Question

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));

Answer 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

}

See also (in the JPA 2.0 specification)

• 2.6 – Collections of Embeddable Classes and Basic Types
• 2.7 Map Collections
• 10.1.11 – ElementCollection Annotation
• 11.1.29 MapKeyColumn Annotation