Introduction:

In this quick tutorial, we’ll bootstrap a simple Spring Boot application backed by an in-memory H2 database. We’ll use the Spring Data JPA to interact with our database.

Project Setup:

Firstly, let’s generate our project template using Spring Initializr:

On clicking the ‘Generate the project’ link, our project files will get downloaded.

Now, if we look closely at the generated POM file, we’ll have below added dependencies:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>runtime</scope>
</dependency>

H2 Default Properties:

Since we have added H2 database dependency, Spring Boot will auto-configure its related properties. The default configurations include:

spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=
spring.h2.console.enabled=false

Let’s override a few of these properties by defining those in our application.properties file:

spring.h2.console.enabled=true
spring.h2.console.path=/h2
spring.datasource.url=jdbc:h2:mem:university

Here, our database name will be a university. We have also enabled the H2 Console and have set its context path.

Defining Entities:

Now, we’ll define a Student entity:

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    private String name;
    
    public Student(String name) {
        this.name = name;   
    }
 
    //getters, setters

    public String toString() {
        return "{id=" + id + ", name=" + name + "}";
    }
}

And its corresponding Spring Data JPA repository:

@Repository
public interface StudentRepository
  extends CrudRepository<Student, Integer> {
}

The Student entity will get mapped to the database table with exactly the same name. If we want, we can specify a different table name with @Table annotation.

The Application Class:

Finally, let’s implement our UniversityApplication class:

@SpringBootApplication
public class UniversityApplication {

    public static void main(String[] args) {
        SpringApplication.run(UniversityApplication.class, args);      
    }

    @Bean
    public CommandLineRunner testApp(StudentRepository repo) {
        return args -> {
            repo.save(new Student("James"));
            repo.save(new Student("Selena"));

            List<Student> allStudents = repo.findAll();
            System.out.println("All students in DB: " + allStudents);

            Student james = repo.findById(1);
            System.out.println("James: " + james);
        };
    }
}

This class is the starting point for our Spring Boot application. Here, the @SpringBootApplication annotation is equivalent to having @ComponentScan, @EnableAutoConfiguration, and @SpringConfiguration together.

We have also defined an instance of CommandLineRunner. And so, when we run our application, our console logs will have:

UniversityApplication:All students in DB: [{id=1, name=James}
 , {id=2, name=Selena}]
James: {id=1, name=James}
...

Please note that in Spring Boot, all entities should ideally be defined at either the same package level or at lower levels(in sub-packages) to that of the main application class. If so, Spring Boot will auto-scan all those entities.

Accessing the H2 Console:

We can also check the database entries on the H2 console.

To do that, we’ll open the URL: http://localhost:8080/h2 on any browser and use our database configurations to login. With it, we’ll be able to see all our created tables and entries easily on a UI Console dashboard.

Conclusion:

In this tutorial, we bootstrapped a very simple Spring Boot application with a single entity. This application is integrated with the H2 database and uses Spring Data JPA.
We can easily extend it to cater to a much wider application scope.