• What is the DefaultServlet
• Where is it declared?
• What can I change?
• How do I customize directory listings?
• How do I secure directory listings?

The default servlet is the servlet which serves static resources as well as serves the directory listings (if directory listings are enabled).

It is declared globally in $CATALINA_BASE/conf/web.xml. By default here is it's declaration:

    <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>false</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>

...

    <servlet-mapping>
        <servlet-name>default</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

So by default, the default servlet is loaded at webapp startup and directory listings are disabled and debugging is turned off.

The DefaultServlet allows the following initParamters:

Property	Description
debug	Debugging level. It is not very useful unless you are a tomcat developer. As of this writing, useful values are 0, 1, 11, 1000. [0]
listings	If no welcome file is present, can a directory listing be shown? value may be true or false [false] Welcome files are part of the servlet api. WARNING: Listings of directories containing many entries are expensive. Multiple requests for large directory listings can consume significant proportions of server resources.
readmeFile	If a directory listing is presented, a readme file may also be presented with the listing. This file is inserted as is so it may contain HTML.
globalXsltFile	If you wish to customize your directory listing, you can use an XSL transformation. This value is a relative file name (to either $CATALINA_BASE/conf/ or $CATALINA_HOME/conf/) which will be used for all directory listings. This can be overridden per context and/or per directory. See contextXsltFile and localXsltFile below. The format of the xml is shown below.
contextXsltFile	You may also customize your directory listing by context by configuring contextXsltFile. This must be a context relative path (e.g.: /path/to/context.xslt) to a file with a .xsl or .xslt extension. This overrides globalXsltFile. If this value is present but a file does not exist, then globalXsltFile will be used. If globalXsltFile does not exist, then the default directory listing will be shown.
localXsltFile	You may also customize your directory listing by directory by configuring localXsltFile. This must be a file in the directory where the listing will take place to with a .xsl or .xslt extension. This overrides globalXsltFile and contextXsltFile. If this value is present but a file does not exist, then contextXsltFile will be used. If contextXsltFile does not exist, then globalXsltFile will be used. If globalXsltFile does not exist, then the default directory listing will be shown.
input	Input buffer size (in bytes) when reading resources to be served. [2048]
output	Output buffer size (in bytes) when writing resources to be served. [2048]
readonly	Is this context "read only", so HTTP commands like PUT and DELETE are rejected? [true]
fileEncoding	File encoding to be used when reading static resources. [platform default]
sendfileSize	If the connector used supports sendfile, this represents the minimal file size in KB for which sendfile will be used. Use a negative value to always disable sendfile. [48]
useAcceptRanges	If true, the Accept-Ranges header will be set when appropriate for the response. [true]
showServerInfo	Should server information be presented in the response sent to clients when directory listing is enabled. [true]

You can override DefaultServlet with you own implementation and use that in your web.xml declaration. If you can understand what was just said, we will assume you can read the code to DefaultServlet servlet and make the appropriate adjustments. (If not, then that method isn't for you)

You can use either localXsltFile or globalXsltFile and DefaultServlet will create an xml document and run it through an xsl transformation based on the values provided in localXsltFile and globalXsltFile. localXsltFile is first checked, followed by globalXsltFile, then default behaviors takes place.

Format:

    <listing>
     <entries>
      <entry type='file|dir' urlPath='aPath' size='###' date='gmt date'>
        fileName1
      </entry>
      <entry type='file|dir' urlPath='aPath' size='###' date='gmt date'>
        fileName2
      </entry>
      ...
     </entries>
     <readme></readme>
    </listing>

• size will be missing if type='dir'
• Readme is a CDATA entry

The following is a sample xsl file which mimics the default tomcat behavior:

<?xml version="1.0" encoding="UTF-8"?>

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="3.0">

  <xsl:output method="html" html-version="5.0"
    encoding="UTF-8" indent="no"
    doctype-system="about:legacy-compat"/>

  <xsl:template match="listing">
   <html>
    <head>
      <title>
        Sample Directory Listing For
        <xsl:value-of select="@directory"/>
      </title>
      <style>
        h1 {color : white;background-color : #0086b2;}
        h3 {color : white;background-color : #0086b2;}
        body {font-family : sans-serif,Arial,Tahoma;
             color : black;background-color : white;}
        b {color : white;background-color : #0086b2;}
        a {color : black;} HR{color : #0086b2;}
        table td { padding: 5px; }
      </style>
    </head>
    <body>
      <h1>Sample Directory Listing For
            <xsl:value-of select="@directory"/>
      </h1>
      <hr style="height: 1px;" />
      <table style="width: 100%;">
        <tr>
          <th style="text-align: left;">Filename</th>
          <th style="text-align: center;">Size</th>
          <th style="text-align: right;">Last Modified</th>
        </tr>
        <xsl:apply-templates select="entries"/>
        </table>
      <xsl:apply-templates select="readme"/>
      <hr style="height: 1px;" />
      <h3>Apache Tomcat/8.0</h3>
    </body>
   </html>
  </xsl:template>


  <xsl:template match="entries">
    <xsl:apply-templates select="entry"/>
  </xsl:template>

  <xsl:template match="readme">
    <hr style="height: 1px;" />
    <pre><xsl:apply-templates/></pre>
  </xsl:template>

  <xsl:template match="entry">
    <tr>
      <td style="text-align: left;">
        <xsl:variable name="urlPath" select="@urlPath"/>
        <a href="{$urlPath}">
          <pre><xsl:apply-templates/></pre>
        </a>
      </td>
      <td style="text-align: right;">
        <pre><xsl:value-of select="@size"/></pre>
      </td>
      <td style="text-align: right;">
        <pre><xsl:value-of select="@date"/></pre>
      </td>
    </tr>
  </xsl:template>

</xsl:stylesheet>

Use web.xml in each individual webapp. See the security section of the Servlet specification.