JAVA NIO和MINA发送数据过程解析
NIO发送数据过程：
1 将信道写入操作加锁保证其他线程不对信道写入（文档中称“是如果另一个线程已经在此通道上发起了一个写入操作，则在该操作完成前此方法的调用被阻塞。”）
2 如果缓冲区为非直接缓冲区，则复制缓冲区内容到直接缓冲区，防止外界对缓冲区内容修改导致发送数据损坏
??? * 复制过程分配的内存将被捆绑在线程上，在线程关闭之前，这部分内存不被回收，等IO操作完成后，可重用这部分内存，减少内存分配过程，其性能消耗更多在复制内容上。
??? * 直接缓冲区与非直接缓冲区参见ByteBuffer类文档
3 通过JNI调用本地方法直接将缓冲区地址和长度传递给操作系统，由系统进行异步IO操作
由此可见jvm与操作系统共用直接缓冲区。
MINA发送数据过程：
?MINA发送数据过程在
??? org.apache.mina.core.polling.AbstractPollingIoProcessor.flushNow(T, long)
??? org.apache.mina.core.polling.AbstractPollingIoProcessor.writeBuffer(T, WriteRequest, boolean, int, long)
??? org.apache.mina.transport.socket.nio.NioProcessor.write(NioSession, IoBuffer, int)
?方法中。
?
??? flashNow方法负责清空IoSession.writeRequestQueue列队。为保证传输效率，flashNow方法尽量将输出长度匹配到接收缓冲区长度的1.5倍。
??? flashNow方法循环调用writeBuffer，直到所调用writeBuffer方法返回值总和大于等于接收缓冲区容量的1.5倍、writeBuffer返回0或全部消息发送成功。
??? writeBuffer方法负责发送一条消息数据，write方法返回0，writeBuffer将连续尝试256次。
??? 如果消息完整发送，将调用IoFilterChain.fireMessageSent(req)方法。如果有数据发送，将返回实际发送数据长度，否则返回0。
??? ??? * 如果连接支持碎片拼接（如TCP连接），则writeBuffer方法可以决定输出消息的一部分，保证输出数据长度不超过输入缓冲区的1.5倍。
??? ??? * 如果连接不支持碎片拼接（如UDP连接），则writterBuffer方法必须将消息完整输出。
??? write方法调用信道的write方法，将IoBuffer包装的ByteBuffer放入操作系统缓冲区，并返回结果。
???
由此可见，在基本NIO中，如果系统底层缓冲区填满，SocketChannel.write方法将返回0，业务逻辑必须处理此情况，否则将丢失发送数据。
在MINA中，使用消息分批或拆分方式将最大效率利用系统缓冲区，并保证由于系统缓冲区装满而未发送的消息将重新发送。
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相关代码：
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sun.nio.ch.SocketChannelImpl.write(ByteBuffer)关键代码：
??? ??? .
??? ??? .
??? ??? .
??? ??? for (;;) {
??? ??? ??? n = IOUtil.write(fd, buf, -1, nd, writeLock);
??? ??? ??? if ((n == IOStatus.INTERRUPTED) && isOpen())
??? ??? ??? continue;
??? ??? ??? return IOStatus.normalize(n);
??? ??? }
??? ??? .
??? ??? .
??? ??? .
??? ???
sun.nio.ch.IOUtil.write(FileDescriptor, ByteBuffer[], int, int, NativeDispatcher) 关键代码：
??? ??? .
??? ??? .
??? ??? .
??? ???
??? IOVecWrapper vec = IOVecWrapper.get(length);
??? .
??? .
??????? ByteBuffer shadow = Util.getTemporaryDirectBuffer(rem);
??????? hadow.put(buf);
??????? shadow.flip();
??????? vec.setShadow(iov_len, shadow);
??? .
??? .
??? .
??? long bytesWritten = nd.writev(fd, vec.address, iov_len);
???
SocketDispatcher.writev:
??? long writev(FileDescriptor fd, long address, int len) throws IOException {
??????? return writev0(fd, address, len);
??? }
?
特殊返回值定义在：
final class IOStatus {
??? static final int EOF = -1;??? ??? ??? // End of file
??? static final int UNAVAILABLE = -2;??? ??? // Nothing available (non-blocking)
??? static final int INTERRUPTED = -3;??? ??? // System call interrupted
??? static final int UNSUPPORTED = -4;??? ??? // Operation not supported
??? static final int THROWN = -5;??? ??? // Exception thrown in JNI code
??? static final int UNSUPPORTED_CASE = -6; // This case not supported
???
??? static int normalize(int n) {
??? ??? if (n == UNAVAILABLE)
??? ??? ??? return 0;
??? ??? return n;
??? }
}
windows下JNI调用：
Java_sun_nio_ch_SocketDispatcher_writev0(JNIEnv *env, jclass clazz,
???????????????????????????????????????? jobject fdo, jlong address, jint len)
{
??? /* set up */
??? int i = 0;
??? DWORD written = 0;
??? jint fd = fdval(env, fdo);
??? struct iovec *iovp = (struct iovec *)address;
??? WSABUF *bufs = malloc(len * sizeof(WSABUF));
??? if (bufs == 0) {
??????? JNU_ThrowOutOfMemoryError(env, 0);
??????? return IOS_THROWN;
??? }
??? if ((isNT() == JNI_FALSE) && (len > 16)) {
??????? len = 16;
??? }
??? /* copy iovec into WSABUF */
??? for(i=0; i<len; i++) {
??????? bufs[i].buf = (char *)iovp[i].iov_base;
??????? bufs[i].len = (u_long)iovp[i].iov_len;
??? }
???
??? /* read into the buffers */
??? i = WSASend((SOCKET)fd, /* Socket */
??????????? bufs,?????????? /* pointers to the buffers */
??????????? (DWORD)len,???? /* number of buffers to process */
??????????? &written,?????? /* receives number of bytes written */
??????????? 0,????????????? /* no flags */
??????????? 0,????????????? /* no overlapped sockets */
??????????? 0);???????????? /* no completion routine */
??? /* clean up */
??? free(bufs);
??? if (i != 0) {
??????? int theErr = (jint)WSAGetLastError();
??????? if (theErr == WSAEWOULDBLOCK) {
??????????? return IOS_UNAVAILABLE;
??????? }
??????? JNU_ThrowIOExceptionWithLastError(env, "Vector write failed");
??????? return IOS_THROWN;
??? }
??? return convertLongReturnVal(env, (jlong)written, JNI_FALSE);
}
windows本地接口
int WSASend (
SOCKET s，
LPWSABUF lpBuffers
DWORD dwBufferCount，
LPDWORD lpNumberOfBytesSent，
DWORD dwFlags，
LPWSAOVERLAPPED lpOverlapped，
LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
)；
返回值WSAEWOULDBLOCK??? Windows NT: Overlapped sockets: There are too many outstanding overlapped I/O requests. Nonoverlapped sockets: The socket is marked as nonblocking and the send operation cannot be completed immediately.
