Common library class (with injection) for different EJB projects

Question

I want to write a utility class named DBUtil
with methods like
bool checkExist(String tableName, String columnName, String value);This class will use a EntityManager reference to have access to a database.
The point is that I would like it to be injected the EntityManager corresponding
to the EJB project where it is used. This is,
if DBUtil is used in project EJB-A with EntityManager-A then DBUtil accesses
EntityManager-A, but it accesses EntityManager-B if used in project EJB-B.
What is the best approach?
I tried DBUtil having a constructor receiving an EntityManager ref.
But I failed to instantiate DBUtil from the default constructor of a SessionBean
in a EJB project. (It is like that default constructor is not used).
Perhaps I have to deepen into SessioBean lifecycle.
Any suggestion will be very appreciated.
JordiBM

Answer 1

I respond to myself:
JordiBM wrote:
I want to write a utility class named DBUtil
with methods like
bool checkExist(String tableName, String columnName, String value);This class will use a EntityManager reference to have access to a database.
The point is that I would like it to be injected the EntityManager corresponding
to the EJB project where it is used. This is,
if DBUtil is used in project EJB-A with EntityManager-A then DBUtil accesses
EntityManager-A, but it accesses EntityManager-B if used in project EJB-B.
What is the best approach?
I tried DBUtil having a constructor receiving an EntityManager ref.
But I failed to instantiate DBUtil from the default constructor of a SessionBean
in a EJB project. (It is like that default constructor is not used).
Yes. The constructor of the SessionBean is not the right place to do that.
The right place is to write a method in the SessionBean class
annotated with @PostConstruct
This method is called after the injection has taken place:
@Stateful
public class MySessionBean {
   @PersistencyContext
   private Entitymanager em; // the EM of *this* project
   private DBUtil myUtil;  // the utility class of many projects
  @PostConstruct
  public void my_init () {
    myUtil = new DBUtil (em); // we pass it this project's EM.
  public void aMethod () {
    // use myUtil
    myUtil.doSomething();
}