后面文章说了，ChannelHandlerContext#write只是将数据缓存到ChannelOutboundBuffer，等到ChannelHandlerContext#flush时，再将ChannelOutboundBuffer缓存的数据写到Channel中。
本文分享Netty中ChannelOutboundBuffer的实现以及Flush过程。
源码剖析基于Netty 4.1
每个Channel的AbstractUnsafe#outboundBuffer 都保护了一个ChannelOutboundBuffer。
ChannelOutboundBuffer，出站数据缓冲区，负责缓存ChannelHandlerContext#write的数据。通过链表治理数据，链表节点为外部类Entry。
关键字段如下
Entry tailEntry; // 链表最初一个节点，新增的节点增加其后。
Entry unflushedEntry; // 链表中第一个未刷新的节点
Entry flushedEntry; // 链表中第一个已刷新但数据未写入的节点
int flushed; // 已刷新但数据未写入的节点数ChannelHandlerContext#flush操作前，须要先刷新一遍待处理的节点（次要是统计本次ChannelHandlerContext#flush操作能够写入多少个节点数据），从unflushedEntry开始。刷新实现后应用flushedEntry标记第一个待写入的节点，flushed为待写入节点数。
后面分享Netty读写过程的文章说过，AbstractUnsafe#write解决写操作时，会调用ChannelOutboundBuffer#addMessage将数据缓存起来
public void addMessage(Object msg, int size, ChannelPromise promise) {
// #1
Entry entry = Entry.newInstance(msg, size, total(msg), promise);
if (tailEntry == null) {
flushedEntry = null;
} else {
Entry tail = tailEntry;
tail.next = entry;
}
tailEntry = entry;
if (unflushedEntry == null) {
unflushedEntry = entry;
}
incrementPendingOutboundBytes(entry.pendingSize, false);
}#1 构建一个Entry，留神，这里应用了对象池RECYCLER，前面有文章具体解析。
次要是更新tailEntry和unflushedEntry#2 如果以后缓存数量超过阀值WriteBufferWaterMark#high，更新unwritable标记为true，并触发pipeline.fireChannelWritabilityChanged()办法。
因为ChannelOutboundBuffer链表没有大小限度，一直累积数据可能导致 OOM，
为了防止这个问题，咱们能够在unwritable标记为true时，不再持续缓存数据。
Netty只会更新unwritable标记，并不阻止数据缓存，咱们能够依据须要实现该性能。示例如下
if (ctx.channel().isActive() && ctx.channel().isWritable()) {
ctx.writeAndFlush(responseMessage);
} else {
...
}addFlush办法负责刷新节点（ChannelHandlerContext#flush操作前调用该办法统计可写入节点数据数）
public void addFlush() {
// #1
Entry entry = unflushedEntry;
if (entry != null) {
// #2
if (flushedEntry == null) {
// there is no flushedEntry yet, so start with the entry
flushedEntry = entry;
}
do {
// #3
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;
// #4
} while (entry != null);
// All flushed so reset unflushedEntry
// #5
unflushedEntry = null;
}
}#1 从unflushedEntry节点开始解决#2 赋值flushedEntry为unflushedEntry。
ChannelHandlerContext#flush写入实现后会置空flushedEntry#3 减少flushed
设置节点的ChannelPromise不可勾销#4 从unflushedEntry开始，遍历前面节点#5 置空unflushedEntry，示意以后所有节点都已刷新。
nioBuffers办法负责将以后缓存的ByteBuf转发为(jvm)ByteBuffer
public ByteBuffer[] nioBuffers(int maxCount, long maxBytes) {
assert maxCount > 0;
assert maxBytes > 0;
long nioBufferSize = 0;
int nioBufferCount = 0;
// #1
final InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
ByteBuffer[] nioBuffers = NIO_BUFFERS.get(threadLocalMap);
Entry entry = flushedEntry;
while (isFlushedEntry(entry) && entry.msg instanceof ByteBuf) {
if (!entry.cancelled) {
ByteBuf buf = (ByteBuf) entry.msg;
final int readerIndex = buf.readerIndex();
final int readableBytes = buf.writerIndex() - readerIndex;
if (readableBytes > 0) {
// #2
if (maxBytes - readableBytes < nioBufferSize && nioBufferCount != 0) {
break;
}
nioBufferSize += readableBytes;
// #3
int count = entry.count;
if (count == -1) {
//noinspection ConstantValueVariableUse
entry.count = count = buf.nioBufferCount();
}
int neededSpace = min(maxCount, nioBufferCount + count);
if (neededSpace > nioBuffers.length) {
nioBuffers = expandNioBufferArray(nioBuffers, neededSpace, nioBufferCount);
NIO_BUFFERS.set(threadLocalMap, nioBuffers);
}
// #4
if (count == 1) {
ByteBuffer nioBuf = entry.buf;
if (nioBuf == null) {
// cache ByteBuffer as it may need to create a new ByteBuffer instance if its a
// derived buffer
entry.buf = nioBuf = buf.internalNioBuffer(readerIndex, readableBytes);
}
nioBuffers[nioBufferCount++] = nioBuf;
} else {
...
}
if (nioBufferCount == maxCount) {
break;
}
}
}
entry = entry.next;
}
this.nioBufferCount = nioBufferCount;
this.nioBufferSize = nioBufferSize;
return nioBuffers;
}#1 从线程缓存中获取nioBuffers变量，这样能够防止重复结构ByteBuffer数组的性能损耗#2 maxBytes，即本次操作最大的字节数。maxBytes - readableBytes < nioBufferSize，示意如果本次操作后将超出maxBytes，退出#3
buf.nioBufferCount()，获取ByteBuffer数量，CompositeByteBuf可能有多个ByteBuffer组成。
neededSpace，即nioBuffers数组中ByteBuffer数量，nioBuffers长度不够时须要扩容。#4buf.internalNioBuffer(readerIndex, readableBytes)，应用readerIndex, readableBytes结构一个ByteBuffer。
这里波及ByteBuf相干常识，前面有文章具体解析。
ChannelHandlerContext#flush实现后，须要移除对应的缓存节点。
public void removeBytes(long writtenBytes) {
for (;;) {
// #1
Object msg = current();
if (!(msg instanceof ByteBuf)) {
assert writtenBytes == 0;
break;
}
final ByteBuf buf = (ByteBuf) msg;
final int readerIndex = buf.readerIndex();
final int readableBytes = buf.writerIndex() - readerIndex;
// #2
if (readableBytes <= writtenBytes) {
if (writtenBytes != 0) {
progress(readableBytes);
writtenBytes -= readableBytes;
}
remove();
} else { // readableBytes > writtenBytes
// #3
if (writtenBytes != 0) {
buf.readerIndex(readerIndex + (int) writtenBytes);
progress(writtenBytes);
}
break;
}
}
clearNioBuffers();
}#1
current办法返回flushedEntry节点缓存数据。
后果null时，退出循环#2 以后节点的数据曾经全副写入，
progress办法唤醒数据节点上ChannelProgressivePromise的监听者
writtenBytes减去对应字节数
remove()办法移除节点，开释ByteBuf，flushedEntry标记后移。#3 以后节点的数据局部写入，它应该是本次ChannelHandlerContext#flush操作的最初一个节点
更新ByteBuf的readerIndex，下次从这里开始读取数据。
退出
移除数据节点
public boolean remove() {
Entry e = flushedEntry;
if (e == null) {
clearNioBuffers();
return false;
}
Object msg = e.msg;
ChannelPromise promise = e.promise;
int size = e.pendingSize;
// #1
removeEntry(e);
if (!e.cancelled) {
// only release message, notify and decrement if it was not canceled before.
// #2
ReferenceCountUtil.safeRelease(msg);
safeSuccess(promise);
decrementPendingOutboundBytes(size, false, true);
}
// recycle the entry
// #3
e.recycle();
return true;
}#1
flushed减1
当flushed为0时，flushedEntry赋值为null，否则flushedEntry指向后一个节点。#2 开释ByteBuf#3 以后节点返回对象池中，以便复用。
上面来看一下ChannelHandlerContext#flush操作过程。
ChannelHandlerContext#flush -> HeadContext#flush -> AbstractUnsafe#flush
public final void flush() {
assertEventLoop();
ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
if (outboundBuffer == null) {
return;
}
// #1
outboundBuffer.addFlush();
// #2
flush0();
}#1 刷新outboundBuffer中数据节点#2 写入操作
flush -> NioSocketChannel#doWrite
protected void doWrite(ChannelOutboundBuffer in) throws Exception {
SocketChannel ch = javaChannel();
int writeSpinCount = config().getWriteSpinCount();
do {
// #1
if (in.isEmpty()) {
clearOpWrite();
return;
}
// #2
int maxBytesPerGatheringWrite = ((NioSocketChannelConfig) config).getMaxBytesPerGatheringWrite();
ByteBuffer[] nioBuffers = in.nioBuffers(1024, maxBytesPerGatheringWrite);
int nioBufferCnt = in.nioBufferCount();
switch (nioBufferCnt) {
case 0:
// #3
writeSpinCount -= doWrite0(in);
break;
case 1: {
// #4
ByteBuffer buffer = nioBuffers[0];
int attemptedBytes = buffer.remaining();
final int localWrittenBytes = ch.write(buffer);
if (localWrittenBytes <= 0) {
// #5
incompleteWrite(true);
return;
}
adjustMaxBytesPerGatheringWrite(attemptedBytes, localWrittenBytes, maxBytesPerGatheringWrite);
// #6
in.removeBytes(localWrittenBytes);
--writeSpinCount;
break;
}
default: {
// #7
...
}
}
} while (writeSpinCount > 0);
incompleteWrite(writeSpinCount < 0);
}#1 通过ChannelOutboundBuffer#flushed判断是否没有数据能够写，没有数据则革除关注事件OP_WRITE，间接返回。#2 获取ChannelOutboundBuffer中ByteBuf保护的(jvm)ByteBuffer，并统计nioBufferSize，nioBufferCount。#3 这时没有ByteBuffer，然而可能有其余类型的数据(如FileRegion类型)，调用doWrite0持续解决，这里不再深刻#4 只有一个ByteBuffer，调用SocketChannel#write将数据写入Channel。#5 如果写入数据数量小于等于0，阐明数据没有被写出去(可能是因为套接字的缓冲区满了等起因)，那么就须要关注该Channel上的OP_WRITE事件，不便下次EventLoop将Channel轮询进去的时候，能持续写数据。#6 移除ChannelOutboundBuffer缓存数据节点。#7 有多个ByteBuffer，调用SocketChannel#write(ByteBuffer[] srcs, int offset, int length)，批量写入，与上一种状况解决相似
回顾之前文章《事件循环机制实现原理》中对NioEventLoop#processSelectedKey办法的解析
...
if ((readyOps & SelectionKey.OP_WRITE) != 0) {
ch.unsafe().forceFlush();
}这里会调用forceFlush办法，再次写入数据。
FlushConsolidationHandler
ChannelHandlerContext#flush是很低廉的操作，可能触发零碎调用，但数据又不能缓存太久，应用FlushConsolidationHandler能够尽量达到写入提早与吞吐量之间的衡量。
FlushConsolidationHandler中保护了explicitFlushAfterFlushes变量，
在ChannelOutboundHandler#channelRead中调用flush，如果调用次数小于explicitFlushAfterFlushes， 会拦挡flush操作不执行。
在channelReadComplete后调用flush，则不会拦挡flush操作。
本文波及ByteBuf组件，它是Netty中的内存缓冲区，前面有文章解析。
如果您感觉本文不错，欢送关注我的微信公众号，您的关注是我保持的能源！
发表回复