Question

java如何实现http长连接

Answer 1

通过轮询来实现长连接
轮询：隔一段时间访问服务器，服务器不管有没有新消息都立刻返回。
http长连接实现代码：
客户端：
package houlei.csdn.keepalive;

import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.concurrent.ConcurrentHashMap;

/**
* C/S架构的客户端对象，持有该对象，可以随时向服务端发送消息。
* <p>
* 创建时间：2010-7-18 上午12:17:25
* @author HouLei
* @since 1.0
*/
public class Client {

/**
* 处理服务端发回的对象，可实现该接口。
*/
public static interface ObjectAction{
void doAction(Object obj,Client client);
}
public static final class DefaultObjectAction implements ObjectAction{
public void doAction(Object obj,Client client) {
System.out.println("处理：\t"+obj.toString());//诊断程序是否正常
}
}
public static void main(String[] args) throws UnknownHostException, IOException {
String serverIp = "127.0.0.1";
int port = 65432;
Client client = new Client(serverIp,port);
client.start();
}

private String serverIp;
private int port;
private Socket socket;
private boolean running=false;
private long lastSendTime;
private ConcurrentHashMap<Class, ObjectAction> actionMapping = new ConcurrentHashMap<Class,ObjectAction>();

public Client(String serverIp, int port) {
this.serverIp=serverIp;this.port=port;
}

public void start() throws UnknownHostException, IOException {
if(running)return;
socket = new Socket(serverIp,port);
System.out.println("本地端口："+socket.getLocalPort());
lastSendTime=System.currentTimeMillis();
running=true;
new Thread(new KeepAliveWatchDog()).start();
new Thread(new ReceiveWatchDog()).start();
}

public void stop(){
if(running)running=false;
}

/**
* 添加接收对象的处理对象。
* @param cls 待处理的对象，其所属的类。
* @param action 处理过程对象。
*/
public void addActionMap(Class<Object> cls,ObjectAction action){
actionMapping.put(cls, action);
}

public void sendObject(Object obj) throws IOException {
ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
oos.writeObject(obj);
System.out.println("发送：\t"+obj);
oos.flush();
}

class KeepAliveWatchDog implements Runnable{
long checkDelay = 10;
long keepAliveDelay = 2000;
public void run() {
while(running){
if(System.currentTimeMillis()-lastSendTime>keepAliveDelay){
try {
Client.this.sendObject(new KeepAlive());
} catch (IOException e) {
e.printStackTrace();
Client.this.stop();
}
lastSendTime = System.currentTimeMillis();
}else{
try {
Thread.sleep(checkDelay);
} catch (InterruptedException e) {
e.printStackTrace();
Client.this.stop();
}
}
}
}
}

class ReceiveWatchDog implements Runnable{
public void run() {
while(running){
try {
InputStream in = socket.getInputStream();
if(in.available()>0){
ObjectInputStream ois = new ObjectInputStream(in);
Object obj = ois.readObject();
System.out.println("接收：\t"+obj);//接受数据
ObjectAction oa = actionMapping.get(obj.getClass());
oa = oa==null?new DefaultObjectAction():oa;
oa.doAction(obj, Client.this);
}else{
Thread.sleep(10);
}
} catch (Exception e) {
e.printStackTrace();
Client.this.stop();
}
}
}
}

}
服务端：
package houlei.csdn.keepalive;

import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.concurrent.ConcurrentHashMap;

/**
* C/S架构的服务端对象。
* <p>
* 创建时间：2010-7-18 上午12:17:37
* @author HouLei
* @since 1.0
*/
public class Server {

/**
* 要处理客户端发来的对象，并返回一个对象，可实现该接口。
*/
public interface ObjectAction{
Object doAction(Object rev);
}

public static final class DefaultObjectAction implements ObjectAction{
public Object doAction(Object rev) {
System.out.println("处理并返回："+rev);//确认长连接状况
return rev;
}
}

public static void main(String[] args) {
int port = 65432;
Server server = new Server(port);
server.start();
}

private int port;
private volatile boolean running=false;
private long receiveTimeDelay=3000;
private ConcurrentHashMap<Class, ObjectAction> actionMapping = new ConcurrentHashMap<Class,ObjectAction>();
private Thread connWatchDog;

public Server(int port) {
this.port = port;
}

public void start(){
if(running)return;
running=true;
connWatchDog = new Thread(new ConnWatchDog());
connWatchDog.start();
}

@SuppressWarnings("deprecation")
public void stop(){
if(running)running=false;
if(connWatchDog!=null)connWatchDog.stop();
}

public void addActionMap(Class<Object> cls,ObjectAction action){
actionMapping.put(cls, action);
}

class ConnWatchDog implements Runnable{
public void run(){
try {
ServerSocket ss = new ServerSocket(port,5);
while(running){
Socket s = ss.accept();
new Thread(new SocketAction(s)).start();
}
} catch (IOException e) {
e.printStackTrace();
Server.this.stop();
}

}
}

class SocketAction implements Runnable{
Socket s;
boolean run=true;
long lastReceiveTime = System.currentTimeMillis();
public SocketAction(Socket s) {
this.s = s;
}
public void run() {
while(running && run){
if(System.currentTimeMillis()-lastReceiveTime>receiveTimeDelay){
overThis();
}else{
try {
InputStream in = s.getInputStream();
if(in.available()>0){
ObjectInputStream ois = new ObjectInputStream(in);
Object obj = ois.readObject();
lastReceiveTime = System.currentTimeMillis();
System.out.println("接收：\t"+obj);
ObjectAction oa = actionMapping.get(obj.getClass());
oa = oa==null?new DefaultObjectAction():oa;
Object out = oa.doAction(obj);
if(out!=null){
ObjectOutputStream oos = new ObjectOutputStream(s.getOutputStream());
oos.writeObject(out);
oos.flush();
}
}else{
Thread.sleep(10);
}
} catch (Exception e) {
e.printStackTrace();
overThis();
}
}
}
}

private void overThis() {
if(run)run=false;
if(s!=null){
try {
s.close();
} catch (IOException e) {
e.printStackTrace();
}
}
System.out.println("关闭："+s.getRemoteSocketAddress());//关闭长连接
}

}

}
长连接的维持，是要客户端程序，定时向服务端程序，发送一个维持连接包的。
如果，长时间未发送维持连接包，服务端程序将断开连接。

Answer 2

目前web上的消息通讯方式主要有以下几种。
轮询，长连接，websocket
轮询：隔一段时间访问服务器，服务器不管有没有新消息都立刻返回。
长连接：页面向服务器发出请求，由服务器决定什么时候返回。（如果有新消息则立刻返回，没有的话就保持连接，直到有新消息才返回）
websocket：类似Java Socket，由Http请求模拟实现的socket。

要实现长连接的关键就是： 由服务器端决定什么时候返回数据。比如在servlet中。

doGet(...){
...
Thread.sleep(30000);
return ...
}
这就是一个长连接的例子，只是没有任何意义而已。

你要做的就是在doGet中阻塞住，
while(!hasNewMsg){
sleep(500)
}
return newMsg...

当然你的ajax超时时间要设置长一点。

如果可以的话，最好可以使用websocket。