Is it possible to use partial mocking for private static methods in PowerMock?

Question

From the examples on the PowerMock homepage, I see the following example for partially mocking a private method with Mockito:

@RunWith(PowerMockRunner.class) // We prepare PartialMockClass for test because it's final or we need to mock private or static methods @PrepareForTest(PartialMockClass.class) public class YourTestCase { @Test public void privatePartialMockingWithPowerMock() {             PartialMockClass classUnderTest = PowerMockito.spy(new PartialMockClass());      // use PowerMockito to set up your expectation     PowerMockito.doReturn(value).when(classUnderTest, "methodToMock", "parameter1");      // execute your test     classUnderTest.execute();      // Use PowerMockito.verify() to verify result     PowerMockito.verifyPrivate(classUnderTest, times(2)).invoke("methodToMock", "parameter1"); } 

However, this approach does not seem to work when the private method we wish to mock is static. I wish to create a partial mock of the below class, with the readFile method mocked:

package org.rich.powermockexample;  import java.io.File; import java.io.IOException; import java.nio.charset.Charset; import java.util.List;  import static com.google.common.io.Files.readLines;  public class DataProvider {      public static List<String> getData() {         List<String> data = null;         try {             data = readFile();         } catch (IOException e) {             e.printStackTrace();         }         return data;     }      private static List<String> readFile() throws IOException {         File file = new File("/some/path/to/file");         List<String> lines = readLines(file, Charset.forName("utf-8"));         return lines;     }  } 

Please could someone let me know how this can be achieved?

Answer 1

After doing a bit more research, it seems that PowerMockito.spy() and PowerMockito.doReturn() are what is required here:

package com.richashworth.powermockexample;  import org.junit.Before; import org.junit.BeforeClass; import org.junit.Test; import org.junit.runner.RunWith; import org.powermock.api.mockito.PowerMockito; import org.powermock.core.classloader.annotations.PrepareForTest; import org.powermock.modules.junit4.PowerMockRunner;  import java.util.ArrayList; import java.util.List;  import static org.junit.Assert.assertEquals;   @RunWith(PowerMockRunner.class) @PrepareForTest({DataProvider.class}) public class ResultsWriterTest {      private static List<String> mockData = new ArrayList<String>();     private ResultsWriter resultsWriter;      @BeforeClass     public static void setUpOnce() {         final String firstLine = "Line 1";         final String secondLine = "Line 2";         mockData.add(firstLine);         mockData.add(secondLine);     }      @Before     public void setUp() {         resultsWriter = new ResultsWriter();     }      @Test     public void testGetDataAsString() throws Exception {         PowerMockito.spy(DataProvider.class);         PowerMockito.doReturn(mockData).when(DataProvider.class, "readFile");          final String expectedData = "Line 1\nLine 2\n";         final String returnedString = resultsWriter.getDataAsString();          assertEquals(expectedData, returnedString);     }  } 

For further details and the complete code listing, check out my blog post here: https://richashworth.com/post/turbocharge-your-mocking-framework-with-powermock/

Answer 2

Test class:

@RunWith(PowerMockRunner.class) @PrepareForTest(DataProvider.class) public class DataProviderTest {      @Test     public void testGetDataWithMockedRead() throws Exception {         mockStaticPartial(DataProvider.class, "readFile");          Method[] methods = MemberMatcher.methods(DataProvider.class, "readFile");         expectPrivate(DataProvider.class, methods[0]).andReturn(Arrays.asList("ohai", "kthxbye"));         replay(DataProvider.class);          List<String> theData = DataProvider.getData();         assertEquals("ohai", theData.get(0));         assertEquals("kthxbye", theData.get(1));     }  } 

Class being tested (basically yours):

public class DataProvider {      public static List<String> getData() {         try {             return readFile();         } catch (IOException e) {             e.printStackTrace();             return null;         }     }      private static List<String> readFile() throws IOException {         File file = new File("/some/path/to/file");         return readLines(file, Charset.forName("utf-8"));     }  }