Composite IDs in Hibernate

Consider the following database table which contains a list of users and the times they last accessed a certain website:

FirstName	LastName	Website	AccessTime
Arthur	Dent	Google	2012-03-30 07:09:00.0
Peter	Griffin	Yahoo!	2012-03-30 10:36:00.0

Let's say that you want to use both the FirstName and LastName fields to uniquely identify records in the table and perform searches/updates using Hibernate. There are different ways to define a "composite primary key" and, in this post, I will show you an approach I have used successfully in the past, which involves creating an EmbeddedId.

The @EmbeddedId approach:
The code is shown below and is pretty self-explanatory. It consists of a User class which contains a static nested UserId class. The latter holds the first name and the last name attributes and thus represents the composite ID.

import java.io.Serializable;

import javax.persistence.Embeddable;
import javax.persistence.EmbeddedId;
import javax.persistence.Entity;
import javax.persistence.Table;

import org.hibernate.annotations.Type;
import org.joda.time.DateTime;

/**
 * The User entity which contains an embedded UserId.
 */
@Entity
@Table(name = "Users")
public class User {

  @EmbeddedId
  private UserId id;

  private String website;

  @Type(type = "org.joda.time.contrib.hibernate.PersistentDateTime")
  private DateTime accessTime;

  /**
   * Default constructor required by Hibernate.
   */
  public User() {
  }

  /**
   * @param id the user id
   */
  public User(UserId id) {
    this.id = id;
  }

  /**
   * @return the id
   */
  public UserId getId() {
    return id;
  }

  /**
   * @param id the id to set
   */
  public void setId(UserId id) {
    this.id = id;
  }

  /**
   * @return the website
   */
  public String getWebsite() {
    return website;
  }

  /**
   * @param website the website to set
   */
  public void setWebsite(String website) {
    this.website = website;
  }

  /**
   * @return the accessTime
   */
  public DateTime getAccessTime() {
    return accessTime;
  }

  /**
   * @param accessTime the accessTime to set
   */
  public void setAccessTime(DateTime accessTime) {
    this.accessTime = accessTime;
  }

  /**
   * This represents a "composite primary key" for the Users table.
   * It contains all the columns that form a unique id.
   * Must implement equals() and hashcode() and be serializable.
   * https://community.jboss.org/wiki/EqualsAndHashCode
   */
  @Embeddable
  public static class UserId  implements Serializable {

    private static final long serialVersionUID = 1L;

    private String firstName;
    private String lastName;

    /**
     * Default constructor required by Hibernate.
     */
    public UserId() {
    }

    /**
     * @param firstName the first name
     * @param lastName the last name
     */
    public UserId(String firstName, String lastName) {
      this.firstName = firstName;
      this.lastName = lastName;
    }

    /**
     * @return the firstName
     */
    public String getFirstName() {
      return firstName;
    }

    /**
     * @param firstName the firstName to set
     */
    public void setFirstName(String firstName) {
      this.firstName = firstName;
    }

    /**
     * @return the lastName
     */
    public String getLastName() {
      return lastName;
    }

    /**
     * @param lastName the lastName to set
     */
    public void setLastName(String lastName) {
      this.lastName = lastName;
    }

    /** (non-Javadoc)
     * @see java.lang.Object#hashCode()
     */
    @Override
    public int hashCode() {
      final int prime = 31;
      int result = 1;
      result = prime * result + ((firstName == null) ? 0 : firstName.hashCode());
      result = prime * result + ((lastName == null) ? 0 : lastName.hashCode());
      return result;
    }

    /** (non-Javadoc)
     * @see java.lang.Object#equals(java.lang.Object)
     */
    @Override
    public boolean equals(Object obj) {
      if (this == obj)
        return true;
      if (obj == null)
        return false;
      if (getClass() != obj.getClass())
        return false;
      UserId other = (UserId) obj;
      if (firstName == null) {
        if (other.firstName != null)
          return false;
      } else if (!firstName.equals(other.firstName))
        return false;
      if (lastName == null) {
        if (other.lastName != null)
          return false;
      } else if (!lastName.equals(other.lastName))
        return false;
      return true;
    }
  }
}

Usage:
The code snippet below shows how you would search for a user and update its access time:

UserId userId = new UserId("Peter", "Griffin");

// search for a user
User user = entityManager.find(User.class, userId);
if (user == null) {
    // the user doesn't exist, so create one
    user = new User(userId);
}
user.setAccessTime(new DateTime());

// update the user
entityManager.merge(user);