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You will build an application that stores Customer POJOs (Plain Old Java Objects) in a memory-based database.

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You can use this pre-initialized project and click Generate to download a ZIP file. This project is configured to fit the examples in this tutorial.

To manually initialize the project:

1. Navigate to https://start.spring.io. This service pulls in all the dependencies you need for an application and does most of the setup for you.

2. Choose either Gradle or Maven and the language you want to use. This guide assumes that you chose Java.

3. Click Dependencies and select Spring Data JPA and then H2 Database.

4. Click Generate.

5. Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.

In this example, you store Customer objects, each annotated as a JPA entity. The following listing shows the Customer class (in src/main/java/com/example/accessingdatajpa/Customer.java):

Here you have a Customer class with three attributes: id, firstName, and lastName. You also have two constructors. The default constructor exists only for the sake of JPA. You do not use it directly, so it is designated as protected. The other constructor is the one you use to create instances of Customer to be saved to the database.

The Customer class is annotated with @Entity, indicating that it is a JPA entity. (Because no @Table annotation exists, it is assumed that this entity is mapped to a table named Customer.)

The Customer object’s id property is annotated with @Id so that JPA recognizes it as the object’s ID. The id property is also annotated with @GeneratedValue to indicate that the ID should be generated automatically.

The other two properties, firstName and lastName, are left unannotated. It is assumed that they are mapped to columns that share the same names as the properties themselves.

The convenient toString() method print outs the customer’s properties.

Spring Data JPA focuses on using JPA to store data in a relational database. Its most compelling feature is the ability to create repository implementations automatically, at runtime, from a repository interface.

To see how this works, create a repository interface that works with Customer entities as the following listing (in src/main/java/com/example/accessingdatajpa/CustomerRepository.java) shows:

CustomerRepository extends the CrudRepository interface. The type of entity and ID that it works with, Customer and Long, are specified in the generic parameters on CrudRepository. By extending CrudRepository, CustomerRepository inherits several methods for working with Customer persistence, including methods for saving, deleting, and finding Customer entities.

Spring Data JPA also lets you define other query methods by declaring their method signature. For example, CustomerRepository includes the findByLastName() method.

In a typical Java application, you might expect to write a class that implements CustomerRepository. However, that is what makes Spring Data JPA so powerful: You need not write an implementation of the repository interface. Spring Data JPA creates an implementation when you run the application.

Now you can wire up this example and see what it looks like!

Spring Initializr creates a simple class for the application. The following listing shows the class that Initializr created for this example (in src/main/java/com/example/accessingdatajpa/AccessingDataJpaApplication.java):

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Now you need to modify the simple class that the Initializr created for you. To get output (to the console, in this example), you need to set up a logger. Then you need to set up some data and use it to generate output. The following listing shows the finished AccessingDataJpaApplication class (in src/main/java/com/example/accessingdatajpa/AccessingDataJpaApplication.java):

The AccessingDataJpaApplication class includes a demo() method that puts the CustomerRepository through a few tests. First, it fetches the CustomerRepository from the Spring application context. Then it saves a handful of Customer objects, demonstrating the save() method and setting up some data to work with. Next, it calls findAll() to fetch all Customer objects from the database. Then it calls findById() to fetch a single Customer by its ID. Finally, it calls findByLastName() to find all customers whose last name is "Bauer". The demo() method returns a CommandLineRunner bean that automatically runs the code when the application launches.

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When you run your application, you should see output similar to the following:

Congratulations! You have written a simple application that uses Spring Data JPA to save objects to and fetch them from a database, all without writing a concrete repository implementation.

The following guides may also be helpful:

• Accessing JPA Data with REST

• Accessing Data with Gemfire

• Accessing Data with MongoDB

• Accessing data with MySQL

• Accessing Data with Neo4j

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