09-04 00:18:36.660: E/NetTcpFileSendThread(1728): null
09-04 00:18:36.660: E/NetTcpFileSendThread(1728): java.net.SocketException: Connection reset by peer
09-04 00:18:36.660: E/NetTcpFileSendThread(1728): at org.apache.harmony.luni.platform.OSNetworkSystem.writeDirect(Native Method)
09-04 00:18:36.660: E/NetTcpFileSendThread(1728): at dalvik.system.BlockGuard$WrappedNetworkSystem.writeDirect(BlockGuard.java:290)
09-04 00:18:36.660: E/NetTcpFileSendThread(1728): at org.apache.harmony.nio.internal.SocketChannelImpl.writeImpl(SocketChannelImpl.java:454)
09-04 00:18:36.660: E/NetTcpFileSendThread(1728): at org.apache.harmony.nio.internal.SocketChannelImpl.write(SocketChannelImpl.java:402)
09-04 00:18:36.660: E/NetTcpFileSendThread(1728): at com.ccf.feige.net.NetTcpFileSendThread.sendFile(NetTcpFileSendThread.java:246)
09-04 00:18:36.660: E/NetTcpFileSendThread(1728): at com.ccf.feige.net.NetTcpFileSendThread.SocketListening(NetTcpFileSendThread.java:147)
09-04 00:18:36.660: E/NetTcpFileSendThread(1728): at com.ccf.feige.net.NetTcpFileSendThread.run(NetTcpFileSendThread.java:72)
09-04 00:18:36.660: E/NetTcpFileSendThread(1728): at java.lang.Thread.run(Thread.java:1019)
09-04 00:18:36.660: W/System.err(1728): java.net.SocketException: Connection reset by peer
09-04 00:18:36.660: W/System.err(1728): at org.apache.harmony.luni.platform.OSNetworkSystem.writeDirect(Native Method)
09-04 00:18:36.660: W/System.err(1728): at dalvik.system.BlockGuard$WrappedNetworkSystem.writeDirect(BlockGuard.java:290)
09-04 00:18:36.670: W/System.err(1728): at org.apache.harmony.nio.internal.SocketChannelImpl.writeImpl(SocketChannelImpl.java:454)
09-04 00:18:36.670: W/System.err(1728): at org.apache.harmony.nio.internal.SocketChannelImpl.write(SocketChannelImpl.java:402)
09-04 00:18:36.670: W/System.err(1728): at com.ccf.feige.net.NetTcpFileSendThread.sendFile(NetTcpFileSendThread.java:246)
09-04 00:18:36.670: W/System.err(1728): at com.ccf.feige.net.NetTcpFileSendThread.SocketListening(NetTcpFileSendThread.java:147)
09-04 00:18:36.670: W/System.err(1728): at com.ccf.feige.net.NetTcpFileSendThread.run(NetTcpFileSendThread.java:72)
09-04 00:18:36.670: W/System.err(1728): at java.lang.Thread.run(Thread.java:1019)
问题：
the answer was that it's the server's fault - it had to close the connection after each request .
作为服务器的一端出问题了。服务器一段单方面关闭了连接，导致请求无效。
it might be that android keeps a pool of connections and use the old one or something like that .
可能是Android系统保留了一个老的连接池，然后使用了老的连接，导致失效。
解决方法：我是利用Nio来进行数据传输的，在Nio中，Accepted、Writable、Readable在同一个线程中，每次连接的时候就创建一个新的线程。但是没有注意到及时销毁老的进程，这样当老的连接线程因为监听而阻塞的时候，新的线程也在监听阻塞。两个线程可能公用一个SocketChannel。这样当回收老的进程的时候，新的进程的SocketChannel也被回收了。
导致连接重置（ Connection reset by peer）。
以上是我的猜想。当我及时干掉老线程的时候，发现这个错误就没有了。