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1. Introduction
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In JPA version 2.0 and below, there's no convenient way to map Enum values to a database column. Each option has its limitations and drawbacks. These issues can be avoided by using JPA 2.1. features.
In this tutorial, we'll take a look at the different possibilities we have to persist enums in a database using JPA. We'll also describe their advantages and disadvantages as well as provide simple code examples.
2. Using @Enumerated Annotation
The most common option to map an enum value to and from its database representation in JPA before 2.1. is to use the @Enumerated annotation. This way, we can instruct a JPA provider to convert an enum to its ordinal or String value.
We'll explore both options in this section.
But first, let's create a simple @Entity that we'll be using throughout this tutorial:
2.1. Mapping Ordinal Value
If we put the @Enumerated(EnumType.ORDINAL) annotation on the enum field, JPA will use the Enum.ordinal() value when persisting a given entity in the database.
Let's introduce the first enum:
Next, let's add it to the Article class and annotate it with @Enumerated(EnumType.ORDINAL):
Now, when persisting an Article entity:
JPA will trigger the following SQL statement:
A problem with this kind of mapping arises when we need to modify our enum. If we add a new value in the middle or rearrange the enum's order, we'll break the existing data model.
Such issues might be hard to catch, as well as problematic to fix, as we would have to update all the database records.
2.2. Mapping String Value
Analogously, JPA will use the Enum.name() value when storing an entity if we annotate the enum field with @Enumerated(EnumType.STRING).
Let's create the second enum:
And let's add it to our Article class and annotate it with @Enumerated(EnumType.STRING):
Now, when persisting an Article entity:
JPA will execute the following SQL statement:
With @Enumerated(EnumType.STRING), we can safely add new enum values or change our enum's order. However, renaming an enum value will still break the database data.
Additionally, even though this data representation is far more readable compared to the @Enumerated(EnumType.ORDINAL) option, it also consumes a lot more space than necessary. This might turn out to be a significant issue when we need to deal with a high volume of data.
3. Using @PostLoad and @PrePersist Annotations
Another option we have to deal with persisting enums in a database is to use standard JPA callback methods. We can map our enums back and forth in the @PostLoad and @PrePersist events.
The idea is to have two attributes in an entity. The first one is mapped to a database value, and the second one is a @Transient field that holds a real enum value. The transient attribute is then used by the business logic code.
To better understand the concept, let's create a new enum and use its int value in the mapping logic:
We've also added the Priority.of() method to make it easy to get a Priority instance based on its int value.
Now, to use it in our Article class, we need to add two attributes and implement callback methods:
Now, when persisting an Article entity:
JPA will trigger the following SQL query:
Even though this option gives us more flexibility in choosing the database value's representation compared to previously described solutions, it's not ideal. It just doesn't feel right to have two attributes representing a single enum in the entity. Additionally, if we use this type of mapping, we aren't able to use enum's value in JPQL queries.
4. Using JPA 2.1 @Converter Annotation
To overcome the limitations of the solutions shown above, JPA 2.1 release introduced a new standardized API that can be used to convert an entity attribute to a database value and vice versa. All we need to do is to create a new class that implements javax.persistence.AttributeConverter and annotate it with @Converter.
Let's see a practical example. But first, as usual, we'll create a new enum:
We also need to add it to the Article class:
Now, let's create a new CategoryConverter:
We've set the @Converter‘s value of autoApply to true so that JPA will automatically apply the conversion logic to all mapped attributes of a Category type. Otherwise, we'd have to put the @Converter annotation directly on the entity's field.
Let's now persist an Article entity:
Then JPA will execute the following SQL statement:
As we can see, we can simply set our own rules of converting enums to a corresponding database value if we use the AttributeConverter interface. Moreover, we can safely add new enum values or change the existing ones without breaking the already persisted data.
The overall solution is simple to implement and addresses all the drawbacks of the options presented in the earlier sections.
5. Using Enums in JPQL
Let's now see how easy it is to use enums in the JPQL queries.
To find all Article entities with Category.SPORT category, we need to execute the following statement:
It's important to note, that in this case, we need to use a fully qualified enum name.
Of course, we're not limited to static queries. It's perfectly legal to use the named parameters:
The above example presents a very convenient way to form dynamic queries.
Additionally, we don't need to use fully qualified names.
6. Conclusion
In this tutorial, we've covered various ways of persisting enum values in a database. We've presented options we have when using JPA in version 2.0 and below, as well as a new API available in JPA 2.1 and above.
It's worth noting that these aren't the only possibilities to deal with enums in JPA. Some databases, like PostgreSQL, provide a dedicated column type to store enum values. However, such solutions are outside the scope of this article.
As a rule of thumb, we should always use the AttributeConverter interface and @Converter annotation if we're using JPA 2.1 or later.
As usual, all the code examples are available over on our GitHub repository.
