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org/apache/commons/httpclient/MultiThreadedHttpConnectionManager.java
/*
 * $Header: /home/cvs/jakarta-commons/httpclient/src/java/org/apache/commons/httpclient/MultiThreadedHttpConnectionManager.java,v 1.17.2.8 2004/03/28 21:06:24 mbecke Exp $
 * $Revision: 1.17.2.8 $
 * $Date: 2004/03/28 21:06:24 $
 *
 * ====================================================================
 *
 *  Copyright 2002-2004 The Apache Software Foundation
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * <http://www.apache.org/>.
 *
 * [Additional notices, if required by prior licensing conditions]
 *
 */

package org.apache.commons.httpclient;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.net.InetAddress;
import java.net.SocketException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Map;
import java.util.WeakHashMap;

import org.apache.commons.httpclient.protocol.Protocol;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Manages a set of HttpConnections for various HostConfigurations.
 *
 @author <a href="mailto:becke@u.washington.edu">Michael Becke</a>
 @author Eric Johnson
 @author <a href="mailto:mbowler@GargoyleSoftware.com">Mike Bowler</a>
 @author Carl A. Dunham
 *
 @since 2.0
 */
public class MultiThreadedHttpConnectionManager implements HttpConnectionManager {

    // -------------------------------------------------------- Class Variables
    /** Log object for this class. */
    private static final Log LOG = LogFactory.getLog(MultiThreadedHttpConnectionManager.class);

    /** The default maximum number of connections allowed per host */
    public static final int DEFAULT_MAX_HOST_CONNECTIONS = 2;   // Per RFC 2616 sec 8.1.4

    /** The default maximum number of connections allowed overall */
    public static final int DEFAULT_MAX_TOTAL_CONNECTIONS = 20;

    /**
     * A mapping from Reference to ConnectionSource.  Used to reclaim resources when connections
     * are lost to the garbage collector.
     */
    private static final Map REFERENCE_TO_CONNECTION_SOURCE = new HashMap();
    
    /**
     * The reference queue used to track when HttpConnections are lost to the
     * garbage collector
     */
    private static final ReferenceQueue REFERENCE_QUEUE = new ReferenceQueue();

    /**
     * The thread responsible for handling lost connections.
     */
    private static ReferenceQueueThread REFERENCE_QUEUE_THREAD;
    
    /**
     * Holds references to all active instances of this class.
     */    
    private static WeakHashMap ALL_CONNECTION_MANAGERS = new WeakHashMap();
    
    /**
     * Shuts down and cleans up resources used by all instances of 
     * MultiThreadedHttpConnectionManager. All static resources are released, all threads are 
     * stopped, and {@link #shutdown()} is called on all live instaces of 
     * MultiThreadedHttpConnectionManager.
     *
     @see #shutdown()
     */
    public static void shutdownAll() {

        synchronized (REFERENCE_TO_CONNECTION_SOURCE) {
            // shutdown all connection managers
            synchronized (ALL_CONNECTION_MANAGERS) {
                Iterator connIter = ALL_CONNECTION_MANAGERS.keySet().iterator();
                while (connIter.hasNext()) {
                    MultiThreadedHttpConnectionManager connManager = 
                        (MultiThreadedHttpConnectionManagerconnIter.next();
                    connIter.remove();
                    connManager.shutdown();
                }
            }
            
            // shutdown static resources
            if (REFERENCE_QUEUE_THREAD != null) {
                REFERENCE_QUEUE_THREAD.shutdown();
                REFERENCE_QUEUE_THREAD = null;
            }
            REFERENCE_TO_CONNECTION_SOURCE.clear();
        }        
    }
    
    /**
     * Stores the reference to the given connection along with the hostConfig and connection pool.  
     * These values will be used to reclaim resources if the connection is lost to the garbage 
     * collector.  This method should be called before a connection is released from the connection 
     * manager.
     
     <p>A static reference to the connection manager will also be stored.  To ensure that
     * the connection manager can be GCed {@link #removeReferenceToConnection(HttpConnection)}
     * should be called for all connections that the connection manager is storing a reference
     * to.</p>
     
     @param connection the connection to create a reference for
     @param hostConfiguration the connection's host config
     @param connectionPool the connection pool that created the connection
     
     @see #removeReferenceToConnection(HttpConnection)
     */
    private static void storeReferenceToConnection(
        HttpConnectionWithReference connection,
        HostConfiguration hostConfiguration,
        ConnectionPool connectionPool
    ) {
        
        ConnectionSource source = new ConnectionSource();
        source.connectionPool = connectionPool;
        source.hostConfiguration = hostConfiguration;
        
        synchronized (REFERENCE_TO_CONNECTION_SOURCE) {
            
            // start the reference queue thread if needed
            if (REFERENCE_QUEUE_THREAD == null) {
                REFERENCE_QUEUE_THREAD = new ReferenceQueueThread();
                REFERENCE_QUEUE_THREAD.start();
            }
            
            REFERENCE_TO_CONNECTION_SOURCE.put(
                connection.reference,
                source
            );
        }
    }
    
    /**
     * Closes and releases all connections currently checked out of the given connection pool.
     @param connectionPool the connection pool to shutdown the connections for
     */
    private static void shutdownCheckedOutConnections(ConnectionPool connectionPool) {

        // keep a list of the connections to be closed
        ArrayList connectionsToClose = new ArrayList()
        
        synchronized (REFERENCE_TO_CONNECTION_SOURCE) {
            
            Iterator referenceIter = REFERENCE_TO_CONNECTION_SOURCE.keySet().iterator();
            while (referenceIter.hasNext()) {
                Reference ref = (ReferencereferenceIter.next();
                ConnectionSource source = 
                    (ConnectionSourceREFERENCE_TO_CONNECTION_SOURCE.get(ref);
                if (source.connectionPool == connectionPool) {
                    referenceIter.remove();
                    HttpConnection connection = (HttpConnectionref.get();
                    if (connection != null) {
                        connectionsToClose.add(connection);
                    }
                }
            }
        }

        // close and release the connections outside of the synchronized block to
        // avoid holding the lock for too long
        for (Iterator i = connectionsToClose.iterator(); i.hasNext();) {
            HttpConnection connection = (HttpConnectioni.next();
            connection.close();
            // remove the reference to the connection manager. this ensures
            // that the we don't accidentally end up here again
            connection.setHttpConnectionManager(null);
            connection.releaseConnection();
        }
    }
    
    /**
     * Removes the reference being stored for the given connection.  This method should be called
     * when the connection manager again has a direct reference to the connection.
     
     @param connection the connection to remove the reference for
     
     @see #storeReferenceToConnection(HttpConnection, HostConfiguration, ConnectionPool)
     */
    private static void removeReferenceToConnection(HttpConnectionWithReference connection) {
        
        synchronized (REFERENCE_TO_CONNECTION_SOURCE) {
            REFERENCE_TO_CONNECTION_SOURCE.remove(connection.reference);
        }
    }
    
    // ----------------------------------------------------- Instance Variables
    /** Maximum number of connections allowed per host */
    private int maxHostConnections = DEFAULT_MAX_HOST_CONNECTIONS;

    /** Maximum number of connections allowed overall */
    private int maxTotalConnections = DEFAULT_MAX_TOTAL_CONNECTIONS;

    /** The value to set when calling setStaleCheckingEnabled() on each connection */
    private boolean connectionStaleCheckingEnabled = true;

    private boolean shutdown = false;
    
    /** Connection Pool */
    private ConnectionPool connectionPool;

    /**
     * No-args constructor
     */
    public MultiThreadedHttpConnectionManager() {
        this.connectionPool = new ConnectionPool();
        synchronized(ALL_CONNECTION_MANAGERS) {
            ALL_CONNECTION_MANAGERS.put(this, null);
        }
    }

    /**
     * Shuts down the connection manager and releases all resources.  All connections associated 
     * with this class will be closed and released. 
     
     <p>The connection manager can no longer be used once shutdown.  
     
     <p>Calling this method more than once will have no effect.
     */
    public synchronized void shutdown() {
        synchronized (connectionPool) {
            if (!shutdown) {
                shutdown = true;
                connectionPool.shutdown();
            }
        }
    }
    
    /**
     * Gets the staleCheckingEnabled value to be set on HttpConnections that are created.
     
     @return <code>true</code> if stale checking will be enabled on HttpConections
     
     @see HttpConnection#isStaleCheckingEnabled()
     */
    public boolean isConnectionStaleCheckingEnabled() {
        return connectionStaleCheckingEnabled;
    }

    /**
     * Sets the staleCheckingEnabled value to be set on HttpConnections that are created.
     
     @param connectionStaleCheckingEnabled <code>true</code> if stale checking will be enabled 
     * on HttpConections
     
     @see HttpConnection#setStaleCheckingEnabled(boolean)
     */
    public void setConnectionStaleCheckingEnabled(boolean connectionStaleCheckingEnabled) {
        this.connectionStaleCheckingEnabled = connectionStaleCheckingEnabled;
    }

    /**
     * Sets the maximum number of connections allowed for a given
     * HostConfiguration. Per RFC 2616 section 8.1.4, this value defaults to 2.
     *
     @param maxHostConnections the number of connections allowed for each
     * hostConfiguration
     */
    public void setMaxConnectionsPerHost(int maxHostConnections) {
        this.maxHostConnections = maxHostConnections;
    }

    /**
     * Gets the maximum number of connections allowed for a given
     * hostConfiguration.
     *
     @return The maximum number of connections allowed for a given
     * hostConfiguration.
     */
    public int getMaxConnectionsPerHost() {
        return maxHostConnections;
    }

    /**
     * Sets the maximum number of connections allowed in the system.
     *
     @param maxTotalConnections the maximum number of connections allowed
     */
    public void setMaxTotalConnections(int maxTotalConnections) {
        this.maxTotalConnections = maxTotalConnections;
    }

    /**
     * Gets the maximum number of connections allowed in the system.
     *
     @return The maximum number of connections allowed
     */
    public int getMaxTotalConnections() {
        return maxTotalConnections;
    }

    /**
     @see HttpConnectionManager#getConnection(HostConfiguration)
     */
    public HttpConnection getConnection(HostConfiguration hostConfiguration) {

        while (true) {
            try {
                return getConnection(hostConfiguration, 0);
            catch (HttpException e) {
                // we'll go ahead and log this, but it should never happen. HttpExceptions
                // are only thrown when the timeout occurs and since we have no timeout
                // it should never happen.
                LOG.debug(
                    "Unexpected exception while waiting for connection",
                    e
                );
            };
        }
    }

    /**
     @see HttpConnectionManager#getConnection(HostConfiguration, long)
     */
    public HttpConnection getConnection(HostConfiguration hostConfiguration, 
        long timeoutthrows HttpException {

        LOG.trace("enter HttpConnectionManager.getConnection(HostConfiguration, long)");

        if (hostConfiguration == null) {
            throw new IllegalArgumentException("hostConfiguration is null");
        }

        if (LOG.isDebugEnabled()) {
            LOG.debug("HttpConnectionManager.getConnection:  config = "
                + hostConfiguration + ", timeout = " + timeout);
        }

        final HttpConnection conn = doGetConnection(hostConfiguration, timeout);

        // wrap the connection in an adapter so we can ensure it is used 
        // only once
        return new HttpConnectionAdapter(conn);
    }

    /**
     * Gets a connection or waits if one is not available.  A connection is
     * available if one exists that is not being used or if fewer than
     * maxHostConnections have been created in the connectionPool, and fewer
     * than maxTotalConnections have been created in all connectionPools.
     *
     @param hostConfiguration The host configuration.
     @param timeout the number of milliseconds to wait for a connection, 0 to
     * wait indefinitely
     *
     @return HttpConnection an available connection
     *
     @throws HttpException if a connection does not become available in
     * 'timeout' milliseconds
     */
    private HttpConnection doGetConnection(HostConfiguration hostConfiguration, 
        long timeoutthrows HttpException {

        HttpConnection connection = null;

        synchronized (connectionPool) {

            // we clone the hostConfiguration
            // so that it cannot be changed once the connection has been retrieved
            hostConfiguration = new HostConfiguration(hostConfiguration);
            HostConnectionPool hostPool = connectionPool.getHostPool(hostConfiguration);
            WaitingThread waitingThread = null;

            boolean useTimeout = (timeout > 0);
            long timeToWait = timeout;
            long startWait = 0;
            long endWait = 0;

            while (connection == null) {

                if (shutdown) {
                    throw new IllegalStateException("Connection factory has been shutdown.");
                }
                
                // happen to have a free connection with the right specs
                //
                if (hostPool.freeConnections.size() 0) {
                    connection = connectionPool.getFreeConnection(hostConfiguration);

                // have room to make more
                //
                else if ((hostPool.numConnections < maxHostConnections
                    && (connectionPool.numConnections < maxTotalConnections)) {

                    connection = connectionPool.createConnection(hostConfiguration);

                // have room to add host connection, and there is at least one free
                // connection that can be liberated to make overall room
                //
                else if ((hostPool.numConnections < maxHostConnections
                    && (connectionPool.freeConnections.size() 0)) {

                    connectionPool.deleteLeastUsedConnection();
                    connection = connectionPool.createConnection(hostConfiguration);

                // otherwise, we have to wait for one of the above conditions to
                // become true
                //
                else {
                    // TODO: keep track of which hostConfigurations have waiting
                    // threads, so they avoid being sacrificed before necessary

                    try {
                        
                        if (useTimeout && timeToWait <= 0) {
                            throw new HttpException("Timeout waiting for connection");
                        }
                        
                        if (LOG.isDebugEnabled()) {
                            LOG.debug("Unable to get a connection, waiting..., hostConfig=" + hostConfiguration);
                        }
                        
                        if (waitingThread == null) {
                            waitingThread = new WaitingThread();
                            waitingThread.hostConnectionPool = hostPool;
                            waitingThread.thread = Thread.currentThread();
                        }
                                    
                        if (useTimeout) {
                            startWait = System.currentTimeMillis();
                        }
                        
                        hostPool.waitingThreads.addLast(waitingThread);
                        connectionPool.waitingThreads.addLast(waitingThread);
                        connectionPool.wait(timeToWait);
                        
                        // we have not been interrupted so we need to remove ourselves from the 
                        // wait queue
                        hostPool.waitingThreads.remove(waitingThread);
                        connectionPool.waitingThreads.remove(waitingThread);
                    catch (InterruptedException e) {
                        // do nothing
                    finally {
                        if (useTimeout) {
                            endWait = System.currentTimeMillis();
                            timeToWait -= (endWait - startWait);
                        }
                    }
                }
            }
        }
        return connection;
    }

    /**
     * Gets the number of connections in use for this configuration.
     *
     @param hostConfiguration the key that connections are tracked on
     @return the number of connections in use
     */
    public int getConnectionsInUse(HostConfiguration hostConfiguration) {
        synchronized (connectionPool) {
            HostConnectionPool hostPool = connectionPool.getHostPool(hostConfiguration);
            return hostPool.numConnections;
        }
    }

    /**
     * Gets the total number of connections in use.
     
     @return the total number of connections in use
     */
    public int getConnectionsInUse() {
        synchronized (connectionPool) {
            return connectionPool.numConnections;
        }
    }

    /**
     * Make the given HttpConnection available for use by other requests.
     * If another thread is blocked in getConnection() that could use this
     * connection, it will be woken up.
     *
     @param conn the HttpConnection to make available.
     */
    public void releaseConnection(HttpConnection conn) {
        LOG.trace("enter HttpConnectionManager.releaseConnection(HttpConnection)");

        if (conn instanceof HttpConnectionAdapter) {
            // connections given out are wrapped in an HttpConnectionAdapter
            conn = ((HttpConnectionAdapterconn).getWrappedConnection();
        else {
            // this is okay, when an HttpConnectionAdapter is released
            // is releases the real connection
        }

        // make sure that the response has been read.
        SimpleHttpConnectionManager.finishLastResponse(conn);

        connectionPool.freeConnection(conn);
    }

    /**
     * Gets the host configuration for a connection.
     @param conn the connection to get the configuration of
     @return a new HostConfiguration
     */
    private HostConfiguration configurationForConnection(HttpConnection conn) {

        HostConfiguration connectionConfiguration = new HostConfiguration();
        
        connectionConfiguration.setHost(
            conn.getHost()
            conn.getVirtualHost()
            conn.getPort()
            conn.getProtocol()
        );
        if (conn.getLocalAddress() != null) {
            connectionConfiguration.setLocalAddress(conn.getLocalAddress());
        }
        if (conn.getProxyHost() != null) {
            connectionConfiguration.setProxy(conn.getProxyHost(), conn.getProxyPort());
        }

        return connectionConfiguration;
    }
    

    /**
     * Global Connection Pool, including per-host pools
     */
    private class ConnectionPool {
        
        /** The list of free connections */
        private LinkedList freeConnections = new LinkedList();

        /** The list of WaitingThreads waiting for a connection */
        private LinkedList waitingThreads = new LinkedList();

        /**
         * Map where keys are {@link HostConfiguration}s and values are {@link
         * HostConnectionPool}s
         */
        private final Map mapHosts = new HashMap();

        /** The number of created connections */
        private int numConnections = 0;

        /**
         * Cleans up all connection pool resources.
         */
        public synchronized void shutdown() {
            
            // close all free connections
            Iterator iter = freeConnections.iterator();
            while (iter.hasNext()) {
                HttpConnection conn = (HttpConnectioniter.next();
                iter.remove();
                conn.close();
            }
            
            // close all connections that have been checked out
            shutdownCheckedOutConnections(this);
            
            // interrupt all waiting threads
            iter = waitingThreads.iterator();
            while (iter.hasNext()) {
                WaitingThread waiter = (WaitingThreaditer.next();
                iter.remove();
                waiter.thread.interrupt();
            }
            
            // clear out map hosts
            mapHosts.clear();
        }
        
        /**
         * Creates a new connection and returns is for use of the calling method.
         *
         @param hostConfiguration the configuration for the connection
         @return a new connection or <code>null</code> if none are available
         */
        public synchronized HttpConnection createConnection(HostConfiguration hostConfiguration) {
            
            HttpConnectionWithReference connection = null;

            HostConnectionPool hostPool = getHostPool(hostConfiguration);

            if ((hostPool.numConnections < getMaxConnectionsPerHost()) 
                && (numConnections < getMaxTotalConnections())) {

                if (LOG.isDebugEnabled()) {
                    LOG.debug("Allocating new connection, hostConfig=" + hostConfiguration);
                }
                connection = new HttpConnectionWithReference(hostConfiguration);
                connection.setStaleCheckingEnabled(connectionStaleCheckingEnabled);
                connection.setHttpConnectionManager(MultiThreadedHttpConnectionManager.this);
                numConnections++;
                hostPool.numConnections++;
                
                // store a reference to this connection so that it can be cleaned up
                // in the event it is not correctly released
                storeReferenceToConnection(connection, hostConfiguration, this);
                
            else if (LOG.isDebugEnabled()) {
                if (hostPool.numConnections >= getMaxConnectionsPerHost()) {
                    LOG.debug("No connection allocated, host pool has already reached "
                        "maxConnectionsPerHost, hostConfig=" + hostConfiguration
                        ", maxConnectionsPerhost=" + getMaxConnectionsPerHost());
                else {
                    LOG.debug("No connection allocated, maxTotalConnections reached, "
                        "maxTotalConnections=" + getMaxTotalConnections());
                }
            }
            
            return connection;
        }
    
        /**
         * Handles cleaning up for a lost connection with the given config.  Decrements any 
         * connection counts and notifies waiting threads, if appropriate.
         
         @param config the host configuration of the connection that was lost
         */
        public synchronized void handleLostConnection(HostConfiguration config) {
            HostConnectionPool hostPool = getHostPool(config);
            hostPool.numConnections--;

            numConnections--;
            notifyWaitingThread(config);
        }

        /**
         * Get the pool (list) of connections available for the given hostConfig.
         *
         @param hostConfiguration the configuraton for the connection pool
         @return a pool (list) of connections available for the given config
         */
        public synchronized HostConnectionPool getHostPool(HostConfiguration hostConfiguration) {
            LOG.trace("enter HttpConnectionManager.ConnectionPool.getHostPool(HostConfiguration)");

            // Look for a list of connections for the given config
            HostConnectionPool listConnections = (HostConnectionPool
                mapHosts.get(hostConfiguration);
            if (listConnections == null) {
                // First time for this config
                listConnections = new HostConnectionPool();
                listConnections.hostConfiguration = hostConfiguration;
                mapHosts.put(hostConfiguration, listConnections);
            }
            
            return listConnections;
        }

        /**
         * If available, get a free connection for this host
         *
         @param hostConfiguration the configuraton for the connection pool
         @return an available connection for the given config
         */
        public synchronized HttpConnection getFreeConnection(HostConfiguration hostConfiguration) {

            HttpConnectionWithReference connection = null;
            
            HostConnectionPool hostPool = getHostPool(hostConfiguration);

            if (hostPool.freeConnections.size() 0) {
                connection = (HttpConnectionWithReferencehostPool.freeConnections.removeFirst();
                freeConnections.remove(connection);
                // store a reference to this connection so that it can be cleaned up
                // in the event it is not correctly released
                storeReferenceToConnection(connection, hostConfiguration, this);
                if (LOG.isDebugEnabled()) {
                    LOG.debug("Getting free connection, hostConfig=" + hostConfiguration);
                }
            else if (LOG.isDebugEnabled()) {
                LOG.debug("There were no free connections to get, hostConfig=" 
                    + hostConfiguration);
            }
            return connection;
        }

        /**
         * Close and delete an old, unused connection to make room for a new one.
         */
        public synchronized void deleteLeastUsedConnection() {

            HttpConnection connection = (HttpConnectionfreeConnections.removeFirst();

            if (connection != null) {
                HostConfiguration connectionConfiguration = configurationForConnection(connection);

                if (LOG.isDebugEnabled()) {
                    LOG.debug("Reclaiming unused connection, hostConfig=" 
                        + connectionConfiguration);
                }

                connection.close();

                HostConnectionPool hostPool = getHostPool(connectionConfiguration);
                
                hostPool.freeConnections.remove(connection);
                hostPool.numConnections--;
                numConnections--;
            else if (LOG.isDebugEnabled()) {
                LOG.debug("Attempted to reclaim an unused connection but there were none.");
            }
        }

        /**
         * Notifies a waiting thread that a connection for the given configuration is 
         * available.
         @param configuration the host config to use for notifying
         @see #notifyWaitingThread(HostConnectionPool)
         */
        public synchronized void notifyWaitingThread(HostConfiguration configuration) {
            notifyWaitingThread(getHostPool(configuration));
        }

        /**
         * Notifies a waiting thread that a connection for the given configuration is 
         * available.  This will wake a thread witing in tis hostPool or if there is not
         * one a thread in the ConnectionPool will be notified.
         
         @param hostPool the host pool to use for notifying
         */
        public synchronized void notifyWaitingThread(HostConnectionPool hostPool) {

            // find the thread we are going to notify, we want to ensure that each
            // waiting thread is only interrupted once so we will remove it from 
            // all wait queues before interrupting it
            WaitingThread waitingThread = null;
                
            if (hostPool.waitingThreads.size() 0) {
                if (LOG.isDebugEnabled()) {
                    LOG.debug("Notifying thread waiting on host pool, hostConfig=" 
                        + hostPool.hostConfiguration);
                }                
                waitingThread = (WaitingThreadhostPool.waitingThreads.removeFirst();
                waitingThreads.remove(waitingThread);
            else if (waitingThreads.size() 0) {
                if (LOG.isDebugEnabled()) {
                    LOG.debug("No-one waiting on host pool, notifying next waiting thread.");
                }
                waitingThread = (WaitingThreadwaitingThreads.removeFirst();
                waitingThread.hostConnectionPool.waitingThreads.remove(waitingThread);
            else if (LOG.isDebugEnabled()) {
                LOG.debug("Notifying no-one, there are no waiting threads");
            }
                
            if (waitingThread != null) {
                waitingThread.thread.interrupt();
            }
        }

        /**
         * Marks the given connection as free.
         @param conn a connection that is no longer being used
         */
        public void freeConnection(HttpConnection conn) {

            HostConfiguration connectionConfiguration = configurationForConnection(conn);

            if (LOG.isDebugEnabled()) {
                LOG.debug("Freeing connection, hostConfig=" + connectionConfiguration);
            }

            synchronized (this) {
                
                if (shutdown) {
                    // the connection manager has been shutdown, release the connection's
                    // resources and get out of here
                    conn.close();
                    return;
                }
                
                HostConnectionPool hostPool = getHostPool(connectionConfiguration);

                // Put the connect back in the available list and notify a waiter
                hostPool.freeConnections.add(conn);
                if (hostPool.numConnections == 0) {
                    // for some reason this connection pool didn't already exist
                    LOG.error("Host connection pool not found, hostConfig=" 
                              + connectionConfiguration);
                    hostPool.numConnections = 1;
                }

                freeConnections.add(conn);
                // we can remove the reference to this connection as we have control over
                // it again.  this also ensures that the connection manager can be GCed
                removeReferenceToConnection((HttpConnectionWithReferenceconn);
                if (numConnections == 0) {
                    // for some reason this connection pool didn't already exist
                    LOG.error("Host connection pool not found, hostConfig=" 
                              + connectionConfiguration);
                    numConnections = 1;
                }
                
                notifyWaitingThread(hostPool);
            }
        }
    }

    /**
     * A simple struct-like class to combine the objects needed to release a connection's
     * resources when claimed by the garbage collector.
     */
    private static class ConnectionSource {
        
        /** The connection pool that created the connection */
        public ConnectionPool connectionPool;

        /** The connection's host configuration */
        public HostConfiguration hostConfiguration;
    }
    
    /**
     * A simple struct-like class to combine the connection list and the count
     * of created connections.
     */
    private static class HostConnectionPool {
        /** The hostConfig this pool is for */
        public HostConfiguration hostConfiguration;
        
        /** The list of free connections */
        public LinkedList freeConnections = new LinkedList();
        
        /** The list of WaitingThreads for this host */
        public LinkedList waitingThreads = new LinkedList();

        /** The number of created connections */
        public int numConnections = 0;
    }
    
    /**
     * A simple struct-like class to combine the waiting thread and the connection 
     * pool it is waiting on.
     */
    private static class WaitingThread {
        /** The thread that is waiting for a connection */
        public Thread thread;
        
        /** The connection pool the thread is waiting for */
        public HostConnectionPool hostConnectionPool;
    }

    /**
     * A thread for listening for HttpConnections reclaimed by the garbage
     * collector.
     */
    private static class ReferenceQueueThread extends Thread {

        private boolean shutdown = false;
        
        /**
         * Create an instance and make this a daemon thread.
         */
        public ReferenceQueueThread() {
            setDaemon(true);
            setName("MultiThreadedHttpConnectionManager cleanup");
        }

        public void shutdown() {
            this.shutdown = true;
        }
        
        /**
         * Handles cleaning up for the given connection reference.
         
         @param ref the reference to clean up
         */
        private void handleReference(Reference ref) {
            
            ConnectionSource source = null;
            
            synchronized (REFERENCE_TO_CONNECTION_SOURCE) {
                source = (ConnectionSourceREFERENCE_TO_CONNECTION_SOURCE.remove(ref);
            }
            // only clean up for this reference if it is still associated with 
            // a ConnectionSource
            if (source != null) {
                if (LOG.isDebugEnabled()) {
                    LOG.debug(
                        "Connection reclaimed by garbage collector, hostConfig=" 
                        + source.hostConfiguration);
                }
                
                source.connectionPool.handleLostConnection(source.hostConfiguration);
            }
        }

        /**
         * Start execution.
         */
        public void run() {
            while (!shutdown) {
                try {
                    // remove the next reference and process it, a timeout 
                    // is used so that the thread does not block indefinitely 
                    // and therefore keep the thread from shutting down
                    Reference ref = REFERENCE_QUEUE.remove(1000);
                    if (ref != null) {
                        handleReference(ref);
                    }
                catch (InterruptedException e) {
                    LOG.debug("ReferenceQueueThread interrupted", e);
                }
            }
        }

    }
    
    /**
     * A connection that keeps a reference to itself.
     */
    private static class HttpConnectionWithReference extends HttpConnection {
        
        public WeakReference reference = new WeakReference(this, REFERENCE_QUEUE);
        
        /**
         @param hostConfiguration
         */
        public HttpConnectionWithReference(HostConfiguration hostConfiguration) {
            super(hostConfiguration);
        }

    }
    
    /**
     * An HttpConnection wrapper that ensures a connection cannot be used
     * once released.
     */
    private static class HttpConnectionAdapter extends HttpConnection {

        // the wrapped connection
        private HttpConnection wrappedConnection;

        /**
         * Creates a new HttpConnectionAdapter.
         @param connection the connection to be wrapped
         */
        public HttpConnectionAdapter(HttpConnection connection) {
            super(connection.getHost(), connection.getPort(), connection.getProtocol());
            this.wrappedConnection = connection;
        }

        /**
         * Tests if the wrapped connection is still available.
         @return boolean
         */
        protected boolean hasConnection() {
            return wrappedConnection != null;
        }

        /**
         @return HttpConnection
         */
        HttpConnection getWrappedConnection() {
            return wrappedConnection;
        }
        
        public void close() {
            if (hasConnection()) {
                wrappedConnection.close();
            else {
                // do nothing
            }
        }

        public InetAddress getLocalAddress() {
            if (hasConnection()) {
                return wrappedConnection.getLocalAddress();
            else {
                return null;
            }
        }

        public boolean isStaleCheckingEnabled() {
            if (hasConnection()) {
                return wrappedConnection.isStaleCheckingEnabled();
            else {
                return false;
            }
        }

        public void setLocalAddress(InetAddress localAddress) {
            if (hasConnection()) {
                wrappedConnection.setLocalAddress(localAddress);
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void setStaleCheckingEnabled(boolean staleCheckEnabled) {
            if (hasConnection()) {
                wrappedConnection.setStaleCheckingEnabled(staleCheckEnabled);
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public String getHost() {
            if (hasConnection()) {
                return wrappedConnection.getHost();
            else {
                return null;
            }
        }

        public HttpConnectionManager getHttpConnectionManager() {
            if (hasConnection()) {
                return wrappedConnection.getHttpConnectionManager();
            else {
                return null;
            }
        }

        public InputStream getLastResponseInputStream() {
            if (hasConnection()) {
                return wrappedConnection.getLastResponseInputStream();
            else {
                return null;
            }
        }

        public int getPort() {
            if (hasConnection()) {
                return wrappedConnection.getPort();
            else {
                return -1;
            }
        }

        public Protocol getProtocol() {
            if (hasConnection()) {
                return wrappedConnection.getProtocol();
            else {
                return null;
            }
        }

        public String getProxyHost() {
            if (hasConnection()) {
                return wrappedConnection.getProxyHost();
            else {
                return null;
            }
        }

        public int getProxyPort() {
            if (hasConnection()) {
                return wrappedConnection.getProxyPort();
            else {
                return -1;
            }
        }

        public OutputStream getRequestOutputStream()
            throws IOException, IllegalStateException {
            if (hasConnection()) {
                return wrappedConnection.getRequestOutputStream();
            else {
                return null;
            }
        }

        public OutputStream getRequestOutputStream(boolean useChunking)
            throws IOException, IllegalStateException {
            if (hasConnection()) {
                return wrappedConnection.getRequestOutputStream(useChunking);
            else {
                return null;
            }
        }

        public InputStream getResponseInputStream()
            throws IOException, IllegalStateException {
            if (hasConnection()) {
                return wrappedConnection.getResponseInputStream();
            else {
                return null;
            }
        }

        public InputStream getResponseInputStream(HttpMethod method)
            throws IOException, IllegalStateException {
            if (hasConnection()) {
                return wrappedConnection.getResponseInputStream(method);
            else {
                return null;
            }
        }

        public boolean isOpen() {
            if (hasConnection()) {
                return wrappedConnection.isOpen();
            else {
                return false;
            }
        }

        public boolean isProxied() {
            if (hasConnection()) {
                return wrappedConnection.isProxied();
            else {
                return false;
            }
        }

        public boolean isResponseAvailable() throws IOException {
            if (hasConnection()) {
                return  wrappedConnection.isResponseAvailable();
            else {
                return false;
            }
        }

        public boolean isResponseAvailable(int timeoutthrows IOException {
            if (hasConnection()) {
                return  wrappedConnection.isResponseAvailable(timeout);
            else {
                return false;
            }
        }

        public boolean isSecure() {
            if (hasConnection()) {
                return wrappedConnection.isSecure();
            else {
                return false;
            }
        }

        public boolean isTransparent() {
            if (hasConnection()) {
                return wrappedConnection.isTransparent();
            else {
                return false;
            }
        }

        public void open() throws IOException {
            if (hasConnection()) {
                wrappedConnection.open();
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void print(String data)
            throws IOException, IllegalStateException, HttpRecoverableException {
            if (hasConnection()) {
                wrappedConnection.print(data);
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void printLine()
            throws IOException, IllegalStateException, HttpRecoverableException {
            if (hasConnection()) {
                wrappedConnection.printLine();
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void printLine(String data)
            throws IOException, IllegalStateException, HttpRecoverableException {
            if (hasConnection()) {
                wrappedConnection.printLine(data);
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public String readLine() throws IOException, IllegalStateException {
            if (hasConnection()) {
                return wrappedConnection.readLine();
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void releaseConnection() {
            if (hasConnection()) {
                HttpConnection wrappedConnection = this.wrappedConnection;
                this.wrappedConnection = null;
                wrappedConnection.releaseConnection();
            else {
                // do nothing
            }
        }

        public void setConnectionTimeout(int timeout) {
            if (hasConnection()) {
                wrappedConnection.setConnectionTimeout(timeout);
            else {
                // do nothing
            }
        }

        public void setHost(String hostthrows IllegalStateException {
            if (hasConnection()) {
                wrappedConnection.setHost(host);
            else {
                // do nothing
            }
        }

        public void setHttpConnectionManager(HttpConnectionManager httpConnectionManager) {
            if (hasConnection()) {
                wrappedConnection.setHttpConnectionManager(httpConnectionManager);
            else {
                // do nothing
            }
        }

        public void setLastResponseInputStream(InputStream inStream) {
            if (hasConnection()) {
                wrappedConnection.setLastResponseInputStream(inStream);
            else {
                // do nothing
            }
        }

        public void setPort(int portthrows IllegalStateException {
            if (hasConnection()) {
                wrappedConnection.setPort(port);
            else {
                // do nothing
            }
        }

        public void setProtocol(Protocol protocol) {
            if (hasConnection()) {
                wrappedConnection.setProtocol(protocol);
            else {
                // do nothing
            }
        }

        public void setProxyHost(String hostthrows IllegalStateException {
            if (hasConnection()) {
                wrappedConnection.setProxyHost(host);
            else {
                // do nothing
            }
        }

        public void setProxyPort(int portthrows IllegalStateException {
            if (hasConnection()) {
                wrappedConnection.setProxyPort(port);
            else {
                // do nothing
            }
        }

        public void setSecure(boolean securethrows IllegalStateException {
            if (hasConnection()) {
                wrappedConnection.setSecure(secure);
            else {
                // do nothing
            }
        }

        public void setSoTimeout(int timeout)
            throws SocketException, IllegalStateException {
            if (hasConnection()) {
                wrappedConnection.setSoTimeout(timeout);
            else {
                // do nothing
            }
        }

        public void shutdownOutput() {
            if (hasConnection()) {
                wrappedConnection.shutdownOutput();
            else {
                // do nothing
            }
        }

        public void tunnelCreated() throws IllegalStateException, IOException {
            if (hasConnection()) {
                wrappedConnection.tunnelCreated();
            else {
                // do nothing
            }
        }

        public void write(byte[] data, int offset, int length)
            throws IOException, IllegalStateException, HttpRecoverableException {
            if (hasConnection()) {
                wrappedConnection.write(data, offset, length);
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void write(byte[] data)
            throws IOException, IllegalStateException, HttpRecoverableException {
            if (hasConnection()) {
                wrappedConnection.write(data);
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void writeLine()
            throws IOException, IllegalStateException, HttpRecoverableException {
            if (hasConnection()) {
                wrappedConnection.writeLine();
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void writeLine(byte[] data)
            throws IOException, IllegalStateException, HttpRecoverableException {
            if (hasConnection()) {
                wrappedConnection.writeLine(data);
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void flushRequestOutputStream() throws IOException {
            if (hasConnection()) {
                wrappedConnection.flushRequestOutputStream();
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public int getSoTimeout() throws SocketException {
            if (hasConnection()) {
                return wrappedConnection.getSoTimeout();
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public String getVirtualHost() {
            if (hasConnection()) {
                return wrappedConnection.getVirtualHost();
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void setVirtualHost(String hostthrows IllegalStateException {
            if (hasConnection()) {
                wrappedConnection.setVirtualHost(host);
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public int getSendBufferSize() throws SocketException {
            if (hasConnection()) {
                return wrappedConnection.getSendBufferSize();
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

        public void setSendBufferSize(int sendBufferSizethrows SocketException {
            if (hasConnection()) {
                wrappedConnection.setSendBufferSize(sendBufferSize);
            else {
                throw new IllegalStateException("Connection has been released");
            }
        }

    }

}