// This Source Code is in the Public Domain per: http://litesoft.org/License.txt
package org.litesoft.servlet.versionedstaticcontentfilter;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

/**
 * This is a filter which enforces proper caching of a "LiteSoft Versioned" web app
 * w/ "common" redirection.  It requires that you serve your GWT application via a
 * Java servlet container.
 * <p/>
 * Many thanks to Mat Gessel <mat.gessel@gmail.com> for providing an implementation of
 * the standard GWT Caching mechanism as a basis.
 * <p/>
 * To use, add the jar to <code>WEB-INF/lib</code> and add the
 * following to your deployment descriptor (web.xml):
 * <p/>
 * <pre>
 * &lt;filter&gt;
 *   &lt;filter-name&gt;VersionedStaticContentFilter&lt;/filter-name&gt;
 *   &lt;filter-class&gt;org.litesoft.servlet.versionedstaticcontentfilter.VersionedStaticContentFilter&lt;/filter-class&gt;
 * &lt;/filter&gt;
 *
 * &lt;filter-mapping&gt;
 *   &lt;filter-name&gt;VersionedStaticContentFilter&lt;/filter-name&gt;
 *   &lt;url-pattern&gt;/*&lt;/url-pattern&gt;
 * &lt;/filter-mapping&gt;</pre>
 *
 * A "LiteSoft Versioned" web app is one where the majority of the DOM is built via
 * JavaScript (e.g. a GWT app) and that the path has a version number in it (e.g. /v1/).
 * This means that everything can be cached "forever" EXCEPT certain JavaScript (e.g.
 * the versioned bootstrap JavaScript).  So this code set the caching headers to "forever"
 * for everything EXCEPT files that end with ".nocache.js".
 *
 * Additionally, it maps requests to a common path under the versioned path to just the
 * common path (e.g. "/v1/common/images/fred.jpg" becomes "/common/images/fred.jpg").  This
 * makes it so that when the app is "versioned", all the common resources do not need to be
 * duplicated for each version.  Note: this also means that older versions will be accessing
 * the newer resources, but as the proper behavior of the older versions (in a "LiteSoft
 * Versioned" web app) are to immediately forward to the current version, there should be
 * little to no deleterious effects.
 *
 * Usage notes
 * <ul>
 * <li>You can verify that the filter is being applied with Firefox's Web
 * Developer Extension. Click Tools > Web Developer > Information > View
 * Response Headers.
 * <li>If you are running an Apache httpd/Jk/Tomcat server configuration you
 * need to ensure that Tomcat is serving HTML files, otherwise the filter will
 * not be applied.
 * <li>One reason that this filter exists is that you cannot use <code>*.nocache.html</code> or
 * <code>*.cache.html</code> for url patterns. According to the 2.3 servlet
 * spec, an extension is defined as the characters after the <strong>last</strong>
 * period.
 * <li>The header is modified <em>before</em> passing control down the filter chain.
 * </ul>
 *
 * @see <a
 *      href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9">Cache-control
 *      directive</a>
 * @see <a
 *      href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.21">Expires
 *      directive</a>
 * @see <a
 *      href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.32">Pragma
 *      directive</a>
 */
@SuppressWarnings({"UnusedDeclaration"})
public class VersionedStaticContentFilter implements Filter
{
    @Override
    public void init( FilterConfig filterConfig )
            throws ServletException
    {
    }

    @Override
    public void doFilter( ServletRequest request, ServletResponse response, FilterChain chain )
            throws IOException, ServletException
    {
        if ( (request instanceof HttpServletRequest) && (response instanceof HttpServletResponse) )
        {
            HttpServletResponse zResponse = (HttpServletResponse) response;
            HttpServletRequest zRequest = (HttpServletRequest) request;
            String zURI = zRequest.getRequestURI();

            if ( zURI.endsWith( ".nocache.js" ) )
            {
                zResponse.setHeader( "Cache-Control", "no-cache no-store must-revalidate" );
                zResponse.setHeader( "Pragma", "no-cache" ); // HTTP/1.0
                zResponse.setDateHeader( "Expires", 86400000 ); // January 2, 1970
            }
            else
            {
                // Everything else is cache "forever".
                //
                // the w3c spec requires a maximum age of 1 year
                // Firefox 3+ needs 'public' to cache this resource when received via SSL
                zResponse.setHeader( "Cache-Control", "public max-age=31536000" );

                // necessary to overwrite "Pragma: no-cache" header
                zResponse.setHeader( "Pragma", "temp" );
                zResponse.setHeader( "Pragma", "" );
                zResponse.setDateHeader( "Expires", System.currentTimeMillis() + 31536000000l );
            }
            // System.out.println( "VersionedStaticContentFilter.doFilter: " + zURI );
            int at = zURI.indexOf( "/common/" );
            if ( (at != -1) && zURI.startsWith( "/v" ) )
            {
                if ( isDigits( zURI.substring( 2, at ) ) )
                {
                    request.getRequestDispatcher( zURI.substring( at ) ).forward( request, response );
                    return;
                }
            }
        }
        chain.doFilter( request, response ); // Goes to default servlet.
    }

    private boolean isDigits( String pString )
    {
        if ( pString.length() == 0 )
        {
            return false;
        }
        for ( int i = 0; i < pString.length(); i++ )
        {
            char c = pString.charAt( i );
            if ( (c < '0') || ('9' < c) )
            {
                return false;
            }
        }
        return true;
    }

    @Override
    public void destroy()
    {
    }
}