最近在写一个FTP上传工具，用到了Apache的FTPClient，为了提高上传效率，我采用了多线程的方式，但是每个线程频繁的创建和销毁FTPClient对象势必会造成不必要的开销，因此，此处最好使用一个FTPClient连接池。仔细翻了一下Apache的api，发现它并没有一个FTPClientPool的实现，所以，不得不自己写一个FTPClientPool。下面就大体介绍一下开发连接池的整个过程，供大家参考。
关于对象池
有些对象的创建开销是比较大的，比如数据库连接等。为了减少频繁创建、销毁对象带来的性能消耗，我们可以利用对象池的技术来实现对象的复用。对象池提供了一种机制，它可以管理对象池中对象的生命周期，提供了获取和释放对象的方法，可以让客户端很方便的使用对象池中的对象。
如果我们要自己实现一个对象池，一般需要完成如下功能：
1. 如果池中有可用的对象，对象池应当能返回给客户端
2. 客户端把对象放回池里后，可以对这些对象进行重用
3. 对象池能够创建新的对象来满足客户端不断增长的需求
4. 需要有一个正确关闭池的机制来结束对象的生命周期
Apache的对象池工具包
为了方便我们开发自己的对象池，Apache 提供的common-pool工具包，里面包含了开发通用对象池的一些接口和实现类。其中最基本的两个接口是ObjectPool 和PoolableObjectFactory。
ObjectPool接口中有几个最基本的方法：
1. addObject() : 添加对象到池
2. borrowObject()：客户端从池中借出一个对象
3. returnObject()：客户端归还一个对象到池中
4. close()：关闭对象池，清理内存释放资源等
5. setFactory(ObjectFactory factory)：需要一个工厂来制造池中的对象
PoolableObjectFactory接口中几个最基本的方法：
1. makeObject()：制造一个对象
2. destoryObject()：销毁一个对象
3. validateObject()：验证一个对象是否还可用
通过以上两个接口我们就可以自己实现一个对象池了。
实例：开发一个FTPClient对象池
最近在开发一个项目，需要把hdfs中的文件上传到一组ftp服务器，为了提高上传效率，自然考虑到使用多线程的方式进行上传。我上传ftp用的工具是Apache common-net包中的FTPClient，但Apache并没有提供FTPClientPool，于是为了减少FTPClient的创建销毁次数，我们就自己开发一个FTPClientPool来复用FTPClient连接。
通过上面的介绍，我们可以利用Apache提供的common-pool包来协助我们开发连接池。而开发一个简单的对象池，仅需要实现common-pool 包中的ObjectPool和PoolableObjectFactory两个接口即可。下面就看一下我写的实现：
写一个ObjectPool接口的实现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;/*** 实现了一个FTPClient连接池* @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; /** * 初始化连接池，需要注入一个工厂来提供FTPClient实例 * @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); } /** * 初始化连接池，需要注入一个工厂来提供FTPClient实例 * @param maxPoolSize * @throws Exception */ private void initPool(int maxPoolSize) throws Exception { for(int i=0;i<maxPoolSize;i++){ //往池中添加对象 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)){//验证不通过 //使对象在池中失效 invalidateObject(client); //制造并添加新对象到池中 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 { //移除无效的客户端 pool.remove(client); } /* (non-Javadoc) * @see org.apache.commons.pool.ObjectPool#addObject() */ public void addObject() throws Exception, IllegalStateException, UnsupportedOperationException { //插入对象到队列 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 { }}再写一个PoolableObjectFactory接口的实现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工厂类，通过FTPClient工厂提供FTPClient实例的创建和销毁* @author heaven*/public class FtpClientFactory implements PoolableObjectFactory<FTPClient> {private static Logger logger = LoggerFactory.getLogger("file"); private FTPClientConfigure config; //给工厂传入一个参数对象，方便配置FTPClient的相关参数 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登陆失败! 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代码中断开连接，否则会导致占用ftp连接情况 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 { }}最后，我们最好给工厂传递一个参数对象，方便我们设置FTPClient的一些参数
package org.apache.commons.pool.impl.contrib;/** * FTPClient配置类，封装了FTPClient的相关配置 * * @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连接池类管理FTPClient对象的生命周期，负责对象的借出、规划、池的销毁等；FTPClientPool类依赖于FtpClientFactory类，由这个工程类来制造和销毁对象；FtpClientFactory又依赖FTPClientConfigure类，FTPClientConfigure负责封装FTPClient的配置参数。至此，我们的FTPClient连接池就开发完成了。
需要注意的是，FTPClientPool中用到了一个阻塞队列ArrayBlockingQueue来管理存放FTPClient对象，关于阻塞队列，请参考我的这篇文章： 【Java并发之】BlockingQueue
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持武林网。