Storing a Map using JPA

问题

I am wondering if it is possible using annotations to persist the attributes map in the following class using JPA2

public class Example {
    long id;
    // ....
    Map<String, String> attributes = new HashMap<String, String>();
    // ....
}

As we already have a pre existing production database, so ideally the values of attributes could map to the following existing table:

create table example_attributes {
    example_id bigint,
    name varchar(100),
    value varchar(100));

回答1:

JPA 2.0 supports collections of primitives through the @ElementCollection annotation that you can use in conjunction with the support of java.util.Map collections. Something like this should work:

@Entity
public class Example {
    @Id long id;
    // ....
    @ElementCollection
    @MapKeyColumn(name="name")
    @Column(name="value")
    @CollectionTable(name="example_attributes", joinColumns=@JoinColumn(name="example_id"))
    Map<String, String> attributes = new HashMap<String, String>(); // maps from attribute name to value

}

回答2:

  @ElementCollection(fetch = FetchType.LAZY)
  @CollectionTable(name = "raw_events_custom", joinColumns = @JoinColumn(name =     "raw_event_id"))
  @MapKeyColumn(name = "field_key", length = 50)
  @Column(name = "field_val", length = 100)
  @BatchSize(size = 20)
  private Map<String, String> customValues = new HashMap<String, String>();

This is an example on how to set up a map with control over column and table names and field length.

来源：https://stackoverflow.com/questions/3393649/storing-a-mapstring-string-using-jpa

标签

java