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
Create an Entity
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.
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
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
You can also place it on class.
An extra attributeName must be specified. You can declare several Converters for properties on an Entity class.
Converters also can be applied on: @Embeddable key of a OneToMany Map type property. @Embeded property @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.
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.
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:
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.