Connecting to an RDBMS from a Java application is done through a JDBC driver. Most (if not all) database vendors include JDBC drivers for their RDBMS. A JDBC driver, in practice, is just a Java dependency (a JAR) in your project. If, for example, you need to connect your application to a PostgreSQL database, you will need to add the postgresql-x.x.x.jar file to your classpath. This, of course, can also be done with Maven. It's through this JDBC driver that your Java application communicates with the RDBMS, and it does so by establishing connections and executing SQL statements to retrieve data.

In this book, we are going to use an H2 database. H2 is a popular open source database engine that doesn't require you to install anything on your computer. All the concepts apply to other RDBMS as well, and we'll include snippets or commented sections in the accompanying code that show the specifics for MySQL and PostgreSQL, in case you want to experiment with these databases by yourself.

Adding a JDBC driver is as simple as including the right dependency in your project. For example, to include the H2 JDBC driver, add the following dependency to your pom.xml file:

<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <version>1.4.196</version>
</dependency>

Or if you want to use MySQL or PostgreSQL, add the following dependencies:

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>6.0.6</version>
</dependency>

<dependency>
    <groupId>org.postgresql</groupId>
    <artifactId>postgresql</artifactId>
    <version>42.1.4</version>
</dependency>

You can, of course, include several JDBC drivers in the same project. The chapter-05 application includes all of the previous drivers.