与为每个连接创建一个新的线程不同，服务器在启动时创建一个由固定数量线程组成的线程池(thread pool)，
当一个新的客户端连接请求传入服务器，它将交给线程池中的一个线程处理。当该线程处理完这个客户端后，又返回线程池，并为下一次请求处理好准备。如果连接请求到达服务器时，线程池中的所有线程都已经被占用，它们则在一个队列中等待，直到有空闲的线程可用。
服务器的简单示例代码如下
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.logging.Level;
import java.util.logging.Logger;
public class TCPEchoServerPool {
public static void main(String[] args) throws IOException {
if(args.length != 2){
throw new IllegalArgumentException("Parameter(s): ");
}
int echoServPort = Integer.parseInt(args[0]);
int threadPoolSize = Integer.parseInt(args[1]);
final ServerSocket servSock = new ServerSocket(echoServPort);
final Logger logger = Logger.getLogger("practical");
for (int i = 0; i < threadPoolSize; i++){
Thread thread= new Thread() {
public void run() {
while(true) {
try {
Socket clntSock = servSock.accept();
EchoProtocol.handleEchoClient(clntSock, logger);
}catch (IOException ex){
logger.log(Level.WARNING, "Client accept failed", ex);
}
}
}
};
thread.start();
logger.info("Created and started Thread = " + thread.getName());
}
}
}
运行后，设置有2个线程
E:\socket>java TCPEchoServerPool 9080 2
2012-4-13 0:12:12 TCPEchoServerPool main
信息: Created and started Thread = Thread-1
2012-4-13 0:12:12 TCPEchoServerPool main
信息: Created and started Thread = Thread-2
然后使用3个客户端连接，客户端加个sleep()语句，保持一段时间的连接不关闭socket，那么前两个客户端可以正常连接服务器，第三个客户端只能阻塞，前2个释放后就可以连接了。客户端程序如下
import java.net.Socket;
import java.net.SocketException;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
public class TCPEchoClient2{
public static void main(String[] args) throws IOException{
if((args.length < 2) || (args.length > 3))
throw new IllegalArgumentException("Parameter(s): [Port]");
String server = args[0];
byte[] data = args[1].getBytes();
int servPort = (args.length == 3) ? Integer.parseInt(args[2]) : 7;
Socket socket = new Socket(server,servPort);
System.out.println("Connected to server ... sending echo string");
InputStream in = socket.getInputStream();
OutputStream ut = socket.getOutputStream();
out.write(data);
int totalBytesRcvd = 0;
int bytesRcvd;
while(totalBytesRcvd < data.length) {
if((bytesRcvd = in.read(data, totalBytesRcvd,data.length - totalBytesRcvd)) == -1)
throw new SocketException("Connection closed premaurely");
totalBytesRcvd += bytesRcvd;
}
System.out.println("Received: " + new String(data));
try{
Thread.sleep(20000);
}catch(InterruptedException e){
}
socket.close();
}
}
运行方式
java TCPEchoClient2 192.168.1.100 test3 9080
