envContext = new InitialContext();
DataSource ds = (DataSource) envContext.lookup( "java:/comp/env/jdbc/Database" );
con = ds.getConnection();

<resource-ref>
    <description>DB Connection</description>
    <res-ref-name>jdbc/Database</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
  </resource-ref>

<?xml version="1.0" encoding="UTF-8"?>
 <!-- The contents of this file will be loaded for each web application -->
  <Context crossContext="true">

    <!-- Default set of monitored resources -->
    <WatchedResource>WEB-INF/web.xml</WatchedResource>

   <Resource name="jdbc/Database" auth="Container"
        type="javax.sql.DataSource"
        maxActive="100" maxIdle="30" maxWait="10000"
        username="user"
        password="password"
        driverClassName="com.mysql.jdbc.Driver"
        url="jdbc:mysql://localhost:3306/configerror_db"/>
</Context>

Hashtable env = new Hashtable();
env.put(Context.INITIAL_CONTEXT_FACTORY,
    "<initialContextFactory>");
env.put(Context.PROVIDER_URL, "<url>");
env.put(Context.SECURITY_PRINCIPAL, "<user>");
env.put(Context.SECURITY_CREDENTIALS, "<password>");
ctx = new InitialContext(env);