NullPointerException 在构造函数定义

Question

我正在尝试测试以下课程：

public final class ClassA {

  private final ClassB member;

  public ClassA() {
    this.member = new ClassB();
  }

}

以下是测试类：

@RunWith(PowerMockRunner.class)
@PrepareForTest({ClassA.class, ClassB.class})
public final class ClassATest {

  @Mock
  private ClassB mocked;

  private ClassA testClass;

  @Before
  public void initTest() {
    PowerMockito.whenNew(ClassB.class).withNoArguments().thenReturn(mocked);
    testClass = new ClassA();
  }

现在，问题是我得到了一个空指针异常，它的堆栈跟踪看起来像：

java.lang.NullPointerException
  [junit]     at a.a.i.ClassA.<init>(ClassA.java:44)
  [junit]     at a.a.i.ClassATest.init(ClassATest.java:53)
  [junit]     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  [junit]     at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
  [junit]     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  [junit]     at java.lang.reflect.Method.invoke(Method.java:606)
  [junit]     at org.junit.internal.runners.MethodRoadie.runBefores(MethodRoadie.java:132)
  [junit]     at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:95)
  [junit]     at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.executeTest(PowerMockJUnit44RunnerDelegateImpl.java:294)
  [junit]     at org.powermock.modules.junit4.internal.impl.PowerMockJUnit47RunnerDelegateImpl$PowerMockJUnit47MethodRunner.executeTestInSuper(PowerMockJUnit47RunnerDelegateImpl.java:127)
  [junit]     at org.powermock.modules.junit4.internal.impl.PowerMockJUnit47RunnerDelegateImpl$PowerMockJUnit47MethodRunner.executeTest(PowerMockJUnit47RunnerDelegateImpl.java:82)
  [junit]     at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runBeforesThenTestThenAfters(PowerMockJUnit44RunnerDelegateImpl.java:282)
  [junit]     at org.junit.internal.runners.MethodRoadie.runTest(MethodRoadie.java:86)
  [junit]     at org.junit.internal.runners.MethodRoadie.run(MethodRoadie.java:49)
  [junit]     at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.invokeTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:207)
  [junit]     at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.runMethods(PowerMockJUnit44RunnerDelegateImpl.java:146)
  [junit]     at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$1.run(PowerMockJUnit44RunnerDelegateImpl.java:120)
  [junit]     at org.junit.internal.runners.ClassRoadie.runUnprotected(ClassRoadie.java:33)
  [junit]     at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:45)
  [junit]     at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.run(PowerMockJUnit44RunnerDelegateImpl.java:122)
  [junit]     at org.powermock.modules.junit4.common.internal.impl.JUnit4TestSuiteChunkerImpl.run(JUnit4TestSuiteChunkerImpl.java:106)
  [junit]     at org.powermock.modules.junit4.common.internal.impl.AbstractCommonPowerMockRunner.run(AbstractCommonPowerMockRunner.java:53)
  [junit]     at org.powermock.modules.junit4.PowerMockRunner.run(PowerMockRunner.java:59)
  [junit]     at junit.framework.JUnit4TestAdapter.run(JUnit4TestAdapter.java:38)
  [junit]     at org.apache.tools.ant.taskdefs.optional.junit.JUnitTestRunner.run(JUnitTestRunner.java:532)
  [junit]     at org.apache.tools.ant.taskdefs.optional.junit.JUnitTestRunner.launch(JUnitTestRunner.java:1165)
  [junit]     at org.apache.tools.ant.taskdefs.optional.junit.JUnitTestRunner.main(JUnitTestRunner.java:988)

A 类的第 44 行是 公共类A() {

ClassATest 的第 52 行是 testClass = new ClassA();

我以前从未在构造函数定义中遇到过 NullPointerException，所以我不知道该怎么做。任何帮助将不胜感激。

Answer 1

我知道这个问题是关于 NPE 的，但与其解决这个问题，不如重新考虑你的测试策略。为什么？

如果 ClassA 负责 ClassB 实例的创建和生命周期，而您决定不注入 ClassB 的对象，为什么还要尝试模拟它？您的测试不应依赖于实现细节，因此如果您没有在任何地方注入 ClassB，则不应模拟 ClassB。

Answer 2

这种方法根本不值得推荐。对象不应该负责创建自己的依赖项。它们应该在它们的构造函数中提供（或通过 setter 方法）。

我建议你像这样改变你的构造函数

public final class ClassA {

  private final ClassB member;

  public ClassA(ClassB member) {
    this.member = member;
  }

}

如果您想保留您的代码，您可以添加一个用于测试的构造函数（尽管也不推荐仅出于测试目的添加代码）

public final class ClassA {

  private final ClassB member;

  public ClassA() {
    this(new ClassB());
  }

  // Default visibility so it's only visible in its package
  // Added for testing purposes
  ClassA(ClassB member) {
    this.member = member;
  }

}

Answer 3

我猜当你将 member 声明为 final 时，你应该立即实例化它 在去构造函数之前尝试这样做，看看会发生什么