上一章节中，分析了Netty如何处理read事件，本节分析Netty如何把数据写会客户端。
把数据返回客户端，需要经历三个步骤：
1、申请一块缓存buf，写入数据。
2、将buf保存到ChannelOutboundBuffer中。
3、将ChannelOutboundBuffer中的buff输出到socketChannel中。
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
ReferenceCountUtil.release(msg);
ByteBuf buf1 = ctx.alloc().buffer(4);
buf1.writeInt(1);
ByteBuf buf2 = ctx.alloc().buffer(4);
buf2.writeInt(2);
ByteBuf buf3 = ctx.alloc().buffer(4);
buf3.writeInt(3);
ctx.write(buf1);
ctx.write(buf2);
ctx.write(buf3);
ctx.flush();
}
为什么需要把buf保存到ChannelOutboundBuffer？
ctx.write()实现：
//AbstractChannelHandlerContext.java
public ChannelFuture write(Object msg) {
return write(msg, newPromise());
}
private void write(Object msg, boolean flush, ChannelPromise promise) {
AbstractChannelHandlerContext next = findContextOutbound();
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
next.invokeWrite(msg, promise);
if (flush) {
next.invokeFlush();
}
} else {
AbstractWriteTask task;
if (flush) {
task = WriteAndFlushTask.newInstance(next, msg, promise);
} else {
task = WriteTask.newInstance(next, msg, promise);
}
safeExecute(executor, task, promise, msg);
}
}
默认情况下，findContextOutbound()会找到pipeline的head节点，触发write方法。
//HeadContext.java
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
unsafe.write(msg, promise);
}
//AbstractUnsafe
public final void write(Object msg, ChannelPromise promise) {
ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
if (outboundBuffer == null) {
safeSetFailure(promise, CLOSED_CHANNEL_EXCEPTION);
ReferenceCountUtil.release(msg);
return;
}
int size;
try {
msg = filterOutboundMessage(msg);
size = estimatorHandle().size(msg);
if (size < 0) {
size = 0;
}
} catch (Throwable t) {
safeSetFailure(promise, t);
ReferenceCountUtil.release(msg);
return;
}
outboundBuffer.addMessage(msg, size, promise);
}
outboundBuffer 随着Unsafe一起实例化，最终将msg通过outboundBuffer封装起来。
ChannelOutboundBuffer内部维护了一个Entry链表，并使用Entry封装msg。
1、unflushedEntry：指向链表头部
2、tailEntry：指向链表尾部
3、totalPendingSize：保存msg的字节数
4、unwritable：不可写标识
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;
}
// increment pending bytes after adding message to the unflushed arrays.
// See https://github.com/netty/netty/issues/1619
incrementPendingOutboundBytes(size, false);
}
通过Entry.newInstance返回Entry实例，Netty对Entry采用了缓存策略，使用完的Entry实例需要清空并回收，难道是因为Entry实例化比较耗时？
新的entry默认插入链表尾部，并让tailEntry指向它。
Paste_Image.png
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);
}
}
方法incrementPendingOutboundBytes主要采用CAS更新totalPendingSize字段，并判断当前totalPendingSize是否超过阈值writeBufferHighWaterMark，默认是65536。如果totalPendingSize >= 65536，则采用CAS更新unwritable为1，并触发ChannelWritabilityChanged事件。
到此为止，全部的buf数据已经保存在outboundBuffer中。
ctx.flush()实现：
public ChannelHandlerContext flush() {
final AbstractChannelHandlerContext next = findContextOutbound();
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
next.invokeFlush();
} else {
Runnable task = next.invokeFlushTask;
if (task == null) {
next.invokeFlushTask = task = new Runnable() {
@Override
public void run() {
next.invokeFlush();
}
};
}
safeExecute(executor, task, channel().voidPromise(), null);
}
return this;
}
默认情况下，findContextOutbound()会找到pipeline的head节点，触发flush方法。
//HeadContext.java
public void flush(ChannelHandlerContext ctx) throws Exception {
unsafe.flush();
}
//AbstractUnsafe
public final void flush() {
assertEventLoop();
ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
if (outboundBuffer == null) {
return;
}
outboundBuffer.addFlush();
flush0();
}
方法addFlush主要对write过程添加的msg进行flush标识，其实我不清楚，这个标识过程有什么意义。
直接看flush0方法：
protected final void flush0() {
// Flush immediately only when there's no pending flush.
// If there's a pending flush operation, event loop will call forceFlush() later,
// and thus there's no need to call it now.
if (isFlushPending()) {
return;
}
super.flush0();
}
private boolean isFlushPending() {
SelectionKey selectionKey = selectionKey();
return selectionKey.isValid() && (selectionKey.interestOps() & SelectionKey.OP_WRITE) != 0;
}
1、如果当前selectionKey 是写事件，说明有线程执行flush过程，则直接返回。
2、否则直接执行flush操作。
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(NOT_YET_CONNECTED_EXCEPTION, true);
} else {
// Do not trigger channelWritabilityChanged because the channel is closed already.
outboundBuffer.failFlushed(CLOSED_CHANNEL_EXCEPTION, false);
}
} finally {
inFlush0 = false;
}
return;
}
try {
doWrite(outboundBuffer);
} 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, false);
} else {
outboundBuffer.failFlushed(t, true);
}
} finally {
inFlush0 = false;
}
}
public boolean isActive() {
SocketChannel ch = javaChannel();
return ch.isOpen() && ch.isConnected();
}
1、如果当前socketChannel已经关闭，或断开连接，则执行失败操作。
2、否则执行doWrite把数据写入到socketChannel。
protected void doWrite(ChannelOutboundBuffer in) throws Exception {
for (;;) {
int size = in.size();
if (size == 0) {
// All written so clear OP_WRITE
clearOpWrite();
break;
}
long writtenBytes = 0;
boolean done = false;
boolean setOpWrite = false;
// Ensure the pending writes are made of ByteBufs only.
ByteBuffer[] nioBuffers = in.nioBuffers();
int nioBufferCnt = in.nioBufferCount();
long expectedWrittenBytes = in.nioBufferSize();
SocketChannel ch = javaChannel();
// Always us nioBuffers() to workaround data-corruption.
// See https://github.com/netty/netty/issues/2761
switch (nioBufferCnt) {
case 0:
// We have something else beside ByteBuffers to write so fallback to normal writes.
super.doWrite(in);
return;
case 1:
// Only one ByteBuf so use non-gathering write
ByteBuffer nioBuffer = nioBuffers[0];
for (int i = config().getWriteSpinCount() - 1; i >= 0; i --) {
final int localWrittenBytes = ch.write(nioBuffer);
if (localWrittenBytes == 0) {
setOpWrite = true;
break;
}
expectedWrittenBytes -= localWrittenBytes;
writtenBytes += localWrittenBytes;
if (expectedWrittenBytes == 0) {
done = true;
break;
}
}
break;
default:
for (int i = config().getWriteSpinCount() - 1; i >= 0; i --) {
final long localWrittenBytes = ch.write(nioBuffers, 0, nioBufferCnt);
if (localWrittenBytes == 0) {
setOpWrite = true;
break;
}
expectedWrittenBytes -= localWrittenBytes;
writtenBytes += localWrittenBytes;
if (expectedWrittenBytes == 0) {
done = true;
break;
}
}
break;
}
// Release the fully written buffers, and update the indexes of the partially written buffer.
in.removeBytes(writtenBytes);
if (!done) {
// Did not write all buffers completely.
incompleteWrite(setOpWrite);
break;
}
}
}
1、size方法返回outboundBuffer有多少Entry实例。
2、in.nioBuffers()负责把Entry中保存的ByteBuf类型的msg，重新返回Nio的ByteBuffer实例，并返回ByteBuffer数组nioBuffers，其实msg和ByteBuffer实例指向的是同一块内存，因为在UnpooledDirectByteBuf实现类中，已经维护了ByteBuffer的实例。
3、socketChannel.write()方法把nioBuffers的数据写到socket中，这是Nio中的实现。
到此为止，nioBuffers的数据都flush到socket，客户端可以准备接收了。
END。
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