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This guide walks you through the process of building an application that uses Spring Data JPA to store and retrieve data in a relational database.
You’ll build an application that stores Customer POJOs in a memory-based database.
In this example, you store Customer objects, annotated as a JPA entity.
src/main/java/hello/Customer.java
link:complete/src/main/java/hello/Customer.java[role=include]Here you have a Customer class with three attributes, the id, the firstName, and the lastName. You also have two constructors. The default constructor only exists for the sake of JPA. You won’t use it directly, so it is designated as protected. The other constructor is the one you’ll use to create instances of Customer to be saved to the database.
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In this guide, the typical getters and setters have been left out for brevity. |
The Customer class is annotated with @Entity, indicating that it is a JPA entity. For lack of a @Table annotation, it is assumed that this entity will be mapped to a table named Customer.
The Customer’s `id property is annotated with @Id so that JPA will recognize 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’ll be mapped to columns that share the same name as the properties themselves.
The convenient toString() method will print out 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:
src/main/java/hello/CustomerRepository.java
link:complete/src/main/java/hello/CustomerRepository.java[role=include]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 allows you to define other query methods by simply declaring their method signature. In the case of CustomerRepository, this is shown with a findByLastName() method.
In a typical Java application, you’d expect to write a class that implements CustomerRepository. But that’s what makes Spring Data JPA so powerful: You don’t have to write an implementation of the repository interface. Spring Data JPA creates an implementation on the fly when you run the application.
Let’s wire this up and see what it looks like!
Here you create an Application class with all the components.
src/main/java/hello/Application.java
link:complete/src/main/java/hello/Application.java[role=include]In the configuration, you need to add the @EnableAutoConfiguration annotation. This annotation tells Spring Boot to configure JPA repositories and entities from the current package. So any interface that extends org.springframework.data.repository.Repository will automatically have an implementation generated. By extending JpaRepository, your CustomerRepository interface transitively extends Repository. Therefore, Spring Data JPA will find it and create an implementation for you.
Application includes a main() 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 findOne() to fetch a single Customer by its ID. Finally, it calls findByLastName() to find all customers whose last name is "Bauer".
You should see something like this:
== Customers found with findAll():
Customer[id=1, firstName='Jack', lastName='Bauer']
Customer[id=2, firstName='Chloe', lastName='O'Brian']
Customer[id=3, firstName='Kim', lastName='Bauer']
Customer[id=4, firstName='David', lastName='Palmer']
Customer[id=5, firstName='Michelle', lastName='Dessler']
== Customer found with findOne(1L):
Customer[id=1, firstName='Jack', lastName='Bauer']
== Customer found with findByLastName('Bauer'):
Customer[id=1, firstName='Jack', lastName='Bauer']
Customer[id=3, firstName='Kim', lastName='Bauer']
Congratulations! You’ve written a simple application that uses Spring Data JPA to save objects to a database and to fetch them — all without writing a concrete repository implementation.
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If you’re interesting in exposing JPA repositories with a hypermedia-based RESTful front end with little effort, you might want to read Accessing JPA Data with REST. |