It depends on what server you are using. For example, for jetty, you can use a context file (in xml) to conceal/hide a directory's content...
<?xml version="1.0"?>
<!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "http://www.eclipse.org/jetty/configure.dtd">
<Configure class="org.eclipse.jetty.server.handler.ContextHandler">
<Call class="org.eclipse.jetty.util.log.Log" name="debug">
<Arg>Configure sarbot_files.xml</Arg>
</Call>
<Set name="contextPath">/some_context</Set>
<Set name="resourceBase">/path/to/files</Set>
<Set name="handler">
<New class="org.eclipse.jetty.server.handler.ResourceHandler">
<!-- Set to true or false here -->
<Set name="directoriesListed">true</Set>
<Set name="welcomeFiles">
<Array type="String">
<Item>index.html</Item>
</Array>
</Set>
<Set name="cacheControl">max-age=3600,public</Set>
</New>
</Set>
</Configure>