Can anyone explain to me why the sample in db2-ds.xml only works with java: prepended to the jboss-web.xml?

here's my web.xml (snippet):

<resource-ref>
JDBC
<res-ref-name>jdbc/TestSource</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>CONTAINER</res-auth>
</resource-ref>

here's my jboss-web.xml:

<!DOCTYPE jboss-web PUBLIC "-//JBoss//DTD Web Application 2.3V2//EN"
"http://www.jboss.org/j2ee/dtd/jboss-web_3_2.dtd">
<jboss-web>
<resource-ref >
<res-ref-name>jdbc/TestSource</res-ref-name>
<jndi-name>java:jdbc/DB2DS</jndi-name> <!-- why is (java:) needed here??? -->
</resource-ref>
</jboss-web>


here's my code snippet:

Context ctx = new InitialContext();
Context envctx = (Context)ctx.lookup("java:comp/env");
DataSource source = (DataSource)envctx.lookup("jdbc/TestSource");


here's my db2-ds.xml:


<local-tx-datasource>
<jndi-name>jdbc/DB2DS</jndi-name>
<connection-url>jdbc:db2:test</connection-url>
<driver-class>COM.ibm.db2.jdbc.app.DB2Driver</driver-class>
<user-name>xyz</user-name>
pdq
<min-pool-size>0</min-pool-size>
</local-tx-datasource>