So, JPA is ready in the classpath. What could be the next logical step? It makes sense to define a connection to the database next. The simplest way of doing this is by creating a persistence.xml file. JPA will automatically read this file in the META-INF directory in your classpath. In a Maven project, the location is resources/META-INF/persistence.xml. Inside this file, you can define one or more persistence units (database connections) and its connection properties. The following is an example of a minimal persistence.xml  file you can use to connect to the H2 database:

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://java.sun.com/xml/ns/persistence   
           http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
             version="2.0">
        <persistence-unit name="jpa-example-pu">
          <properties>
            <property name="javax.persistence.jdbc.url" 
            value="jdbc:h2:tcp://localhost/~/h2-databases/demo" />
           <property name="javax.persistence.jdbc.user" value="sa" />
           <property name="javax.persistence.jdbc.password" value="" />
          </properties>
        </persistence-unit>
</persistence>

Since you can define multiple persistence units (when your application needs to connect to multiple databases, for example), each persistence unit must have a name that identifies it. We have used jpa-example-pu for ours. Notice how we used the same connection properties (URL, user, password) we used previously with plain JDBC.