17.2. Accessing Data

Now that we have connected our Java application to a MySQL database, we need to set up our Java classes and interfaces. In the previous lessons, we learned about performing CRUD operations on a database and its tables. One of the reasons we use ORM is so that now we can write Java code in our application to manage our relational database.

17.2.1. Persistent Classes

Our JPA needs to know what Java class is going to be converted to a table in the MySQL database. A Java class that models a persistent data store is called a persistent class or entity class.

@Entity denotes an entity class. Entity classes look very similar to any other Java class, except entity classes have two constructors. The first constructor creates an instance of the class. The second one is protected and has no arguments and/or no return statement. While you must set up this second constructor, it will only be used by the JPA to create a new instance.

Since an entity class determines the structure of a table in our relational database, you can add fields to the class to create ids and primary keys. @Id is an annotation that denotes that an integer id field is to be used as an id in the corresponding table in the database. @GeneratedValue is used in conjunction with the @Id annotation to create a primary key for the entity.

Example

In the case of the previous example, we want to make our ContactInfo class an entity class.

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@Entity
public class ContactInfo {
   @Id
   @GeneratedValue
   private int id;

   private String name;

   private String email;

   public ContactInfo(String n, String e) {
      this.name = n;
      this.email = e;
   }

   public ContactInfo() {}
}

This class declaration makes use of @Id and @GeneratedValue for the id field. Every time we instantiate a new object of the ContactInfo class, the object will have an id that translates to the primary key of the contactinfo table. We also have two constructors: the first is what we will use to instantiate an object for Frank, and the second is what the JPA uses to instantiate an object.

17.2.2. Repositories

While entity classes determine the structure of a table in our relational database, a repository gets at the data in the table. Repositories are not models and are stored in a separate folder from models. Repositories are also interfaces, not classes. We use the @Repository annotation to denote a repository.

When we create repository interfaces, we are not going to be extending a particular class. Instead, we will be extending CrudRepository. Extending the CrudRepository interface gives us access to methods to perform all of the CRUD operations that we made happen in SQL.

Note

CrudRepository has a number of different methods. You might want to bookmark the documentation for later reference about the different methods.

Example

To get at the data in the contactinfo table, we need to create a ContactInfoRepository interface.

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@Repository
public interface ContactInfoRepository extends CrudRepository<ContactInfo, Integer> {
}

This code creates a repository called ContactInfoRepository so we can fetch data from the contactinfo table.

On the next page, we will cover how to use a controller and CrudRepository methods to get info in and out of the tables of our relational database.

17.2.3. Creating Persistent Models - Video

The following video explains how we can add an entity class and a repository to our coding-events application. The accompanying text is a quick rundown of what happens in the video. To get started, create a branch off of your db-config branch.

Note

The starter code for this video is found at the db-config branch of the coding-events-demo repo. The final code presented in this video is found on the persistent-model branch. As always, code along to the videos on your own coding-events project.

Remember when switching branches to update your application.properties file to reflect your schema and authentication settings.

17.2.3.1. Creating Persistent Models - Text

In summary, now that we have connected MySQL and coding-events, let’s create a persistent class and a repository.

Create a persistent class for an event called Events.

Create the following fields for Events:

  1. id
  2. name
  3. description
  4. contactemail
  5. type

For Events, create all of the getters and setters for these fields and both constructors.

After making Events, we can add a repository to data called EventRepository. EventRepository extends CrudRepository. We will add more methods and info to make use of EventRepository soon.

17.2.4. Check Your Understanding

Question

Entity classes are _________ and repositories are ____________.

  1. classes, interfaces
  2. interfaces, classes
  3. classes, classes
  4. interfaces, interfaces