It’s often convenient to develop applications using an in-memory embedded database. Obviously, in-memory databases do not provide persistent storage; you will need to populate your database when your application starts and be prepared to throw away data when your application ends.

	Tip
	The ‘How-to’ section includes a section on how to initialize a database

Spring Boot can auto-configure embedded H2, HSQL and Derby databases. You don’t need to provide any connection URLs, simply include a build dependency to the embedded database that you want to use.

For example, typical POM dependencies would be:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
    <groupId>org.hsqldb</groupId>
    <artifactId>hsqldb</artifactId>
    <scope>runtime</scope>
</dependency>

	Note
	You need a dependency on spring-jdbc for an embedded database to be auto-configured. In this example it’s pulled in transitively via spring-boot-starter-data-jpa.

Tip

If, for whatever reason, you do configure the connection URL for an embedded database, care should be taken to ensure that the database’s automatic shutdown is disabled. If you’re using H2 you should use DB_CLOSE_ON_EXIT=FALSE to do so. If you’re using HSQLDB, you should ensure that shutdown=true is not used. Disabling the database’s automatic shutdown allows Spring Boot to control when the database is closed, thereby ensuring that it happens once access to the database is no longer needed.

Production database connections can also be auto-configured using a pooling DataSource. Here’s the algorithm for choosing a specific implementation:

If you use the spring-boot-starter-jdbc or spring-boot-starter-data-jpa ‘starters’ you will automatically get a dependency to tomcat-jdbc.

	Note
	You can bypass that algorithm completely and specify the connection pool to use via the spring.datasource.type property. This is especially important if you are running your application in a Tomcat container as tomcat-jdbc is provided by default.

	Tip
	Additional connection pools can always be configured manually. If you define your own DataSource bean, auto-configuration will not occur.

DataSource configuration is controlled by external configuration properties in spring.datasource.*. For example, you might declare the following section in application.properties:

spring.datasource.url=jdbc:mysql://localhost/test
spring.datasource.username=dbuser
spring.datasource.password=dbpass
spring.datasource.driver-class-name=com.mysql.jdbc.Driver

	Tip
	You often won’t need to specify the driver-class-name since Spring boot can deduce it for most databases from the url.

	Note
	For a pooling DataSource to be created we need to be able to verify that a valid Driver class is available, so we check for that before doing anything. I.e. if you set spring.datasource.driver-class-name=com.mysql.jdbc.Driver then that class has to be loadable.

See DataSourceProperties for more of the supported options. These are the standard options that work regardless of the actual implementation. It is also possible to fine-tune implementation-specific settings using their respective prefix (spring.datasource.tomcat.*, spring.datasource.hikari.*, spring.datasource.dbcp.* and spring.datasource.dbcp2.*). Refer to the documentation of the connection pool implementation you are using for more details.

For instance, if you are using the Tomcat connection pool you could customize many additional settings:

# Number of ms to wait before throwing an exception if no connection is available.
spring.datasource.tomcat.max-wait=10000

# Maximum number of active connections that can be allocated from this pool at the same time.
spring.datasource.tomcat.max-active=50

# Validate the connection before borrowing it from the pool.
spring.datasource.tomcat.test-on-borrow=true

If you are deploying your Spring Boot application to an Application Server you might want to configure and manage your DataSource using your Application Servers built-in features and access it using JNDI.

The spring.datasource.jndi-name property can be used as an alternative to the spring.datasource.url, spring.datasource.username and spring.datasource.password properties to access the DataSource from a specific JNDI location. For example, the following section in application.properties shows how you can access a JBoss AS defined DataSource:

spring.datasource.jndi-name=java:jboss/datasources/customers

Traditionally, JPA ‘Entity’ classes are specified in a persistence.xml file. With Spring Boot this file is not necessary and instead ‘Entity Scanning’ is used. By default all packages below your main configuration class (the one annotated with @EnableAutoConfiguration or @SpringBootApplication) will be searched.

Any classes annotated with @Entity, @Embeddable or @MappedSuperclass will be considered. A typical entity class would look something like this:

package com.example.myapp.domain;

import java.io.Serializable;
import javax.persistence.*;

@Entity
public class City implements Serializable {

    @Id
    @GeneratedValue
    private Long id;

    @Column(nullable = false)
    private String name;

    @Column(nullable = false)
    private String state;

    // ... additional members, often include @OneToMany mappings

    protected City() {
        // no-args constructor required by JPA spec
        // this one is protected since it shouldn't be used directly
    }

    public City(String name, String state) {
        this.name = name;
        this.country = country;
    }

    public String getName() {
        return this.name;
    }

    public String getState() {
        return this.state;
    }

    // ... etc

}

	Tip
	You can customize entity scanning locations using the @EntityScan annotation. See the Section 75.4, “Separate @Entity definitions from Spring configuration” how-to.

Spring Data JPA repositories are interfaces that you can define to access data. JPA queries are created automatically from your method names. For example, a CityRepository interface might declare a findAllByState(String state) method to find all cities in a given state.

For more complex queries you can annotate your method using Spring Data’s Query annotation.

Spring Data repositories usually extend from the Repository or CrudRepository interfaces. If you are using auto-configuration, repositories will be searched from the package containing your main configuration class (the one annotated with @EnableAutoConfiguration or @SpringBootApplication) down.

Here is a typical Spring Data repository:

package com.example.myapp.domain;

import org.springframework.data.domain.*;
import org.springframework.data.repository.*;

public interface CityRepository extends Repository<City, Long> {

    Page<City> findAll(Pageable pageable);

    City findByNameAndCountryAllIgnoringCase(String name, String country);

}

	Tip
	We have barely scratched the surface of Spring Data JPA. For complete details check their reference documentation.

By default, JPA databases will be automatically created only if you use an embedded database (H2, HSQL or Derby). You can explicitly configure JPA settings using spring.jpa.* properties. For example, to create and drop tables you can add the following to your application.properties.

spring.jpa.hibernate.ddl-auto=create-drop

	Note
	Hibernate’s own internal property name for this (if you happen to remember it better) is hibernate.hbm2ddl.auto. You can set it, along with other Hibernate native properties, using spring.jpa.properties.* (the prefix is stripped before adding them to the entity manager). Example:

spring.jpa.properties.hibernate.globally_quoted_identifiers=true

passes hibernate.globally_quoted_identifiers to the Hibernate entity manager.

By default the DDL execution (or validation) is deferred until the ApplicationContext has started. There is also a spring.jpa.generate-ddl flag, but it is not used if Hibernate autoconfig is active because the ddl-auto settings are more fine-grained.

By default the console will be available at /h2-console. You can customize the console’s path using the spring.h2.console.path property.

When Spring Security is on the classpath and basic auth is enabled, the H2 console will be automatically secured using basic auth. The following properties can be used to customize the security configuration: