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The implementation of a converter is pretty simple; just implement the AttributeConverter interface, annotate the class with the Converter annotation, and implement the two interface methods: convertToDatabaseColumn - converts the Java class to its database representation convertToEntityAttribute - conversely, converts the column back to an object.
Implement a custom enum mapping You can see a simple example in the following code snippet. Now you may be wondering, why would you implement your own conversion, given that JPA already supports two options to persist an enum: 1 the String representation or 2 the ordinal value of the particular enum value.
Both options have their disadvantages if you need to change the enum. If you persist an enum as a String, you need to update your database if you decide to change the name of any enum value.
On the other hand, if you persist the ordinal value, which represents the position of the value in the enum definition, then if you change the order of the enum values, or if you add new values anywhere but as the last entry, or if you remove any of the enum values besides the last one, you will again need to update your persisted data.
Otherwise your persistence provider will not be able to map the existing values, or will map them to the wrong enum value.
You can avoid that by implementing a custom mapping with an AttributeConverter, which gives you control of the mapping and allows you to avoid any changes to the existing mappings if you need to refactor your enum. If, however you are aiming for reuse, JPA 2. Unfortunately, JPA 2.
Before the release of JPA 2. Date , for example. With JPA 2. Objects; import javax. Entity; import javax.
GeneratedValue; import javax. GenerationType; import javax. Id; import javax.
The Table annotation specifies the primary table for the annotated entity. AUTO private Long id; The Id annotation specifies the primary key of an entity while the GeneratedValue provides for the specification of generation strategies for the values of primary keys. City; import java.
List; import javax. EntityManager; import javax. EntityManagerFactory; import javax.
Persistence; import javax.