前言
PowerMock是一个单元测试框架，能够模仿静态方法，公有办法和final办法等来简化单元测试的编写。本篇文章将联合简略例子对PowerMock的罕用办法进行阐明。
筹备工作
一. 注解增加与应用场景
在应用PowerMock时须要针对不同场景增加对应注解，次要是@RunWith和@PrepareForTest注解。注解增加和场景对应如下所示。
| 场景 | 注解 |
|---|---|
| 模仿final办法 | @PrepareForTest，@RunWith |
| 模仿静态方法 | @PrepareForTest，@RunWith |
| 模仿公有办法 | @PrepareForTest |
| 应用whenNew | @PrepareForTest，@RunWith |
二. 应用PowerMock须要增加的依赖
须要引入的依赖如下所示。
<dependency> <groupId>org.mockito</groupId> <artifactId>mockito-core</artifactId> <version>2.23.0</version> <scope>test</scope></dependency><dependency> <groupId>org.powermock</groupId> <artifactId>powermock-api-mockito2</artifactId> <version>2.0.2</version> <scope>test</scope></dependency><dependency> <groupId>org.powermock</groupId> <artifactId>powermock-module-junit4</artifactId> <version>2.0.2</version> <scope>test</scope></dependency>引入mockito-core是为了提供Mockito性能，次要应用到org.mockito.ArgumentMatchers参数占位符，局部状况须要应用到org.mockito.BDDMockito。引入powermock-api-mockito2和powermock-module-junit4是为了提供PowerMock性能，其中powermock-module-junit4中还引入了hamcrest-core，次要是应用其提供的org.hamcrest.MatcherAssert.assertThat和org.hamcrest.Matchers.is进行断言判断。
在引入依赖时，须要留神核查Mockito和PowerMock的版本对应关系，否则会报java.lang.ClassNotFoundException: org.mockito.exceptions.Reporter谬误。版本对应关系能够去PowerMock官网进行查问：PowerMock官网，通常状况下，如果引入的mockito-core版本为2.x，则PowerMock的api须要应用powermock-api-mockito2。
注释
一. mock public办法
public class Mock { public boolean isTrue_1() { return true; }}public class PowerMockTest { @Test public void mockPublic() { Mock mock = PowerMockito.mock(Mock.class); PowerMockito.when(mock.isTrue_1()).thenReturn(false); assertThat(mock.isTrue_1(), is(false)); }}mock public办法时须要应用PowerMockito.mock(办法所在类.class)获取mock进去的对象，这里称之为mock实例，mock实例的办法均为假办法，不对mock实例进行任何操作的状况下，调用mock实例的办法会返回（如果有返回值的话）返回值类型的默认值（零值，比方String返回null，Integer返回0）。如果想要调用mock实例的办法时使其执行实在办法，那么打桩时须要应用thenCallRealMethod()，如下所示。
public class Mock { public boolean isTrue_1() { return true; }}public class PowerMockTest { @Test public void mockPublicThenCallRealMethod() { Mock mock = PowerMockito.mock(Mock.class); PowerMockito.when(mock.isTrue_1()).thenCallRealMethod(); assertThat(mock.isTrue_1(), is(true)); }}同时能够应用whenNew()来实现在程序中new一个对象时失去一个mock实例，如下所示。
public class Mock { public boolean isTrue_1() { return true; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Mock.class)public class PowerMockTest { @Test public void mockWhenNew() throws Exception { Mock mock_1 = PowerMockito.mock(Mock.class); PowerMockito.when(mock_1.isTrue_1()).thenReturn(false); PowerMockito.whenNew(Mock.class).withAnyArguments().thenReturn(mock_1); Mock mock_2 = new Mock(); assertThat(mock_2.isTrue_1(), is(false)); }}下面的例子中实现了在调用Mock对象的构造函数时返回预置好的mock实例（下面例子中为mock_1），因而mock_2实例理论指向了mock_1实例，调用mock_2.isTrue_1()办法时会返回false。
二. mock final public办法
和mock public办法操作统一。如下所示。
public class Mock { public final boolean isTure_2() { return true; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Mock.class)public class PowerMockTest { @Test public void mockFinalPublic() { Mock mock = PowerMockito.mock(Mock.class); PowerMockito.when(mock.isTure_2()).thenReturn(false); assertThat(mock.isTure_2(), is(false)); }}三. mock private办法
public class Mock { public boolean isTrue_3() { return returnTrue_1(); } private boolean returnTrue_1() { return true; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Mock.class)public class PowerMockTest { @Test public void mockPrivate() throws Exception { Mock mock = PowerMockito.mock(Mock.class); PowerMockito.when(mock, "returnTrue_1").thenReturn(false); PowerMockito.when(mock.isTrue_3()).thenCallRealMethod(); assertThat(mock.isTrue_3(), is(false)); }}mock private办法打桩时，须要应用PowerMockito.when(mock实例，"公有办法名").thenReturn(冀望返回值)的模式设置mock实例的公有办法的返回值，如果公有办法有参数，还须要在公有办法名前面增加参数占位符，比方PowerMockito.when(mock实例，"公有办法名"，anyInt()).thenReturn(冀望返回值)。下面例子中进行断言时，调用公有办法采取了调用公共办法来间接调用公有办法的模式，单元测试代码对业务代码造成了入侵，因而如果仅仅只是为了验证一个公有办法，能够应用Whitebox来不便的调用公有办法，如下所示。
public class Mock { private boolean returnTrue_1() { return true; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Mock.class)public class PowerMockTest { @Test public void mockPrivate() throws Exception { Mock mock = PowerMockito.mock(Mock.class); PowerMockito.when(mock, "returnTrue_1").thenReturn(false); assertThat(Whitebox.invokeMethod(mock, "returnTrue_1"), is(false)); }}四. mock static public办法
通常是针对工具类进行mock。如下所示。
public class Mock { public static boolean isTrue_4() { return true; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Mock.class)public class PowerMockTest { @Test public void mockStaticPublic() { PowerMockito.mockStatic(Mock.class); PowerMockito.when(Mock.isTrue_4()).thenReturn(false); assertThat(Mock.isTrue_4(), is(false)); }}五. mock static private办法
public class Mock { public static boolean isTrue_5() { return returnTrue_2(); } private static boolean returnTrue_2() { return true; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Mock.class)public class PowerMockTest { @Test public void mockStaticPrivate() throws Exception { PowerMockito.mockStatic(Mock.class); PowerMockito.when(Mock.class, "returnTrue_2").thenReturn(false); PowerMockito.when(Mock.isTrue_5()).thenCallRealMethod(); assertThat(Mock.isTrue_5(), is(false)); }}同样也能够应用Whitebox来不便的调用动态公有办法，如下所示。
public class Mock { private static boolean returnTrue_2() { return true; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Mock.class)public class PowerMockTest { @Test public void mockStaticPrivate() throws Exception { PowerMockito.mockStatic(Mock.class); PowerMockito.when(Mock.class, "returnTrue_2").thenReturn(false); assertThat(Whitebox.invokeMethod(Mock.class, "returnTrue_2"), is(false)); }}六. Whitebox
应用Whitebox能够不便的设置对象（动态）公有属性值，如下所示。
public class Mock { private boolean flag = true; public boolean isTrue_6() { return flag; }}public class PowerMockTest { @Test public void whiteboxPrivateField() { Mock mock = new Mock(); Whitebox.setInternalState(mock, "flag", false); assertThat(mock.isTrue_6(), is(false)); }}仅应用Whitebox时不须要增加@RunWith和@PrepareForTest注解，同时对于下面例子如果flag是动态变量，那么设置动态变量值时须要应用Whitebox.setInternalState(Mock.class, "flag", false)。应用Whitebox也能够不便的调用对象（动态）公有办法，如下所示。
public class Mock { private boolean isTrue_7() { return true; }}public class PowerMockTest { @Test public void whiteboxPrivateMethod() throws Exception { Mock mock = new Mock(); assertThat(Whitebox.invokeMethod(mock, "isTrue_7"), is(true)); }}对于下面例子，如果isTrue_7()是动态公有办法，那么调用动态公有办法时的语句为：assertThat(Whitebox.invokeMethod(Mock.class, "isTrue_7").is(true))。
七. Answer-mock
针对同一办法屡次被调用且不同入参须要mock不同出参的状况，能够应用Answer，如下所示。
public class Mock { public String isTrue_8(int num) { return ""; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Mock.class)public class PowerMockTest { @Test public void answer() { Mock mock = PowerMockito.mock(Mock.class); Answer<String> answer = new Answer<String>() { @Override public String answer(InvocationOnMock invocation) { int num = (Integer) invocation.getArguments()[0]; if (num == 0) { return "zero"; } else if (num == 1) { return "one"; } return null; } }; PowerMockito.when(mock.isTrue_8(anyInt())).thenAnswer(answer); assertThat(mock.isTrue_8(0), is("zero")); assertThat(mock.isTrue_8(1), is("one")); }}其中Answer的泛型类型须要与answer()办法的返回值类型统一，且通过InvocationOnMock的getArguments()能够获取mock实例调用的办法所有入参，callRealMethod()能够调用实在办法，getMethod()能够获取mock实例调用的办法，getMock()能够获取mock实例。同时，还能够应用org.mockito.BDDMockito.given来实现雷同的成果，如下所示。
public class Mock { public String isTrue_8(int num) { return ""; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Mock.class)public class PowerMockTest { @Test public void answer() { Mock mock = PowerMockito.mock(Mock.class); Answer<String> answer = new Answer<String>() { @Override public String answer(InvocationOnMock invocation) { int num = (Integer) invocation.getArguments()[0]; if (num == 0) { return "zero"; } else if (num == 1) { return "one"; } return null; } }; given(mock.isTrue_8(anyInt())).willAnswer(answer); assertThat(mock.isTrue_8(0), is("zero")); assertThat(mock.isTrue_8(1), is("one")); }}八. spy public办法
public class Spy { public boolean isTrue_1() { return true; } public boolean isTrue_2() { return true; }}public class SpyTest() { @Test public void spyPublic() { Spy spy = PowerMockito.spy(new Spy()); PowerMockito.doReturn(false).when(spy).isTrue_1(); assertThat(spy.isTrue_1(), is(false)); assertThat(spy.isTrue_2(), is(true)); }}spy public办法时须要应用PowerMockito.spy(办法所在类的实例)获取spy进去的对象，这里称之为spy实例，不对spy实例进行任何操作的状况下，spy实例与实在实例是齐全一样的。同时因为spy实例与实在实例齐全一样，因而在对spy实例进行打桩时应用doReturn()和thenReturn()是存在差异的：应用doReturn(返回值)时不会执行实在形式，间接返回返回值；应用thenReturn(返回值)时会先执行一遍实在办法，而后返回返回值。通常状况下spy须要配合doReturn()应用，用于克制实在办法的执行，避免执行实在办法时报错。
同时，打桩时应用doReturn()和thenReturn()的语法存在差异，下面例子中打桩时如果应用的语句为PowerMockito.doReturn(false).when(spy.isTrue_1())，会导致编译时失常，运行时报错的景象。下表对打桩时doReturn()和thenReturn()的语法进行了比照。
| 应用场景 | doReturn() | thenReturn() |
|---|---|---|
| 打桩public办法 | PowerMockito.doReturn(false).when(spy).isTrue_1() | PowerMockito.when(spy.isTrue_1()).thenReturn(false) |
| 打桩private办法 | PowerMockito.doReturn(false).when(spy, "returnTrue_1") | PowerMockito.when(spy, "returnTrue_1").thenReturn(false) |
| 打桩static办法 | PowerMockito.doReturn(false).when(Spy.class, "isTrue_5") | PowerMockito.when(Spy.isTrue_5()).thenReturn(false) |
| 打桩static private办法 | PowerMockito.doReturn(false).when(Spy.class, "returnTrue") | PowerMockito.when(Spy.class, "returnTrue").thenReturn(false) |
最初，spy也和mock一样，能够配合whenNew()进行应用。
九. spy private办法
public class Spy { public boolean isTrue_4() { return returnTrue_1(); } private boolean returnTrue_1() { return true; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Spy.class)public class SpyTest() { @Test public void spyPrivate() throws Exception { Spy spy = PowerMockito.spy(new Spy()); PowerMockito.doReturn(false).when(spy, "returnTrue_1"); assertThat(spy.isTrue_4(), is(false)); }}十. spy static public办法
public class Spy { public static boolean isTrue_5() { return true; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Spy.class)public class SpyTest() { @Test public void spyStaticPublic() throws Exception { PowerMockito.spy(Spy.class); PowerMockito.doReturn(false).when(Spy.class, "isTrue_5"); assertThat(Spy.isTrue_5(), is(false)); }}十一. spy static private办法
public class Spy { public static boolean isTrue_6() { return returnTrue(); } private static boolean returnTrue_2() { return true; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Spy.class)public class SpyTest() { @Test public void spyStaticPrivate() throws Exception { PowerMockito.spy(Spy.class); PowerMockito.doReturn(false).when(Spy.class, "returnTrue_2"); assertThat(Spy.isTrue_6(), is(false)); }}十二. Answer-spy
public class Spy { public String isTrue_7(int num) { return ""; }}@RunWith(PowerMockRunner.class)@PrepareForTest(Spy.class)public class SpyTest() { @Test public void answer() { Spy spy = PowerMockito.spy(new Spy()); Answer<String> answer = new Answer<String>() { @Override public String answer(InvocationOnMock invocation) { int num = (Integer) invocation.getArguments()[0]; if (num == 0) { return "zero"; } else if (num == 1) { return "one"; } return ""; } }; PowerMockito.doAnswer(answer).when(spy).isTrue_7(anyInt()); assertThat(spy.isTrue_7(0), is("zero")); assertThat(spy.isTrue_7(1), is("one")); } @Test public void given_when_then_bdd() { Spy spy = PowerMockito.spy(new Spy()); Answer<String> answer = new Answer<String>() { @Override public String answer(InvocationOnMock invocation) { int num = (Integer) invocation.getArguments()[0]; if (num == 0) { return "zero"; } else if (num == 1) { return "one"; } return ""; } }; given(spy.isTrue_7(anyInt())).willAnswer(answer); assertThat(spy.isTrue_7(0), is("zero")); assertThat(spy.isTrue_7(1), is("one")); }}总结
正当应用PowerMock能够解决单元测试编写中的一些常见难题，在解依赖时能够帮忙咱们躲避例如数据库，kafka等与内部存在交互的组件的影响。