AbstractProxyServlet.java

//
//  ========================================================================
//  Copyright (c) 1995-2020 Mort Bay Consulting Pty Ltd and others.
//  ------------------------------------------------------------------------
//  All rights reserved. This program and the accompanying materials
//  are made available under the terms of the Eclipse Public License v1.0
//  and Apache License v2.0 which accompanies this distribution.
//
//      The Eclipse Public License is available at
//      http://www.eclipse.org/legal/epl-v10.html
//
//      The Apache License v2.0 is available at
//      http://www.opensource.org/licenses/apache2.0.php
//
//  You may elect to redistribute this code under either of these licenses.
//  ========================================================================
//

package org.eclipse.jetty.proxy;

import java.net.InetAddress;
import java.net.URI;
import java.net.UnknownHostException;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Locale;
import java.util.Set;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeoutException;
import javax.servlet.AsyncContext;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.UnavailableException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.eclipse.jetty.client.ContinueProtocolHandler;
import org.eclipse.jetty.client.HttpClient;
import org.eclipse.jetty.client.ProtocolHandlers;
import org.eclipse.jetty.client.api.Request;
import org.eclipse.jetty.client.api.Response;
import org.eclipse.jetty.client.http.HttpClientTransportOverHTTP;
import org.eclipse.jetty.http.HttpField;
import org.eclipse.jetty.http.HttpHeader;
import org.eclipse.jetty.http.HttpHeaderValue;
import org.eclipse.jetty.http.HttpStatus;
import org.eclipse.jetty.util.HttpCookieStore;
import org.eclipse.jetty.util.ProcessorUtils;
import org.eclipse.jetty.util.StringUtil;
import org.eclipse.jetty.util.log.Log;
import org.eclipse.jetty.util.log.Logger;
import org.eclipse.jetty.util.thread.QueuedThreadPool;

/**
 * <p>Abstract base class for proxy servlets.</p>
 * <p>Forwards requests to another server either as a standard web reverse
 * proxy or as a transparent reverse proxy (as defined by RFC 7230).</p>
 * <p>To facilitate JMX monitoring, the {@link HttpClient} instance is set
 * as ServletContext attribute, prefixed with this servlet's name and
 * exposed by the mechanism provided by
 * {@link ServletContext#setAttribute(String, Object)}.</p>
 * <p>The following init parameters may be used to configure the servlet:</p>
 * <ul>
 * <li>preserveHost - the host header specified by the client is forwarded to the server</li>
 * <li>hostHeader - forces the host header to a particular value</li>
 * <li>viaHost - the name to use in the Via header: Via: http/1.1 &lt;viaHost&gt;</li>
 * <li>whiteList - comma-separated list of allowed proxy hosts</li>
 * <li>blackList - comma-separated list of forbidden proxy hosts</li>
 * </ul>
 * <p>In addition, see {@link #createHttpClient()} for init parameters
 * used to configure the {@link HttpClient} instance.</p>
 * <p>NOTE: By default the Host header sent to the server by this proxy
 * servlet is the server's host name. However, this breaks redirects.
 * Set {@code preserveHost} to {@code true} to make redirects working,
 * although this may break server's virtual host selection.</p>
 * <p>The default behavior of not preserving the Host header mimics
 * the default behavior of Apache httpd and Nginx, which both have
 * a way to be configured to preserve the Host header.</p>
 */
public abstract class AbstractProxyServlet extends HttpServlet
{
    protected static final String CLIENT_REQUEST_ATTRIBUTE = "org.eclipse.jetty.proxy.clientRequest";
    protected static final Set<String> HOP_HEADERS;

    static
    {
        Set<String> hopHeaders = new HashSet<>();
        hopHeaders.add("connection");
        hopHeaders.add("keep-alive");
        hopHeaders.add("proxy-authorization");
        hopHeaders.add("proxy-authenticate");
        hopHeaders.add("proxy-connection");
        hopHeaders.add("transfer-encoding");
        hopHeaders.add("te");
        hopHeaders.add("trailer");
        hopHeaders.add("upgrade");
        HOP_HEADERS = Collections.unmodifiableSet(hopHeaders);
    }

    private final Set<String> _whiteList = new HashSet<>();
    private final Set<String> _blackList = new HashSet<>();
    protected Logger _log;
    private boolean _preserveHost;
    private String _hostHeader;
    private String _viaHost;
    private HttpClient _client;
    private long _timeout;

    @Override
    public void init() throws ServletException
    {
        _log = createLogger();

        ServletConfig config = getServletConfig();

        _preserveHost = Boolean.parseBoolean(config.getInitParameter("preserveHost"));

        _hostHeader = config.getInitParameter("hostHeader");

        _viaHost = config.getInitParameter("viaHost");
        if (_viaHost == null)
            _viaHost = viaHost();

        try
        {
            _client = createHttpClient();

            // Put the HttpClient in the context to leverage ContextHandler.MANAGED_ATTRIBUTES
            getServletContext().setAttribute(config.getServletName() + ".HttpClient", _client);

            String whiteList = config.getInitParameter("whiteList");
            if (whiteList != null)
                getWhiteListHosts().addAll(parseList(whiteList));

            String blackList = config.getInitParameter("blackList");
            if (blackList != null)
                getBlackListHosts().addAll(parseList(blackList));
        }
        catch (Exception e)
        {
            throw new ServletException(e);
        }
    }

    @Override
    public void destroy()
    {
        try
        {
            _client.stop();
        }
        catch (Exception x)
        {
            if (_log.isDebugEnabled())
                _log.debug(x);
        }
    }

    public String getHostHeader()
    {
        return _hostHeader;
    }

    public String getViaHost()
    {
        return _viaHost == null ? viaHost() : _viaHost;
    }

    private static String viaHost()
    {
        try
        {
            return InetAddress.getLocalHost().getHostName();
        }
        catch (UnknownHostException x)
        {
            return "localhost";
        }
    }

    public long getTimeout()
    {
        return _timeout;
    }

    public void setTimeout(long timeout)
    {
        this._timeout = timeout;
    }

    public Set<String> getWhiteListHosts()
    {
        return _whiteList;
    }

    public Set<String> getBlackListHosts()
    {
        return _blackList;
    }

    /**
     * @return a logger instance with a name derived from this servlet's name.
     */
    protected Logger createLogger()
    {
        String servletName = getServletConfig().getServletName();
        servletName = StringUtil.replace(servletName, '-', '.');
        if ((getClass().getPackage() != null) && !servletName.startsWith(getClass().getPackage().getName()))
        {
            servletName = getClass().getName() + "." + servletName;
        }
        return Log.getLogger(servletName);
    }

    /**
     * <p>Creates a {@link HttpClient} instance, configured with init parameters of this servlet.</p>
     * <p>The init parameters used to configure the {@link HttpClient} instance are:</p>
     * <table>
     * <caption>Init Parameters</caption>
     * <thead>
     * <tr>
     * <th>init-param</th>
     * <th>default</th>
     * <th>description</th>
     * </tr>
     * </thead>
     * <tbody>
     * <tr>
     * <td>maxThreads</td>
     * <td>256</td>
     * <td>The max number of threads of HttpClient's Executor.  If not set, or set to the value of "-", then the
     * Jetty server thread pool will be used.</td>
     * </tr>
     * <tr>
     * <td>maxConnections</td>
     * <td>32768</td>
     * <td>The max number of connections per destination, see {@link HttpClient#setMaxConnectionsPerDestination(int)}</td>
     * </tr>
     * <tr>
     * <td>idleTimeout</td>
     * <td>30000</td>
     * <td>The idle timeout in milliseconds, see {@link HttpClient#setIdleTimeout(long)}</td>
     * </tr>
     * <tr>
     * <td>timeout</td>
     * <td>60000</td>
     * <td>The total timeout in milliseconds, see {@link Request#timeout(long, java.util.concurrent.TimeUnit)}</td>
     * </tr>
     * <tr>
     * <td>requestBufferSize</td>
     * <td>HttpClient's default</td>
     * <td>The request buffer size, see {@link HttpClient#setRequestBufferSize(int)}</td>
     * </tr>
     * <tr>
     * <td>responseBufferSize</td>
     * <td>HttpClient's default</td>
     * <td>The response buffer size, see {@link HttpClient#setResponseBufferSize(int)}</td>
     * </tr>
     * <tr>
     * <td>selectors</td>
     * <td>cores / 2</td>
     * <td>The number of NIO selectors used by {@link HttpClient}</td>
     * </tr>
     * </tbody>
     * </table>
     *
     * @return a {@link HttpClient} configured from the {@link #getServletConfig() servlet configuration}
     * @throws ServletException if the {@link HttpClient} cannot be created
     * @see #newHttpClient()
     */
    protected HttpClient createHttpClient() throws ServletException
    {
        ServletConfig config = getServletConfig();

        HttpClient client = newHttpClient();

        // Redirects must be proxied as is, not followed.
        client.setFollowRedirects(false);

        // Must not store cookies, otherwise cookies of different clients will mix.
        client.setCookieStore(new HttpCookieStore.Empty());

        Executor executor;
        String value = config.getInitParameter("maxThreads");
        if (value == null || "-".equals(value))
        {
            executor = (Executor)getServletContext().getAttribute("org.eclipse.jetty.server.Executor");
            if (executor == null)
                throw new IllegalStateException("No server executor for proxy");
        }
        else
        {
            QueuedThreadPool qtp = new QueuedThreadPool(Integer.parseInt(value));
            String servletName = config.getServletName();
            int dot = servletName.lastIndexOf('.');
            if (dot >= 0)
                servletName = servletName.substring(dot + 1);
            qtp.setName(servletName);
            executor = qtp;
        }

        client.setExecutor(executor);

        value = config.getInitParameter("maxConnections");
        if (value == null)
            value = "256";
        client.setMaxConnectionsPerDestination(Integer.parseInt(value));

        value = config.getInitParameter("idleTimeout");
        if (value == null)
            value = "30000";
        client.setIdleTimeout(Long.parseLong(value));

        value = config.getInitParameter("timeout");
        if (value == null)
            value = "60000";
        _timeout = Long.parseLong(value);

        value = config.getInitParameter("requestBufferSize");
        if (value != null)
            client.setRequestBufferSize(Integer.parseInt(value));

        value = config.getInitParameter("responseBufferSize");
        if (value != null)
            client.setResponseBufferSize(Integer.parseInt(value));

        try
        {
            client.start();

            // Content must not be decoded, otherwise the client gets confused.
            client.getContentDecoderFactories().clear();

            // Pass traffic to the client, only intercept what's necessary.
            ProtocolHandlers protocolHandlers = client.getProtocolHandlers();
            protocolHandlers.clear();
            protocolHandlers.put(new ProxyContinueProtocolHandler());

            return client;
        }
        catch (Exception x)
        {
            throw new ServletException(x);
        }
    }

    /**
     * The servlet init parameter 'selectors' can be set for the number of
     * selector threads to be used by the HttpClient.
     *
     * @return a new HttpClient instance
     */
    protected HttpClient newHttpClient()
    {
        int selectors = Math.max(1, ProcessorUtils.availableProcessors() / 2);
        String value = getServletConfig().getInitParameter("selectors");
        if (value != null)
            selectors = Integer.parseInt(value);
        return new HttpClient(new HttpClientTransportOverHTTP(selectors), null);
    }

    protected HttpClient getHttpClient()
    {
        return _client;
    }

    private Set<String> parseList(String list)
    {
        Set<String> result = new HashSet<>();
        String[] hosts = list.split(",");
        for (String host : hosts)
        {
            host = host.trim();
            if (host.length() == 0)
                continue;
            result.add(host);
        }
        return result;
    }

    /**
     * Checks the given {@code host} and {@code port} against whitelist and blacklist.
     *
     * @param host the host to check
     * @param port the port to check
     * @return true if it is allowed to be proxy to the given host and port
     */
    public boolean validateDestination(String host, int port)
    {
        String hostPort = host + ":" + port;
        if (!_whiteList.isEmpty())
        {
            if (!_whiteList.contains(hostPort))
            {
                if (_log.isDebugEnabled())
                    _log.debug("Host {}:{} not whitelisted", host, port);
                return false;
            }
        }
        if (!_blackList.isEmpty())
        {
            if (_blackList.contains(hostPort))
            {
                if (_log.isDebugEnabled())
                    _log.debug("Host {}:{} blacklisted", host, port);
                return false;
            }
        }
        return true;
    }

    protected String rewriteTarget(HttpServletRequest clientRequest)
    {
        if (!validateDestination(clientRequest.getServerName(), clientRequest.getServerPort()))
            return null;

        StringBuffer target = clientRequest.getRequestURL();
        String query = clientRequest.getQueryString();
        if (query != null)
            target.append("?").append(query);
        return target.toString();
    }

    /**
     * <p>Callback method invoked when the URI rewrite performed
     * in {@link #rewriteTarget(HttpServletRequest)} returns null
     * indicating that no rewrite can be performed.</p>
     * <p>It is possible to use blocking API in this method,
     * like {@link HttpServletResponse#sendError(int)}.</p>
     *
     * @param clientRequest the client request
     * @param proxyResponse the client response
     */
    protected void onProxyRewriteFailed(HttpServletRequest clientRequest, HttpServletResponse proxyResponse)
    {
        sendProxyResponseError(clientRequest, proxyResponse, HttpStatus.FORBIDDEN_403);
    }

    protected boolean hasContent(HttpServletRequest clientRequest)
    {
        return clientRequest.getContentLength() > 0 ||
            clientRequest.getContentType() != null ||
            clientRequest.getHeader(HttpHeader.TRANSFER_ENCODING.asString()) != null;
    }

    protected boolean expects100Continue(HttpServletRequest request)
    {
        return HttpHeaderValue.CONTINUE.is(request.getHeader(HttpHeader.EXPECT.asString()));
    }

    protected void copyRequestHeaders(HttpServletRequest clientRequest, Request proxyRequest)
    {
        // First clear possibly existing headers, as we are going to copy those from the client request.
        proxyRequest.getHeaders().clear();

        Set<String> headersToRemove = findConnectionHeaders(clientRequest);

        for (Enumeration<String> headerNames = clientRequest.getHeaderNames(); headerNames.hasMoreElements(); )
        {
            String headerName = headerNames.nextElement();
            String lowerHeaderName = headerName.toLowerCase(Locale.ENGLISH);

            if (HttpHeader.HOST.is(headerName) && !_preserveHost)
                continue;

            // Remove hop-by-hop headers.
            if (HOP_HEADERS.contains(lowerHeaderName))
                continue;
            if (headersToRemove != null && headersToRemove.contains(lowerHeaderName))
                continue;

            for (Enumeration<String> headerValues = clientRequest.getHeaders(headerName); headerValues.hasMoreElements(); )
            {
                String headerValue = headerValues.nextElement();
                if (headerValue != null)
                    proxyRequest.header(headerName, headerValue);
            }
        }

        // Force the Host header if configured
        if (_hostHeader != null)
            proxyRequest.header(HttpHeader.HOST, _hostHeader);
    }

    protected Set<String> findConnectionHeaders(HttpServletRequest clientRequest)
    {
        // Any header listed by the Connection header must be removed:
        // http://tools.ietf.org/html/rfc7230#section-6.1.
        Set<String> hopHeaders = null;
        Enumeration<String> connectionHeaders = clientRequest.getHeaders(HttpHeader.CONNECTION.asString());
        while (connectionHeaders.hasMoreElements())
        {
            String value = connectionHeaders.nextElement();
            String[] values = value.split(",");
            for (String name : values)
            {
                name = name.trim().toLowerCase(Locale.ENGLISH);
                if (hopHeaders == null)
                    hopHeaders = new HashSet<>();
                hopHeaders.add(name);
            }
        }
        return hopHeaders;
    }

    protected void addProxyHeaders(HttpServletRequest clientRequest, Request proxyRequest)
    {
        addViaHeader(clientRequest, proxyRequest);
        addXForwardedHeaders(clientRequest, proxyRequest);
    }

    /**
     * Adds the HTTP Via header to the proxied request.
     *
     * @deprecated Use {@link #addViaHeader(HttpServletRequest, Request)} instead.
     * @param proxyRequest the request being proxied
     */
    @Deprecated
    protected void addViaHeader(Request proxyRequest)
    {
        proxyRequest.header(HttpHeader.VIA, "http/1.1 " + getViaHost());
    }

    /**
     * Adds the HTTP Via header to the proxied request, taking into account data present in the client request.
     * This method considers the protocol of the client request when forming the proxied request. If it
     * is HTTP, then the protocol name will not be included in the Via header that is sent by the proxy, and only
     * the protocol version will be sent. If it is not, the entire protocol (name and version) will be included. 
     * If the client request includes a Via header, the result will be appended to that to form a chain.
     *
     * @since 9.4.32
     * @param clientRequest the client request
     * @param proxyRequest the request being proxied
     * @see <a href="https://tools.ietf.org/html/rfc7230#section-5.7.1">RFC 7230 section 5.7.1</a>
     */
    protected void addViaHeader(HttpServletRequest clientRequest, Request proxyRequest)
    {
        String protocol = clientRequest.getProtocol();
        String[] parts = protocol.split("/", 2);
        String protocolName = parts.length == 2 && "HTTP".equals(parts[0]) ? parts[1] : protocol;
        String viaHeaderValue = "";
        String clientViaHeader = clientRequest.getHeader(HttpHeader.VIA.name());

        if (clientViaHeader != null)
            viaHeaderValue = clientViaHeader;

        viaHeaderValue += protocolName + " " + getViaHost();

        proxyRequest.header(HttpHeader.VIA, viaHeaderValue);
    }

    protected void addXForwardedHeaders(HttpServletRequest clientRequest, Request proxyRequest)
    {
        proxyRequest.header(HttpHeader.X_FORWARDED_FOR, clientRequest.getRemoteAddr());
        proxyRequest.header(HttpHeader.X_FORWARDED_PROTO, clientRequest.getScheme());
        proxyRequest.header(HttpHeader.X_FORWARDED_HOST, clientRequest.getHeader(HttpHeader.HOST.asString()));
        proxyRequest.header(HttpHeader.X_FORWARDED_SERVER, clientRequest.getLocalName());
    }

    protected void sendProxyRequest(HttpServletRequest clientRequest, HttpServletResponse proxyResponse, Request proxyRequest)
    {
        if (_log.isDebugEnabled())
        {
            StringBuilder builder = new StringBuilder(clientRequest.getMethod());
            builder.append(" ").append(clientRequest.getRequestURI());
            String query = clientRequest.getQueryString();
            if (query != null)
                builder.append("?").append(query);
            builder.append(" ").append(clientRequest.getProtocol()).append(System.lineSeparator());
            for (Enumeration<String> headerNames = clientRequest.getHeaderNames(); headerNames.hasMoreElements(); )
            {
                String headerName = headerNames.nextElement();
                builder.append(headerName).append(": ");
                for (Enumeration<String> headerValues = clientRequest.getHeaders(headerName); headerValues.hasMoreElements(); )
                {
                    String headerValue = headerValues.nextElement();
                    if (headerValue != null)
                        builder.append(headerValue);
                    if (headerValues.hasMoreElements())
                        builder.append(",");
                }
                builder.append(System.lineSeparator());
            }
            builder.append(System.lineSeparator());

            _log.debug("{} proxying to upstream:{}{}{}{}{}",
                getRequestId(clientRequest),
                System.lineSeparator(),
                builder,
                proxyRequest,
                System.lineSeparator(),
                proxyRequest.getHeaders().toString().trim());
        }

        proxyRequest.send(newProxyResponseListener(clientRequest, proxyResponse));
    }

    protected abstract Response.CompleteListener newProxyResponseListener(HttpServletRequest clientRequest, HttpServletResponse proxyResponse);

    protected void onClientRequestFailure(HttpServletRequest clientRequest, Request proxyRequest, HttpServletResponse proxyResponse, Throwable failure)
    {
        boolean aborted = proxyRequest.abort(failure);
        if (!aborted)
        {
            int status = clientRequestStatus(failure);
            sendProxyResponseError(clientRequest, proxyResponse, status);
        }
    }

    protected int clientRequestStatus(Throwable failure)
    {
        return failure instanceof TimeoutException
            ? HttpStatus.REQUEST_TIMEOUT_408
            : HttpStatus.INTERNAL_SERVER_ERROR_500;
    }

    protected void onServerResponseHeaders(HttpServletRequest clientRequest, HttpServletResponse proxyResponse, Response serverResponse)
    {
        for (HttpField field : serverResponse.getHeaders())
        {
            String headerName = field.getName();
            String lowerHeaderName = headerName.toLowerCase(Locale.ENGLISH);
            if (HOP_HEADERS.contains(lowerHeaderName))
                continue;

            String newHeaderValue = filterServerResponseHeader(clientRequest, serverResponse, headerName, field.getValue());
            if (newHeaderValue == null || newHeaderValue.trim().length() == 0)
                continue;

            proxyResponse.addHeader(headerName, newHeaderValue);
        }

        if (_log.isDebugEnabled())
        {
            StringBuilder builder = new StringBuilder(System.lineSeparator());
            builder.append(clientRequest.getProtocol()).append(" ").append(proxyResponse.getStatus())
                .append(" ").append(serverResponse.getReason()).append(System.lineSeparator());
            for (String headerName : proxyResponse.getHeaderNames())
            {
                builder.append(headerName).append(": ");
                for (Iterator<String> headerValues = proxyResponse.getHeaders(headerName).iterator(); headerValues.hasNext(); )
                {
                    String headerValue = headerValues.next();
                    if (headerValue != null)
                        builder.append(headerValue);
                    if (headerValues.hasNext())
                        builder.append(",");
                }
                builder.append(System.lineSeparator());
            }
            _log.debug("{} proxying to downstream:{}{}{}{}{}",
                getRequestId(clientRequest),
                System.lineSeparator(),
                serverResponse,
                System.lineSeparator(),
                serverResponse.getHeaders().toString().trim(),
                System.lineSeparator(),
                builder);
        }
    }

    protected String filterServerResponseHeader(HttpServletRequest clientRequest, Response serverResponse, String headerName, String headerValue)
    {
        return headerValue;
    }

    protected void onProxyResponseSuccess(HttpServletRequest clientRequest, HttpServletResponse proxyResponse, Response serverResponse)
    {
        if (_log.isDebugEnabled())
            _log.debug("{} proxying successful", getRequestId(clientRequest));

        AsyncContext asyncContext = clientRequest.getAsyncContext();
        asyncContext.complete();
    }

    protected void onProxyResponseFailure(HttpServletRequest clientRequest, HttpServletResponse proxyResponse, Response serverResponse, Throwable failure)
    {
        if (_log.isDebugEnabled())
            _log.debug(getRequestId(clientRequest) + " proxying failed", failure);

        int status = proxyResponseStatus(failure);
        int serverStatus = serverResponse == null ? status : serverResponse.getStatus();
        if (expects100Continue(clientRequest) && serverStatus >= HttpStatus.OK_200)
            status = serverStatus;
        sendProxyResponseError(clientRequest, proxyResponse, status);
    }

    protected int proxyResponseStatus(Throwable failure)
    {
        return failure instanceof TimeoutException
            ? HttpStatus.GATEWAY_TIMEOUT_504
            : HttpStatus.BAD_GATEWAY_502;
    }

    protected int getRequestId(HttpServletRequest clientRequest)
    {
        return System.identityHashCode(clientRequest);
    }

    protected void sendProxyResponseError(HttpServletRequest clientRequest, HttpServletResponse proxyResponse, int status)
    {
        try
        {
            if (!proxyResponse.isCommitted())
            {
                proxyResponse.resetBuffer();
                proxyResponse.setHeader(HttpHeader.CONNECTION.asString(), HttpHeaderValue.CLOSE.asString());
            }
            proxyResponse.sendError(status);
        }
        catch (Exception e)
        {
            _log.ignore(e);
            try
            {
                proxyResponse.sendError(-1);
            }
            catch (Exception e2)
            {
                _log.ignore(e2);
            }
        }
        finally
        {
            if (clientRequest.isAsyncStarted())
                clientRequest.getAsyncContext().complete();
        }
    }

    protected void onContinue(HttpServletRequest clientRequest, Request proxyRequest)
    {
        if (_log.isDebugEnabled())
            _log.debug("{} handling 100 Continue", getRequestId(clientRequest));
    }

    /**
     * <p>Utility class that implement transparent proxy functionalities.</p>
     * <p>Configuration parameters:</p>
     * <ul>
     * <li>{@code proxyTo} - a mandatory URI like http://host:80/context to which the request is proxied.</li>
     * <li>{@code prefix} - an optional URI prefix that is stripped from the start of the forwarded URI.</li>
     * </ul>
     * <p>For example, if a request is received at "/foo/bar", the {@code proxyTo} parameter is
     * "http://host:80/context" and the {@code prefix} parameter is "/foo", then the request would
     * be proxied to "http://host:80/context/bar".
     */
    protected static class TransparentDelegate
    {
        private final AbstractProxyServlet proxyServlet;
        private String _proxyTo;
        private String _prefix;

        protected TransparentDelegate(AbstractProxyServlet proxyServlet)
        {
            this.proxyServlet = proxyServlet;
        }

        protected void init(ServletConfig config) throws ServletException
        {
            _proxyTo = config.getInitParameter("proxyTo");
            if (_proxyTo == null)
                throw new UnavailableException("Init parameter 'proxyTo' is required.");

            String prefix = config.getInitParameter("prefix");
            if (prefix != null)
            {
                if (!prefix.startsWith("/"))
                    throw new UnavailableException("Init parameter 'prefix' must start with a '/'.");
                _prefix = prefix;
            }

            // Adjust prefix value to account for context path
            String contextPath = config.getServletContext().getContextPath();
            _prefix = _prefix == null ? contextPath : (contextPath + _prefix);

            if (proxyServlet._log.isDebugEnabled())
                proxyServlet._log.debug(config.getServletName() + " @ " + _prefix + " to " + _proxyTo);
        }

        protected String rewriteTarget(HttpServletRequest request)
        {
            String path = request.getRequestURI();
            if (!path.startsWith(_prefix))
                return null;

            StringBuilder uri = new StringBuilder(_proxyTo);
            if (_proxyTo.endsWith("/"))
                uri.setLength(uri.length() - 1);
            String rest = path.substring(_prefix.length());
            if (!rest.isEmpty())
            {
                if (!rest.startsWith("/"))
                    uri.append("/");
                uri.append(rest);
            }

            String query = request.getQueryString();
            if (query != null)
            {
                // Is there at least one path segment ?
                String separator = "://";
                if (uri.indexOf("/", uri.indexOf(separator) + separator.length()) < 0)
                    uri.append("/");
                uri.append("?").append(query);
            }
            URI rewrittenURI = URI.create(uri.toString()).normalize();

            if (!proxyServlet.validateDestination(rewrittenURI.getHost(), rewrittenURI.getPort()))
                return null;

            return rewrittenURI.toString();
        }
    }

    class ProxyContinueProtocolHandler extends ContinueProtocolHandler
    {
        @Override
        protected void onContinue(Request request)
        {
            HttpServletRequest clientRequest = (HttpServletRequest)request.getAttributes().get(CLIENT_REQUEST_ATTRIBUTE);
            AbstractProxyServlet.this.onContinue(clientRequest, request);
        }
    }
}