Netty Data Stream Handling - write
上篇文章中介绍了Netty是读数据的流程：EventLoop不停的select IO；一旦发现OP_READ可用则利用Channel.Unsafe读取数据，并把数据传给Pipeline；Pipeline拿到数据，并交由内部的Handler处理。还有上篇文章也说了Pipeline中的两种数据流向：inbound和outbound。read操作符合inbound流向；而write则符合outbound刘翔。这些知识对我们理解write操作大有裨益，因为write操作在Pipeline中是read的反向操作。
那我们开始介绍Netty的write写数据流程。先找到EchoServerHandler的channelRead方法，在这个示例中，它会把读到的数据再写回客户端：
public void channelRead(ChannelHandlerContext ctx, Object msg) {
ctx.write(msg);
}
这个方法直接调用了ChannelHandlerContext的write方法：
@Override
public ChannelFuture write(Object msg) {
return write(msg, newPromise());
}
没什么好解释的，给出write的调用链：
@Override
public ChannelFuture write(final Object msg, final ChannelPromise promise) {
if (msg == null) {
throw new NullPointerException("msg");
}
try {
if (isNotValidPromise(promise, true)) {
ReferenceCountUtil.release(msg);
// cancelled
return promise;
}
} catch (RuntimeException e) {
ReferenceCountUtil.release(msg);
throw e;
}
write(msg, false, promise);
return promise;
}
private void write(Object msg, boolean flush, ChannelPromise promise) {
AbstractChannelHandlerContext next = findContextOutbound();
final Object m = pipeline.touch(msg, next);
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
if (flush) {
next.invokeWriteAndFlush(m, promise);
} else {
next.invokeWrite(m, promise);
}
} else {
AbstractWriteTask task;
if (flush) {
task = WriteAndFlushTask.newInstance(next, m, promise);
} else {
task = WriteTask.newInstance(next, m, promise);
}
safeExecute(executor, task, promise, m);
}
}
private void invokeWrite(Object msg, ChannelPromise promise) {
if (invokeHandler()) {
invokeWrite0(msg, promise);
} else {
write(msg, promise);
}
}
private void invokeWrite0(Object msg, ChannelPromise promise) {
try {
((ChannelOutboundHandler) handler()).write(this, msg, promise);
} catch (Throwable t) {
notifyOutboundHandlerException(t, promise);
}
}
invokeWrite0接下来会调用HeadContext中的write方法（没什么好解释的了，read的反向操作）：
@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
unsafe.write(msg, promise);
}
这个方法调用了Unsafe的write方法，至此write操作从Pipeline中走完了，接下来才是重头戏。我们来看这个AbstractUnsafe实现的write方法：
@Override
public final void write(Object msg, ChannelPromise promise) {
assertEventLoop();
// 1
ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
if (outboundBuffer == null) {
safeSetFailure(promise, WRITE_CLOSED_CHANNEL_EXCEPTION);
ReferenceCountUtil.release(msg);
return;
}
int size;
try {
// 2
msg = filterOutboundMessage(msg);
size = pipeline.estimatorHandle().size(msg);
if (size < 0) {
size = 0;
}
} catch (Throwable t) {
safeSetFailure(promise, t);
ReferenceCountUtil.release(msg);
return;
}
// 3
outboundBuffer.addMessage(msg, size, promise);
}
直接调用write写数据的时候，并不是直接写到channel中，而是先写到缓冲区里，也就是ChannelOutboundBuffer。当调用调用flushflush才开始向channel写数据。ChannelOutboundBuffer是一个无界链表，如果不停的向缓冲区写入数据可能会导致内存溢出。因此ChannelOutboundBuffer检测当newWriteBufferSize > channel.config().getWriteBufferHighWaterMark()时，buffer便不可写。同时通知Pipeline Writablity Changed：pipeline.fireChannelWritabilityChanged();。我们在实现自己的Handler时可以重新实现该方法。
我们接着看下filterOutboundMessage这个方法：
@Override
protected final Object filterOutboundMessage(Object msg) {
if (msg instanceof ByteBuf) {
ByteBuf buf = (ByteBuf) msg;
if (buf.isDirect()) {
return msg;
}
return newDirectBuffer(buf);
}
if (msg instanceof FileRegion) {
return msg;
}
throw new UnsupportedOperationException(
"unsupported message type: " + StringUtil.simpleClassName(msg) + EXPECTED_TYPES);
}
在讲解ByteBuf的文章中提到过关于使用直接内存还是堆内存的最佳实践：在IO通信的线程中操作ByteBuf应使用DirectBuffer(省去内存拷贝的成本)，在后端业务逻辑中操作ByteBuf应使用HeapBuffer(不用担心内存泄露)。
这个方法首先检查msg类型是不是ByteBuf，如果是ByteBuf则检查是不是使用了直接内存，如果没有则把基于堆的内存换成直接内存。
我们接着分析ChannelOutboundBuffer的addMessage方法：
public void addMessage(Object msg, int size, ChannelPromise promise) {
Entry entry = Entry.newInstance(msg, size, total(msg), promise);
if (tailEntry == null) {
flushedEntry = null;
tailEntry = entry;
} else {
Entry tail = tailEntry;
tail.next = entry;
tailEntry = entry;
}
if (unflushedEntry == null) {
unflushedEntry = entry;
}
incrementPendingOutboundBytes(entry.pendingSize, false);
}
这个方法做了两件事情：把msg加入到链表中；调用incrementPendingOutboundBytes方法。我们来看这个方法：
private void incrementPendingOutboundBytes(long size, boolean invokeLater) {
if (size == 0) {
return;
}
long newWriteBufferSize = TOTAL_PENDING_SIZE_UPDATER.addAndGet(this, size);
if (newWriteBufferSize > channel.config().getWriteBufferHighWaterMark()) {
setUnwritable(invokeLater);
}
}
首先，先更新缓冲区大小，接着判断缓冲区是否大于最高水位，如果是则设置buffer为unWritable（默认的高水位线大小是64K）。setUnwritable方法内通知了Pipeline Writablity Changed，这里不贴代码了自己看去吧。
到这的话write方法是执行完了，但是数据仍然留在内存中。接下来我们看flush方法。跳过Pipeline直接来看AbstractUnsafe的flush：
@Override
public final void flush() {
assertEventLoop();
ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
if (outboundBuffer == null) {
return;
}
outboundBuffer.addFlush();
flush0();
}
这个方法首先调用了outboundBuffer的addFlush方法，我们先看下addFlush做了什么：
public void addFlush() {
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()) {
int pending = entry.cancel();
decrementPendingOutboundBytes(pending, false, true);
}
entry = entry.next;
} while (entry != null);
unflushedEntry = null;
}
}
unflushedEntry表示缓冲区链表中第一个未被flush的元素。如果这个变量为null的话，表示当前缓冲区已被flush。for循环中首先把entry的promise设置为Uncancellable（Promise继承自Future，Future是可以cancel的），然后增加flushed计数。decrementPendingOutboundBytes方法中有逻辑检查目前Buffer是否低于低水位，如果是则重置Buffer为可写。
我们接着看flush0方法，flush0主要是调用NioSocketChannel的doWrite方法：
protected void flush0() {
///...
doWrite(outboundBuffer);
///...
}
protected void doWrite(ChannelOutboundBuffer in) throws Exception {
for (;;) {
int size = in.size();
if (size == 0) {
clearOpWrite();
break;
}
long writtenBytes = 0;
boolean done = false;
boolean setOpWrite = false;
// 1
ByteBuffer[] nioBuffers = in.nioBuffers();
int nioBufferCnt = in.nioBufferCount();
long expectedWrittenBytes = in.nioBufferSize();
SocketChannel ch = javaChannel();
switch (nioBufferCnt) {
case 0:
super.doWrite(in);
return;
case 1:
ByteBuffer nioBuffer = nioBuffers[0];
for (int i = config().getWriteSpinCount() - 1; i >= 0; i --) {
// 2
final int localWrittenBytes = ch.write(nioBuffer);
if (localWrittenBytes == 0) {
setOpWrite = true;
break;
}
expectedWrittenBytes -= localWrittenBytes;
writtenBytes += localWrittenBytes;
if (expectedWrittenBytes == 0) {
done = true;
break;
}
}
break;
default:
///...
}
in.removeBytes(writtenBytes);
if (!done) {
// Did not write all buffers completely.
incompleteWrite(setOpWrite);
break;
}
}
}
doWrite方法首先获取Buffer中缓存的消息，并将Netty ByteBuf转成Nio ByteBuffer；然后把数据写入Nio SocketChannel。写数据的操作放到了一个for循环中。因为Netty的一个缓冲数据可能不会一次性的刷到Channel中，如果只作一次write操作就返回，那么很有可能余下的数据要等到下次OP_WRITE Selection Key可用才能全部写完。这期间经历了一次昂贵的IO操作，所以这个for循环又是CPU时间和IO时间的一个精心分配。
至此，Netty写数据的流程分析完毕。