调用返回固定值在官方说法中称为stub。
when(mockedList.get(0)).thenReturn("first");
// 此时打印输出first
System.out.println(mockedList.get(0));

模拟方法调用抛出异常

// 模拟获取第二个元素时，抛出RuntimeException  
when(mockedList.get(1)).thenThrow(new RuntimeException());
// 此时将会抛出RuntimeException  
System.out.println(mockedList.get(1));

如果一个函数没有返回值类型，那么可以使用此方法模拟异常抛出

doThrow(new RuntimeException("clear exception")).when(mockedList).clear();
mockedList.clear();

模拟调用方法时的参数匹配

// anyInt()匹配任何int参数，这意味着参数为任意值，其返回值均是element  
when(mockedList.get(anyInt())).thenReturn("element");
// 此时打印是element
System.out.println(mockedList.get(999));

模拟方法调用次数

// 调用add一次
mockedList.add("once"); 
// 下面两个写法验证效果一样，均验证add方法是否被调用了一次
verify(mockedList).add("once");
verify(mockedList, times(1)).add("once");

校验行为

 // mock creation
 List mockedList = mock(List.class);
 // using mock object
 mockedList.add("one");
 mockedList.clear();
 //verification
 verify(mockedList).add("one");
 verify(mockedList).clear();

模拟方法调用(Stubbing)

 //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));

 verify(mockedList).get(0);

参数匹配

 //stubbing using built-in anyInt() argument matcher
 when(mockedList.get(anyInt())).thenReturn("element");
 //stubbing using custom matcher (let's say isValid() returns your own matcher implementation):
 when(mockedList.contains(argThat(isValid()))).thenReturn("element");
 //following prints "element"
 System.out.println(mockedList.get(999));
 //you can also verify using an argument matcher
 verify(mockedList).get(anyInt());
 //argument matchers can also be written as Java 8 Lambdas
 verify(mockedList).add(someString -> someString.length() > 5);

校验方法调用次数

 //using mock
 mockedList.add("once");

 mockedList.add("twice");
 mockedList.add("twice");

 mockedList.add("three times");
 mockedList.add("three times");
 mockedList.add("three times");
 //following two verifications work exactly the same - times(1) is used by default
 verify(mockedList).add("once");
 verify(mockedList, times(1)).add("once");
 //exact number of invocations verification
 verify(mockedList, times(2)).add("twice");
 verify(mockedList, times(3)).add("three times");
 //verification using never(). never() is an alias to times(0)
 verify(mockedList, never()).add("never happened");
 //verification using atLeast()/atMost()
 verify(mockedList, atLeastOnce()).add("three times");
 verify(mockedList, atLeast(2)).add("five times");
 verify(mockedList, atMost(5)).add("three times");

模拟无返回方法抛出异常

doThrow(new RuntimeException()).when(mockedList).clear();
//following throws RuntimeException:
mockedList.clear();

校验方法调用顺序

 // A. Single mock whose methods must be invoked in a particular order
 List singleMock = mock(List.class);
 //using a single mock
 singleMock.add("was added first");
 singleMock.add("was added second");
 //create an inOrder verifier for a single mock
 InOrder inOrder = inOrder(singleMock);
 //following will make sure that add is first called with "was added first, then with "was added second"
 inOrder.verify(singleMock).add("was added first");
 inOrder.verify(singleMock).add("was added second");

 // B. Multiple mocks that must be used in a particular order
 List firstMock = mock(List.class);
 List secondMock = mock(List.class);
 //using mocks
 firstMock.add("was called first");
 secondMock.add("was called second");
 //create inOrder object passing any mocks that need to be verified in order
 InOrder inOrder = inOrder(firstMock, secondMock);
 //following will make sure that firstMock was called before secondMock
 inOrder.verify(firstMock).add("was called first");
 inOrder.verify(secondMock).add("was called second");
 // Oh, and A + B can be mixed together at will

校验方法是否从未调用

 //using mocks - only mockOne is interacted
 mockOne.add("one");
 //ordinary verification
 verify(mockOne).add("one");
 //verify that method was never called on a mock
 verify(mockOne, never()).add("two");
 //verify that other mocks were not interacted
 verifyZeroInteractions(mockTwo, mockThree);

快速创建Mock对象

public class ArticleManagerTest {
   @Mock private ArticleCalculator calculator;
      @Mock private ArticleDatabase database;
      @Mock private UserProvider userProvider;
      @Before
      public void before(){
          MockitoAnnotations.initMocks(this);
      }
}

自定义返回不同结果

when(mock.someMethod("some arg"))
   .thenThrow(new RuntimeException())  // 第一次会抛出异常
   .thenReturn("foo"); // 第二次会返回这个结果
//First call: throws runtime exception:
mock.someMethod("some arg"); // 第一次
//Second call: prints "foo"
System.out.println(mock.someMethod("some arg")); // 第二次
//Any consecutive call: prints "foo" as well (last stubbing wins).
System.out.println(mock.someMethod("some arg")); // 第n次(n> 2)，依旧以最后返回最后一个配置

对返回结果进行拦截

when(mock.someMethod(anyString())).thenAnswer(new Answer() {
    Object answer(InvocationOnMock invocation) {
        Object[] args = invocation.getArguments();
        Object mock = invocation.getMock();
        return "called with arguments: " + args;
    }
});
//the following prints "called with arguments: foo"
System.out.println(mock.someMethod("foo"));

Mock函数操作

可以通过doThrow(), doAnswer(), doNothing(), doReturn() and doCallRealMethod() 来自定义函数操作。

暗中调用真实对象

List list = new LinkedList();
List spy = spy(list);
//optionally, you can stub out some methods:
when(spy.size()).thenReturn(100);
//using the spy calls *real* methods
spy.add("one");
spy.add("two");
//prints "one" - the first element of a list
System.out.println(spy.get(0));
//size() method was stubbed - 100 is printed
System.out.println(spy.size());
//optionally, you can verify
verify(spy).add("one");
   verify(spy).add("two");

改变默认返回值

Foo mock = mock(Foo.class, Mockito.RETURNS_SMART_NULLS);
Foo mockTwo = mock(Foo.class, new YourOwnAnswer());

捕获函数的参数值

ArgumentCaptor<Person> argument = ArgumentCaptor.forClass(Person.class);
verify(mock).doSomething(argument.capture());
assertEquals("John", argument.getValue().getName());

部分Mock

//you can create partial mock with spy() method:
List list = spy(new LinkedList());
//you can enable partial mock capabilities selectively on mocks:
Foo mock = mock(Foo.class);
//Be sure the real implementation is 'safe'.
//If real implementation throws exceptions or depends on specific state of the object then you're in trouble.
when(mock.someMethod()).thenCallRealMethod();

重置Mock

List mock = mock(List.class);
when(mock.size()).thenReturn(10);
mock.add(1);
reset(mock);
//at this point the mock forgot any interactions & stubbing

序列化

List<Object> list = new ArrayList<Object>();
List<Object> spy = mock(ArrayList.class, withSettings()
                 .spiedInstance(list)
                 .defaultAnswer(CALLS_REAL_METHODS)
                 .serializable());

检查超时

//passes when someMethod() is called within given time span
verify(mock, timeout(100)).someMethod();
//above is an alias to:
verify(mock, timeout(100).times(1)).someMethod();
//passes when som`eMethod() is called *exactly* 2 times within given time span
verify(mock, timeout(100).times(2)).someMethod();
//passes when someMethod() is called *at least* 2 times within given time span
verify(mock, timeout(100).atLeast(2)).someMethod();
//verifies someMethod() within given time span using given verification mode
//useful only if you have your own custom verification modes.
verify(mock, new Timeout(100, yourOwnVerificationMode)).someMethod();

Mock详情

Mockito.mockingDetails(someObject).isMock();
Mockito.mockingDetails(someObject).isSpy();

原文转自：http://www.jianshu.com/p/77db26b4fb54