How to mock Autowired constructor dependency in groovy spock

I have a class for which i am writing groovy test case. This class has constructor autowiring and calls another method to initialise the fields.:

@Service
public class ServiceA {

private final PrincipalDao principalDao;
    @Autowired
    public ServiceA(final PrincipalDao principalDao){
		this.principalDao=principalDao;
        this.serviceMap = getMap();           
    }
	    	
    private Map<> getMap() {
        final List<ClassA> classAList = principalDao.findAll(); //this line returns null
       
    }    	
}

this line final List<ClassA> classAList = principalDao.findAll(); returns null and I cant mock it in groovy like following:

principalDao.findAll() >> list

because its called even before my above line is called in groovy test case

Without seeing you unit test, it's hard to tell what the issue is, but a common misconception I run into sometimes is the timing of the mock.

Here's a sample code elaborating what I mean. Since parts of the code missing from your question, going to assume some parts of it, but I'm sure it explains the point I'm making and hopefully should help you out in finding an answer:

Service class:

@Service
public class ServiceA{

  private final PrincipalDao principalDao;
  private final Map<String, String> serviceMap;

  @Autowired
  public ServiceA(final PrincipalDao principalDao){
    this.principalDao = principalDao;
    this.serviceMap = getMap();
  }

  private Map<String, String> getMap(){
    final HashMap<String, String> stringStringHashMap = new HashMap<String, String>();
    stringStringHashMap.put("hello", principalDao.khello());
    return stringStringHashMap;
  }

  public String printServiceMap() {
    return serviceMap.get("hello");
  }
}

class ServiceATest extends Specification {

  def "some test"() {
    given:
    PrincipalDao principalDao = Mock()
    principalDao.khello() >> "khello" // this is the key here. Mocking is happening before instantiation of ServiceA
    ServiceA someService = new ServiceA(principalDao)

    expect:
    "khello" == someService.printServiceMap()
  }
}

This unit test passes successfully. The point here is the timing of the mocking. principalDao.khello() >> "khello" happens before new ServiceA(principalDao).

If you don't need to modify serviceMap per unit test, you can also do this so you don't have to define it per unit test:

class ServiceATest extends Specification {

  @Shared
  PrincipalDao principalDao = Mock()
  ServiceA someService = new ServiceA(principalDao)

  def setupSpec() {
    principalDao.khello() >> "khello"
  }

  def "some test"() {
    expect:
    "khello" == someService.printServiceMap()
  }
}