Java 8 brought lots of great features and one of the most important and most anticipated ones was the new Date and Time API. There were lots of issues with the old API and I won’t get into any details on why we needed a new one. I’m sure you had to struggle often enough with it yourself.
All these issues are gone with Java 8. The new Date and Time API is well designed, easy to use and (finally) immutable. The only issue that remains is, that you cannot use it with JPA.
Well, that’s not completely correct. You can use it, but JPA will map it to a BLOB instead of a DATE or TIMESTAMP. That means the database is not aware of the date object and cannot apply any optimization for it. That’s not the way we should or want to do it.
Why does JPA not support LocalDate and LocalDateTime?
The answer is simple, JPA 2.1 was released before Java 8 and the Date and Time API simply didn’t exist at that point in time. Therefore the @Temporal annotation can only be applied to attributes of type java.util.Date and java.util.Calendar.
If you want to store a LocalDate attribute in a DATE column or a LocalDateTime in a TIMESTAMP column, you need to define the mapping to java.sql.Date or java.sql.Timestamp yourself. Thanks to the attribute converter, one of several new features in JPA 2.1, this can be achieved with just a few lines of code.
In the following examples, I will show you how to create an attribute converter for LocalDate and LocalDateTime. If you want to learn more about attribute converter, have a look at How to implement a JPA 2.1 Attribute Converter or one of the other usage examples like a better way to persist enums or encrypting data.
The most important things you need to remember about attribute converter are also described in the free “New Features in JPA 2.1” cheat sheet.
Before we create the attribute converters, lets have a look at the example entity for this post.
Attribute converter are part of the JPA 2.1 specification and can therefore be used with any JPA 2.1 implementation, e.g. Hibernate or EclipseLink. I used Wildfly 8.2 with Hibernate 4.3 for the following examples.
As you can see in the following code snippet, there isn’t much you need to do to create an attribute converter for LocalDate.
You need to implement the AttributeConverter<LocalDate, Date> interface with its two methods convertToDatabaseColumn and convertToEntityAttribute. As you can see on the method names, one of them defines the conversion from the type of the entity attribute (LocalDate) to the database column type (Date) and the other one the inverse conversion. The conversion itself is very simple because java.sql.Date already provides the methods to do the conversion to and from a LocalDate.
Additionally the attribute converter needs to be annotated with the @Converter annotation. Due to the optional autoApply=true property, the converter will be applied to all attributes of type LocalDate. Have a look here, if you want to define the usage of the converter for each attribute individually.
The conversion of the attribute is transparent to the developer and the LocalDate attribute can be used as any other entity attribute. You can use it as a query parameter for example.
The attribute converter for LocalDateTime is basically the same. You need to implement the AttributeConverter<LocalDateTime, Timestamp> interface and the converter needs to be annotated with the @Converter annotation. Similar to the LocalDateConverter, the conversion between a LocalDateTime and an java.sql.Timestamp is done with the conversion methods of Timestamp.
JPA 2.1 was released before Java 8 and therefore doesn’t support the new Date and Time API. If you want to use the new classes (in the right way), you need to define the conversion to java.sql.Date and java.sql.Timestamp yourself. This can be easily done by implementing the AttributeConverter<EntityType, DatabaseType> interface and annotating the class with @Converter(autoApply=true). By setting autoApply=true, the converter will be applied to all attributes of the EntityType and no changes on the entity are required.
As far as I know, the next JPA release will support the Date and Time API and the different implementations will probably support it even earlier. Hibernate 5 for example will support it as a proprietary feature.