Netty深入浅出系列：Netty read
boss线程主要负责监听并处理accept事件，将socketChannel注册到work线程的selector，由worker线程来监听并处理read事件，本节主要分析Netty如何处理read事件。
accept->read
当work线程的selector检测到OP_READ事件发生时，触发read操作。
//NioEventLoop
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
unsafe.read();
if (!ch.isOpen()) {
// Connection already closed - no need to handle write.
return;
}
}该read方法定义在类NioByteUnsafe中。
//AbstractNioByteChannel.NioByteUnsafe
public final void read() {
final ChannelConfig config = config();
if (!config.isAutoRead() && !isReadPending()) {
// ChannelConfig.setAutoRead(false) was called in the meantime
removeReadOp();
return;
}
final ChannelPipeline pipeline = pipeline();
final ByteBufAllocator allocator = config.getAllocator();
final int maxMessagesPerRead = config.getMaxMessagesPerRead();
RecvByteBufAllocator.Handle allocHandle = this.allocHandle;
if (allocHandle == null) {
this.allocHandle = allocHandle = config.getRecvByteBufAllocator().newHandle();
}
ByteBuf byteBuf = null;
int messages = 0;
boolean close = false;
try {
int totalReadAmount = 0;
boolean readPendingReset = false;
do {
byteBuf = allocHandle.allocate(allocator);
int writable = byteBuf.writableBytes();
int localReadAmount = doReadBytes(byteBuf);
if (localReadAmount <= 0) {
// not was read release the buffer
byteBuf.release();
byteBuf = null;
close = localReadAmount < 0;
break;
}
if (!readPendingReset) {
readPendingReset = true;
setReadPending(false);
}
pipeline.fireChannelRead(byteBuf);
byteBuf = null;
if (totalReadAmount >= Integer.MAX_VALUE - localReadAmount) {
// Avoid overflow.
totalReadAmount = Integer.MAX_VALUE;
break;
}
totalReadAmount += localReadAmount;
// stop reading
if (!config.isAutoRead()) {
break;
}
if (localReadAmount < writable) {
// Read less than what the buffer can hold,
// which might mean we drained the recv buffer completely.
break;
}
} while (++ messages < maxMessagesPerRead);
pipeline.fireChannelReadComplete();
allocHandle.record(totalReadAmount);
if (close) {
closeOnRead(pipeline);
close = false;
}
} catch (Throwable t) {
handleReadException(pipeline, byteBuf, t, close);
} finally {
// Check if there is a readPending which was not processed yet.
// This could be for two reasons:
// * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
// * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
//
// See https://github.com/netty/netty/issues/2254
if (!config.isAutoRead() && !isReadPending()) {
removeReadOp();
}
}
}1、allocHandle负责自适应调整当前缓存分配的大小，以防止缓存分配过多或过少，先看看AdaptiveRecvByteBufAllocator内部实现：
public class AdaptiveRecvByteBufAllocator implements RecvByteBufAllocator {
static final int DEFAULT_MINIMUM = 64;
static final int DEFAULT_INITIAL = 1024;
static final int DEFAULT_MAXIMUM = 65536;
private static final int INDEX_INCREMENT = 4;
private static final int INDEX_DECREMENT = 1;
private static final int[] SIZE_TABLE;
}SIZE_TABLE：按照从小到大的顺序预先存储可以分配的缓存大小。
从16开始，每次累加16，直到496，接着从512开始，每次增大一倍，直到溢出。
DEFAULT_MINIMUM：最小缓存（64），在SIZE_TABLE中对应的下标为3。
**DEFAULT_MAXIMUM **：最大缓存（65536），在SIZE_TABLE中对应的下标为38。
**DEFAULT_INITIAL **：初始化缓存大小，第一次分配缓存时，由于没有上一次实际收到的字节数做参考，需要给一个默认初始值。
INDEX_INCREMENT：上次预估缓存偏小，下次index的递增值。
**INDEX_DECREMENT **：上次预估缓存偏大，下次index的递减值。
2、allocHandle.allocate(allocator) 申请一块指定大小的内存。
//AdaptiveRecvByteBufAllocator.HandleImpl
public ByteBuf allocate(ByteBufAllocator alloc) {
return alloc.ioBuffer(nextReceiveBufferSize);
}通过ByteBufAllocator的ioBuffer方法申请缓存。
//AbstractByteBufAllocator
public ByteBuf ioBuffer(int initialCapacity) {
if (PlatformDependent.hasUnsafe()) {
return directBuffer(initialCapacity);
}
return heapBuffer(initialCapacity);
}根据平台是否支持unsafe，选择使用直接物理内存还是堆上内存。
direct buffer方案：
//AbstractByteBufAllocator
public ByteBuf directBuffer(int initialCapacity) {
return directBuffer(initialCapacity, Integer.MAX_VALUE);
}
public ByteBuf directBuffer(int initialCapacity, int maxCapacity) {
if (initialCapacity == 0 && maxCapacity == 0) {
return emptyBuf;
}
validate(initialCapacity, maxCapacity);
return newDirectBuffer(initialCapacity, maxCapacity);
}
//UnpooledByteBufAllocator
protected ByteBuf newDirectBuffer(int initialCapacity, int maxCapacity) {
ByteBuf buf;
if (PlatformDependent.hasUnsafe()) {
buf = new UnpooledUnsafeDirectByteBuf(this, initialCapacity, maxCapacity);
} else {
buf = new UnpooledDirectByteBuf(this, initialCapacity, maxCapacity);
}
return toLeakAwareBuffer(buf);
}UnpooledUnsafeDirectByteBuf是如何实现缓存管理的？对Nio的ByteBuffer进行了封装，通过ByteBuffer的allocateDirect方法实现缓存的申请。
protected UnpooledUnsafeDirectByteBuf(ByteBufAllocator alloc, ByteBuffer initialBuffer, int maxCapacity) {
//判断逻辑已经忽略
this.alloc = alloc;
setByteBuffer(allocateDirect(initialCapacity));
}
protected ByteBuffer allocateDirect(int initialCapacity) {
return ByteBuffer.allocateDirect(initialCapacity);
}
private void setByteBuffer(ByteBuffer buffer) {
ByteBuffer oldBuffer = this.buffer;
if (oldBuffer != null) {
if (doNotFree) {
doNotFree = false;
} else {
freeDirect(oldBuffer);
}
}
this.buffer = buffer;
memoryAddress = PlatformDependent.directBufferAddress(buffer);
tmpNioBuf = null;
capacity = buffer.remaining();
}memoryAddress = PlatformDependent.directBufferAddress(buffer) 获取buffer的address字段值，指向缓存地址。
capacity = buffer.remaining() 获取缓存容量。
方法toLeakAwareBuffer(buf)对申请的buf又进行了一次包装：
protected static ByteBuf toLeakAwareBuffer(ByteBuf buf) {
ResourceLeak leak;
switch (ResourceLeakDetector.getLevel()) {
case SIMPLE:
leak = AbstractByteBuf.leakDetector.open(buf);
if (leak != null) {
buf = new SimpleLeakAwareByteBuf(buf, leak);
}
break;
case ADVANCED:
case PARANOID:
leak = AbstractByteBuf.leakDetector.open(buf);
if (leak != null) {
buf = new AdvancedLeakAwareByteBuf(buf, leak);
}
break;
}
return buf;
}Netty中使用引用计数机制来管理资源，ByteBuf实现了ReferenceCounted接口，当实例化一个ByteBuf时，引用计数为1， 代码中需要保持一个该对象的引用时需要调用retain方法将计数增1，对象使用完时调用release将计数减1。当引用计数变为0时，对象将释放所持有的底层资源或将资源返回资源池。
3、方法doReadBytes(byteBuf) 将socketChannel数据写入缓存。
//NioSocketChannel
@Override
protected int doReadBytes(ByteBuf byteBuf) throws Exception {
return byteBuf.writeBytes(javaChannel(), byteBuf.writableBytes());
}
//WrappedByteBuf
@Override
public int writeBytes(ScatteringByteChannel in, int length) throws IOException {
return buf.writeBytes(in, length);
}
//AbsractByteBuf
@Override
public int writeBytes(ScatteringByteChannel in, int length) throws IOException {
ensureAccessible();
ensureWritable(length);
int writtenBytes = setBytes(writerIndex, in, length);
if (writtenBytes > 0) {
writerIndex += writtenBytes;
}
return writtenBytes;
}
//UnpooledUnsafeDirectByteBuf
@Override
public int setBytes(int index, ScatteringByteChannel in, int length) throws IOException {
ensureAccessible();
ByteBuffer tmpBuf = internalNioBuffer();
tmpBuf.clear().position(index).limit(index + length);
try {
return in.read(tmpBuf);
} catch (ClosedChannelException ignored) {
return -1;
}
}
private ByteBuffer internalNioBuffer() {
ByteBuffer tmpNioBuf = this.tmpNioBuf;
if (tmpNioBuf == null) {
this.tmpNioBuf = tmpNioBuf = buffer.duplicate();
}
return tmpNioBuf;
}最终底层采用ByteBuffer实现read操作，这里有一块逻辑不清楚，为什么要用tmpNioBuf？
int localReadAmount = doReadBytes(byteBuf);
1、如果返回0，则表示没有读取到数据，则退出循环。
2、如果返回-1，表示对端已经关闭连接，则退出循环。
3、否则，表示读取到了数据，数据读入缓存后，触发pipeline的ChannelRead事件，byteBuf作为参数进行后续处理，这时自定义Inbound类型的handler就可以进行业务处理了。
static class DiscardServerHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
ByteBuf in = (ByteBuf) msg;
try {
while (in.isReadable()) { // (1)
System.out.print((char) in.readByte());
System.out.flush();
}
} finally {
ReferenceCountUtil.release(msg); // (2)
}
}
}其中参数msg，就是对应的byteBuf，当请求的数据量比较大时，会多次触发channelRead事件，默认最多触发16次，可以通过maxMessagesPerRead字段进行配置。
如果客户端传输的数据过大，可能会分成好几次传输，因为TCP一次传输内容大小有上限，所以同一个selectKey会触发多次read事件，剩余的数据会在下一轮select操作继续读取。
在实际应用中，应该把所有请求数据都缓存起来再进行业务处理。
所有数据都处理完，触发pipeline的ChannelReadComplete事件，并且allocHandle记录这次read的字节数，进行下次处理时缓存大小的调整。
到此为止，整个NioSocketChannel的read事件已经处理完成。
