Make application layer loosely coupled

You approach is fine, it is look like some sort of Facade design pattern, but I would recommend more traditional way.

You already have spring as IoC container, so all you have to do is to extract interfaces from your *DBManager-classes and tell spring about your implementations. Then you will be able to inject an actual implementation using an interface, which in turn might resides in a totally different package. Therefore coupling will cease to exist.

Here is an example.

DataStorage contract:

package root;import java.util.List;public interface UserDataStorage {    void saveUserData(List<String> rows);}

DocumentStorage contract:

package root;public interface UserDocumentStorage {    void saveUserDocs(String docs);}

DataStorage implementation:

package root.domain;import org.springframework.stereotype.Service;import root.UserDataStorage;import java.util.List;@Servicepublic class MysqlDBManager implements UserDataStorage {    @Override    public void saveUserData(List<String> rows) {        System.out.println("Saving data....");    }}

DocumentStorage implementation:

package root.domain;import org.springframework.stereotype.Service;import root.UserDocumentStorage;@Servicepublic class MongoDBManager implements UserDocumentStorage {    @Override    public void saveUserDocs(String docs) {        System.out.println("Saving docs....");    }}

Then your service classes might look like this (please pay attention there is no reference to domain package:

package root.service;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Service;import root.UserDataStorage;import root.UserDocumentStorage;import java.util.Collections;@Servicepublic class MyService {    private final UserDataStorage dataStorage;    private final UserDocumentStorage documentStorage;    @Autowired    public MyService(UserDataStorage dataStorage, UserDocumentStorage documentStorage) {        this.dataStorage = dataStorage;        this.documentStorage = documentStorage;    }    public void callMe() {        this.dataStorage.saveUserData(Collections.emptyList());        this.documentStorage.saveUserDocs("Document1");    }}

IMHO there is a better way, The interfaces help you to have loosely couple layers, here is an example:

interface DocsDBManager {   void saveUserDocs(String docs);}interface UserDBManager {   void saveUserData(List<String> rows);}interface DBManager extends DocsDBManager, UserDBManager {}

And you must to implement them:

class  MongoDBManager implements DocsDBManager {   public void saveUserDocs(String docs) {     //.....   }}class  MysqlDBManager implements UserDBManager {   public void saveUserData(List<String> rows) {     //.....   }}

And:

class  DBManagerBase implements DBManager  {       @Autowired        private DocsDBManager docsDBManager;       @Autowired        private UserDBManager userDBManager;       public void saveUserData(List<String> rows) {            userDBManager.saveUserData(rows);       }       public void saveUserDocs(String docs) {           docsDBManager.saveUserDocs(docs);       } }

And finally:

class Service {    @Autowired     private DBManager dBManager;    //....... code here    dBManager.saveUserData(rows);    dBManager.saveUserDocs(docs);}