The Java EE 6 Tutorial, Volume I

Managing the order Application’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.

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,

The ConfigBean singleton session bean is used to initialize the data in order. ConfigBean is annotated with @Startup, which indicates that the EJB container should create ConfigBean when order is deployed. The createData method is annotated with @PostConstruct, and it creates the initial entities used by order by calling RequestsBean's business methods.

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,

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

Using Queries

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

    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(

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.

    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(
    .setParameter("id", vendorId)

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);