powermock. мокаем конструктор внутри класса

build.gradle

plugins {
    id 'java'
    id 'org.springframework.boot' version '1.5.10.RELEASE'
}

group 'ru.test'
version '1.0-SNAPSHOT'

buildDir = 'target'
sourceCompatibility = 1.8

repositories {
    mavenCentral()
}

dependencies {
    compile("org.springframework.boot:spring-boot-starter")
    testCompile("cglib:cglib-nodep:3.2.4")

    compileOnly 'org.projectlombok:lombok'
    testCompile 'junit:junit:4.12'
    testCompile 'org.codehaus.groovy:groovy-all:2.4.10'
    testCompile 'org.hamcrest:hamcrest-core:1.3'
    testCompile 'org.mockito:mockito-all:1.10.19'
    testCompile 'org.powermock:powermock-module-junit4:1.7.3'
    testCompile 'org.powermock:powermock-api-mockito:1.7.3'
    testCompile 'org.spockframework:spock-core:1.1-groovy-2.4'
}

task wrapper(type: Wrapper) {
    gradleVersion = '4.4'
}

Child.java

package ru;

import lombok.Data;

/**
 * Created by Dmitry Tkachenko on 4/7/18
 */
@Data
public class Child {

    private int age;

    private String name;

    public void init(int age, String name) {
        this.age = age;
        this.name = name;
        System.out.println("---------ТЫ-НЕ-ДОЛЖЕН-УВИДЕТЬ-ЭТОГО-СООБЩЕНИЯ--------");
    }
}

Parent.java

package ru;

import lombok.Data;

/**
 * Created by Dmitry Tkachenko on 4/7/18
 */
@Data
public class Parent {

    private Child child;

    public void init(int age, String name) {
        this.child = new Child();
        this.child.init(age, name);
        System.out.println("-----E-N-D------");
    }
}

ParentTest.groovy

package ru

import org.junit.runner.RunWith
import org.mockito.Matchers
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.modules.junit4.PowerMockRunnerDelegate
import org.spockframework.runtime.Sputnik
import spock.lang.Specification

/**
 * Created by Dmitry Tkachenko on 4/7/18
 */
@RunWith(PowerMockRunner.class)
@PowerMockRunnerDelegate(Sputnik.class)
@PrepareForTest([Parent.class])
class ParentTest extends Specification {

    Parent parent

    def setup() {
//        parent = Spy(Parent) // не будет работать!
        parent = Mockito.spy(new Parent()) // или так Mockito.spy(Parent.class)
    }

    def test() {
        setup:
        Child childMock = Mockito.mock(Child.class) // не Child childMock = Mock()
//         иначе получим исключение:
//        org.mockito.exceptions.misusing.NotAMockException:
//        Argument passed to when() is not a mock!
//                Example of correct stubbing:
//        doThrow(new RuntimeException()).when(mock).someMethod();

        PowerMockito.whenNew(Child.class).withNoArguments().thenReturn(childMock)
        Mockito.doNothing().when(childMock).init(Matchers.any(Integer.class), Matchers.anyString()) // эту строку можно не писать. и так работает


        when:
        parent.init(3, 'rom')


        then:
        parent.child.age == 0
        parent.child.name == null
    }
}