How to access in memory h2 database of one spring boot application from another spring boot application

You can setup H2 Server as Spring Bean.

First edit pom.xml – delete <scope>runtime</scope> from h2 dependency:

<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
</dependency>

Then add H2 server bean to SpringBootApplication or Configuration class:

@SpringBootApplication
public class Application {

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

    /**
     * Start internal H2 server so we can query the DB from IDE
     *
     * @return H2 Server instance
     * @throws SQLException
     */
    @Bean(initMethod = "start", destroyMethod = "stop")
    public Server h2Server() throws SQLException {
        return Server.createTcpServer("-tcp", "-tcpAllowOthers", "-tcpPort", "9092");
    }
}

Last – edit application.properties – set the name of the database:

spring.datasource.url=jdbc:h2:mem:dbname
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=
spring.jpa.hibernate.ddl-auto=create

Then you can connect to this H2 Server from outside (e.g. to your application with H2 DB) using this connection:

jdbc:h2:tcp://localhost:9092/mem:dbname

As a bonus using this url you can connect to the database of your app right from your IDE.

UPDATE

There is a chance of getting an error when trying to connect to the H2 for Spring Boot app of 1.5.x version. In this case just change a version of H2 to previous one, for example:

<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <version>1.4.193</version>
</dependency>

UPDATE 2

If you need to run several apps with H2 simultaneously on the same host you should set the different H2 ports on them in Server.createTcpServer mothod, for example: 9092, 9093, etc..

// First App
@Bean(initMethod = "start", destroyMethod = "stop")
public Server h2Server() throws SQLException {
    return Server.createTcpServer("-tcp", "-tcpAllowOthers", "-tcpPort", "9092");
}

// Second App
@Bean(initMethod = "start", destroyMethod = "stop")
public Server h2Server() throws SQLException {
    return Server.createTcpServer("-tcp", "-tcpAllowOthers", "-tcpPort", "9093");
}

Then you can connect to the H2 DB of these apps with following urls:

App1 H2: jdbc:h2:tcp://localhost:9092/mem:dbname
App2 H2: jdbc:h2:tcp://localhost:9093/mem:dbname