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

Question

此SocketException在ObjectInputStream.readObject（）方法中引发，这是什么原因引起的呢？此外，客户端和服务器套接字的soTimeout值均为0，而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)

从有关setSoTimeout方法的JAVA API文档中，如果此方法设置为非零值，则当时间到期时，仅引发SocketTimeoutException，而不引发SocketException：Connection超时，因此，此异常不应与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.

但是，由PlainSocketImpl（此处为SSLSocketImpl）.getTimeout（）返回的值是通过setSoTimeout（）方法传输的，因此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，而setTimeout方法仅与SocketTimedoutException相关。

从tcpdump捕获的消息跟踪中，如下所示：

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

虽然此日志引发java.net.SocketException：连接在调用的writeObject约15分钟后在22:54超时。

因此，我们可以得出结论，此异常是由以下原因引起的：tcp连接可能由于长时间（约30分钟）没有流量而无法使用，同时没有调用socket.close（）方法来关闭实例套接字，然后抛出此异常。

因此，为避免此异常，应使用Socket.setKeepAlive（）方法使连接保持活动状态，尽管可能未使用setTimeout（）方法，这意味着要求套接字无限量地接收；

或在您的应用程序中应用心跳机制。