How to implement a JPA Attribute Converter

JPA 2.1 brings several improvements. One of them is the Attribute Converter. It allows the developer to specify methods to convert between the database and the Java representation of an attribute.

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 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:

Please leave a comment with your idea on how to use an Attribute Converter.
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Links

JPA 2.1 Specification
Java Doc javax.persistence

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Comments

  1. says

    Hi

    Thanks for the article.

    I am using JBoss AS 7.1. I set the “” in persistence.xml. But it looks like the converer is not getting called in my case. Could you please help for the same. If the request comes from tomcat, then the converter is being called but not if the request is from jboss.

    Your help would be highly appreciable

    Thanks

  2. Anonymous says

    Is there also a way to use the Converter implicitly with a javax.persistence.Query? When I add the Object as a query parameter then I get an Exception.

  3. Anonymous says

    Hi Thorben and others,

    I have a project where I need to create many different (70+) enums and hence repeatedly define 70+ AttributeConverters. Even though using Java8, there is no neat way of doing things at compile time.

    Anyone knows if we could define those converters at runtime?

    Karyn

  4. Raghu says

    I followed the same steps as above..
    But my Converter never being called.
    I am using hibernate-jpa-2.1-api-1.0.0.Final.jar
    on Jboss 7.1.1 server…

    PLs help me

  5. Raghu says

    Its not working even after changing hibernate-jpa 2.0 module of Jboss to hibernate-jpa-2.1-api-1.0.0.Final.jar… any idea? why its not being called?

    • Thorben Janssen says

      Hi Raghu,
      you need to update the used Hibernate module to a version >4.3.0 or switch to Wildfly 8. The hibernate-jpa-2.1-api-1.0.0.Final.jar provides only the JPA 2.1 APIs but not the implementation.
      Regards,
      Thorben

  6. Stefan Bley says

    Is there any reason why ID attributes can not be converted?
    Hibernate User Types can be used with IDs, though.

    • Thorben Janssen says

      Hi Stefan,

      that’s the way it’s defined in the JPA spec. To be honest, I don’t know why they excluded IDs. One reason might be, that the conversion has to be biuniquely and that there is no way the spec or persistence provider can assure that.

      Regards,
      Thorben

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