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：http://blog.csdn.net/suifeng3051/article/details/48807423
