1.概念
/**
* (Transport implementors only) an internal data structure used by {@link AbstractChannel} to store its pending
* outbound write requests.
*/
public final class ChannelOutboundBuffer {
}
使用一个内部数据结构来实现出站写请求
2.内部数据结构
// 要被写入的数据
private Entry flushedEntry;
// 未被flush的头部数据节点,处于pending状态
private Entry unflushedEntry;
// 尾部节点数据
private Entry tailEntry;
3.unflushedEntry数据的写入
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
ctx.write(Unpooled.copiedBuffer("hello", CharsetUtil.UTF_8));
ctx.write(Unpooled.copiedBuffer("netty", CharsetUtil.UTF_8));
}
当调用ChannelHandlerContext的write方法,最终会调用ChannelOutboundBuffer的addMessage方法,将以链表的形式将数据添加到tailEntry尾部节点后面
addMessage方法实现
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(entry.pendingSize, false);
}
数据结构示意：
4.unflushedEntry到flushedEntry的转换
当ChannelHandlerContext调用flush方法时,实际会调用ChannelOutboundBuffer的addFlush方法，该方式实现了unflushedEntry到flushedEntry的转换过程
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;
}
}
5.缓冲区大小限制
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
for (int i=0;i<100000;i++)
{
ctx.write(Unpooled.copiedBuffer("hello", CharsetUtil.UTF_8));
ctx.write(Unpooled.copiedBuffer("netty", CharsetUtil.UTF_8));
}
}
当向缓冲区不断写入数据时,会有一个阈值,超过了则会出现告警事件,即告诉调用者请不要再写入了,等flush后,会将阈值递减恢复,并发出事件
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);
}
}
private void decrementPendingOutboundBytes(long size, boolean invokeLater, boolean notifyWritability) {
if (size == 0) {
return;
}
long newWriteBufferSize = TOTAL_PENDING_SIZE_UPDATER.addAndGet(this, -size);
if (notifyWritability && newWriteBufferSize < channel.config().getWriteBufferLowWaterMark()) {
setWritable(invokeLater);
}
}
private void setWritable(boolean invokeLater) {
for (;;) {
final int oldValue = unwritable;
final int newValue = oldValue & ~1;
if (UNWRITABLE_UPDATER.compareAndSet(this, oldValue, newValue)) {
if (oldValue != 0 && newValue == 0) {
fireChannelWritabilityChanged(invokeLater);
}
break;
}
}
}
private void setUnwritable(boolean invokeLater) {
for (;;) {
final int oldValue = unwritable;
final int newValue = oldValue | 1;
if (UNWRITABLE_UPDATER.compareAndSet(this, oldValue, newValue)) {
if (oldValue == 0 && newValue != 0) {
fireChannelWritabilityChanged(invokeLater);
}
break;
}
}
}
private void setWritable(boolean invokeLater) {
for (;;) {
final int oldValue = unwritable;
final int newValue = oldValue & ~1;
if (UNWRITABLE_UPDATER.compareAndSet(this, oldValue, newValue)) {
if (oldValue != 0 && newValue == 0) {
fireChannelWritabilityChanged(invokeLater);
}
break;
}
}
}
private void setUnwritable(boolean invokeLater) {
for (;;) {
final int oldValue = unwritable;
final int newValue = oldValue | 1;
if (UNWRITABLE_UPDATER.compareAndSet(this, oldValue, newValue)) {
if (oldValue == 0 && newValue != 0) {
fireChannelWritabilityChanged(invokeLater);
}
break;
}
}
}
5.write的流程
nioBufferCnt是flush区域的数据块数量,根据不同的数量来调用SocketChannel不同的write方法,写入完后,再调用ChannelOutboundBuffer的removeBytes方法移除已写入的ByteBuf数据块
@Override
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;
}
}
}
参考:
Netty 出站缓冲区 ChannelOutboundBuffer 源码解析（isWritable 属性的重要性）
netty学习系列五：write&flush
