blocks|key|590920|text|在各种消息传递协议中，通常的做法是保持彼此的心跳(保持发送ping数据包)，数据包不需要很大。探测机制将允许你在TCP找出之前就检测到断开的客户端(TCP超时要高得多)发送一个探测并等待例如5秒的回复，如果你没有看到后续2-3个探测的回复，那么你的播放器是断开的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|590921|还有，related+question|offset|length|590922|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/485855/how-do-you-handle-socket-disconnecting-in-java^0|0|3|G|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

It is general practice in various messaging protocols to keep heartbeating each other (keep sending ping packets) the packet does not need to be very large. The probing mechanism will allow you to detect the disconnected client even before TCP figures it out in general (TCP timeout is far higher) Send a probe and wait for say 5 seconds for a reply, if you do not see reply for say 2-3 subsequent probes, your player is disconnected.

Also, <a href="https://stackoverflow.com/questions/485855/how-do-you-handle-socket-disconnecting-in-java">related question</a>

blocks|key|375205|text|我认为这是tcp连接的本质，在该标准中，在传输过程中需要大约6分钟的静默，然后我们才能得出输出连接消失的结论！所以我不认为你能找到这个问题的确切解决方案。也许更好的方法是编写一些方便的代码来猜测服务器应该在什么时候假设用户连接关闭。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|375206|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I think this is nature of tcp connections, in that standards it takes about 6 minutes of silence in transmission before we conclude that out connection is gone!
So I don`t think you can find an exact solution for this problem. Maybe the better way is to write some handy code to guess when server should suppose a user connection is closed.

blocks|key|375275|text|在Linux上，当write()到一个套接字时，你不知道的另一端，关闭它会引起一个SIGPIPE信号/异常，无论你想怎么调用它。但是，如果您不想被SIGPIPE捕获，那么可以使用带有标志MSG_NOSIGNAL的send()。send()调用将返回-1，在这种情况下，您可以检查errno，它将告诉您，您试图使用值EPIPE写入一个断开的管道(在本例中是一个套接字)，根据errno.h，该值等于32。作为对EPIPE的反应，您可以返回并尝试重新打开套接字，然后尝试再次发送您的信息。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|375276|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

On Linux when write()ing into a socket which the other side, unknown to you, closed will provoke a SIGPIPE signal/exception however you want to call it. However if you don't want to be caught out by the SIGPIPE you can use send() with the flag MSG_NOSIGNAL. The send() call will return with -1 and in this case you can check errno which will tell you that you tried to write a broken pipe (in this case a socket) with the value EPIPE which according to errno.h is equivalent to 32. As a reaction to the EPIPE you could double back and try to reopen the socket and try to send your information again.

blocks|key|591116|text|正如@user207421所说，由于TCP/IP协议体系结构模型的原因，没有办法知道连接的当前状态。因此，服务器必须在关闭连接之前注意到您，或者您自己检查它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|591117|这是一个简单的示例，展示了如何知道socket已被服务器关闭：|591118|sockAdr+=+new+InetSocketAddress(SERVER_HOSTNAME,+SERVER_PORT);
socket+=+new+Socket();
timeout+=+5000;
socket.connect(sockAdr,+timeout);
reader+=+new+BufferedReader(new+InputStreamReader(socket.getInputStream());
while+((data+=+reader.readLine())!=null)+
++++++log.e(TAG,+"received+->+"+%2B+data);
log.e(TAG,+"Socket+closed+!");|code-block|syntax|javascript|591119|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

As @user207421 say there is no way to know the current state of the connection because of the TCP/IP Protocol Architecture Model. So the server has to notice you before closing the connection or you check it by yourself. 
This is a simple example that shows how to know the socket is closed by the server:

<pre><code>sockAdr = new InetSocketAddress(SERVER_HOSTNAME, SERVER_PORT);
socket = new Socket();
timeout = 5000;
socket.connect(sockAdr, timeout);
reader = new BufferedReader(new InputStreamReader(socket.getInputStream());
while ((data = reader.readLine())!=null) 
 log.e(TAG, "received -&gt; " + data);
log.e(TAG, "Socket closed !");
</code></pre>

blocks|key|375388|text|我就是这么处理的|type|unstyled|depth|inlineStyleRanges|entityRanges|data|375389|+while(true)+{
++++++++if((receiveMessage+=+receiveRead.readLine())+!=+null+)+{++

++++++++System.out.println("first+message+same+:"%2BreceiveMessage);
++++++++System.out.println(receiveMessage);++++++

++++++++}
++++++++else+if(receiveRead.readLine()==null)
++++++++{

++++++++System.out.println("Client+has+disconected:+"%2Bsock.isClosed());+
++++++++System.exit(1);
+++++++++}++++}+|code-block|syntax|javascript|375390|如果result.code+==为null|375391|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Thats how I handle it 

<pre><code> while(true) {
 if((receiveMessage = receiveRead.readLine()) != null ) { 

 System.out.println("first message same :"+receiveMessage);
 System.out.println(receiveMessage); 

 }
 else if(receiveRead.readLine()==null)
 {

 System.out.println("Client has disconected: "+sock.isClosed()); 
 System.exit(1);
 } } 
</code></pre>

if the result.code == null

blocks|key|375438|text|我也遇到过类似的问题。在我的例子中，客户端必须定期发送数据。我希望你有同样的要求。然后我设置了SO_TIMEOUT+socket.setSoTimeout(1000+*+60+*+5);，当指定的时间到期时抛出java.net.SocketTimeoutException。这样我就可以很容易地检测到死掉的客户端。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|375439|entityMap^0|1M|Z|2X|V|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

I faced similar problem. In my case client must send data periodically. I hope you have same requirement. Then I set SO_TIMEOUT <code>socket.setSoTimeout(1000 * 60 * 5);</code> which is throw <code>java.net.SocketTimeoutException</code> when specified time is expired. Then I can detect dead client easily.

blocks|key|375490|text|这里是任何数据类型的另一个通用解决方案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|375491|int+offset+=+0;
byte[]+buffer+=+new+byte[8192];

try+{
++++do+{
++++++++int+b+=+inputStream.read();

++++++++if+(b+==+-1)
+++++++++++break;

++++++++buffer[offset%2B%2B]+=+(byte)+b;

++++++++//check+offset+with+buffer+length+and+reallocate+array+if+needed
++++}+while+(inputStream.available()+>+0);
}+catch+(SocketException+e)+{
++++//connection+was+lost
}

//process+buffer|code-block|syntax|javascript|375492|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Here you are another general solution for any data type.
<pre class="lang-java prettyprint-override"><code>int offset = 0;
byte[] buffer = new byte[8192];

try {
 do {
 int b = inputStream.read();

 if (b == -1)
 break;

 buffer[offset++] = (byte) b;

 //check offset with buffer length and reallocate array if needed
 } while (inputStream.available() &gt; 0);
} catch (SocketException e) {
 //connection was lost
}

//process buffer
</code></pre>

I am running into some issues with the Java socket API. I am trying to display the number of players currently connected to my game. It is easy to determine when a player has connected. However, it seems unnecessarily difficult to determine when a player has disconnected using the socket API.

Calling <code>isConnected()</code> on a socket that has been disconnected remotely always seems to return <code>true</code>. Similarly, calling <code>isClosed()</code> on a socket that has been closed remotely always seems to return <code>false</code>. I have read that to actually determine whether or not a socket has been closed, data must be written to the output stream and an exception must be caught. This seems like a really unclean way to handle this situation. We would just constantly have to spam a garbage message over the network to ever know when a socket had closed.

Is there any other solution?

Java socket API: How to tell if a connection has been closed?

服务器

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我遇到了Java套接字API的一些问题。我正在尝试显示当前连接到我的游戏的玩家数量。很容易确定玩家何时已连接。但是，使用套接字API确定播放器何时断开连接似乎是不必要的困难。在远程断开的套接字上调用isConnected()似乎总是返回true。类似地，在远程关闭的套接字上调用isClosed()似乎总是返回false...

问Java套接字API:如何判断连接是否已关闭？
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