@Entity
@Table(name="tabs")
public class TabObject implements Serializable {
   @Id
   @Column(name="tab_id")
   @GeneratedValue
   private int id;
   
   @ManyToOne
   @JoinColumn(name="site_id")
   private SiteObject site;
   
   @ManyToOne
   @JoinColumn(name="category_id")
   private CategoryObject category;

   // Set and Get methods...
}

@Entity
@Table(name="categories")
public class CategoryObject {
   @Id
   @Column(name="category_id")
   @GeneratedValue
   private int id;
   
   @Column(nullable = false)
   private String name;

   // Set and Get methods...
}

@Entity
@Table(name="sites")
public class SiteObject {
   @Id
   @Column(name="site_id")
   @GeneratedValue
   private int id;
   
   @Column(nullable = false)
   private String name;

   // Set and Get methods...
}

@Entity
@Table(name="categories")
public class CategoryObject {
   @Id
   @Column(name="category_id")
   @GeneratedValue
   private int id;
   
   private SiteObject site;
   
   @Column(nullable = false)
   private String name ;
   
   public void setId(int id) {
      this.id = id ;
   }
   
   public int getId() {
      return id ;
   }
   
   public void setName(String name) {
      this.name = name ;
   }
   
   public String getName() {
      return name ;
   }
   
   public void setSite(SiteObject site) {
      this.site = site ;
   }
   
   @ManyToMany
   @JoinTable(name="sites", joinColumns={@JoinColumn(name="site_id")})
   public SiteObject getSite() {
      return site ;
   }
}

@Entity
@Table(name="sites")
public class SiteObject {
   @Id
   @Column(name="site_id")
   @GeneratedValue
   private int id;
   
   private CategoryObject category;
   
   @Column(nullable = false)
   private String name ;
   
   @Column(nullable = false)
   private String link ;
   
   @Column(nullable = false)
   private String logo ;
   
   public void setId(int id) {
      this.id = id ;
   }
   
   public int getId() {
      return id ;
   }
   
   public void setName(String name) {
      this.name = name ;
   }
   
   public String getName() {
      return name ;
   }
   
   public void setLink(String link) {
      this.link = link ;
   }
   
   public String getLink() {
      return link ;
   }
   
   public void setLogo(String logo) {
      this.logo = logo ;
   }
   
   public String getLogo() {
      return logo ;
   }
   
   public void setCategory(CategoryObject category) {
      this.category = category ;
   }
   
   @ManyToMany
   @JoinTable(name="categories", joinColumns={@JoinColumn(name="category_id")})
   public CategoryObject getCategory() {
      return category ;
   }
}

org.hibernate.MappingException: Could not determine type for:
SiteObject, for columns: [org.hibernate.mapping.Column(site)]
        at org.hibernate.mapping.SimpleValue.getType(SimpleValue.java:266)
        at org.hibernate.mapping.SimpleValue.isValid(SimpleValue.java:253)
        at org.hibernate.mapping.Property.isValid(Property.java:185)
        at org.hibernate.mapping.PersistentClass.validate(PersistentClass.java:4
40)