JPA 2.1: Attribute Converter

JPA 2.1: Attribute Converter

If you are using Hibernate, and want a customized type is supported in your Entity class, you could write a custom Hibernate Type.

JPA 2.1 brings a new feature named attribute converter, which can help you convert your custom class type to JPA supported type.

Reuse the Post entity class as example.

Create an Entity

@Entity
@Table(name="POSTS")
public class Post implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="ID")
    private Long id;

    @Column(name="TITLE")
    private String title;

    @Column(name="BODY")
    private String body;

    @Temporal(javax.persistence.TemporalType.DATE)
    @Column(name="CREATED")
    private Date created;

    @Column(name="TAGS")
    private List

 

 
  
 
  
  tags=new ArrayList<>();
}


 

 

Create an attribute converter

A new property tags is added, I want to use store tags list in to one column.

Create an attribute converter.

@Converter
public class ListToStringConveter implements AttributeConverter

 

 
  
 
  
 , String> {

    @Override
    public String convertToDatabaseColumn(List
 
  
 
  
    attribute) { if (attribute == null || attribute.isEmpty()) { return ""; } return StringUtils.join(attribute, ","); } @Override public List 
   
     convertToEntityAttribute(String dbData) { if (dbData == null || dbData.trim().length() == 0) { return new ArrayList 
    
      (); } String[] data = dbData.split(","); return Arrays.asList(data); } } 
     
    
  

 

 

It is easy to understand, the tags property will be converted into a comma based string when store it into database, and tags property will be converted into a List when fetch it from database.

Apply Converter

You can use the autoApply attribute of the Converter to apply the converter to any supported type.

@Converter(autoApply=true)
public class ListToStringConveter implements AttributeConverter

 

 
  
 
  
 , String> {...}


 

 

It is dangerous in a real world project when there are some List you do not want to be converted.

Luckily, you can apply it on the property via @Convert annotation.

@Column(name="TAGS")
@Convert(converter = ListToStringConveter.class)
private List

 

 
  
 
  
  tags=new ArrayList<>();


 

 

You can also place it on class.

@Converts(value={
    @Convert(attributeName="tags", converter = ListToStringConveter.class)
})
public class Post implements Serializable {...}

An extra attributeName must be specified. You can declare several Converters for properties on an Entity class.

Converters also can be applied on:

1. @Embeddable key of a OneToMany Map type property.
2. @Embeded property
3. @ElementCollection property

Summary

This feature is very useful when you want use a JPA supported type to store your custom class, especially, convert an unsupported type to JPA support type, for example, such as Joda Datetime/Java 8 new Date objects are not supported in JPA 2.1 yet, you can use a converter to convert it to java.util.Date type which is supported by JPA.

The sample codes are hosted on my github.com account, check out and play it yourself.

https://github.com/hantsy/ee7-sandbox

When you run the project(jpa-converter) on Glassfish 4.0 and you could get an exception.

java.lang.RuntimeException: unable to create policy context directory.

This is a known issue of Glassfish 4.0, the fix should be included in the next release. I am using an Nightly version to overcome this barrier.