Overview of the order Application

order is a simple inventory and ordering application for maintaining a catalog of parts and placing an itemized order of those parts. It has entities that represent parts, vendors, orders, and line items. These entities are accessed using a stateful session bean that holds the business logic of the application. A simple command-line client adds data to the entities, manipulates the data, and displays data from the catalog.

The information contained in an order can be divided into different elements. What is the order number? What parts are included in the order? What parts make up that part? Who makes the part? What are the specifications for the part? Are there any schematics for the part? order is a simplified version of an ordering system that has all these elements.

order consists of two modules: order-ejb, an enterprise bean JAR file containing the entities, the support classes, and a stateful session bean that accesses the data in the entitiess; and order-app-client, the application client that populates the entities with data and manipulates the data, displaying the results in a terminal.

Entity Relationships in order

order demonstrates several types of entity relationships: one-to-many, many-to-one, one-to-one, unidirectional, and self-referential relationships.

Self-Referential Relationships

A self-referential relationship is a relationship between relationship fields in the same entity. Part has a field bomPart that has a one-to-many relationship with the field parts, which is also in Part. That is, a part can be made up of many parts, and each of those parts has exactly one bill-of-material part.

The primary key for Part is a compound primary key, a combination of the partNumber and revision fields. It is mapped to the PARTNUMBER and REVISION columns in the EJB_ORDER_PART table. 

...
@ManyToOne
@JoinColumns({
  @JoinColumn(name="BOMPARTNUMBER",
    referencedColumnName="PARTNUMBER"),
  @JoinColumn(name="BOMREVISION",
    referencedColumnName="REVISION")
})
public Part getBomPart() {
  return bomPart;
}
...
@OneToMany(mappedBy="bomPart")
public Collection<Part> getParts() {
  return parts;
}
...

One-to-One Relationships

Part has a field, vendorPart, that has a one-to-one relationship with VendorPart's part field. That is, each part has exactly one vendor part, and vice versa.

Here is the relationship mapping in Part: 

@OneToOne(mappedBy="part")
public VendorPart getVendorPart() {
  return vendorPart;
}

Here is the relationship mapping in VendorPart: 

@OneToOne
@JoinColumns({
  @JoinColumn(name="PARTNUMBER",
    referencedColumnName="PARTNUMBER"),
  @JoinColumn(name="PARTREVISION",
    referencedColumnName="REVISION")
})
public Part getPart() {
  return part;
}

Note that, because Part uses a compound primary key, the @JoinColumns annotation is used to map the columns in the EJB_ORDER_VENDOR_PART table to the columns in EJB_ORDER_PART. EJB_ORDER_VENDOR_PART's PARTREVISION column refers to EJB_ORDER_PART's REVISION column.

One-to-Many Relationship Mapped to Overlapping Primary and Foreign Keys

Order has a field, lineItems, that has a one-to-many relationship with LineItem's field order. That is, each order has one or more line item.

LineItem uses a compound primary key that is made up of the orderId and itemId fields. This compound primary key maps to the ORDERID and ITEMID columns in the EJB_ORDER_LINEITEM database table. ORDERID is a foreign key to the ORDERID column in the EJB_ORDER_ORDER table. This means that the ORDERID column is mapped twice: once as a primary key field, orderId; and again as a relationship field, order.

Here's the relationship mapping in Order: 

@OneToMany(cascade=ALL, mappedBy="order")
  public Collection<LineItem> getLineItems() {
  return lineItems;
}

Here is the relationship mapping in LineItem: 

@ManyToOne
  public Order getOrder() {
  return order;
}

Unidirectional Relationships

LineItem has a field, vendorPart, that has a unidirectional many-to-one relationship with VendorPart. That is, there is no field in the target entity in this relationship. 

@ManyToOne
  public VendorPart getVendorPart() {
  return vendorPart;
}

Primary Keys in order

order uses several types of primary keys: single-valued primary keys, compound primary keys, and generated primary keys.

Generated Primary Keys

VendorPart uses a generated primary key value. That is, the application does not assign primary key values for the entities, but instead relies on the persistence provider to generate the primary key values. The @GeneratedValue annotation is used to specify that an entity will use a generated primary key.

In VendorPart, the following code specifies the settings for generating primary key values: 

@TableGenerator(
  name="vendorPartGen",
  table="EJB_ORDER_SEQUENCE_GENERATOR",
  pkColumnName="GEN_KEY",
  valueColumnName="GEN_VALUE",
  pkColumnValue="VENDOR_PART_ID",
  allocationSize=10)
@Id
@GeneratedValue(strategy=GenerationType.TABLE,
  generator="vendorPartGen")
public Long getVendorPartNumber() {
  return vendorPartNumber;
}

The @TableGenerator annotation is used in conjunction with @GeneratedValue's strategy=TABLE element. That is, the strategy used to generate the primary keys is use a table in the database. @TableGenerator is used to configure the settings for the generator table. The name element sets the name of the generator, which is vendorPartGen in VendorPart. The EJB_ORDER_SEQUENCE_GENERATOR table, which has two columns GEN_KEY and GEN_VALUE, will store the generated primary key values. This table could be used to generate other entity's primary keys, so the pkColumnValue element is set to VENDOR_PART_ID to distinguish this entity's generated primary keys from other entity's generated primary keys. The allocationSize element specifies the amount to increment when allocating primary key values In this case, each VendorPart's primary key will increment by 10.

The primary key field vendorPartNumber is of type Long, as the generated primary key's field must be an integral type.

Compound Primary Keys

A compound primary key is made up of multiple fields and follows the requirements described in Primary Key Classes (page 775). To use a compound primary key, you must create a wrapper class.

In order, two entities use compound primary keys: Part and LineItem.

Part uses the PartKey wrapper class. Part's primary key is a combination of the part number and the revision number. PartKey encapsulates this primary key.

LineItem uses the LineItemKey class. LineItem's primary key is a combination of the order number and the item number. LineItemKey encapsulates this primary key. This is the LineItemKey compound primary key wrapper class: 

package order.entity;

public final class LineItemKey implements 
      java.io.Serializable {

  private Integer orderId;
  private int itemId;

  public int hashCode() {
    return ((this.getOrderId()==null
            ?0:this.getOrderId().hashCode()) 
        ^ ((int) this.getItemId()));
  }

  public boolean equals(Object otherOb) {
    if (this == otherOb) {
      return true;
    }
    if (!(otherOb instanceof LineItemKey)) {
      return false;
    }
    LineItemKey other = (LineItemKey) otherOb;
    return ((this.getOrderId()==null
            ?other.orderId==null:this.getOrderId().equals
        (other.orderId)) && (this.getItemId ==
          other.itemId));
  }

  public String toString() {
    return "" + orderId + "-" + itemId;
  }
}

The @IdClass annotation is used to specify the primary key class in the entity class. In LineItem, @IdClass is used as follows: 

@IdClass(order.entity.LineItemKey.class)
@Entity
...
public class LineItem {
...
}

The two fields in LineItem are tagged with the @Id annotation to mark those fields as part of the compound primary key: 

@Id
public int getItemId() {
  return itemId;
}
...
@Id
@Column(name="ORDERID", nullable=false,
  insertable=false, updatable=false)
public Integer getOrderId() {
  return orderId;
}

For orderId, we also use the @Column annotation to specify the column name in the table, and that this column should not be inserted or updated, as it is an overlapping foreign key pointing at the EJB_ORDER_ORDER table's ORDERID column (see One-to-Many Relationship Mapped to Overlapping Primary and Foreign Keys). That is, orderId will be set by the Order entity.

In LineItem's constructor, the line item number (LineItem.itemId) is set using the Order.getNextId method. 

public LineItem(Order order, int quantity, VendorPart
    vendorPart) {
  this.order = order;
  this.itemId = order.getNextId();
  this.orderId = order.getOrderId();
  this.quantity = quantity;
  this.vendorPart = vendorPart;
}

Order.getNextId counts the number of current line items, adds one, and returns that number. 

public int getNextId() {
  return this.lineItems.size() + 1;
}

Part doesn't require the @Column annotation on the two fields that comprise Part's compound primary key. This is because Part's compound primary key is not an overlapping primary key/foreign key. 

@IdClass(order.entity.PartKey.class)
@Entity
...
public class Part {
...
  @Id
  public String getPartNumber() {
    return partNumber;
  }
...
  @Id
  public int getRevision() {
    return revision;
  }
...
}

Entity Mapped to More Than One Database Table

Part's fields map to more than one database table: EJB_ORDER_PART and EJB_ORDER_PART_DETAIL. The EJB_ORDER_PART_DETAIL table holds the specification and schematics for the part. The @SecondaryTable is used to specify the secondary table. 

...
@Entity
@Table(name="EJB_ORDER_PART")
@SecondaryTable(name="EJB_ORDER_PART_DETAIL", pkJoinColumns={
  @PrimaryKeyJoinColumn(name="PARTNUMBER",
    referencedColumnName="PARTNUMBER"),
  @PrimaryKeyJoinColumn(name="REVISION",
    referencedColumnName="REVISION")
})
public class Part {
...
}

EJB_ORDER_PART_DETAIL shares the same primary key values as EJB_ORDER_PART. The pkJoinColumns element of @SecondaryTable is used to specify that EJB_ORDER_PART_DETAIL's primary key columns are foreign keys to EJB_ORDER_PART. The @PrimaryKeyJoinColumn sets the primary key column names and specifies which column in the primary table the column refers to. In this case, the primary key column names for both EJB_ORDER_PART_DETAIL and EJB_ORDER_PART are the same: PARTNUMBER and REVISION, respectively.

Cascade Operations in order

Entities that have relationships to other entities often have dependencies on the existence of the other entity in the relationship. For example, a line item is part of an order, and if the order is deleted, then the line item should also be deleted. This is called a cascade delete relationship.

In order, there are two cascade delete dependencies in the entity relationships. If the Order to which a LineItem is related is deleted, then the LineItem should also be deleted. If the Vendor to which a VendorPart is related is deleted, then the VendorPart should also be deleted.

You specify the cascade operations for entity relationships by setting the cascade element in the inverse (non-owning) side of the relationship. The cascade element is set to ALL in the case of Order.lineItems. This means that all persistence operations (deletes, updates, and so on) are cascaded from orders to line items.

Here is the relationship mapping in Order: 

@OneToMany(cascade=ALL, mappedBy="order")
public Collection<LineItem> getLineItems() {
  return lineItems;
}

Here is the relationship mapping in LineItem: 

@ManyToOne
  public Order getOrder() {
  return order;
}

BLOB and CLOB Database Types in order

The PARTDETAIL table in the database has a column, DRAWING, of type BLOB. BLOB stands for binary large objects, which are used for storing binary data such as an image. The DRAWING column is mapped to the field Part. drawing of type java.io.Serializable. The @Lob annotation is used to denote that the field is large object. 

@Column(table="EJB_ORDER_PART_DETAIL")
@Lob
public Serializable getDrawing() {
  return drawing;
}

PARTDETAIL also has a column, SPECIFICATION, of type CLOB. CLOB stands for character large objects, which are used to store string data too large to be stored in a VARCHAR column. SPECIFICATION is mapped to the field Part.specification of type java.lang.String. The @Lob annotation is also used here to denote that the field is a large object. 

@Column(table="EJB_ORDER_PART_DETAIL")
@Lob
public String getSpecification() {
  return specification;
}

Both of these fields use the @Column annotation and set the table element to the secondary table.

Temporal Types in order

The Order.lastUpdate persistent property, which is of type java.util.Date, is mapped to the EJB_ORDER_ORDER.LASTUPDATE database field, which is of the SQL type TIMESTAMP. To ensure the proper mapping between these types, you must use the @Temporal annotation with the proper temporal type specified in @Temporal's element. @Temporal's elements are of type javax.persistence.TemporalType. The possible values are:

Here is the relevant section of Order: 

@Temporal(TIMESTAMP)
public Date getLastUpdate() {
  return lastUpdate;
}

Managing order's Entities

The RequestBean stateful session bean contains the business logic and manages the entities of order.

RequestBean uses the @PersistenceContext annotation to retrieve an entity manager instance which is used to manage order's entities in RequestBean's business methods. 

@PersistenceContext
private EntityManager em;

This EntityManager instance is a container-managed entity manager, so the container takes care of all the transactions involved in the managing order's entities.

Creating Entities

The RequestBean.createPart business method creates a new Part entity. The EntityManager.persist method is used to persist the newly created entity to the database. 

Part part = new Part(partNumber,
  revision,
  description,
  revisionDate,
  specification,
  drawing);
em.persist(part);

Finding Entities

The RequestBean.getOrderPrice business method returns the price of a given order, based on the orderId. The EntityManager.find method is used to retrieve the entity from the database. 

Order order = em.find(Order.class, orderId);

The first argument of EntityManager.find is the entity class, and the second is the primary key.

Setting Entity Relationships

The RequestBean.createVendorPart business method creates a VendorPart associated with a particular Vendor. The EntityManager.persist method is used to persist the newly created VendorPart entity to the database, and the VendorPart.setVendor and Vendor.setVendorPart methods are used to associate the VendorPart with the Vendor. 

PartKey pkey = new PartKey();
pkey.partNumber = partNumber;
pkey.revision = revision;

Part part = em.find(Part.class, pkey);
VendorPart vendorPart = new VendorPart(description, price,
  part);
em.persist(vendorPart);

Vendor vendor = em.find(Vendor.class, vendorId);
vendor.addVendorPart(vendorPart);
vendorPart.setVendor(vendor);

Using Queries

The RequestBean.adjustOrderDiscount business method updates the discount applied to all orders. It uses the findAllOrders named query, defined in Order: 

@NamedQuery(
  name="findAllOrders",
  query="SELECT o FROM Order o"
)

The EntityManager.createNamedQuery method is used to run the query. Because the query returns a List of all the orders, the Query.getResultList method is used. 

List orders = em.createNamedQuery(
  "findAllOrders")
  .getResultList();

The RequestBean.getTotalPricePerVendor business method returns the total price of all the parts for a particular vendor. It uses a named parameter, id, defined in the named query findTotalVendorPartPricePerVendor defined in VendorPart. 

@NamedQuery(
  name="findTotalVendorPartPricePerVendor",
  query="SELECT SUM(vp.price) " +
  "FROM VendorPart vp " +
  "WHERE vp.vendor.vendorId = :id"
)

When running the query, the Query.setParameter method is used to set the named parameter id to the value of vendorId, the parameter to RequestBean.getTotalPricePerVendor. 

return (Double) em.createNamedQuery(
  "findTotalVendorPartPricePerVendor")
  .setParameter("id", vendorId)
  .getSingleResult();

The Query.getSingleResult method is used for this query because the query returns a single value.

Removing Entities

The RequestBean.removeOrder business method deletes a given order from the database. It uses the EntityManager.remove method to delete the entity from the database. 

Order order = em.find(Order.class, orderId);
em.remove(order);