The following situation does not seem to work.

1.) In a Context, add an alias to a directory.
2.) Enable directory listings on the DefaultServlet.
3.) Try to view the directory listing of the alias directory.

Example:

conf/context.xml:

  <Context aliases="/files=/my/downloads">
  ...
  </Context>

conf/web.xml:

    ...
    <servlet>
        <servlet-name>default</servlet-name>
        <servlet-class>org.apache.catalina.servlets.DefaultServlet</servlet-class>
        <init-param>
            <param-name>debug</param-name>
            <param-value>0</param-value>
        </init-param>
        <init-param>
            <param-name>listings</param-name>
            <param-value>true</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    ...

contents of the directory "/my/downloads":

  -rw-r--r--@ 1 daniel  staff  8156824 Sep 10 09:15 apache-tomcat-7.0.30.zip

URL works:

http://localhost:8080/files/apache-tomcat-7.0.30.zip

URL does not work:

http://localhost:8080/files/

The URL that does not work fails with the error:

SEVERE: Servlet.service() for servlet [default] in context with path [/] threw exception [Error accessing resource] with root cause
javax.naming.NameNotFoundException: Resource /files/ not found
	at org.apache.naming.resources.FileDirContext.list(FileDirContext.java:303)
	at org.apache.naming.resources.ProxyDirContext.list(ProxyDirContext.java:512)
	at org.apache.catalina.servlets.DefaultServlet.renderHtml(DefaultServlet.java:1458)
	at org.apache.catalina.servlets.DefaultServlet.render(DefaultServlet.java:1254)
	at org.apache.catalina.servlets.DefaultServlet.serveResource(DefaultServlet.java:951)
	at org.apache.catalina.servlets.DefaultServlet.doGet(DefaultServlet.java:412)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
	at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
	at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1002)
	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:585)
	at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
	at java.lang.Thread.run(Thread.java:680)