JUnit change class or method behaviour in integration tests

I found a solution that works very well and it is very simple!

The solution is PowerMock (https://github.com/powermock/powermock) and in particular replace the creation of an instance of a class with another: https://github.com/powermock/powermock/wiki/mockito#how-to-mock-construction-of-new-objects

There is only one problem in my project and it is JUnit 5. PowerMock support JUnit 4 and for this reason, only for some tests of the solution are using it. In order to do this there is the needed to replace

import org.junit.jupiter.api.Test;

with

import org.junit.Test;

In order to use teh "whenNew()" methods I had extented the class that in tests must be replaced and I have overwritten only methods that are necessary for the integration test. The big benefit of this solution is that my code is untouched and I can use this approach also on old code without the risk of introducing regressions during the refactor of the code.

Regarding the code of a integration test, here an example:

import org.junit.jupiter.api.DisplayName;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PowerMockIgnore;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PowerMockIgnore({"javax.crypto.*" }) // https://github.com/powermock/powermock/issues/294
@PrepareForTest(LegacyCoreNetworkClassPlg.class) // it is the class that contains the "new SOAPCallHelper(..)" code that I want to intercept and replace with a stub
public class ITestExample extends InitTestSuite {
    @Test
    @DisplayName("Test the update of a document status")
    public void iTestStubLegacyNetworkCall() throws Exception {

        // I'm using JUnit 4
        // I need to call @BeforeAll defined in InitTestSuite.init();
        // that works only with JUnit 5
        init();

        LOG.debug("IN stubbing...");
        SOAPCallHelperStub stub = new SOAPCallHelperStub("empty");
        PowerMockito.whenNew(SOAPCallHelper.class).withAnyArguments().thenReturn(stub);
        LOG.debug("OUT stubbing!!!");

        LOG.debug("IN iTestStubLegacyNetworkCall");
        ...
        // Here I can create any instance of every class, but when an instance of 
        // LegacyCoreNetworkClassPlg.class is created directly or indirectly, PowerMock
        // is checking it and when LegacyCoreNetworkClassPlg.class will create a new
        // instance of SOAPCallHelper it will change it with the 
        // SOAPCallHelperStub instance.
        ...
        LOG.debug("OUT iTestStubLegacyNetworkCall");
    }
}

Here the configuration of the pom.xml

    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <junit.jupiter.version>5.5.2</junit.jupiter.version>
    <junit.vintage.version>5.5.2</junit.vintage.version>
    <junit.platform.version>1.3.2</junit.platform.version>
    <junit.platform.engine.version>1.5.2</junit.platform.engine.version>
    <powermock.version>2.0.2</powermock.version>

    <!-- FOR TEST -->
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-api</artifactId>
        <version>${junit.jupiter.version}</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <scope>test</scope>
    </dependency>
    <!-- Only required to run tests in an IDE that bundles an older version -->
    <dependency>
        <groupId>org.junit.platform</groupId>
        <artifactId>junit-platform-launcher</artifactId>
        <version>${junit.platform.version}</version>
        <scope>test</scope>
    </dependency>
    <!-- Only required to run tests in an IDE that bundles an older version -->
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-engine</artifactId>
        <version>${junit.jupiter.version}</version>
        <scope>test</scope>
    </dependency>
    <!-- Only required to run tests in an IDE that bundles an older version -->
    <dependency>
        <groupId>org.junit.vintage</groupId>
        <artifactId>junit-vintage-engine</artifactId>
        <version>${junit.vintage.version}</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.junit.platform</groupId>
        <artifactId>junit-platform-engine</artifactId>
        <version>${junit.platform.engine.version}</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-params</artifactId>
        <version>${junit.vintage.version}</version>
        <scope>test</scope>
    </dependency>

    <dependency>
        <groupId>org.powermock</groupId>
        <artifactId>powermock-module-junit4</artifactId>
        <version>${powermock.version}</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.powermock</groupId>
        <artifactId>powermock-api-mockito2</artifactId>
        <version>${powermock.version}</version>
        <scope>test</scope>
    </dependency>

I think the main headache in your case is that you have tightly coupled dependencies between class A, B and NotifierHandler. I would start with:

class A {
  private B b;

  public A(B b) {
    this.b = b;
  }

  Result mainFunction(Request r){
      ....
      b.logEvent(someParameters)
      ....
  }
}

class B {
  private NotifierHandler nh;

  public B(NotifierHandler nh) {
    this.nh = nh;
  }

  int logEvent(Object someParameters){
      ....
      nh.sendNotification(json);
      ....
  }
}

Make NotifierHanlder an interface:

interface NotifierHandler {
  String sendNotification(String json);
}

and make two implementations: one for a real use case, and one fake that you can stub whatever you want:

class FakeNotifierHandler implements NotifierHandler {

  @Override
  public String sendNotification(String json) {
    // whatever is needed for you
  }
}

Inject FakeNotifierHandler in your test.

I hope this helps you.