NullPointerException is thrown when executing org.apache.coyote.http11.InternalNioOutputBuffer.writeToSocket

[HU E]

Dear all,

During performance and stability run, we encountered one tomcat issue, i.e. 
sometimes a NullPointerException is thrown when executing 
org.apache.coyote.http11.InternalNioOutputBuffer.writeToSocket. This issue was 
happened very rarely.

Following is the stack:
PRS0: Http response exception : java.lang.NullPointerException
org.apache.coyote.http11.InternalNioOutputBuffer.writeToSocket(InternalNioOutputBuffer.java:463)
org.apache.coyote.http11.InternalNioOutputBuffer.flushBuffer(InternalNioOutputBuffer.java:800)
org.apache.coyote.http11.InternalNioOutputBuffer.endRequest(InternalNioOutputBuffer.java:412)
org.apache.coyote.http11.Http11NioProcessor.action(Http11NioProcessor.java:1097)
org.apache.coyote.Response.action(Response.java:183)
org.apache.coyote.Response.finish(Response.java:305)
org.apache.catalina.connector.OutputBuffer.close(OutputBuffer.java:284)
org.apache.catalina.connector.CoyoteWriter.close(CoyoteWriter.java:108)
....

The tomcat version is 6.0.35. I checked the source code, the exception occurred 
at line 463 of InternalNioOutputBuffer.java, which is marked in red.

private synchronized int writeToSocket(ByteBuffer bytebuffer, boolean block, 
boolean flip) throws IOException {
......
......
try {
            written = getSelectorPool().write(bytebuffer, socket, selector, 
writeTimeout, block,lastWrite);
            //make sure we are flushed
            do {
                if (socket.flush(true,selector,writeTimeout,lastWrite)) break;
            }while ( true );
        }finally {
            if ( selector != null ) getSelectorPool().put(selector);
        }

......
......
}

Before executing line 460 (i.e. written = getSelectorPool().write(bytebuffer, 
socket, selector, writeTimeout, block,lastWrite)),socket object should not be 
null.

So I guess socket is recycled after executing line 460 so that 
NullPointException is thrown. But I am not sure why the socket is recycled. 
Client closes the TCP connection? TCP write timeout? TCP is reset?

I appreciate your help and thanks for your time in advance.

Thanks
Eric