Mock FileInputStream in FileReader can't work using PowerMock - filereader

Question

I noticed that in FileReader constructor, the FileInputStream is created. So I what to Mock it in the FileReader class, but it can't work. Can anyone figure it out?
The code it like below:
package util;
import java.io.BufferedReader;
import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import org.junit.Assert;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mockito;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
#RunWith(PowerMockRunner.class)
#PrepareForTest({ FileReader.class, ContentReader.class})
public class FileReaderTest {
#Test
public void testGetContent() throws Exception {
File file = PowerMockito.mock(File.class);
InputStreamReader isr = new InputStreamReader(new ByteArrayInputStream("123".getBytes()));
PowerMockito.whenNew(InputStreamReader.class)
.withArguments(Mockito.any(FileInputStream.class)).thenReturn(isr);
Assert.assertEquals("123", ContentReader.getContent(file));
}
}
class ContentReader {
public static String getContent(File file) throws IOException {
String content = "unknown";
BufferedReader in = null;
in = new BufferedReader(new FileReader(file));
content = in.readLine();
in.close();
return content;
}
}

Answer 1

Shot answer - it's impossible, because to mock system classes the PowerMock should be able to modified a client class which uses system class. In your case both classes: who uses and what is used are system classes. More you can read here (it's about static calls of system classes, but same true for mocking constructor call)
Also, please check this good point: don't mock what you don't own. For you it means:
You should wrap reading data from file via util class which you can mock
Write an integration test for your util class. If ContentReader
is an util class then you shouldn't write unit test for it.