Question

Given the following code:

    LinkedList list = mock(LinkedList.class);
    doCallRealMethod().when(list).clear();
    list.clear();

by executing this test, a NullPointerException is thrown from first line in LinkedList#clear:

public void clear() {
    Entry<E> e = header.next;
    while (e != header) {
        Entry<E> next = e.next;
        //Code omitted. 

but header has been instantiated before:

private transient Entry<E> header = new Entry<E>(null, null, null);

Could someone please explain what’s happening during mock creation?

####### UPDATE. ######

Having read all answers especially Ajay’s one, I looked into Objenesis source code and find out that it’s using Reflection API to create the proxy instance (through CGLIB) and therefore bypassing all constructors in the hierarchy until java.lang.Object.

Here is the sample code to simulate the issue:

public class ReflectionConstructorTest {

    @Test
    public void testAgain() {

        try {
            //java.lang.Object default constructor
            Constructor javaLangObjectConstructor = Object.class
                    .getConstructor((Class[]) null);
            Constructor mungedConstructor = ReflectionFactory
                    .getReflectionFactory()
                    .newConstructorForSerialization(CustomClient.class, javaLangObjectConstructor);

            mungedConstructor.setAccessible(true);

            //Creates new client instance without calling its constructor
            //Thus "name" is not initialized.
            Object client = mungedConstructor.newInstance((Object[]) null);

            //this will print "CustomClient" 
            System.out.println(client.getClass());
            //this will print "CustomClient: null". name is null.
            System.out.println(client.toString());

        } catch(Exception e) {
            e.printStackTrace();
        }
    }
}


class CustomClient {
    private String name;

    CustomClient() {
        System.out.println(this.getClass().getSimpleName() + " - Constructor");
        this.name = "My Name";
    }

    @Override
    public String toString() {
        return this.getClass().getSimpleName() + ": " + name;
    }
}

Answer 1

Your reasoning is flawless.
The key issue is that you are not operating on the actual LinkedList object. Here is what is happening behind the scenes:

The object that you are given by Mockito’s mock() is an Enhancer object from the CGLIB library.

For me it is something like java.util.LinkedList$$EnhancerByMockitoWithCGLIB$$cae81a28

which kind of acts like a Proxy, albeit with the fields set to default values. (null,0 etc)