什麼原因導致：java.net.SocketException：連接超時，不是SocketTimeoutException

Question

這個SocketException拋出ObjectInputStream.readObject（）方法，是什麼原因導致這個excetpion？此外，客戶端和服務器套接字的soTimeout值均爲0，並且KeepAlive值爲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) 

大約從setSoTimeout方法的Java API文件，如果該方法被設置無零值，則次期滿時，僅SocketTimeoutException被拋出，而不是SocketException：連接超時，於是，這異常不應該與setSotimeoutMethod相關。

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（在這裏，是SSLSocketImpl）.getTimeout（）由setSoTimeout（）方法轉移，那麼它仍然是一個非常奇怪的方法。

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：連接超時異常與setTimeout方法無關，只與SocketTimedoutException有關。

從由tcpdump的捕獲的消息痕跡，象下面這樣：

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 

而此日誌拋出java.net.SocketException異常：連接在22點54超時，以後約15分鐘從的writeObject調用。

所以我們可以得出這樣的結論：這個異常是由於可能tcp連接由於很長時間（大約30分鐘）沒有流量而不可用，同時沒有socket.close（）方法被調用來關閉套接字在實例中，然後拋出這個異常。

所以爲了避免這種異常，你應該使用方法Socket.setKeepAlive（）方法保持連接的活動狀態，儘管你可能沒有使用方法setTimeout（），這意味着要求套接字無限制地接收;

或在您的應用程序中應用心跳機制。