<Resource name="global" auth="Container" type="javax.sql.DataSource"
          maxTotal="100" maxIdle="30" maxWaitMillis="10000"
          username="user" password="betterThanThis"
          factory="org.apache.tomcat.jdbc.pool.DataSourceFactory"
          driverClassName="org.apache.derby.jdbc.EmbeddedDriver"
          url="jdbc:derby:memory:test;create=true"/>

<ResourceLink name="local"
              auth="Container"
              global="global"
              type="javax.sql.DataSource" />

<resource-ref>
    <res-ref-name>local</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
</resource-ref>

DataSource ds = InitialContext.doLookup("java:/comp/env/local");
ds.getConnection();

java.sql.SQLException: Cannot create JDBC driver of class '' for connect URL 'null'
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createConnectionFactory(BasicDataSource.java:2167) ~[tomcat8-dbcp-8.5.14.jar:8.5.14]
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createDataSource(BasicDataSource.java:2037) ~[tomcat8-dbcp-8.5.14.jar:8.5.14]
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.getConnection(BasicDataSource.java:1543) ~[tomcat8-dbcp-8.5.14.jar:8.5.14]