我们项目使用的是 Apache的（commons-net-3.2.jar） FTPClient，但是系统偶尔会有异常，趁着刚解决完，总结一下。

日志中提示是类似

java.lang.Exception: FTP response 421 received.  Server closed connection.

上网上一查，说是FTP服务端连接数已满了，出现这种问题一般是，部分连接占用的时间太久，导致新连接获取不到，从而抛出的异常，找到问题就方便下手了（^_^）；

我的思路是，既然连接会出问题，那就从这里入手，

  1. 使用FTP连接池，管理连接
  2. 初始化连接池，设置超时时间，
  3. 连接数满时，等待连接释放

我的连接池这一块采用的也是Apache的（commons-pool2-2.2.jar）GenericObjectPool，大概有一下几个类

  1. 配置类

    package com.i1stcs.mvs.utils.ftp;

//FTPClient配置类
class FTPConfig {

    private String host;
    private int port;
    private String username;
    private String password;
    private String encoding;
    private String workPath;

    private int maxTotal;
    private int maxWaitMillis;

    public String getHost() {
        return host;
    }

    public void setHost(String host) {
        this.host = host;
    }

    public int getPort() {
        return port;
    }

    public void setPort(int port) {
        this.port = port;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getEncoding() {
        return encoding;
    }

    public void setEncoding(String encoding) {
        this.encoding = encoding;
    }

    public String getWorkPath() {
        return workPath;
    }

    public void setWorkPath(String workPath) {
        this.workPath = workPath;
    }

    public int getMaxTotal() {
        return maxTotal;
    }

    public void setMaxTotal(int maxTotal) {
        this.maxTotal = maxTotal;
    }

    public int getMaxWaitMillis() {
        return maxWaitMillis;
    }

    public void setMaxWaitMillis(int maxWaitMillis) {
        this.maxWaitMillis = maxWaitMillis;
    }

}
  2. 连接池实现类 
    package com.i1stcs.mvs.utils.ftp;

import org.apache.commons.net.ftp.FTPClient;
import org.apache.commons.pool2.impl.GenericObjectPool;
import org.apache.commons.pool2.impl.GenericObjectPoolConfig;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.i1stcs.mvs.utils.PropertiesUtil;

public class FTPPool {

    private static final GenericObjectPool<FTPClient> internalPool;
    private static Logger logger = LoggerFactory.getLogger(FTPPool.class);

    static {
        String ip = PropertiesUtil.getValue("ftp.server.ip", "");
        String userName = PropertiesUtil.getValue("ftp.server.username", "");
        String password = PropertiesUtil.getValue("ftp.server.password", "");
        String encoding = PropertiesUtil.getValue("ftp.server.encoding", "UTF-8");
        int port = PropertiesUtil.getValue("ftp.server.port", 21);

        int maxTotal = PropertiesUtil.getValue("ftp.server.maxTotal", 8);
        int maxWaitMillis = PropertiesUtil.getValue("ftp.server.maxWaitMillis", 30_000);

        FTPConfig config = new FTPConfig();
        config.setEncoding(encoding);
        config.setHost(ip);
        config.setUsername(userName);
        config.setPassword(password);
        config.setPort(port);
        config.setMaxTotal(maxTotal);
        config.setMaxWaitMillis(maxWaitMillis);

        GenericObjectPoolConfig poolConfig = new GenericObjectPoolConfig();
        poolConfig.setMaxTotal(config.getMaxTotal());// 不设置的话默认是8
        poolConfig.setMaxWaitMillis(config.getMaxWaitMillis());// 不设置默认无限等待

        internalPool = new GenericObjectPool<FTPClient>(new FTPPoolFactory(config), poolConfig);
    }

    /**
     * 获取资源
     *
     * @return
     */
    public static FTPClient getFTPClient() {
        try {
            return internalPool.borrowObject();
        } catch (Exception e) {
            logger.error("获取FTP连接异常：", e);
            return null;
        }
    }

    /**
     * 归还资源
     *
     * @param ftpClient
     */
    public static void returnFTPClient(FTPClient ftpClient) {
        try {
            internalPool.returnObject(ftpClient);
        } catch (Exception e) {
            logger.error("释放FTP连接异常：", e);
        }
    }

    /**
     * 销毁池子
     */
    public static void destroy() {
        try {
            internalPool.close();
        } catch (Exception e) {
            logger.error("销毁FTP数据源异常：", e);
        }
    }

}
  3. 连接池工厂类 
    package com.i1stcs.mvs.utils.ftp;

import java.io.IOException;
import java.net.SocketException;

import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPClient;
import org.apache.commons.net.ftp.FTPClientConfig;
import org.apache.commons.net.ftp.FTPReply;
import org.apache.commons.pool2.PooledObject;
import org.apache.commons.pool2.PooledObjectFactory;
import org.apache.commons.pool2.impl.DefaultPooledObject;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

class FTPPoolFactory implements PooledObjectFactory<FTPClient> {

    private static Logger logger = LoggerFactory.getLogger(FTPPoolFactory.class);

    private FTPConfig ftpConfig;
    private static int BUFF_SIZE = 256000;

    public FTPPoolFactory(FTPConfig ftpConfig) {
        this.ftpConfig = ftpConfig;
    }

    @Override
    public PooledObject<FTPClient> makeObject() throws Exception {
        FTPClient ftpClient = new FTPClient();
        ftpClient.setDefaultPort(ftpConfig.getPort());
        ftpClient.setConnectTimeout(30000);
        ftpClient.setDataTimeout(180000);
        ftpClient.setControlKeepAliveTimeout(60);
        ftpClient.setControlKeepAliveReplyTimeout(60);
        ftpClient.setControlEncoding(ftpConfig.getEncoding());

        FTPClientConfig clientConfig = new FTPClientConfig(FTPClientConfig.SYST_UNIX);
        clientConfig.setServerLanguageCode(ftpConfig.getEncoding());
        ftpClient.configure(clientConfig);

        try {
            ftpClient.connect(ftpConfig.getHost());
        } catch (SocketException exp) {
            logger.warn("connect timeout with FTP server:" + ftpConfig.getHost());
            throw new Exception(exp.getMessage());
        } catch (IOException exp) {
            logger.warn("connect FTP server:" + ftpConfig.getHost() + " meet error:" + exp.getMessage());
            throw new Exception(exp.getMessage());
        }

        int reply = ftpClient.getReplyCode();
        if (!FTPReply.isPositiveCompletion(reply)) {
            ftpClient.disconnect();
            logger.error("FTPServer refused connection");
            return null;
        }
        boolean result = ftpClient.login(ftpConfig.getUsername(), ftpConfig.getPassword());
        if (!result) {
            logger.warn("FTP server refused refused connection.");
            throw new Exception(
                    "login failed with FTP server:" + ftpConfig.getHost() + " may user name and password is wrong");
        }
        ftpClient.setBufferSize(BUFF_SIZE);
        ftpClient.setFileType(FTP.BINARY_FILE_TYPE);
        ftpClient.setFileTransferMode(FTP.COMPRESSED_TRANSFER_MODE);

        ftpClient.changeWorkingDirectory(ftpConfig.getWorkPath());

        return new DefaultPooledObject<FTPClient>(ftpClient);
    }

    @Override
    public void destroyObject(PooledObject<FTPClient> pooledObject) throws Exception {
        FTPClient ftpClient = pooledObject.getObject();
        try {
            ftpClient.logout();
            if (ftpClient.isConnected()) {
                ftpClient.disconnect();
            }
        } catch (IOException e) {
            throw new RuntimeException("Could not disconnect from server.", e);
        }
    }

    @Override
    public boolean validateObject(PooledObject<FTPClient> pooledObject) {
        FTPClient ftpClient = pooledObject.getObject();
        try {
            return ftpClient.sendNoOp();//
        } catch (IOException e) {
            logger.error("Failed to validate client:", e);
            return false;
        }
    }

    @Override
    public void activateObject(PooledObject<FTPClient> p) throws Exception {
        // TODO Auto-generated method stub

    }

    @Override
    public void passivateObject(PooledObject<FTPClient> p) throws Exception {
        // TODO Auto-generated method stub

    }

}
  4. FTP工具类 
    package com.songyz.ftp;

import org.apache.commons.net.ftp.FTPClient;
import org.apache.commons.net.ftp.FTPFile;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.i1stcs.mvs.utils.PropertiesUtil;

public class FTPHelper {
    private static final Logger log = LoggerFactory.getLogger(FTPHelper.class);

    public static FTPCommand createDelDirCmd(String remotePath) {
        return new DeleteDirCommand(remotePath);
    }

    public static FTPCommand createDelFileCmd(String remoteFile) {
        return new DeleteFileCommand(remoteFile);
    }

    public static void execute(FTPCommand... cmds) throws Exception {
        FTPClient client = null;
        try {
            client = FTPPool.getFTPClient();
            String mode = PropertiesUtil.getValue("ftp.server.mode", "localactive");
            String workpath = PropertiesUtil.getValue("ftp.server.workpath", "./");
            for (FTPCommand cmd : cmds) {
                client.changeWorkingDirectory(workpath);
                if (mode.equals("localactive"))
                    client.enterLocalActiveMode();
                else if (mode.equals("localpassive"))
                    client.enterLocalPassiveMode();
                else
                    client.enterRemotePassiveMode();

                cmd.isOK = cmd.doAction(client);
                if (cmd.isOK == false) {
                    if (cmd.error == null || cmd.error.isEmpty()) {
                        String clsName = cmd.getClass().getName();
                        cmd.error = "execute command:" + clsName.substring(clsName.lastIndexOf("$") + 1) + " failed -- "
                                + client.getReplyString();
                    }
                    throw new Exception(cmd.error);
                }
            }
        } finally {
            FTPPool.returnFTPClient(client);
        }
    }

    public static abstract class FTPCommand {
        protected boolean isOK = false;
        protected String error = "";

        public abstract boolean doAction(FTPClient client);

        public boolean isSuccessed() {
            return isOK;
        }

        public String getError() {
            return error;
        }
    }

    static class DeleteDirCommand extends FTPCommand {
        private String remotePath;

        public DeleteDirCommand(String remotePath) {
            this.remotePath = remotePath;
        }

        @Override
        public boolean doAction(FTPClient client) {
            try {
                FTPFile[] files = client.listFiles(remotePath);
                if (files == null || files.length == 0)
                    return client.removeDirectory(remotePath);

                for (FTPFile file : files) {
                    if (file.isDirectory())
                        deleteChildren(client, remotePath + "/" + file.getName());
                    else
                        client.dele(remotePath + "/" + file.getName());
                }
                return client.removeDirectory(remotePath);
            } catch (Exception exp) {
                error = "failed to delete dirctory " + remotePath + " in FTP server because " + exp.getMessage();
                log.warn(error, exp);
                return false;
            }
        }

        private boolean deleteChildren(FTPClient client, String pathName) {
            try {
                FTPFile[] files = client.listFiles(pathName);
                if (files == null || files.length == 0)
                    return client.removeDirectory(pathName);

                for (FTPFile file : files) {
                    if (file.isDirectory())
                        deleteChildren(client, pathName + "/" + file.getName());
                    else
                        client.dele(pathName + "/" + file.getName());
                }
                return client.removeDirectory(pathName);
            } catch (Exception exp) {
                error = "failed to delete dirctory " + remotePath + "in FTP server because " + exp.getMessage();
                log.warn(error, exp);
                return false;
            }
        }
    }

    static class DeleteFileCommand extends FTPCommand {
        private String remoteFilePath;

        public DeleteFileCommand(String remoteFilePath) {
            this.remoteFilePath = remoteFilePath;
        }

        @Override
        public boolean doAction(FTPClient client) {
            try {
                FTPFile[] files = client.listFiles(remoteFilePath);
                if (files == null || files.length == 0)
                    return true;
                else
                    return client.deleteFile(remoteFilePath);
            } catch (Exception exp) {
                error = "failed to delete file " + remoteFilePath + " in FTP server because " + exp.getMessage();
                log.warn(error, exp);
                return false;
            }
        }
    }

}

    特别提示：PropertiesUtil这个类是我自己读取配置文件的类，就不贴代码了，

    至此，大功告成！！！

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