What can be converted?
A Converter supports type conversion of all basic attributes defined by entity classes, mapped superclasses, or embeddable classes. The only exceptions are Id attributes, version attributes, relationship attributes and attributes annotated as Temporal or Enumerated.
How to implement a Converter?
A Converter must implement the javax.persistence.AttributeConverter<X, Y> interface, where X is the class of the entity representation and Y the class of the database representation of the attribute. Additionally a Converter has to be annotated with the javax.persistence.Converter annotation.
The following example shows a Converter implementation that can be used to store a java.awt.Color object as a String with the format red|green|blue|alpha in the database. When reading the entity from the database, the String will be used to instantiate a new Color object.
As you can see the implementation is simple and straight forward. You just need to implement one method for each conversion.
How to use a Converter?
There are two options to define the usage of a Converter. The first one is to set autoapply=true at the @Converter annotation of the Converter class. In this case the JPA provider will use this Converter to convert all entity attributes of the given type.
If autoapply is set to false, you need to add the javax.persistence.Convert annotation to all attributes that shall be converted and specify the Converter class. The following code snippet shows an example for this approach:
That is all that needs to be done to implement a simple Attribute Converter. Don’t forget to grab your free Cheat Sheet about Attribute Converter and all the other features introduced with JPA 2.1.
If you want to try it yourself, you can use Hibernate 4.3.0.Beta4 which is part of the new Wildfly 8.0.0.Beta1 release.
Looking for more information? Read JPA 2.1 – 12 features every developer should know to get an overview about the features introduced with JPA 2.1 or have a look at the other examples on how to use an Attribute Converter:
- JPA 2.1 Attribute Converter – The better way to persist enums
- How to use a JPA Attribute Converter to encrypt your data
- How to persist LocalDate and LocalDateTime with JPA
- Testing with Aliens: How to test a JPA Attribute Converter with Arquillian
Please leave a comment with your idea on how to use an Attribute Converter.
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