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

Question

ObjectInputStream.readObject()方法抛出了这个SocketException，是什么原因导致这个异常？另外，客户端和服务端socket的soTimeout的值都为0，KeepAlive的值为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)

从关于setSoTimeout方法的JAVA API文档中，如果该方法设置了一个非零值，当times过期时，只会抛出SocketTimeoutException，而不是SocketException:Connection timed out，所以，这个异常应该不相关使用 setSotimeoutMethod。

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PlainSocketImpl(here,is SSLSocketImpl).getTimeout()返回的值是通过setSoTimeout()方法传过来的，那么还是一个很奇怪的方法。

 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。

来自 tcpdump 捕获的消息跟踪，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

虽然此日志抛出 java.net.SocketException: Connection timed out at 22:54，大约 15 分钟后调用 writeObject。

所以我们可以断定这个异常可能是由于长时间（大约30分钟）没有流量导致tcp连接不可用，同时没有调用socket.close()方法关闭socket实例，然后抛出此异常。

所以为了避免这个异常，你应该使用方法 Socket.setKeepAlive() 方法保持连接处于活动状态，尽管你可能没有使用方法 setTimeout()，这意味着要求套接字无限阻塞接收；

或者在您的应用程序中应用心跳机制。