context.xml

<?xml version="1.0" encoding="UTF-8"?>
<Context>
  <Resource name="jdbc/myappDS"
            auth="Container"
            type="javax.sql.DataSource"
            initialSize="20" maxTotal="20" maxIdle="20" maxWaitMillis="10000"
            username="postgres" password="postgres" driverClassName="org.postgresql.Driver"
            url="jdbc:postgresql://localhost:5432/test"
            connectionProperties="loginTimeout=10;socketTimeout=310"
            validationQuery="SELECT 1"
            validationQueryTimeout="30" />
</Context>

lookup above datasource in application code:
InitialContext.doLookup("java:comp/env/jdbc/myappDS");