/*
 * Copyright (c) 2009, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package sun.net.ftp;

import java.net.*;
import java.io.*;
import java.util.Date;
import java.util.List;
import java.util.Iterator;

/**
 * A class that implements the FTP protocol according to
 * RFCs <A href="http://www.ietf.org/rfc/rfc0959.txt">959</A>,
 * <A href="http://www.ietf.org/rfc/rfc2228.txt">2228</A>,
 * <A href="http://www.ietf.org/rfc/rfc2389.txt">2389</A>,
 * <A href="http://www.ietf.org/rfc/rfc2428.txt">2428</A>,
 * <A href="http://www.ietf.org/rfc/rfc3659.txt">3659</A>,
 * <A href="http://www.ietf.org/rfc/rfc4217.txt">4217</A>.
 * Which includes support for FTP over SSL/TLS (aka ftps).
 *
 * {@code FtpClient} provides all the functionalities of a typical FTP
 * client, like storing or retrieving files, listing or creating directories.
 * A typical usage would consist of connecting the client to the server,
 * log in, issue a few commands then logout.
 * Here is a code example:
 * <pre>
 * FtpClient cl = FtpClient.create();
 * cl.connect("ftp.gnu.org").login("anonymous", "john.doe@mydomain.com".toCharArray())).changeDirectory("pub/gnu");
 * Iterator&lt;FtpDirEntry&gt; dir = cl.listFiles();
 *     while (dir.hasNext()) {
 *         FtpDirEntry f = dir.next();
 *         System.err.println(f.getName());
 *     }
 *     cl.close();
 * }
 * </pre>
 * <p><b>Error reporting:</b> There are, mostly, two families of errors that
 * can occur during an FTP session. The first kind are the network related issues
 * like a connection reset, and they are usually fatal to the session, meaning,
 * in all likelyhood the connection to the server has been lost and the session
 * should be restarted from scratch. These errors are reported by throwing an
 * {@link IOException}. The second kind are the errors reported by the FTP server,
 * like when trying to download a non-existing file for example. These errors
 * are usually non fatal to the session, meaning more commands can be sent to the
 * server. In these cases, a {@link FtpProtocolException} is thrown.</p>
 * <p>
 * It should be noted that this is not a thread-safe API, as it wouldn't make
 * too much sense, due to the very sequential nature of FTP, to provide a
 * client able to be manipulated from multiple threads.
 *
 * @since 1.7
 */
public abstract class FtpClient implements java.io.Closeable {

    private static final int FTP_PORT = 21;

    public static enum TransferType {

        ASCII, BINARY, EBCDIC
    };

    /**
     * Returns the default FTP port number.
     *
     * @return the port number.
     */
    public static final int defaultPort() {
        return FTP_PORT;
    }

    /**
     * Creates an instance of FtpClient. The client is not connected to any
     * server yet.
     *
     */
    protected FtpClient() {
    }

    /**
     * Creates an instance of {@code FtpClient}. The client is not connected to any
     * server yet.
     *
     * @return the created {@code FtpClient}
     */
    public static FtpClient create() {
        FtpClientProvider provider = FtpClientProvider.provider();
        return provider.createFtpClient();
    }

    /**
     * Creates an instance of FtpClient and connects it to the specified
     * address.
     *
     * @param dest the {@code InetSocketAddress} to connect to.
     * @return The created {@code FtpClient}
     * @throws IOException if the connection fails
     * @see #connect(java.net.SocketAddress)
     */
    public static FtpClient create(InetSocketAddress dest) throws FtpProtocolException, IOException {
        FtpClient client = create();
        if (dest != null) {
            client.connect(dest);
        }
        return client;
    }

    /**
     * Creates an instance of {@code FtpClient} and connects it to the
     * specified host on the default FTP port.
     *
     * @param dest the {@code String} containing the name of the host
     *        to connect to.
     * @return The created {@code FtpClient}
     * @throws IOException if the connection fails.
     * @throws FtpProtocolException if the server rejected the connection
     */
    public static FtpClient create(String dest) throws FtpProtocolException, IOException {
        return create(new InetSocketAddress(dest, FTP_PORT));
    }

    /**
     * Enables, or disables, the use of the <I>passive</I> mode. In that mode,
     * data connections are established by having the client connect to the server.
     * This is the recommended default mode as it will work best through
     * firewalls and NATs. If set to {@code false} the mode is said to be
     * <I>active</I> which means the server will connect back to the client
     * after a PORT command to establish a data connection.
     *
     * <p><b>Note:</b> Since the passive mode might not be supported by all
     * FTP servers, enabling it means the client will try to use it. If the
     * server rejects it, then the client will attempt to fall back to using
     * the <I>active</I> mode by issuing a {@code PORT} command instead.</p>
     *
     * @param passive {@code true} to force passive mode.
     * @return This FtpClient
     * @see #isPassiveModeEnabled()
     */
    public abstract FtpClient enablePassiveMode(boolean passive);

    /**
     * Tests whether passive mode is enabled.
     *
     * @return {@code true} if the passive mode has been enabled.
     * @see #enablePassiveMode(boolean)
     */
    public abstract boolean isPassiveModeEnabled();

    /**
     * Sets the default timeout value to use when connecting to the server,
     *
     * @param timeout the timeout value, in milliseconds, to use for the connect
     *        operation. A value of zero or less, means use the default timeout.
     *
     * @return This FtpClient
     */
    public abstract FtpClient setConnectTimeout(int timeout);

    /**
     * Returns the current default connection timeout value.
     *
     * @return the value, in milliseconds, of the current connect timeout.
     * @see #setConnectTimeout(int)
     */
    public abstract int getConnectTimeout();

    /**
     * Sets the timeout value to use when reading from the server,
     *
     * @param timeout the timeout value, in milliseconds, to use for the read
     *        operation. A value of zero or less, means use the default timeout.
     * @return This FtpClient
     */
    public abstract FtpClient setReadTimeout(int timeout);

    /**
     * Returns the current read timeout value.
     *
     * @return the value, in milliseconds, of the current read timeout.
     * @see #setReadTimeout(int)
     */
    public abstract int getReadTimeout();

    /**
     * Set the {@code Proxy} to be used for the next connection.
     * If the client is already connected, it doesn't affect the current
     * connection. However it is not recommended to change this during a session.
     *
     * @param p the {@code Proxy} to use, or {@code null} for no proxy.
     * @return This FtpClient
     */
    public abstract FtpClient setProxy(Proxy p);

    /**
     * Get the proxy of this FtpClient
     *
     * @return the {@code Proxy}, this client is using, or {@code null}
     * if none is used.
     * @see #setProxy(Proxy)
     */
    public abstract Proxy getProxy();

    /**
     * Tests whether this client is connected or not to a server.
     *
     * @return {@code true} if the client is connected.
     */
    public abstract boolean isConnected();

    /**
     * Connects the {@code FtpClient} to the specified destination server.
     *
     * @param dest the address of the destination server
     * @return this FtpClient
     * @throws IOException if connection failed.
     * @throws SecurityException if there is a SecurityManager installed and it
     * denied the authorization to connect to the destination.
     * @throws FtpProtocolException
     */
    public abstract FtpClient connect(SocketAddress dest) throws FtpProtocolException, IOException;

    /**
     * Connects the FtpClient to the specified destination server.
     *
     * @param dest the address of the destination server
     * @param timeout the value, in milliseconds, to use as a connection timeout
     * @return this FtpClient
     * @throws IOException if connection failed.
     * @throws SecurityException if there is a SecurityManager installed and it
     * denied the authorization to connect to the destination.
     * @throws FtpProtocolException
     */
    public abstract FtpClient connect(SocketAddress dest, int timeout) throws FtpProtocolException, IOException;

    /**
     * Retrieves the address of the FTP server this client is connected to.
     *
     * @return the {@link SocketAddress} of the server, or {@code null} if this
     * client is not connected yet.
     */
    public abstract SocketAddress getServerAddress();

    /**
     * Attempts to log on the server with the specified user name and password.
     *
     * @param user The user name
     * @param password The password for that user
     * @return this FtpClient
     * @throws IOException if an error occured during the transmission
     * @throws FtpProtocolException if the login was refused by the server
     */
    public abstract FtpClient login(String user, char[] password) throws FtpProtocolException, IOException;

    /**
     * Attempts to log on the server with the specified user name, password and
     * account name.
     *
     * @param user The user name
     * @param password The password for that user.
     * @param account The account name for that user.
     * @return this FtpClient
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the login was refused by the server
     */
    public abstract FtpClient login(String user, char[] password, String account) throws FtpProtocolException, IOException;

    /**
     * Closes the current connection. Logs out the current user, if any, by
     * issuing the QUIT command to the server.
     * This is in effect terminates the current
     * session and the connection to the server will be closed.
     * <p>After a close, the client can then be connected to another server
     * to start an entirely different session.</P>
     *
     * @throws IOException if an error occurs during transmission
     */
    public abstract void close() throws IOException;

    /**
     * Checks whether the client is logged in to the server or not.
     *
     * @return {@code true} if the client has already completed a login.
     */
    public abstract boolean isLoggedIn();

    /**
     * Changes to a specific directory on a remote FTP server
     *
     * @param  remoteDirectory path of the directory to CD to.
     * @return this FtpClient
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the command was refused by the server
     */
    public abstract FtpClient changeDirectory(String remoteDirectory) throws FtpProtocolException, IOException;

    /**
     * Changes to the parent directory, sending the CDUP command to the server.
     *
     * @return this FtpClient
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the command was refused by the server
     */
    public abstract FtpClient changeToParentDirectory() throws FtpProtocolException, IOException;

    /**
     * Retrieve the server current working directory using the PWD command.
     *
     * @return a {@code String} containing the current working directory
     * @throws IOException if an error occurs during transmission
     * @throws FtpProtocolException if the command was refused by the server,
     */
    public abstract String getWorkingDirectory() throws FtpProtocolException, IOException;

    /**
     * Sets the restart offset to the specified value.  That value will be
     * sent through a {@code REST} command to server before the next file
     * transfer and has the effect of resuming a file transfer from the
     * specified point. After the transfer the restart offset is set back to
     * zero.
     *
     * @param offset the offset in the remote file at which to start the next
     *        transfer. This must be a value greater than or equal to zero.
     * @return this FtpClient
     * @throws IllegalArgumentException if the offset is negative.
     */
    public abstract FtpClient setRestartOffset(long offset);

    /**
     * Retrieves a file from the ftp server and writes its content to the specified
     * {@code OutputStream}.
     * <p>If the restart offset was set, then a {@code REST} command will be
     * sent before the {@code RETR} in order to restart the tranfer from the specified
     * offset.</p>
     * <p>The {@code OutputStream} is not closed by this method at the end
     * of the transfer. </p>
     * <p>This method will block until the transfer is complete or an exception
     * is thrown.</p>
     *
     * @param name a {@code String} containing the name of the file to
     *        retreive from the server.
     * @param local the {@code OutputStream} the file should be written to.
     * @return this FtpClient
     * @throws IOException if the transfer fails.
     * @throws FtpProtocolException if the command was refused by the server
     * @see #setRestartOffset(long)
     */
    public abstract FtpClient getFile(String name, OutputStream local) throws FtpProtocolException, IOException;

    /**
     * Retrieves a file from the ftp server, using the {@code RETR} command, and
     * returns the InputStream from the established data connection.
     * {@link #completePending()} <b>has</b> to be called once the application
     * is done reading from the returned stream.
     * <p>If the restart offset was set, then a {@code REST} command will be
     * sent before the {@code RETR} in order to restart the tranfer from the specified
     * offset.</p>
     *
     * @param name the name of the remote file
     * @return the {@link java.io.InputStream} from the data connection
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was refused by the server
     * @see #setRestartOffset(long)
     */
    public abstract InputStream getFileStream(String name) throws FtpProtocolException, IOException;

    /**
     * Transfers a file from the client to the server (aka a <I>put</I>)
     * by sending the STOR command, and returns the {@code OutputStream}
     * from the established data connection.
     *
     * A new file is created at the server site if the file specified does
     * not already exist.
     *
     * {@link #completePending()} <b>has</b> to be called once the application
     * is finished writing to the returned stream.
     *
     * @param name the name of the remote file to write.
     * @return the {@link java.io.OutputStream} from the data connection or
     *         {@code null} if the command was unsuccessful.
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public OutputStream putFileStream(String name) throws FtpProtocolException, IOException {
        return putFileStream(name, false);
    }

    /**
     * Transfers a file from the client to the server (aka a <I>put</I>)
     * by sending the STOR or STOU command, depending on the
     * {@code unique} argument, and returns the {@code OutputStream}
     * from the established data connection.
     * {@link #completePending()} <b>has</b> to be called once the application
     * is finished writing to the stream.
     *
     * A new file is created at the server site if the file specified does
     * not already exist.
     *
     * If {@code unique} is set to {@code true}, the resultant file
     * is to be created under a name unique to that directory, meaning
     * it will not overwrite an existing file, instead the server will
     * generate a new, unique, file name.
     * The name of the remote file can be retrieved, after completion of the
     * transfer, by calling {@link #getLastFileName()}.
     *
     * @param name the name of the remote file to write.
     * @param unique {@code true} if the remote files should be unique,
     *        in which case the STOU command will be used.
     * @return the {@link java.io.OutputStream} from the data connection.
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract OutputStream putFileStream(String name, boolean unique) throws FtpProtocolException, IOException;

    /**
     * Transfers a file from the client to the server (aka a <I>put</I>)
     * by sending the STOR or STOU command, depending on the
     * {@code unique} argument. The content of the {@code InputStream}
     * passed in argument is written into the remote file, overwriting any
     * existing data.
     *
     * A new file is created at the server site if the file specified does
     * not already exist.
     *
     * If {@code unique} is set to {@code true}, the resultant file
     * is to be created under a name unique to that directory, meaning
     * it will not overwrite an existing file, instead the server will
     * generate a new, unique, file name.
     * The name of the remote file can be retrieved, after completion of the
     * transfer, by calling {@link #getLastFileName()}.
     *
     * <p>This method will block until the transfer is complete or an exception
     * is thrown.</p>
     *
     * @param name the name of the remote file to write.
     * @param local the {@code InputStream} that points to the data to
     *        transfer.
     * @return this FtpClient
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public FtpClient putFile(String name, InputStream local) throws FtpProtocolException, IOException {
        return putFile(name, local, false);
    }

    /**
     * Transfers a file from the client to the server (aka a <I>put</I>)
     * by sending the STOR command. The content of the {@code InputStream}
     * passed in argument is written into the remote file, overwriting any
     * existing data.
     *
     * A new file is created at the server site if the file specified does
     * not already exist.
     *
     * <p>This method will block until the transfer is complete or an exception
     * is thrown.</p>
     *
     * @param name the name of the remote file to write.
     * @param local the {@code InputStream} that points to the data to
     *        transfer.
     * @param unique {@code true} if the remote file should be unique
     *        (i.e. not already existing), {@code false} otherwise.
     * @return this FtpClient
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     * @see #getLastFileName()
     */
    public abstract FtpClient putFile(String name, InputStream local, boolean unique) throws FtpProtocolException, IOException;

    /**
     * Sends the APPE command to the server in order to transfer a data stream
     * passed in argument and append it to the content of the specified remote
     * file.
     *
     * <p>This method will block until the transfer is complete or an exception
     * is thrown.</p>
     *
     * @param name A {@code String} containing the name of the remote file
     *        to append to.
     * @param local The {@code InputStream} providing access to the data
     *        to be appended.
     * @return this FtpClient
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient appendFile(String name, InputStream local) throws FtpProtocolException, IOException;

    /**
     * Renames a file on the server.
     *
     * @param from the name of the file being renamed
     * @param to the new name for the file
     * @return this FtpClient
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient rename(String from, String to) throws FtpProtocolException, IOException;

    /**
     * Deletes a file on the server.
     *
     * @param name a {@code String} containing the name of the file
     *        to delete.
     * @return this FtpClient
     * @throws IOException if an error occured during the exchange
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient deleteFile(String name) throws FtpProtocolException, IOException;

    /**
     * Creates a new directory on the server.
     *
     * @param name a {@code String} containing the name of the directory
     *        to create.
     * @return this FtpClient
     * @throws IOException if an error occured during the exchange
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient makeDirectory(String name) throws FtpProtocolException, IOException;

    /**
     * Removes a directory on the server.
     *
     * @param name a {@code String} containing the name of the directory
     *        to remove.
     *
     * @return this FtpClient
     * @throws IOException if an error occured during the exchange.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient removeDirectory(String name) throws FtpProtocolException, IOException;

    /**
     * Sends a No-operation command. It's useful for testing the connection
     * status or as a <I>keep alive</I> mechanism.
     *
     * @return this FtpClient
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient noop() throws FtpProtocolException, IOException;

    /**
     * Sends the {@code STAT} command to the server.
     * This can be used while a data connection is open to get a status
     * on the current transfer, in that case the parameter should be
     * {@code null}.
     * If used between file transfers, it may have a pathname as argument
     * in which case it will work as the LIST command except no data
     * connection will be created.
     *
     * @param name an optional {@code String} containing the pathname
     *        the STAT command should apply to.
     * @return the response from the server
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract String getStatus(String name) throws FtpProtocolException, IOException;

    /**
     * Sends the {@code FEAT} command to the server and returns the list of supported
     * features in the form of strings.
     *
     * The features are the supported commands, like AUTH TLS, PROT or PASV.
     * See the RFCs for a complete list.
     *
     * Note that not all FTP servers support that command, in which case
     * a {@link FtpProtocolException} will be thrown.
     *
     * @return a {@code List} of {@code Strings} describing the
     *         supported additional features
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the command is rejected by the server
     */
    public abstract List<String> getFeatures() throws FtpProtocolException, IOException;

    /**
     * Sends the {@code ABOR} command to the server.
     * <p>It tells the server to stop the previous command or transfer. No action
     * will be taken if the previous command has already been completed.</p>
     * <p>This doesn't abort the current session, more commands can be issued
     * after an abort.</p>
     *
     * @return this FtpClient
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient abort() throws FtpProtocolException, IOException;

    /**
     * Some methods do not wait until completion before returning, so this
     * method can be called to wait until completion. This is typically the case
     * with commands that trigger a transfer like {@link #getFileStream(String)}.
     * So this method should be called before accessing information related to
     * such a command.
     * <p>This method will actually block reading on the command channel for a
     * notification from the server that the command is finished. Such a
     * notification often carries extra information concerning the completion
     * of the pending action (e.g. number of bytes transfered).</p>
     * <p>Note that this will return immediately if no command or action
     * is pending</p>
     * <p>It should be also noted that most methods issuing commands to the ftp
     * server will call this method if a previous command is pending.
     * <p>Example of use:
     * <pre>
     * InputStream in = cl.getFileStream("file");
     * ...
     * cl.completePending();
     * long size = cl.getLastTransferSize();
     * </pre>
     * On the other hand, it's not necessary in a case like:
     * <pre>
     * InputStream in = cl.getFileStream("file");
     * // read content
     * ...
     * cl.close();
     * </pre>
     * <p>Since {@link #close()} will call completePending() if necessary.</p>
     * @return this FtpClient
     * @throws IOException if an error occured during the transfer
     * @throws FtpProtocolException if the command didn't complete successfully
     */
    public abstract FtpClient completePending() throws FtpProtocolException, IOException;

    /**
     * Reinitializes the USER parameters on the FTP server
     *
     * @return this FtpClient
     * @throws IOException if an error occurs during transmission
     * @throws FtpProtocolException if the command fails
     */
    public abstract FtpClient reInit() throws FtpProtocolException, IOException;

    /**
     * Changes the transfer type (binary, ascii, ebcdic) and issue the
     * proper command (e.g. TYPE A) to the server.
     *
     * @param type the {@code TransferType} to use.
     * @return This FtpClient
     * @throws IOException if an error occurs during transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient setType(TransferType type) throws FtpProtocolException, IOException;

    /**
     * Changes the current transfer type to binary.
     * This is a convenience method that is equivalent to
     * {@code setType(TransferType.BINARY)}
     *
     * @return This FtpClient
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     * @see #setType(TransferType)
     */
    public FtpClient setBinaryType() throws FtpProtocolException, IOException {
        setType(TransferType.BINARY);
        return this;
    }

    /**
     * Changes the current transfer type to ascii.
     * This is a convenience method that is equivalent to
     * {@code setType(TransferType.ASCII)}
     *
     * @return This FtpClient
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     * @see #setType(TransferType)
     */
    public FtpClient setAsciiType() throws FtpProtocolException, IOException {
        setType(TransferType.ASCII);
        return this;
    }

    /**
     * Issues a {@code LIST} command to the server to get the current directory
     * listing, and returns the InputStream from the data connection.
     *
     * <p>{@link #completePending()} <b>has</b> to be called once the application
     * is finished reading from the stream.</p>
     *
     * @param path the pathname of the directory to list, or {@code null}
     *        for the current working directory.
     * @return the {@code InputStream} from the resulting data connection
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     * @see #changeDirectory(String)
     * @see #listFiles(String)
     */
    public abstract InputStream list(String path) throws FtpProtocolException, IOException;

    /**
     * Issues a {@code NLST path} command to server to get the specified directory
     * content. It differs from {@link #list(String)} method by the fact that
     * it will only list the file names which would make the parsing of the
     * somewhat easier.
     *
     * <p>{@link #completePending()} <b>has</b> to be called once the application
     * is finished reading from the stream.</p>
     *
     * @param path a {@code String} containing the pathname of the
     *        directory to list or {@code null} for the current working directory.
     * @return the {@code InputStream} from the resulting data connection
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract InputStream nameList(String path) throws FtpProtocolException, IOException;

    /**
     * Issues the {@code SIZE [path]} command to the server to get the size of a
     * specific file on the server.
     * Note that this command may not be supported by the server. In which
     * case -1 will be returned.
     *
     * @param path a {@code String} containing the pathname of the
     *        file.
     * @return a {@code long} containing the size of the file or -1 if
     *         the server returned an error, which can be checked with
     *         {@link #getLastReplyCode()}.
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract long getSize(String path) throws FtpProtocolException, IOException;

    /**
     * Issues the {@code MDTM [path]} command to the server to get the modification
     * time of a specific file on the server.
     * Note that this command may not be supported by the server, in which
     * case {@code null} will be returned.
     *
     * @param path a {@code String} containing the pathname of the file.
     * @return a {@code Date} representing the last modification time
     *         or {@code null} if the server returned an error, which
     *         can be checked with {@link #getLastReplyCode()}.
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract Date getLastModified(String path) throws FtpProtocolException, IOException;

    /**
     * Sets the parser used to handle the directory output to the specified
     * one. By default the parser is set to one that can handle most FTP
     * servers output (Unix base mostly). However it may be necessary for
     * and application to provide its own parser due to some uncommon
     * output format.
     *
     * @param p The {@code FtpDirParser} to use.
     * @return this FtpClient
     * @see #listFiles(String)
     */
    public abstract FtpClient setDirParser(FtpDirParser p);

    /**
     * Issues a {@code MLSD} command to the server to get the specified directory
     * listing and applies the internal parser to create an Iterator of
     * {@link java.net.FtpDirEntry}. Note that the Iterator returned is also a
     * {@link java.io.Closeable}.
     * <p>If the server doesn't support the MLSD command, the LIST command is used
     * instead and the parser set by {@link #setDirParser(java.net.FtpDirParser) }
     * is used instead.</p>
     *
     * {@link #completePending()} <b>has</b> to be called once the application
     * is finished iterating through the files.
     *
     * @param path the pathname of the directory to list or {@code null}
     *        for the current working directoty.
     * @return a {@code Iterator} of files or {@code null} if the
     *         command failed.
     * @throws IOException if an error occured during the transmission
     * @see #setDirParser(FtpDirParser)
     * @see #changeDirectory(String)
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract Iterator<FtpDirEntry> listFiles(String path) throws FtpProtocolException, IOException;

    /**
     * Attempts to use Kerberos GSSAPI as an authentication mechanism with the
     * ftp server. This will issue an {@code AUTH GSSAPI} command, and if
     * it is accepted by the server, will followup with {@code ADAT}
     * command to exchange the various tokens until authentication is
     * successful. This conforms to Appendix I of RFC 2228.
     *
     * @return this FtpClient
     * @throws IOException if an error occurs during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient useKerberos() throws FtpProtocolException, IOException;

    /**
     * Returns the Welcome string the server sent during initial connection.
     *
     * @return a {@code String} containing the message the server
     *         returned during connection or {@code null}.
     */
    public abstract String getWelcomeMsg();

    /**
     * Returns the last reply code sent by the server.
     *
     * @return the lastReplyCode or {@code null} if none were received yet.
     */
    public abstract FtpReplyCode getLastReplyCode();

    /**
     * Returns the last response string sent by the server.
     *
     * @return the message string, which can be quite long, last returned
     *         by the server, or {@code null} if no response were received yet.
     */
    public abstract String getLastResponseString();

    /**
     * Returns, when available, the size of the latest started transfer.
     * This is retreived by parsing the response string received as an initial
     * response to a {@code RETR} or similar request.
     *
     * @return the size of the latest transfer or -1 if either there was no
     *         transfer or the information was unavailable.
     */
    public abstract long getLastTransferSize();

    /**
     * Returns, when available, the remote name of the last transfered file.
     * This is mainly useful for "put" operation when the unique flag was
     * set since it allows to recover the unique file name created on the
     * server which may be different from the one submitted with the command.
     *
     * @return the name the latest transfered file remote name, or
     *         {@code null} if that information is unavailable.
     */
    public abstract String getLastFileName();

    /**
     * Attempts to switch to a secure, encrypted connection. This is done by
     * sending the {@code AUTH TLS} command.
     * <p>See <a href="http://www.ietf.org/rfc/rfc4217.txt">RFC 4217</a></p>
     * If successful this will establish a secure command channel with the
     * server, it will also make it so that all other transfers (e.g. a RETR
     * command) will be done over an encrypted channel as well unless a
     * {@link #reInit()} command or a {@link #endSecureSession()} command is issued.
     * <p>This method should be called after a successful {@link #connect(java.net.InetSocketAddress) }
     * but before calling {@link #login(java.lang.String, char[]) }.</p>
     *
     * @return this FtpCLient
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     * @see #endSecureSession()
     */
    public abstract FtpClient startSecureSession() throws FtpProtocolException, IOException;

    /**
     * Sends a {@code CCC} command followed by a {@code PROT C}
     * command to the server terminating an encrypted session and reverting
     * back to a non encrypted transmission.
     *
     * @return this FtpClient
     * @throws IOException if an error occured during transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     * @see #startSecureSession()
     */
    public abstract FtpClient endSecureSession() throws FtpProtocolException, IOException;

    /**
     * Sends the "Allocate" ({@code ALLO}) command to the server telling it to
     * pre-allocate the specified number of bytes for the next transfer.
     *
     * @param size The number of bytes to allocate.
     * @return this FtpClient
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient allocate(long size) throws FtpProtocolException, IOException;

    /**
     * Sends the "Structure Mount" ({@code SMNT}) command to the server. This let the
     * user mount a different file system data structure without altering his
     * login or accounting information.
     *
     * @param struct a {@code String} containing the name of the
     *        structure to mount.
     * @return this FtpClient
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient structureMount(String struct) throws FtpProtocolException, IOException;

    /**
     * Sends a System ({@code SYST}) command to the server and returns the String
     * sent back by the server describing the operating system at the
     * server.
     *
     * @return a {@code String} describing the OS, or {@code null}
     *         if the operation was not successful.
     * @throws IOException if an error occured during the transmission.
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract String getSystem() throws FtpProtocolException, IOException;

    /**
     * Sends the {@code HELP} command to the server, with an optional command, like
     * SITE, and returns the text sent back by the server.
     *
     * @param cmd the command for which the help is requested or
     *        {@code null} for the general help
     * @return a {@code String} containing the text sent back by the
     *         server, or {@code null} if the command failed.
     * @throws IOException if an error occured during transmission
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract String getHelp(String cmd) throws FtpProtocolException, IOException;

    /**
     * Sends the {@code SITE} command to the server. This is used by the server
     * to provide services specific to his system that are essential
     * to file transfer.
     *
     * @param cmd the command to be sent.
     * @return this FtpClient
     * @throws IOException if an error occured during transmission
     * @throws FtpProtocolException if the command was rejected by the server
     */
    public abstract FtpClient siteCmd(String cmd) throws FtpProtocolException, IOException;
}