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Mapping entities
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8.2.4 Mapping CLOBs and BLOBs
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An extremely powerful feature of relational databases is the ability to store very large data as binary large object (BLOB) and character large object (CLOB) types. These correspond to the JDBC java.sql.Blob and java.sql.Clob objects. The @Lob annotation designates a property of field as a CLOB or BLOB. For example, we designate the picture field as a BLOB in listing 8.4:
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@Lob @Basic(fetch=FetchType.LAZY) protected byte[] picture;
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Whether a field or property designated @Lob is a CLOB or a BLOB is determined by its type. If the data is of type char[] or String, the persistence provider maps the data to a CLOB column. Otherwise, the column is mapped as a BLOB. An extremely useful annotation to use in conjunction with @Lob is @Basic. @Basic can be marked on any attribute with direct-to-field mapping. Just as we have done for the picture field, the @Basic(fetch=FetchType.LAZY) specification causes the BLOB or CLOB data to be loaded from the database only when it is first accessed. Postponing of loading of entity data from the database is known as lazy loading. (You will learn more about lazy loading in chapter 9.) This is a great feature since LOB data is usually very memory intensive and should only be loaded if needed. Unfortunately, lazy loading of LOB types is left as optional for vendors by the EJB 3 specification and there is no guarantee that the column will actually be lazily loaded.
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8.2.5 Mapping temporal types
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Most databases support a few different temporal data types with different granularity levels corresponding to DATE (storing day, month, and year), TIME (storing just time and not day, month, or year) and TIMESTAMP (storing time, day, month, and year). The @Temporal annotation specifies which of these data types we want to map a java.util.Date or java.util.Calendar persistent data type to. In listing 8.3, we save the creationDate field into the database as a DATE:
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@Temporal(TemporalType.DATE) protected Date creationDate;
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Note this explicit mapping is redundant while using the java.sql.Date, java. sql.Time or java.sql.Timestamp Java types. If we do not specify a parameter for @Temporal annotation or omit it altogether, the persistence provider will assume the data type mapping to be TIMESTAMP (the smallest possible data granularity).
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Object-relational mapping
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8.2.6 Mapping an entity to multiple tables
This is not often the case for nonlegacy databases, but sometimes an entity s data must come from two different tables. In fact, in some rare situations, this is a very sensible strategy. For example, the User entity in listing 8.4 is stored across the USERS and USER_PICTURES tables, as shown in figure 8.2. This makes excellent sense because the USER_PICTURES table stores large binary images that could significantly slow down queries using the table. However, this approach is rarely used. Isolating the binary images into a separate table in conjunction with the lazy loading technique discussed in section 8.2.4 to deal with the picture field mapped to the USER_PICTURE table can result in a significant boost in application performance. The @SecondaryTable annotation enables us to derive entity data from more than one table and is defined as follows:
Figure 8.2 An entity can be mapped to more than one table; for example, the User entity spans more than one table: USERS and USER_PICTURES. The primary table is mapped using @Table and the secondary table is mapped using @SecondaryTable. The primary and secondary tables must have the same primary key.
Mapping entities
@Target({TYPE}) @Retention(RUNTIME) public @interface SecondaryTable { String name(); String catalog() default ""; String schema() default ""; PrimaryKeyJoinColumn[] pkJoinColumns() default {}; UniqueConstraint[] uniqueConstraints() default {}; }
Notice that other than the pkJoinColumns element, the definition of the annotation is identical to the definition of the @Table annotation. This element is the key to how annotation works. To see what we mean, examine the following code implementing the User entity mapped to two tables:
@Entity @Table(name="USERS") @SecondaryTable(name="USER_PICTURES", pkJoinColumns=@PrimaryKeyJoinColumn(name="USER_ID")) public class User implements Serializable { ..}
Obviously, the two tables in @Table and @SecondaryTable are related somehow and are joined to create the entity. This kind of relationship is implemented by creating a foreign key in the secondary table referencing the primary key in the first table. In this case, the foreign key also happens to be the primary key of the secondary table. To be precise, USER_ID is the primary key of the USER_PICTURES table and it references the primary key of the USERS table. The pkJoinColumns=@PrimaryKeyJoinColumn(name="USER_ID") specification assumes this relationship. The name element points to the USER_ID foreign key in the USER_PICTURES secondary table. The persistence provider is left to figure out what the primary key of the USERS table is, which also happens to be named USER_ID. The provider performs a join using the detected primary key in order to fetch the data for the User entity. In the extremely unlikely case that an entity consists of columns from more than two tables, we may use the @SecondaryTables annotation more than once for the same entity. We won t cover this case here, but encourage you to explore it if needed. Before we conclude the section on mapping entities, let s discuss a vital feature of JPA: primary key generation.
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