最近在寫一個FTP上傳工具，用到了Apache的FTPClient，為了提高上傳效率，我採用了多線程的方式，但是每個線程頻繁的創建和銷毀FTPClient對象勢必會造成不必要的開銷，因此，此處最好使用一個FTPClient連接池。仔細翻了一下Apache的api，發現它並沒有一個FTPClientPool的實現，所以，不得不自己寫一個FTPClientPool。下面就大體介紹一下開發連接池的整個過程，供大家參考。
關於對像池
有些對象的創建開銷是比較大的，比如數據庫連接等。為了減少頻繁創建、銷毀對象帶來的性能消耗，我們可以利用對像池的技術來實現對象的複用。對像池提供了一種機制，它可以管理對像池中對象的生命週期，提供了獲取和釋放對象的方法，可以讓客戶端很方便的使用對像池中的對象。
如果我們要自己實現一個對像池，一般需要完成如下功能：
1. 如果池中有可用的對象，對像池應當能返回給客戶端
2. 客戶端把對象放回池里後，可以對這些對象進行重用
3. 對像池能夠創建新的對象來滿足客戶端不斷增長的需求
4. 需要有一個正確關閉池的機制來結束對象的生命週期
Apache的對像池工具包
為了方便我們開發自己的對像池，Apache 提供的common-pool工具包，裡麵包含了開發通用對像池的一些接口和實現類。其中最基本的兩個接口是ObjectPool 和PoolableObjectFactory。
ObjectPool接口中有幾個最基本的方法：
1. addObject() : 添加對像到池
2. borrowObject()：客戶端從池中藉出一個對象
3. returnObject()：客戶端歸還一個對像到池中
4. close()：關閉對像池，清理內存釋放資源等
5. setFactory(ObjectFactory factory)：需要一個工廠來製造池中的對象
PoolableObjectFactory接口中幾個最基本的方法：
1. makeObject()：製造一個對象
2. destoryObject()：銷毀一個對象
3. validateObject()：驗證一個對像是否還可用
通過以上兩個接口我們就可以自己實現一個對像池了。
實例：開發一個FTPClient對像池
最近在開發一個項目，需要把hdfs中的文件上傳到一組ftp服務器，為了提高上傳效率，自然考慮到使用多線程的方式進行上傳。我上傳ftp用的工具是Apache common-net包中的FTPClient，但Apache並沒有提供FTPClientPool，於是為了減少FTPClient的創建銷毀次數，我們就自己開發一個FTPClientPool來復用FTPClient連接。
通過上面的介紹，我們可以利用Apache提供的common-pool包來協助我們開發連接池。而開發一個簡單的對像池，僅需要實現common-pool 包中的ObjectPool和PoolableObjectFactory兩個接口即可。下面就看一下我寫的實現：
寫一個ObjectPool接口的實現FTPClientPool
import java.io.IOException;import java.util.NoSuchElementException;import java.util.concurrent.ArrayBlockingQueue;import java.util.concurrent.BlockingQueue;import java.util.concurrent.TimeUnit;import org.apache.commons.net.ftp.FTPClient;import org.apache.commons.pool.ObjectPool;import org.apache.commons.pool.PoolableObjectFactory;/*** 實現了一個FTPClient連接池* @author heaven*/public class FTPClientPool implements ObjectPool<FTPClient>{ private static final int DEFAULT_POOL_SIZE = 10; private final BlockingQueue<FTPClient> pool; private final FtpClientFactory factory; /** * 初始化連接池，需要注入一個工廠來提供FTPClient實例* @param factory * @throws Exception */ public FTPClientPool(FtpClientFactory factory) throws Exception{ this(DEFAULT_POOL_SIZE, factory); } /** * * @param maxPoolSize * @param factory * @throws Exception */ public FTPClientPool(int poolSize, FtpClientFactory factory) throws Exception { this.factory = factory; pool = new ArrayBlockingQueue<FTPClient>(poolSize*2); initPool(poolSize); } /** * 初始化連接池，需要注入一個工廠來提供FTPClient實例* @param maxPoolSize * @throws Exception */ private void initPool(int maxPoolSize) throws Exception { for(int i=0;i<maxPoolSize;i++){ //往池中添加對象addObject(); } } /* (non-Javadoc) * @see org.apache.commons.pool.ObjectPool#borrowObject() */ public FTPClient borrowObject() throws Exception, NoSuchElementException, IllegalStateException { FTPClient client = pool.take(); if (client == null) { client = factory.makeObject(); addObject(); }else if(!factory.validateObject(client)){//驗證不通過//使對像在池中失效invalidateObject(client); //製造並添加新對像到池中client = factory.makeObject(); addObject(); } return client; } /* (non-Javadoc) * @see org.apache.commons.pool.ObjectPool#returnObject(java.lang.Object) */ public void returnObject(FTPClient client) throws Exception { if ((client != null) && !pool.offer(client,3,TimeUnit.SECONDS)) { try { factory.destroyObject(client); } catch (IOException e) { e.printStackTrace(); } } } public void invalidateObject(FTPClient client) throws Exception { //移除無效的客戶端pool.remove(client); } /* (non-Javadoc) * @see org.apache.commons.pool.ObjectPool#addObject() */ public void addObject() throws Exception, IllegalStateException, UnsupportedOperationException { //插入對像到隊列pool.offer(factory.makeObject(),3,TimeUnit.SECONDS); } public int getNumIdle() throws UnsupportedOperationException { return 0; } public int getNumActive() throws UnsupportedOperationException { return 0; } public void clear() throws Exception, UnsupportedOperationException { } /* (non-Javadoc) * @see org.apache.commons.pool.ObjectPool#close() */ public void close() throws Exception { while(pool.iterator().hasNext()){ FTPClient client = pool.take(); factory.destroyObject(client); } } public void setFactory(PoolableObjectFactory<FTPClient> factory) throws IllegalStateException, UnsupportedOperationException { }}再寫一個PoolableObjectFactory接口的實現FTPClientFactory
import java.io.IOException;import org.apache.commons.net.ftp.FTPClient;import org.apache.commons.net.ftp.FTPReply;import org.apache.commons.pool.PoolableObjectFactory;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import com.hdfstoftp.util.FTPClientException;/*** FTPClient工廠類，通過FTPClient工廠提供FTPClient實例的創建和銷毀* @author heaven*/public class FtpClientFactory implements PoolableObjectFactory<FTPClient> {private static Logger logger = LoggerFactory.getLogger("file"); private FTPClientConfigure config; //給工廠傳入一個參數對象，方便配置FTPClient的相關參數public FtpClientFactory(FTPClientConfigure config){ this.config=config; } /* (non-Javadoc) * @see org.apache.commons.pool.PoolableObjectFactory#makeObject() */ public FTPClient makeObject() throws Exception { FTPClient ftpClient = new FTPClient(); ftpClient.setConnectTimeout(config.getClientTimeout()); try { ftpClient.connect(config.getHost(), config.getPort()); int reply = ftpClient.getReplyCode(); if (!FTPReply.isPositiveCompletion(reply)) { ftpClient.disconnect(); logger.warn("FTPServer refused connection"); return null; } boolean result = ftpClient.login(config.getUsername(), config.getPassword()); if (!result) { throw new FTPClientException("ftpClient登陸失敗! userName:" + config.getUsername() + " ; password:" + config.getPassword()); } ftpClient.setFileType(config.getTransferFileType()); ftpClient.setBufferSize(1024); ftpClient.setControlEncoding(config.getEncoding()); if (config.getPassiveMode().equals("true")) { ftpClient.enterLocalPassiveMode(); } } catch (IOException e) { e.printStackTrace(); } catch (FTPClientException e) { e.printStackTrace(); } return ftpClient; } /* (non-Javadoc) * @see org.apache.commons.pool.PoolableObjectFactory#destroyObject(java.lang.Object) */ public void destroyObject(FTPClient ftpClient) throws Exception { try { if (ftpClient != null && ftpClient.isConnected()) { ftpClient.logout(); } } catch (IOException io) { io.printStackTrace(); } finally { // 注意,一定要在finally代碼中斷開連接，否則會導致佔用ftp連接情況try { ftpClient.disconnect(); } catch (IOException io) { io.printStackTrace(); } } } /* (non-Javadoc) * @see org.apache.commons.pool.PoolableObjectFactory#validateObject(java.lang.Object) */ public boolean validateObject(FTPClient ftpClient) { try { return ftpClient.sendNoOp(); } catch (IOException e) { throw new RuntimeException("Failed to validate client: " + e, e); } } public void activateObject(FTPClient ftpClient) throws Exception { } public void passivateObject(FTPClient ftpClient) throws Exception { }}最後，我們最好給工廠傳遞一個參數對象，方便我們設置FTPClient的一些參數
package org.apache.commons.pool.impl.contrib;/** * FTPClient配置類，封裝了FTPClient的相關配置* * @author heaven */public class FTPClientConfigure { private String host; private int port; private String username; private String password; private String passiveMode; private String encoding; private int clientTimeout; private int threadNum; private int transferFileType; private boolean renameUploaded; private int retryTimes; 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 getPassiveMode() { return passiveMode; } public void setPassiveMode(String passiveMode) { this. passiveMode = passiveMode; } public String getEncoding() { return encoding; } public void setEncoding(String encoding) { this. encoding = encoding; } public int getClientTimeout() { return clientTimeout; } public void setClientTimeout( int clientTimeout) { this. clientTimeout = clientTimeout; } public int getThreadNum() { return threadNum; } public void setThreadNum( int threadNum) { this. threadNum = threadNum; } public int getTransferFileType() { return transferFileType; } public void setTransferFileType( int transferFileType) { this. transferFileType = transferFileType; } public boolean isRenameUploaded() { return renameUploaded; } public void setRenameUploaded( boolean renameUploaded) { this. renameUploaded = renameUploaded; } public int getRetryTimes() { return retryTimes; } public void setRetryTimes( int retryTimes) { this. retryTimes = retryTimes; } @Override public String toString() { return "FTPClientConfig [host=" + host + "/n port=" + port + "/n username=" + username + "/n password=" + password + "/n passiveMode=" + passiveMode + "/n encoding=" + encoding + "/n clientTimeout=" + clientTimeout + "/n threadNum=" + threadNum + "/n transferFileType=" + transferFileType + "/n renameUploaded=" + renameUploaded + "/n retryTimes=" + retryTimes + "]" ; }}FTPClientPool連接池類管理FTPClient對象的生命週期，負責對象的借出、規劃、池的銷毀等；FTPClientPool類依賴於FtpClientFactory類，由這個工程類來製造和銷毀對象；FtpClientFactory又依賴FTPClientConfigure類，FTPClientConfigure負責封裝FTPClient的配置參數。至此，我們的FTPClient連接池就開發完成了。
需要注意的是，FTPClientPool中用到了一個阻塞隊列ArrayBlockingQueue來管理存放FTPClient對象，關於阻塞隊列，請參考我的這篇文章： 【Java並發之】BlockingQueue
以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持武林網。