NettyClient端的channel为NioSocketChannel，通过writeAndFlush方法将数据发送到NettyServer端。
NioSocketChannel.writeAndFlush->AbstractChannel.writeAndFlush->pipeline.writeAndFlush->tail.writeAndFlush->AbstractChannelHandlerContext.writeAndFlush->AbstractChannelHandlerContext.write
private void write(Object msg, boolean flush, ChannelPromise promise) {
AbstractChannelHandlerContext next = findContextOutbound();（1）
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
if (flush) {
next.invokeWriteAndFlush(msg, promise);（2）
} else {
next.invokeWrite(msg, promise);
}
} else {
AbstractWriteTask task;
if (flush) {
task = WriteAndFlushTask.newInstance(next, msg, promise);
} else {
task = WriteTask.newInstance(next, msg, promise);
}
safeExecute(executor, task, promise, msg);
}
}
(1)处从TailContext向前找第一个outbounnd context，业务handler一般为inbound，因此会找到HeadContext。
(2)执行HeadContext.invokeWriteAndFlush方法。
private void invokeWriteAndFlush(Object msg, ChannelPromise promise) {
if (invokeHandler()) {
invokeWrite0(msg, promise);
invokeFlush0();
} else {
writeAndFlush(msg, promise);
}
}
private void invokeWrite0(Object msg, ChannelPromise promise) {
try {
((ChannelOutboundHandler) handler()).write(this, msg, promise);
} catch (Throwable t) {
notifyOutboundHandlerException(t, promise);
}
}
private void invokeFlush0() {
try {
((ChannelOutboundHandler) handler()).flush(this);
} catch (Throwable t) {
notifyHandlerException(t);
}
}
最终会调用HeadContext的write和flush方法，如下：
@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
unsafe.write(msg, promise);
}
@Override
public void flush(ChannelHandlerContext ctx) throws Exception {
unsafe.flush();
}
先看unsafe的write方法：
@Override
public final void write(Object msg, ChannelPromise promise) {
assertEventLoop();
ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
if (outboundBuffer == null) {
// If the outboundBuffer is null we know the channel was closed and so
// need to fail the future right away. If it is not null the handling of the rest
// will be done in flush0()
// See https://github.com/netty/netty/issues/2362
safeSetFailure(promise, WRITE_CLOSED_CHANNEL_EXCEPTION);
// release message now to prevent resource-leak
ReferenceCountUtil.release(msg);
return;
}
int size;
try {
msg = filterOutboundMessage(msg);
/**
* pipeline.estimatorHandle()会得到DefaultMessageSizeEstimator，再调用size(msg)方法用于计算该msg的可写到网络的字节的数量，即该buf的可读字节数量
* 如果msg为ByteBuf，那么返回该buf的可读字节数，即可以发送出去的字节数
*/
size = pipeline.estimatorHandle().size(msg);
if (size < 0) {
size = 0;
}
} catch (Throwable t) {
safeSetFailure(promise, t);
ReferenceCountUtil.release(msg);
return;
}
outboundBuffer.addMessage(msg, size, promise);(3)
}
(3)处的方法如下，主要是将msg放到ChannelOutboundBuffer的双向链表里：
public void addMessage(Object msg, int size, ChannelPromise promise) {
Entry entry = Entry.newInstance(msg, size, total(msg), promise);
if (tailEntry == null) {
/**
* 添加第1个msg时，将添加的msg包成entry，作为链表tail
* 因为entry还没flush，所以flushedEntry设为null
*/
flushedEntry = null;
tailEntry = entry;
} else {
/**
* tail不为null，说明已经有msg在之前add进来了
* 将新add的entry连在之前tail的后面，作为新的tail
*/
Entry tail = tailEntry;
tail.next = entry;
tailEntry = entry;
}
if (unflushedEntry == null) {
/**
* 第1次add的entry，作为unflushedEntry
*/
unflushedEntry = entry;
}
// increment pending bytes after adding message to the unflushed arrays.
// See https://github.com/netty/netty/issues/1619
incrementPendingOutboundBytes(entry.pendingSize, false);
}
放入第1个entry1后，双向链表为：
entry1->null
指针指向为：
flushedEntry->null
unflushedEntry->entry1(本次添加的entry)
tailEntry->entry1
放入第2个entry2后，双向链表为：
entry1->entry2->null
指针指向为：
flushedEntry->null
unflushedEntry->entry1
tailEntry->entry2(本次添加的entry)
再看unsafe的flush方法：
@Override
public final void flush() {
assertEventLoop();
ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
if (outboundBuffer == null) {
return;
}
outboundBuffer.addFlush();(4)
flush0();(5)
}
(4)处的方法为：
public void addFlush() {
// There is no need to process all entries if there was already a flush before and no new messages
// where added in the meantime.
//
// See https://github.com/netty/netty/issues/2577
Entry entry = unflushedEntry;
if (entry != null) {
if (flushedEntry == null) {
// there is no flushedEntry yet, so start with the entry
flushedEntry = entry;
}
do {
flushed ++;
if (!entry.promise.setUncancellable()) {
// Was cancelled so make sure we free up memory and notify about the freed bytes
int pending = entry.cancel();
decrementPendingOutboundBytes(pending, false, true);
}
entry = entry.next;
} while (entry != null);
// All flushed so reset unflushedEntry
unflushedEntry = null;
}
}
执行addFlush前后，双向链表的指针情况如下：
addFlush前指针指向为：
flushedEntry->null
unflushedEntry->entry1
tailEntry->entry1
addFlush后指针指向为：
flushedEntry->entry1
unflushedEntry->null
tailEntry->entry1
addFlush操作主要是从unflushedEntry开始链表后面扫，直到扫到null，并且置unflushedEntry为null，将扫过的entry的promise的result都置为UNCANCELLABLE。
(5)处的方法为：
protected void flush0() {
if (inFlush0) {
// Avoid re-entrance
return;
}
final ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
if (outboundBuffer == null || outboundBuffer.isEmpty()) {
return;
}
inFlush0 = true;
// Mark all pending write requests as failure if the channel is inactive.
if (!isActive()) {
try {
if (isOpen()) {
outboundBuffer.failFlushed(FLUSH0_NOT_YET_CONNECTED_EXCEPTION, true);
} else {
// Do not trigger channelWritabilityChanged because the channel is closed already.
outboundBuffer.failFlushed(FLUSH0_CLOSED_CHANNEL_EXCEPTION, false);
}
} finally {
inFlush0 = false;
}
return;
}
try {
doWrite(outboundBuffer);(6)
} catch (Throwable t) {
if (t instanceof IOException && config().isAutoClose()) {
/**
* Just call {@link #close(ChannelPromise, Throwable, boolean)} here which will take care of
* failing all flushed messages and also ensure the actual close of the underlying transport
* will happen before the promises are notified.
*
* This is needed as otherwise {@link #isActive()} , {@link #isOpen()} and {@link #isWritable()}
* may still return {@code true} even if the channel should be closed as result of the exception.
*/
close(voidPromise(), t, FLUSH0_CLOSED_CHANNEL_EXCEPTION, false);
} else {
outboundBuffer.failFlushed(t, true);
}
} finally {
inFlush0 = false;
}
}
(6)处的方法为AbstractNioByteChannel的doWrite方法：
@Override
protected void doWrite(ChannelOutboundBuffer in) throws Exception {
int writeSpinCount = -1;
/**
* 当发送缓冲区已经满，没能将完整数据包发送出去时，需要设置setOpWrite为true，
* 这样在因为缓冲区满不能将完整数据包发送出去提前退出循环后重新设置写感兴趣事件，
* 待下次可写事件发生时继续把剩下的数据包发送出去。
*/
boolean setOpWrite = false;
for (;;) {
Object msg = in.current();
if (msg == null) {
// Wrote all messages.
clearOpWrite();
// Directly return here so incompleteWrite(...) is not called.
return;
}
if (msg instanceof ByteBuf) {
ByteBuf buf = (ByteBuf) msg;
int readableBytes = buf.readableBytes();
if (readableBytes == 0) {
in.remove();
continue;
}
boolean done = false;
long flushedAmount = 0;
if (writeSpinCount == -1) {
writeSpinCount = config().getWriteSpinCount();
}
for (int i = writeSpinCount - 1; i >= 0; i --) {
/**
* 这里的localFlushedAmount是真正写入到channel的字节数
*/
int localFlushedAmount = doWriteBytes(buf);
if (localFlushedAmount == 0) {
/**
* 写入的字节数为0，说明channel的写缓冲区已满，不能再写入
*/
setOpWrite = true;
break;
}
flushedAmount += localFlushedAmount;
/**
* buf不可读说明写入buf的数据已经全部发送到channel
*/
if (!buf.isReadable()) {
done = true;
break;
}
}
in.progress(flushedAmount);
if (done) {
in.remove();
} else {
// Break the loop and so incompleteWrite(...) is called.
break;
}
} else if (msg instanceof FileRegion) {
FileRegion region = (FileRegion) msg;
boolean done = region.transfered() >= region.count();
if (!done) {
long flushedAmount = 0;
if (writeSpinCount == -1) {
writeSpinCount = config().getWriteSpinCount();
}
for (int i = writeSpinCount - 1; i >= 0; i--) {
long localFlushedAmount = doWriteFileRegion(region);
if (localFlushedAmount == 0) {
setOpWrite = true;
break;
}
flushedAmount += localFlushedAmount;
if (region.transfered() >= region.count()) {
done = true;
break;
}
}
in.progress(flushedAmount);
}
if (done) {
in.remove();
} else {
// Break the loop and so incompleteWrite(...) is called.
break;
}
} else {
// Should not reach here.
throw new Error();
}
}
incompleteWrite(setOpWrite);
}
doWrite方法执行的操作主要为循环从ChannelOutboundBuffer中取出msg写到channel的socket发送缓冲区，直到写完ChannelOutboundBuffer的所有msg，但有时因为socket发送缓冲区满了不能及时发出去，那么设置setOpWrite为true，标识写半包情况，退出循环后要注册channel的write感兴趣事件，等待下次可写了继续发。
对于server端，在NioEventLoop select循环中获取到读事件，解析好client发送来的数据并响应给client，同样是执行NioSocketChannel.writeAndFlush操作，与client端一样，不在详细阐述。