Can any one help me in mocking a static method which returns an object, and this static method is present in a final class - junit

Question

I need help for below thing,
I have to write a Junit using PowerMock/Mockito for a method which makes a call to a static method of a final class present in an external jar.
The method for which i need to write the JUnit test is:
public class SomeClass {
private PrivateKey privateKeyFromPkcs8(String privateKeyPem) throws IOException {
Reader reader = new StringReader(privateKeyPem);
Section section = PemReader.readFirstSectionAndClose(reader, "PRIVATE KEY");
if (section == null) {
throw new IOException("Invalid PKCS8 data.");
}
byte[] bytes = section.getBase64DecodedBytes();
PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(bytes);
try {
KeyFactory keyFactory = SecurityUtils.getRsaKeyFactory();
PrivateKey privateKey = keyFactory.generatePrivate(keySpec);
return privateKey;
} catch (NoSuchAlgorithmException exception) {
} catch (InvalidKeySpecException exception) {
}
throw new IOException("Unexpected exception reading PKCS data");
}
}
In the above code PemReader is a final class and readFirstSectionAndClose(reader, "PRIVATE KEY") is a static method in PemReader.
I have tried writing the test shown below but Section object(section) is showing as null while debugging. Perhaps the actual code (PemReader.readFirstSectionAndClose(reader, "PRIVATE KEY")) is getting called instead of the mock.
#RunWith(PowerMockRunner.class)
#PrepareForTest({SomeClass.class,PemReader.class})
public class SomeClassTest {
#InjectMocks
SomeClass mockSomeClass;
#Mock
private Reader mockReader;
#Mock
private Section mockSection;
#Test
public void testPrivateKeyFromPkcs8() throws Exception {
PowerMockito.mockStatic(PemReader.class);
Mockito.when(PemReader.readFirstSectionAndClose(mockReader, "PRIVATE KEY")).thenReturn(mockSection);
assertNotNull(mockSomeClass.privateKeyFromPkcs8(dummyPrivateKey));
}
}
Please help me in writing a Junit using powermockito/mockito

Answer 1

You have to prepare the final, static class.
Here's an example using the PowerMock annotations for JUnit:
#RunWith(PowerMockRunner.class)
#PrepareForTest({PemReader.class})
public class PemReaderTest {
#Mock
private Reader mockReader;
#Mock
private Section mockSection;
#Test
public void testMockingStatic() {
PowerMockito.mockStatic(PemReader.class);
Mockito.when(PemReader.readFirstSectionAndClose(mockReader, "PRIVATE KEY")).thenReturn(mockSection);
Assert.assertEquals(mockSection, PemReader.readFirstSectionAndClose(mockReader, "PRIVATE KEY"));
}
}
For completeness, here's the definition of PemReader:
public final class PemReader {
public static Section readFirstSectionAndClose(Reader reader, String key) {
return null;
}
}
The above test passes with the following versions:
JUnit: 4.12
Mockito: 2.7.19
PowerMock: 1.7.0
Update 1: based on your updated question. Your test case will pass (or at least the invocation on PemReader.readFirstSectionAndClose will return something) if you just make this change:
Mockito.when(PemReader.readFirstSectionAndClose(
Mockito.any(Reader.class),
Mockito.eq("PRIVATE KEY"))
).thenReturn(mockSection);
The version of this instruction in your current test case relies on equality matching between the StringReader which your code passes into readFirstSectionAndClose and the mocked Reader which your test case supplies. These are not 'equal' hence the mocked invocation's expectations are not met and your mockSection is not returned.
A few, unrelated, notes:
There is no need to include SomeClass.class in #PrepareForTest, you only need to include the classes which you want to mock in that annotation, since SomeClass is the class you are trying to test there is no mocking required for that class.
Using #InjectMocks to instance SomeClass is a bit odd, since SomeClass has no (mockito provided) mocks to inject into it :) you can replace this declaration with SomeClass someClass = new SomeClass();
In the code you supplied SomeClass.privateKeyFromPkcs8 has private scope so it cannot be tested (or called in any way) from SomeClassTest.