Java EE: Generic Data Access

Here’s a simple yet practical generic approach for accessing and storing data in your enterprise model.

First we define the conceptual abstraction of a specific service which access the data in our persistence layer:

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

public class DataAccessService<T extends AbstractEntity> {
    protected EntityManager em;
    private final Class<T> type;

    protected DataAccessService(Class<T> type) {
        this.type = type;

We keep the type generic since we do want to implement the specifics in separate services, for example:

import javax.ejb.Stateless;

public class SpecificService extends DataAccessService<SpecificEntity> {

    public SpecificService() {

To have generics in our abstract service, we need each specific entity to be a concrete implementation of an abstract entity that bundles common attributes and methods of all entities, for example:

import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MappedSuperclass;

public abstract class AbstractEntity implements Serializable, Comparable<AbstractEntity> {
    private static final long serialVersionUID = 1L;

    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    // hashCode
    // equals
    // toString
    // getId
    // setId

With the a specific entity implementation example:

import javax.persistence.Entity;

public class SpecificEntity extends AbstractEntity {
    // Fields and methods

Now that we are set up, we can implement a more abstract approach to CRUD in our DataAccessService.


public T save(T t) {
    if (t.getId() == null || t.getId() == 0) {
    else {
    return t;


public T find(Object id) {
    return this.em.find(this.type, id);

public List<T> findWithNamedQuery(String namedQuery) {
    return this.em.createNamedQuery(namedQuery, this.type).getResultList();

public List<T> findWithNamedQuery(String namedQuery, HashMap<String, Object> params) {
    TypedQuery query = em.createNamedQuery(namedQuery, this.type);

    for (Map.Entry e : params.entrySet()) {
        query.setParameter((String) e.getKey(), e.getValue());
    return query.getResultList();


public void remove(T t) {
    if (em.contains(t)) {
    else {


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