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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。

img

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。

666. 彩蛋

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  1. 1. 666. 彩蛋