什么原因导致：java.net.SocketException：连接超时，不是SocketTimeoutException

Question

这个SocketException抛出ObjectInputStream.readObject（）方法，是什么原因导致这个excetpion？此外，客户端和服务器套接字的soTimeout值均为0，并且KeepAlive值为false。

{2012-01-09 17:44:13,908} ERROR java.net.SocketException: Connection timed out 
at java.net.SocketInputStream.socketRead0(Native Method) 
at java.net.SocketInputStream.read(SocketInputStream.java:146) 
at sun.security.ssl.InputRecord.readFully(InputRecord.java:312) 
at sun.security.ssl.InputRecord.read(InputRecord.java:350) 
at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:809) 
at sun.security.ssl.SSLSocketImpl.readDataRecord(SSLSocketImpl.java:766) 
at sun.security.ssl.AppInputStream.read(AppInputStream.java:94) 
at sun.security.ssl.AppInputStream.read(AppInputStream.java:69) 
at java.io.ObjectInputStream$PeekInputStream.peek(ObjectInputStream.java:2265) 
at java.io.ObjectInputStream$BlockDataInputStream.peek(ObjectInputStream.java:2558) 
at java.io.ObjectInputStream$BlockDataInputStream.peekByte(ObjectInputStream.java:2568) 
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1314) 
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:368) 

大约从setSoTimeout方法的Java API文件，如果该方法被设置无零值，则次期满时，仅SocketTimeoutException被抛出，而不是SocketException：连接超时，于是，这异常不应该与setSotimeoutMethod相关。

public void setSoTimeout(int timeout) throws SocketException 
Enable/disable SO_TIMEOUT with the specified timeout, in milliseconds. With this 
option set to a non-zero timeout, a read() call on the InputStream associated with 
this Socket will block for only this amount of time. If the timeout expires, a 
java.net.SocketTimeoutException is raised, though the Socket is still valid. The 
option must be enabled prior to entering the blocking operation to have effect. The 
timeout must be > 0. A timeout of zero is interpreted as an infinite timeout. 

然而，PlainSocketImpl返回的值（在这里，是SSLSocketImpl）.getTimeout（）由setSoTimeout（）方法转移，那么它仍然是一个非常奇怪的方法。

SocketInputStream.socketRead0(FileDescriptor fd, byte b[], int off, int len, int timeout) 
    the timeout value is passed in the constructor of SocketInputStream:SocketInputStream(PlainSocketImpl impl) throws IOException {} 
    timeout = impl.getTimeout(); 

Answer 1

java.net.SocketException：连接超时异常与setTimeout方法无关，只与SocketTimedoutException有关。

从由tcpdump的捕获的消息痕迹，象下面这样：

NO.   Time  Source Destination Protocol Info 
14845 2012-02-07 22:37:46 10.* 10.*  DNP 3. len=1,from 52156 to 29466,ACK 
14846 2012-02-07 22:37:46 21.* 10.*  DNP 3. len=1,from 54562 to 50276,ACK 
14848 2012-02-07 22:37:46 32.* 10.*  DNP 3. [TCP Restransmission] len=1,from from 52156 to 29466,ACK 
……(7 times retransmission) 
14849 2012-02-07 22:37:47 10.* 10.*  DNP 3. [TCP Restransmission] len=1,from from 52156 to 29466,ACK 

而此日志抛出java.net.SocketException异常：连接在22点54超时，以后约15分钟从的writeObject调用。

所以我们可以得出这样的结论：这个异常是由于可能tcp连接由于很长时间（大约30分钟）没有流量而不可用，同时没有socket.close（）方法被调用来关闭套接字在实例中，然后抛出这个异常。

所以为了避免这种异常，你应该使用方法Socket.setKeepAlive（）方法保持连接的活动状态，尽管你可能没有使用方法setTimeout（），这意味着要求套接字无限制地接收;

或在您的应用程序中应用心跳机制。