/gs-accessing-data-jpa

Accessing Data with JPA :: Learn how to work with JPA data persistence using Spring Data JPA.

Primary LanguageShell

tags projects
spring-data
jpa
spring-data-jpa
Table of Contents

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.

What you’ll build

You’ll build an application that stores Customer POJOs in a memory-based database.

What you’ll need

Define a simple entity

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.

Note
 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.

Create simple queries

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!

Create an Application class

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]

@SpringBootApplication is a convenience annotation that adds all of the following:

  • @Configuration tags the class as a source of bean definitions for the application context.

  • @EnableAutoConfiguration tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings.

  • Normally you would add @EnableWebMvc for a Spring MVC app, but Spring Boot adds it automatically when it sees spring-webmvc on the classpath. This flags the application as a web application and activates key behaviors such as setting up a DispatcherServlet.

  • @ComponentScan tells Spring to look for other components, configurations, and services in the the hello package, allowing it to find the HelloController.

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".

Note
 By default, Spring Boot will enable JPA repository support and look in the package (and its subpackages) where @SpringBootApplication is located. If your configuration has JPA repository interface definitions located in a package not visible, you can point out alternate packages using @EnableJpaRepositories and its type-safe basePackageClasses=MyRepository.class parameter.

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']

Summary

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.

Note
 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.