Java NullPointerException而mockStatic如果在前whenNew

我有以下测试课程：

@RunWith(PowerMockRunner.class)
@PrepareForTest({BuildCcHeaderProcess.class, IOUtils.class})
public class MyTest {
...
@Before
    public void setUp() throws Exception {  
          ...
          whenNew(StringWriter.class).withNoArguments().thenReturn(stringWriterMock);
          PowerMockito.mockStatic(IOUtils.class); //exception throws here
          ....
     }
     ....
}

调用测试时，我看到以下结果：

java.lang.ExceptionInInitializerError
    at sun.reflect.GeneratedSerializationConstructorAccessor23.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at org.objenesis.instantiator.sun.SunReflectionFactoryInstantiator.newInstance(SunReflectionFactoryInstantiator.java:40)
    at org.objenesis.ObjenesisBase.newInstance(ObjenesisBase.java:59)
    at org.mockito.internal.creation.jmock.ClassImposterizer.createProxy(ClassImposterizer.java:111)
    at org.mockito.internal.creation.jmock.ClassImposterizer.imposterise(ClassImposterizer.java:51)
    at org.powermock.api.mockito.internal.mockcreation.MockCreator.createMethodInvocationControl(MockCreator.java:100)
    at org.powermock.api.mockito.internal.mockcreation.MockCreator.mock(MockCreator.java:58)
    at org.powermock.api.mockito.PowerMockito.mockStatic(PowerMockito.java:70)
    at com.MyTest.setUp(MyTest.java:98)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at org.junit.internal.runners.MethodRoadie.runBefores(MethodRoadie.java:132)
    at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:95)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.executeTest(PowerMockJUnit44RunnerDelegateImpl.java:294)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runBeforesThenTestThenAfters(PowerMockJUnit44RunnerDelegateImpl.java:282)
    at org.junit.internal.runners.MethodRoadie.runTest(MethodRoadie.java:86)
    at org.junit.internal.runners.MethodRoadie.run(MethodRoadie.java:49)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.invokeTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:207)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.runMethods(PowerMockJUnit44RunnerDelegateImpl.java:146)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$1.run(PowerMockJUnit44RunnerDelegateImpl.java:120)
    at org.junit.internal.runners.ClassRoadie.runUnprotected(ClassRoadie.java:33)
    at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:45)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.run(PowerMockJUnit44RunnerDelegateImpl.java:118)
    at org.powermock.modules.junit4.common.internal.impl.JUnit4TestSuiteChunkerImpl.run(JUnit4TestSuiteChunkerImpl.java:102)
    at org.powermock.modules.junit4.common.internal.impl.AbstractCommonPowerMockRunner.run(AbstractCommonPowerMockRunner.java:53)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:160)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:74)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:211)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:67)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
Caused by: java.lang.NullPointerException
    at java.io.Writer.<init>(Writer.java:71)
    at java.io.PrintWriter.<init>(PrintWriter.java:75)
    at java.io.PrintWriter.<init>(PrintWriter.java:62)
    at org.apache.commons.io.IOUtils.<clinit>(IOUtils.java:113)
    ... 37 more

代码开始工作

---

神奇之处是什么？

执行

IOUtils

的静态初始值设定项时会引发异常，如下所示（版本2.4）：

静态{
//避免安全问题
StringBuilderWriter buf=新StringBuilderWriter（4）；
PrintWriter out=new PrintWriter（buf）；//这样我就可以交换两行了！你说的是哪两行？我的猜测很简单，你其余的模拟会弄乱IOUtils初始值设定项的内部工作。调试可能有助于检查在哪一点引入了
null
值。
whenNew(StringWriter.class).withNoArguments().thenReturn(stringWriterMock);

static {
    // avoid security issues
    StringBuilderWriter buf = new StringBuilderWriter(4);
    PrintWriter out = new PrintWriter(buf); //<-- kaboom here
    out.println();
    LINE_SEPARATOR = buf.toString();
    out.close();
}