How to mock a private method called inside another private method

Question

I was trying to mock a private method's output which is being called inside another private method, I have no choice but to test the later private method, so I have added sample test code which I can represent here,

This Sample Class

package com.testableClass;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class TestableClass {
    
     private int initialMockMethod(Object obj)
     {
         
         System.out.println(" ++++ Came Here ++++ ");
         
         String str = getRestString("");
         
         System.out.println("str ="+str);
         return str.length();
     }

     private String getRestString(String abc)
     {
         String output="";
         try {

                URL url = new URL("https://gorest.co.in/public-api/users");//your url i.e fetch data from .
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                conn.setRequestProperty("Accept", "application/json");
                if (conn.getResponseCode() != 200) {
                    throw new RuntimeException("Failed : HTTP Error code : "
                            + conn.getResponseCode());
                }
                InputStreamReader in = new InputStreamReader(conn.getInputStream());
                BufferedReader br = new BufferedReader(in);
                
                while ((output = br.readLine()) != null) {
                    System.out.println(output);
                }
                conn.disconnect();
                
                return output;

            } catch (Exception e) {
                System.out.println("Exception in NetClientGet:- " + e);
            }
        return abc;
     }
     
}

Now This PowerMock Class

package com.testableClass;

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;
import org.powermock.reflect.Whitebox;

import junit.framework.Assert;

@RunWith(PowerMockRunner.class)
@PrepareForTest(fullyQualifiedNames = "com.testableClass.TestableClass")
public class PowerMockTest {
    

    
    @Test
    public void testPrivateMethod() throws Exception
    {
        String message = "Hello PowerMockito";
        String expectation = "Expectation";
 
        TestableClass mock = PowerMockito.spy(new TestableClass());
//      PowerMockito.doReturn(expectation).when(mock, "getRestString", message);
        PowerMockito.when(mock, "getRestString", message).thenReturn(expectation);
        int count = Whitebox.invokeMethod(mock, "initialMockMethod", new Object());
       System.out.println(" +++ Count : "+count+" ++++ ");
        Assert.assertTrue(true);
    }

}

my issue is when I am running my test case then

  PowerMockito.when(mock, "getRestString", message).thenReturn(expectation);

executes original method and returns original output while my requirement is that, when my test case is actually calling private method initialMockMethod it should not call getRestString instead of that it should return my mocked expected output which is "Expectation"

Answer 1

Instead of using reflection and PowerMock, I'd say that do not try to mock a private method. It is a hidden detail of the class.

What I suggest is that, if your method makes an HTTP request, then let it do that. But you can use a mock server to mock the response. But to do that, you need to make your endpoint external, so that it can be injected.

I changed your class a little bit; still the same purpose tho.

public class TestableClass {
    private final String resourceUrl;

    public TestableClass(String resourceUrl) {
        this.resourceUrl = resourceUrl;
    }

    public int publicMethod() {
        return initialMockMethod(null);
    }

    private int initialMockMethod(Object obj) {
        var str = getRestString("");
        return str.length();
    }

    private String getRestString(String abc) {
        try {
            var url = new URL(resourceUrl);
            var conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("GET");
            conn.setRequestProperty("Accept", "application/json");
            if (conn.getResponseCode() != 200) {
                throw new RuntimeException("Failed : HTTP Error code : "
                        + conn.getResponseCode());
            }

            var in = new InputStreamReader(conn.getInputStream());
            var br = new BufferedReader(in);

            var result = br.lines().collect(Collectors.joining("\n"));
            conn.disconnect();

            return result;
        } catch (Exception e) {
            System.out.println("Exception in NetClientGet:- " + e);
        }
        return abc;
    }

}

and here's the test

public class TestingTestableClass {

    @Test
    @SneakyThrows
    public void test() {
        final var server = new MockWebServer();
        server.start(9999);

        var instance = new TestableClass("http://127.0.0.1:9999");

        server.enqueue(
                new MockResponse()
                    .setResponseCode(200)
                    .setBody("this is the response")
        );

        final int result = instance.publicMethod();

        Assertions.assertEquals(
                "this is the response".length(),
                result
        );

        final var record = server.takeRequest();
        final var method = record.getMethod();
        Assertions.assertEquals("GET", method);

        server.shutdown();
    }

}

Check out MockWebServer here