无法使用选项CALLS_REAL_METHODS模拟ArrayList

Question

根据官方文档：http://site.mockito.org/mockito/docs/current/org/mockito/Mockito.html#CALLS_REAL_METHODS

我写了以下测试：

package tk.puncha.study;
import org.junit.Test;
import java.util.ArrayList;
import static org.mockito.Mockito.*;
public class MockitoTest {
  @Test
  public void test() {
    ArrayList arrayList = mock(ArrayList.class, withSettings().defaultAnswer((CALLS_REAL_METHODS)));
    arrayList.add(new Object());
  }
}

但它因以下异常而崩溃：

java.lang.NullPointerException
at java.util.ArrayList.ensureExplicitCapacity(ArrayList.java:234)
at java.util.ArrayList.ensureCapacityInternal(ArrayList.java:227)
at java.util.ArrayList.add(ArrayList.java:458)
at tk.puncha.study.MockitoTest.test(MockitoTest.java:16)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:119)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:42)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:234)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:74)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

是JUnit错误还是我错了？

Answer 1

试试这个：ArrayList ArrayList=mock（ArrayList.class，withSettings（）.useConstructor（）.defaultAnswer（CALLS_REAL_METHODS））；

Answer 2

使用带有CALLS_REAL_METHODS默认答案的mock是危险的。Mockito不会将模拟对象的字段初始化为真实的（非null），因此真实的方法会与无效字段交互，并导致类似您所看到的错误。

这里，ArrayList.ensureExplicitCapacity访问必须为空的elementData数组上的elementData.length，从而导致错误。

如果您使用默认为CALLS_REAL_METHODS行为的spy，Mockito将从您提供的实际构建的对象实例中复制字段值。这通常更安全。

（附带说明：除了玩具示例之外，请记住，模拟您不拥有的类型是危险的，而且几乎从来没有理由模拟List或其实现等经过良好测试的简单库对象。）