Jacoco is adding a $jacocoInit method in my interface when running unit tests

hatellla :

I am getting a unnecessary method $jacocoInit when adding the defaut method in interface. My interface looks like this:

@Value.Immutable
public interface Customer {


    Optional<@NotBlank String> name();

    default Object getObject(final String abc) {
        return null
    }

    default void getObject() {

    }
}

When I am doing

for (Method method : Customer.class.getDeclaredMethods()) {
    System.out.println(method.getName());
}

then, I am getting

name
getObject
$jacocoInit

If I remove the default method from it, then it does not $jacocInit method. I am not sure why it is happening? Can someone please help in this?

Godin :

According to JaCoCo FAQ:

To collect execution data JaCoCo instruments the classes under test which adds two members to the classes: A private static field $jacocoData and a private static method $jacocoInit(). Both members are marked as synthetic.

Please change your code to ignore synthetic members. This is a good practice anyways as also the Java compiler creates synthetic members in certain situation.

Method isSynthetic of class java.lang.reflect.Method

Returns true if this executable is a synthetic construct; returns false otherwise.

So to ignore synthetic members:

for (Method method : Customer.class.getDeclaredMethods()) {
    if (method.isSynthetic()) {
        continue;
    }
    System.out.println(method.getName());
}

And here is one of many examples where Java compiler creates synthetic method:

import java.lang.reflect.*;

class Example {

    interface I {
        Runnable r = () -> {}; // lambda

        void m();
    }

    public static void main(String[] args) {
        for (Method method : I.class.getDeclaredMethods()) {
            System.out.println("name: " + method.getName());
            System.out.println("isSynthetic: " + method.isSynthetic());
            System.out.println();
        }
    }
}

Using JDK 1.8.0_152 execution of

javac Example.java
java Example

produces

name: m
isSynthetic: false

name: lambda$static$0
isSynthetic: true

And here is what Java Virtual Machine Specification states about synthetic members:

A class member that does not appear in the source code must be marked using a Synthetic attribute, [...]

You can read more about JaCoCo implementation in its documentation and in presentations done by JaCoCo team, which also include some other examples of synthetic constructions.