1. JPA: Mapping to an existing database
IS-907 Java EE
JPA: Mapping to an existing database

2. Mapping entities to existing databases
We have seen how JPA automatically maps classes to tables
To map a class model to an existing database more detailed control over the mapping may be needed
package javax.persistence, Annotation Types
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3. Table names
Tablename, schema and catalog can optionally be specified with annotation:
@Entity
@Table(name=“tablename””, schema=“schema”, catalog=“catalog”)
public class Employee {
... }
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4. Columns Column properties are specified with column annotation:
@Entity
public class Employee {
@Id private long empId;
@Column(name=“login”, unique=“true”, nullable=“false”)
private String username;
... }
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5. Large objects
annotation is used to map fields to CLOBs or BLOBs
byte[], Byte[], and Serializable classes are mapped to BLOB
char[], Character[], and String are mapped to CLOB
@Entity
public class Employee {
@Lob byte[] speech;
@Lob Picture picture;
@Lob String biography; }
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6. @Basic annotation
Fetching of field values can be deferred until the field is actually needed, to speed up loading of data:
@Entity
public class Employee {
@Basic(fetch=“FetchType.LAZY”)
@Lob
private byte[] picture;
... }
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7. Temporal types
The mapping of Date and Calendar fields can be controlled annotations:
@Entity public class Employee {
@Temporal(TemporalType.DATE)
private Calendar birthDate;
@Temporal(TemporalType.TIMESTAMP)
private Timestamp lastLogin; }
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8. Foreign keys
The name of foreign key columns can be specified with a JoinColumn annotation:
@Entity public class Employee {
@ManyToOne
@JoinColumn(name=“dep”, nullable=“false”, unique=“false”)
private Department dep;
... }
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9. Join tables @Entity public class Project { @ManyToMany
@JoinTable(name=“proj_emp”
private List<Employee> members;
... }
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10. Ordered list @Entity public class Project { @ManyToMany
@OrderBy(“name ASC”)
private List<Employee> members;
... }
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11. Using a Map instead of a List
@Entity public class private Map<String,Employee> members; ... }
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12. Lazy relationships
Loading of related objects may be deferred by specifying fetch type:
@Entity
public class Employee {
@ManyToOne(fetch=“FetchType.LAZY”)
private Department department;
... }
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13. Cascading @Entity public class Order {
@OneToMany(cascade={CascadeType.PERSIST, CascadType.MERGE})
List<OrderLine> lines; }
ALL
DETACH
MERGE
PERSIST
REFRESH
REMOVE
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