cannot lookup jndi if using following method

[peter tong]

I have set up the datasource in weblogic console with jndi name jdbc/als
in web.xml, I add the following section

<resource-ref>
  <res-ref-name>jdbc/als</res-ref-name>
  <res-type>java.sql.DataSource</res-type>
  <res-auth>Container</res-auth>
</resource-ref>

then I created a weblogic.xml and write

<wls:context-root>ALS</wls:context-root>
<wls:resource-description>
     <wls:res-ref-name>jdbc/asl</wls:res-ref-name>
     <wls:jndi-name>jdbc/asl</wls:jndi-name>
</wls:resource-description>

but when I deploy the application, error

NamingException: Cannot bind null object to jndi with name jdbc/alsJNDI

is thrown, what is the reason?

[peter tong]

But if I remove the section in web.xml and weblogic.xml, then in the DBConnection class, I write

Hashtable<String, String> env = new Hashtable<String, String>();
env.put(Context.INITIAL_CONTEXT_FACTORY, "weblogic.jndi.WLInitialContextFactory");
Context ctx = new InitialContext(env);
DataSource ds = (DataSource) ctx.lookup("jdbc/als");
Connection conn = ds.getConnection();

everything is Ok!!

So what is wrong with the first method (setting in web.xml and weblogic.xml)? I see most online tutorial recommend the first method.