Java Socket 类在连接状态方面撒谎
我们有一个 Java 客户端，它保持与远程服务器打开的持久套接字连接。客户端每 15 秒轮询一次数据库表，如果有新项目，它将序列化并将其写入套接字。
在写入输出流之前，我想检查套接字连接是否仍然良好。对于特定的应用程序逻辑，执行此主动检查比捕获异常并被动地重新连接更简单。
我使用以下代码找出连接断开时哪种方法可以让我知道：
LOG.debug("Socket status: bound=" + _socket.isBound() + ", closed=" + _socket.isClosed() + ", connected=" + _socket.isConnected() + ", outputShutdown=" + _socket.isOutputShutdown() + ", inputShutdown=" + _socket.isOutputShutdown());
我短暂禁用了网络适配器，并且在下一次轮询期间，如预期的那样，写入套接字时出现异常。
但是，调试语句打印了以下内容：
"Socket status: bound=true, closed=false, connected=true, outputShutdown=false, inputShutdown=false"
我预计 close 为 true 或 connect 为 false。我得到的实际值似乎是一个谎言。
有没有办法可靠地检查套接字的连接状态？
We have a Java client that keeps a persistent socket connection open to a remote server. The client polls a DB table every 15 seconds and if there a new item, it serializes and writes it to the socket.
Before writing to the output stream, I would like to check whether socket connection is still good. For the specific application logic doing this proactive check is simpler than catching an exception and reconnecting reactively.
I used following code figure out which method can let me know when the connection is broken:
LOG.debug("Socket status: bound=" + _socket.isBound() + ", closed=" + _socket.isClosed() + ", connected=" + _socket.isConnected() + ", outputShutdown=" + _socket.isOutputShutdown() + ", inputShutdown=" + _socket.isOutputShutdown());
I briefly disable my network adapter and during the next polling, as expected, there was an exception while writing to the socket.
However, the debug statement printed the following:
"Socket status: bound=true, closed=false, connected=true, outputShutdown=false, inputShutdown=false"
I expected either closed to be true or connected to be false. What actual values I get seem to be a lie.
Is there a way to reliably check the connection status of a socket?
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仔细阅读 Socket 类 Javadoc。
isConnected 为 true套接字是否能够连接。方法名称用词不当，如果是 hasConnected 会更准确。
一旦套接字成功连接，它就变为 true 并保持 true。 isBound。您必须尝试套接字操作并检查是否失败。
Read the Socket class Javadoc carefully.
isConnected is true if the socket was able to connect. The method name is a misnomer, it would more accurate if it was hasConnected.
Once the socket successfully connects it becomes true and stays true. Same thing for isBound. You have to try a socket operation and check for failure.
我对此不是 100% 确定，但我非常确定底层 BSD Sockets API 实际上没有一种机制来确定 TCP 流是否仍然打开；
read()或write()失败是判断流已被拆除的非常可靠的方法。I'm not 100% sure about this, but I'm pretty certain that the underlying BSD Sockets API doesn't actually have a mechanism to determine whether or not a TCP stream is still open; having a
read()orwrite()fail is a pretty reliable way to tell that the stream has been torn down.Elliot Rusty Harold 的《Java 网络编程》提供了以下精华：
“...要判断套接字当前是否打开，您需要检查 isConnected() 返回 true 并且 isClosed() 返回 false。例如：
希望这会有所帮助。
Java Network Programming by Elliot Rusty Harold offers the following gem:
"...To tell if a socket is currently open, you need to check that isConnected() returns true and isClosed() returns false. For example:
Hope this helps.