@Entity(access = AccessType.FIELD)
public class TestMap {
    @Column(name = "_id")
    @Id(generate = GeneratorType.AUTO)
    public long id;

    @OneToMany(targetEntity = TestEntry.class)
    @JoinTable(table = @Table(name = "TestMap"), joinColumns = {@JoinColumn(name = "_testId")})
    @MapKey(name = "key")
    public Map map;
}

@Embeddable
public class TestEntry {
    public String key;
    public String value;
}