Storing a Map<String,String> using JPA

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

Answer 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.