public class Customer implements Serializable {
      private Collection<Merchant> preferences;


   @ManyToMany(targetEntity = Merchant.class, cascade = { CascadeType.PERSIST,
         CascadeType.MERGE, CascadeType.ALL }, fetch = FetchType.EAGER)
   @JoinTable(name = "CUSTOMER_MERCHANT", joinColumns = @JoinColumn(name = "CUSTOMER_ID"), inverseJoinColumns = @JoinColumn(name = "MERCHANT_ID"))
   public Collection<Merchant> getPreferences() {
      return preferences;
   }

   public void setPreferences(Collection<Merchant> preferences) {
      this.preferences = preferences;
   }
}

public class Merchant implements Serializable {
private Collection<Customer> cutomers;

@ManyToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE,
         CascadeType.ALL }, fetch = FetchType.EAGER, mappedBy = "preferences", targetEntity = Customer.class)
   public Collection<Customer> getCutomers() {
      return cutomers;
   }

   public void setCutomers(Collection<Customer> cutomers) {
      this.cutomers = cutomers;
   }
}