Error when trying to use @Spy Annotation: java.lang.NullPointerException: Cannot read the array length because "elementData" is null

I am using the @Spy annotation and the following error appear when i'm trying to run the test:
java.lang.NullPointerException: Cannot read the array length because "elementData" is null

This is the code i have:

package com.tddk.junit.service;

import lombok.extern.slf4j.Slf4j;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Spy;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

import java.util.ArrayList;
import java.util.List;

import static org.assertj.core.api.Assertions.*;
import static org.junit.jupiter.api.Assertions.*;
import static org.mockito.Mockito.*;
@Slf4j
@ExtendWith(MockitoExtension.class)
public class StudentServiceTest {
    @Spy
    List<String> spyList = new ArrayList<String>();

    @Test
    void whenUsingTheSpyAnnotation_thenObjectIsSpied() {
        spyList.add("one");
        spyList.add("two");

        verify(spyList).add("one");
        verify(spyList).add("two");

        assertThat(spyList).hasSize(2);
    }
}

The sample code above is from: https://www.baeldung.com/mockito-spy I do exactly as the tutorial said but still getting an error. Can someone show me how to fix it please.

This could be because you're using JDK 17 or plus, and the author of the article is using a lower JDK version.

After the JDK 17, more and more use cases are broken, and Spy is one of them. You can switch to the inline mockmaker to avoid this issue.

Switching to the inline mockmaker

1. Create the resources directory inside the test directory.
2. Then Create a mockito-extensions directory within resources directory.
3. Create a file named org.mockito.plugins.MockMaker within mockito-extensions directory.
4. Finally, add this text mock-maker-inline to the org.mockito.plugins.MockMaker file.

Folder structure should look like this:

Refer here as well: Switch the default mockmaker to the inline mockmaker on JDK 17+

Lastly, my advice is to not spy on lists, but instead perform checks on the real list.