Why is my EntityManagerHelper class returning previous query results and how do I fix it?

Question

This question What is the right way to use entitymanager supplied a link to EntityManagerHelper.java which I added to my code base. When I use this helper class to make subsequent calls to the database it returns previous results to the same query.

The scenario I see this most in is retrieving the lastentry property of my User Class . On the browser I make an AJAX request and the servlet partial below gets the user object and calls a method to return my lastentry .

I've read about .clear() but I get server errors when I added it to my EntityManagerHelper. I would like to avoid creating an EntityManager every time I want to make a call to the db.

How can I fix this problem?

partial from servlet

User user = User.getUser();

response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(user.getLastentry());

User Class

package entities;

import java.io.Serializable;
import java.util.Date;
import javax.persistence.Basic;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.NamedQueries;
import javax.persistence.NamedQuery;
import javax.persistence.Persistence;
import javax.persistence.Table;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;
import javax.xml.bind.annotation.XmlRootElement;
import org.apache.shiro.SecurityUtils;
import responseablees.EntityManagerHelper;

/**
 *
 * @author Christopher Loughnane <chrisloughnane1@gmail.com>
 */
@Entity
@Table(name = "user")
@XmlRootElement
@NamedQueries({
    @NamedQuery(name = "User.findAll", query = "SELECT u FROM User u")
    , @NamedQuery(name = "User.findById", query = "SELECT u FROM User u WHERE u.id = :id")
    , @NamedQuery(name = "User.findByUsername", query = "SELECT u FROM User u WHERE u.username = :username")
    , @NamedQuery(name = "User.getUsernameByUserEmail", query = "SELECT u.username FROM User u WHERE u.useremail = :useremail")
    , @NamedQuery(name = "User.findByUserEmail", query = "SELECT u FROM User u WHERE u.useremail = :useremail")
    , @NamedQuery(name = "User.findByPassword", query = "SELECT u FROM User u WHERE u.password = :password")})
public class User implements Serializable {

    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 2048)
    @Column(name = "lastentry")
    private String lastentry;

    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 128)
    @Column(name = "useremail")
    private String useremail;
    @Basic(optional = false)
    @Column(name="created", insertable = false, updatable = false)
    @Temporal(TemporalType.TIMESTAMP)
    private Date created;

    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id")
    private Integer id;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 100)
    @Column(name = "username")
    private String username;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 100)
    @Column(name = "password")
    private String password;

    public User() {
    }

    public User(Integer id) {
        this.id = id;
    }

    public User(Integer id, String username, String password) {
        this.id = id;
        this.username = username;
        this.password = password;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (id != null ? id.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof User)) {
            return false;
        }
        User other = (User) object;
        if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "DAOs.User[ id=" + id + " ]";
    }

    public String getUseremail() {
        return useremail;
    }

    public void setUseremail(String useremail) {
        this.useremail = useremail;
    }

    public Date getCreated() {
        return created;
    }

    public void setCreated(Date created) {
        this.created = created;
    }

    public String getLastentry() {
        return lastentry;
    }

    public void setLastentry(String lastentry) {
        this.lastentry = lastentry;
    }

    public static User getUser(){

        String currentUser = (String) SecurityUtils.getSubject().getPrincipal();
        User user = (User) em.createNamedQuery("User.findByUserEmail")
                .setParameter("useremail", currentUser)
                .getSingleResult();
        return user;
    }
}

Requested Code

Verbose Approach

    String currentUser = (String) SecurityUtils.getSubject().getPrincipal();

    EntityManagerFactory emfactory = Persistence.createEntityManagerFactory("com.mycompany_responseableES_war_1.0-SNAPSHOTPU");
    EntityManager em = emfactory.createEntityManager();

    User user = (User) em.createNamedQuery("User.findByUserEmail")
            .setParameter("useremail", currentUser)
            .getSingleResult();
    em.getTransaction().begin();
    user.setLastentry(JSON);
    em.getTransaction().commit();

EntityManagerHelper Approach

    User user = User.getUser();
    EntityManager em = EntityManagerHelper.getEntityManager();
    em.getTransaction().begin();
    user.setLastentry(JSON);
    em.getTransaction().commit();

Answer 1

First of all, I would be careful to put ALL my data fetching/modification code between

em.getTransaction().begin()

and

em.getTransaction().commit()

This ensures that your entities are managed and your changes are automatically persisted . In your code it looks like the User instance comes from outside the transaction. I have not worked much with JPA and manual transactions, but I would not be surprised if the user is detached (that is, not managed by the EntityManager ) and so changes are not automatically persisted to the database. If that were the case I guess you could solve it by putting

// merge(...) takes an entity that is not managed, apply changes to
// the persistence context and returns a new *managed* entity.
// The original entity REMAINS *detached* or *non persistent*
User managedUser = em.merge(user);

// I assume that you have something like this to set your user in the
// current session or whatever
User.setUser(managedUser);

just before the commit.

JPA vocabulary can be a bit hard at first, read this for more details about the JPA entity lifecycle (specially the diagram at the bottom).