Question

I have a very basic question about best practice of using try/catch.

I have a simple function (DAO) like this:

public void addVehicle(Vehicle vehicle) {

    em.getTransaction().begin();
    em.persist(vehicle);
    em.getTransaction().commit();
}

and using DAO function inside web service:

@WebMethod(operationName = "addVehicle")
public void addVehicle(Vehicle vehicle) {

    try {
        vehicleDAO.addVehicle(vehicle);
        System.out.print("Vehicle added");
    } catch (Exception e) {
        e.printStackTrace();
    }
}

OR is better using try/catch inside DAO function like this:

public void addVehicle(Vehicle vehicle) {

    try {
        em.getTransaction().begin();
        em.persist(vehicle);
        em.getTransaction().commit();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Answer 1

There is no perfect rule for that.

Often code is clearer and less complex if exceptions are catched as early as needed, but as late as possible.
You should think who has to take an action when that Exception happens, this decides if you catch it inside the method (addVehicle) or if you throw it such that the caller has to catch it.

E.g:

 public void addVehicle(Vehicle vehicle) throws SQLException{
        em.getTransaction().begin();
        em.persist(vehicle);
        em.getTransaction().commit();
 }

In this example the caller has to catch.
Further only in few situations you should catch Exception or RunTimeException, better
catch that specific Exception, like IOException instead of Exception.

Somewhere in your code you will need a “last line of defense” where it make sense to catch (Exception ex). This is needed to handle errors that should not happen.