In-memory database is created/initialized when an application starts up; and destroyed when the application shuts down.

H2 is one of the popular in-memory databases written in Java.

It can be embedded in Java applications or run in the client-server mode.

Spring Boot provides excellent integration support for H2 using simple properties configuration.

H2 also provides a console view to maintain and interact with the database tables and data.

we configure H2 database with Spring boot to create and use an in-memory database in runtime, generally for unit testing or POC purposes.

The in-memory databases are volatile, by default, and all stored data will be lost when we restart the application

Use H2 in Spring boot application

To use H2 in Spring boot application, all we need to do is adding H2 runtime jar into dependencies. The best way to add is through maven.

 pom.xml  <dependency>     <groupId>com.h2database</groupId>     <artifactId>h2</artifactId>     <scope>runtime</scope> </dependency>

H2 Configuration Options

Simple configuration

 application.properties  spring.datasource.url=jdbc:h2:mem:testdb spring.datasource.driverClassName=org.h2.Driver spring.datasource.username=sa spring.datasource.password= spring.jpa.database-platform=org.hibernate.dialect.H2Dialect

Please note by default, Spring Boot configures the in-memory database connection with the username 'sa' and an empty password ' '. If you wish to change these values, override them in above properties options.

Configure data persistence

The in-memory databases are volatile, by default, and all stored data will be lost when we restart the application. In this case, data is written in temporary memory and as soon as JVM is stopped, data is flushed.

To have a persistent data store, which is capable to storing data between application start/stop, we should store the data in files. For this change the spring.datasource.url property.

 application.properties  # temporary data storage spring.datasource.url = jdbc:h2:mem:testdb   # temporary data storage spring.datasource.url = jdbc:h2:file:/data/sample spring.datasource.url = jdbc:h2:file:C:/data/sample (Windows only)

Create schema and insert data on initialization

We may want to initialize database with some fixed schema (DDL) and insert default data (DML) into tables before the application is ready is run business usecases.

We can achieve this by putting sql files into resources folder (/src/main/resources/).

  • schema.sql – To initialize the schema ie. create tables and dependencies.
  • data.sql – To insert default data rows.
 schema.sql  DROP TABLE IF EXISTS TBL_EMPLOYEES;    CREATE TABLE TBL_EMPLOYEES (   id INT AUTO_INCREMENT  PRIMARY KEY,   first_name VARCHAR(250) NOT NULL,   last_name VARCHAR(250) NOT NULL,   email VARCHAR(250) DEFAULT NULL ); 
 data.sql  INSERT INTO TBL_EMPLOYEES (first_name, last_name, email) VALUES   ('Lokesh', 'Gupta', 'abc@gmail.com'),   ('Deja', 'Vu', 'xyz@email.com'),   ('Caption', 'America', 'cap@marvel.com');

H2 Console

Enable H2 console

By default, the console view of H2 database is disabled.

We must enable it to view and access it in browser.

Note that we can customize the URL of H2 console which, by default, is '/h2'.

 application.properties  # Enabling H2 Console spring.h2.console.enabled=true   # Custom H2 Console URL spring.h2.console.path=/h2

Accessing H2 console

Start the spring boot application and access the console in browser with URL : http://localhost:8080/h2

Other configuration options

Spring boot provides two more properties to further customize the behaviour of H2 console. i.e. we can enable/disable the database trace logs and we can enable/disable the remote access of H2 console.

By default both properties are false

 application.properties  # Whether to enable trace output. spring.h2.console.settings.trace=false   # Whether to enable remote access. spring.h2.console.settings.web-allow-others=false
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