Managing entities - EJB 3 in Action - page 401

As mentioned earlier, all persistence operations that require database updates must be in-
voked within the scope of a transaction. If an enclosing transaction isn't present when the
persist method is invoked, a TransactionRequiredException is thrown for a
transaction-scoped entity manager. The same is true for the EntityManager' s flush ,
merge , refresh , and remove methods.
10.2.2. Retrieving entities by key
JPA supports several ways to retrieve entity instances from the database. By far the simplest
way is retrieving an entity by its primary key using the find method we introduced in list-
ing 10.1 . The other ways all involve using the query API and JPQL, which we'll discuss in
chapter 11 . Recall that the find method was used in the addItem method in listing 10.1
to retrieve the Seller instance corresponding to the Item to add:
Seller seller = entityManager.find(Seller.class, sellerId);
In this line of code, the first parameter of the find method specifies the Java type of the
entity to be retrieved. The second parameter specifies the identity value for the entity in-
stance to retrieve. Recall from chapter 9 that an entity identity can either be a simple Java
type identified by the @Id annotation or a composite primary key class specified through
the @EmbeddedId or @IdClass annotation. In the example in listing 10.1 , the find
method is passed a simple java.lang.Long value matching the Seller entity's @Id
annotated identity, sellerId .
Although this isn't the case in listing 10.1 , the find method is fully capable of supporting
composite primary keys. To see how this code might look, assume for the sake of illus-
tration that the identity of the Seller entity consists of the seller's first and last names
instead of a simple numeric identifier. This identity is encapsulated in a composite primary
key class annotated with the @IdClass annotation. The following listing shows how this
identity class can be populated and passed to the find method.
Listing 10.4. Find by primary key using composite keys
SellerPK sellerKey = new SellerPK();
sellerKey.setFirstName(firstName);
sellerKey.setLastName(lastName);
Seller seller = entityManager.find(Seller.class, sellerKey);