NullPointerException when calling mocked method

Question

I try to mock a final method (readChar of class DataInputStream):

MyClassTest

@RunWith(PowerMockRunner.class)
@PrepareForTest(DataInputStream.class)
public class MyClassTest {

    @Test
    public void testMyMethod() throws IOException {
        DataInputStream mockStream = PowerMockito.mock(DataInputStream.class);
        Mockito.when(mockStream.readChar()).thenReturn('a');
        System.out.println(mockStream.readChar());  // OK (print 'a')
        Assert.assertEquals('a', MyClass.myMethod(mockStream));
    }
}

MyClass

public class MyClass {
    public static char myMethod(DataInputStream dis) throws IOException {
        return dis.readChar();  // NPE raises
    }
}

It works when calling the mocked method in testMyMethod() but in myMethod() NullPointerException raises, why?

EDIT :

The complete failure trace :

java.lang.NullPointerException
    at java.io.DataInputStream.readChar(Unknown Source)
    at test.test.MyClass.myMethod(MyClass.java:8)
    at test.test.MyClassTest.testMyMethod(MyClassTest.java:23)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.junit.internal.runners.TestMethod.invoke(TestMethod.java:59)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:310)
    at org.junit.internal.runners.MethodRoadie$2.run(MethodRoadie.java:79)
    at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:87)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$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:77)
    at org.junit.internal.runners.MethodRoadie.run(MethodRoadie.java:42)
    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:27)
    at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:37)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.run(PowerMockJUnit44RunnerDelegateImpl.java:122)
    at org.powermock.modules.junit4.common.internal.impl.JUnit4TestSuiteChunkerImpl.run(JUnit4TestSuiteChunkerImpl.java:104)
    at org.powermock.modules.junit4.common.internal.impl.AbstractCommonPowerMockRunner.run(AbstractCommonPowerMockRunner.java:53)
    at org.powermock.modules.junit4.PowerMockRunner.run(PowerMockRunner.java:53)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

score 2 · Answer 1 · edited May 23 '17 at 12:22

First this code is a mock antipattern : Don't mock types you don't own! (see this answer on StackOverflow)

Second, DataInputStream is a class of the JDK PowerMock cannot use the same hacky classloader that modifies byte code.

For that there's a solution and two possible tricks :

use the interface
encapsulate the call in a class that you own, then prepare this class instead, documented here at google code, or even better make your own mockable class (without the need of powermock)
use an agent, documented here at google code

The first option is clearly the very best, and the first two options also allows one to avoid this mock antipattern.

score 1 · Accepted Answer · answered Mar 28 '15 at 15:31

DataInputStream is a 'system' class from JVM which is probably already loaded by JVM. @PrepareForTest would have to remove final modifier from the methods (to be able to mock), but it can't do so for already-loaded classes (HotSpot JVM doesn't support class signature changes for already-loaded classes), and this is probably why you get this exception.

Luckily there's also DataInput interface implemented by DataInputStream - maybe you can try mocking not DataInputStream but DataInput, for this you don't even need PowerMock, just Mockito.

I have mocked DataInput instead of DataInputStream and it works. Thanks for your clear explanation and your help ! — csa, Mar 28 '15 at 15:42

NullPointerException when calling mocked method

2 Answers2