Netty源码分析 (七)----- read过程 源码分析

2019-09-17 10:30:58来源:博客园 阅读 ()

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Netty源码分析 (七)----- read过程 源码分析

在上一篇文章中,我们分析了processSelectedKey这个方法中的accept过程,本文将分析一下work线程中的read过程。

private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
    final NioUnsafe unsafe = ch.unsafe();
    //检查该SelectionKey是否有效,如果无效,则关闭channel
    if (!k.isValid()) {
        // close the channel if the key is not valid anymore
        unsafe.close(unsafe.voidPromise());
        return;
    }

    try {
        int readyOps = k.readyOps();
        // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
        // to a spin loop
        // 如果准备好READ或ACCEPT则触发unsafe.read() ,检查是否为0,如上面的源码英文注释所说:解决JDK可能会产生死循环的一个bug。
        if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
            unsafe.read();
            if (!ch.isOpen()) {//如果已经关闭,则直接返回即可,不需要再处理该channel的其他事件
                // Connection already closed - no need to handle write.
                return;
            }
        }
        // 如果准备好了WRITE则将缓冲区中的数据发送出去,如果缓冲区中数据都发送完成,则清除之前关注的OP_WRITE标记
        if ((readyOps & SelectionKey.OP_WRITE) != 0) {
            // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
            ch.unsafe().forceFlush();
        }
        // 如果是OP_CONNECT,则需要移除OP_CONNECT否则Selector.select(timeout)将立即返回不会有任何阻塞,这样可能会出现cpu 100%
        if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
            // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
            // See https://github.com/netty/netty/issues/924
            int ops = k.interestOps();
            ops &= ~SelectionKey.OP_CONNECT;
            k.interestOps(ops);

            unsafe.finishConnect();
        }
    } catch (CancelledKeyException ignored) {
        unsafe.close(unsafe.voidPromise());
    }
}

该方法主要是对SelectionKey k进行了检查,有如下几种不同的情况

1)OP_ACCEPT,接受客户端连接

2)OP_READ, 可读事件, 即 Channel 中收到了新数据可供上层读取。

3)OP_WRITE, 可写事件, 即上层可以向 Channel 写入数据。

4)OP_CONNECT, 连接建立事件, 即 TCP 连接已经建立, Channel 处于 active 状态。

本篇博文主要来看下当work 线程 selector检测到OP_READ事件时,内部干了些什么。

if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
    unsafe.read();
    if (!ch.isOpen()) {//如果已经关闭,则直接返回即可,不需要再处理该channel的其他事件
        // Connection already closed - no need to handle write.
        return;
    }
} 

从代码中可以看到,当selectionKey发生的事件是SelectionKey.OP_READ,执行unsafe的read方法。注意这里的unsafe是NioByteUnsafe的实例

为什么说这里的unsafe是NioByteUnsafe的实例呢?在上篇博文Netty源码分析:accept中我们知道Boss NioEventLoopGroup中的NioEventLoop只负责accpt客户端连接,然后将该客户端注册到Work NioEventLoopGroup中的NioEventLoop中,即最终是由work线程对应的selector来进行read等时间的监听,即work线程中的channel为SocketChannel,SocketChannel的unsafe就是NioByteUnsafe的实例

下面来看下NioByteUnsafe中的read方法

@Override
    public 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 {
                //1、分配缓存
                byteBuf = allocHandle.allocate(allocator);
                int writable = byteBuf.writableBytes();//可写的字节容量
                //2、将socketChannel数据写入缓存
                int localReadAmount = doReadBytes(byteBuf);
                if (localReadAmount <= 0) {
                    // not was read release the buffer
                    byteBuf.release();
                    close = localReadAmount < 0;
                    break;
                }
                if (!readPendingReset) {
                    readPendingReset = true;
                    setReadPending(false);
                }
                //3、触发pipeline的ChannelRead事件来对byteBuf进行后续处理
                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 {
            if (!config.isAutoRead() && !isReadPending()) {
                removeReadOp();
            }
        }
    }
} 

下面一一介绍比较重要的代码

allocHandler的实例化过程

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allocHandler的实例化过程

RecvByteBufAllocator.Handle allocHandle = this.allocHandle;
if (allocHandle == null) {
    this.allocHandle = allocHandle = config.getRecvByteBufAllocator().newHandle();
}

其中, config.getRecvByteBufAllocator()得到的是一个 AdaptiveRecvByteBufAllocator实例DEFAULT。

public static final AdaptiveRecvByteBufAllocator DEFAULT = new AdaptiveRecvByteBufAllocator();

而AdaptiveRecvByteBufAllocator中的newHandler()方法的代码如下:

@Override
public Handle newHandle() {
    return new HandleImpl(minIndex, maxIndex, initial);
}

HandleImpl(int minIndex, int maxIndex, int initial) {
    this.minIndex = minIndex;
    this.maxIndex = maxIndex;

    index = getSizeTableIndex(initial);
    nextReceiveBufferSize = SIZE_TABLE[index];
}

其中,上面方法中所用到参数:minIndex maxIndex initial是什么意思呢?含义如下:minIndex是最小缓存在SIZE_TABLE中对应的下标。maxIndex是最大缓存在SIZE_TABLE中对应的下标,initial为初始化缓存大小。

AdaptiveRecvByteBufAllocator的相关常量字段

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; 

上面这些字段的具体含义说明如下:

1)、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。SIZE_TABLE初始化过程如下。

static {
    List<Integer> sizeTable = new ArrayList<Integer>();
    for (int i = 16; i < 512; i += 16) {
        sizeTable.add(i);
    }

    for (int i = 512; i > 0; i <<= 1) {
        sizeTable.add(i);
    }

    SIZE_TABLE = new int[sizeTable.size()];
    for (int i = 0; i < SIZE_TABLE.length; i ++) {
        SIZE_TABLE[i] = sizeTable.get(i);
    }
}

2)、DEFAULT_MINIMUM:最小缓存(64),在SIZE_TABLE中对应的下标为3。

3)、DEFAULT_MAXIMUM :最大缓存(65536),在SIZE_TABLE中对应的下标为38。

4)、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。

5)、INDEX_INCREMENT:上次预估缓存偏小,下次index的递增值。

6)、INDEX_DECREMENT :上次预估缓存偏大,下次index的递减值。

构造函数:

private AdaptiveRecvByteBufAllocator() {
    this(DEFAULT_MINIMUM, DEFAULT_INITIAL, DEFAULT_MAXIMUM);
}

public AdaptiveRecvByteBufAllocator(int minimum, int initial, int maximum) {
    if (minimum <= 0) {
        throw new IllegalArgumentException("minimum: " + minimum);
    }
    if (initial < minimum) {
        throw new IllegalArgumentException("initial: " + initial);
    }
    if (maximum < initial) {
        throw new IllegalArgumentException("maximum: " + maximum);
    }

    int minIndex = getSizeTableIndex(minimum);
    if (SIZE_TABLE[minIndex] < minimum) {
        this.minIndex = minIndex + 1;
    } else {
        this.minIndex = minIndex;
    }

    int maxIndex = getSizeTableIndex(maximum);
    if (SIZE_TABLE[maxIndex] > maximum) {
        this.maxIndex = maxIndex - 1;
    } else {
        this.maxIndex = maxIndex;
    }

    this.initial = initial;
}

该构造函数对参数进行了有效性检查,然后初始化了如下3个字段,这3个字段就是上面用于产生allocHandle对象所要用到的参数。

private final int minIndex;
private final int maxIndex;
private final int initial;

其中,getSizeTableIndex函数的代码如下,该函数的功能为:找到SIZE_TABLE中的元素刚好大于或等于size的位置。

private static int getSizeTableIndex(final int size) {
    for (int low = 0, high = SIZE_TABLE.length - 1;;) {
        if (high < low) {
            return low;
        }
        if (high == low) {
            return high;
        }

        int mid = low + high >>> 1;
        int a = SIZE_TABLE[mid];
        int b = SIZE_TABLE[mid + 1];
        if (size > b) {
            low = mid + 1;
        } else if (size < a) {
            high = mid - 1;
        } else if (size == a) {
            return mid;
        } else { //这里的情况就是 a < size <= b 的情况
            return mid + 1;
        }
    }
}

byteBuf = allocHandle.allocate(allocator);

申请一块指定大小的内存

AdaptiveRecvByteBufAllocator#HandlerImpl

@Override
public ByteBuf allocate(ByteBufAllocator alloc) {
    return alloc.ioBuffer(nextReceiveBufferSize);
}

直接调用了ioBuffer方法,继续看

AbstractByteBufAllocator.java

@Override
public ByteBuf ioBuffer(int initialCapacity) {
    if (PlatformDependent.hasUnsafe()) {
        return directBuffer(initialCapacity);
    }
    return heapBuffer(initialCapacity);
}

ioBuffer函数中主要逻辑为:看平台是否支持unsafe,选择使用直接物理内存还是堆上内存。先看 heapBuffer

AbstractByteBufAllocator.java 

@Override
public ByteBuf heapBuffer(int initialCapacity) {
    return heapBuffer(initialCapacity, Integer.MAX_VALUE);
}

@Override
public ByteBuf heapBuffer(int initialCapacity, int maxCapacity) {
    if (initialCapacity == 0 && maxCapacity == 0) {
        return emptyBuf;
    }
    validate(initialCapacity, maxCapacity);
    return newHeapBuffer(initialCapacity, maxCapacity);
} 

这里的newHeapBuffer有两种实现:至于具体用哪一种,取决于我们对系统属性io.netty.allocator.type的设置,如果设置为: “pooled”,则缓存分配器就为:PooledByteBufAllocator,进而利用对象池技术进行内存分配。如果不设置或者设置为其他,则缓存分配器为:UnPooledByteBufAllocator,则直接返回一个UnpooledHeapByteBuf对象。

UnpooledByteBufAllocator.java

@Override
protected ByteBuf newHeapBuffer(int initialCapacity, int maxCapacity) {
    return new UnpooledHeapByteBuf(this, initialCapacity, maxCapacity);
}

PooledByteBufAllocator.java

@Override
protected ByteBuf newHeapBuffer(int initialCapacity, int maxCapacity) {
    PoolThreadCache cache = threadCache.get();
    PoolArena<byte[]> heapArena = cache.heapArena;

    ByteBuf buf;
    if (heapArena != null) {
        buf = heapArena.allocate(cache, initialCapacity, maxCapacity);
    } else {
        buf = new UnpooledHeapByteBuf(this, initialCapacity, maxCapacity);
    }

    return toLeakAwareBuffer(buf);
}

再看directBuffer

AbstractByteBufAllocator.java

@Override
public ByteBuf directBuffer(int initialCapacity) {
    return directBuffer(initialCapacity, Integer.MAX_VALUE);
}  

@Override
public ByteBuf directBuffer(int initialCapacity, int maxCapacity) {
    if (initialCapacity == 0 && maxCapacity == 0) {
        return emptyBuf;
    }
    validate(initialCapacity, maxCapacity);//参数的有效性检查
    return newDirectBuffer(initialCapacity, maxCapacity);
}

与newHeapBuffer一样,这里的newDirectBuffer方法也有两种实现:至于具体用哪一种,取决于我们对系统属性io.netty.allocator.type的设置,如果设置为: “pooled”,则缓存分配器就为:PooledByteBufAllocator,进而利用对象池技术进行内存分配。如果不设置或者设置为其他,则缓存分配器为:UnPooledByteBufAllocator。这里主要看下UnpooledByteBufAllocator. newDirectBuffer的内部实现

UnpooledByteBufAllocator.java

@Override
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, int initialCapacity, int maxCapacity) {
    super(maxCapacity);
    //省略了部分参数检查的代码
    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();
}

上面代码的主要逻辑为:

1、先利用ByteBuffer的allocateDirect方法分配了大小为initialCapacity的缓存

2、然后判断将旧缓存给free掉

3、最后将新缓存赋给字段buffer上

其中: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。

接下来看toLeakAwareBuffer(buf)方法

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;
}

方法toLeakAwareBuffer(buf)对申请的buf又进行了一次包装。

上面一长串的分析,得到了缓存后,回到AbstractNioByteChannel.read方法,继续看。

doReadBytes方法

下面看下doReadBytes方法:将socketChannel数据写入缓存。

NioSocketChannel.java

@Override
protected int doReadBytes(ByteBuf byteBuf) throws Exception {
    return byteBuf.writeBytes(javaChannel(), byteBuf.writableBytes());
}

将Channel中的数据读入缓存byteBuf中。继续看

WrappedByteBuf.java

@Override
public int writeBytes(ScatteringByteChannel in, int length) throws IOException {
    return buf.writeBytes(in, length);
} 

AbstractByteBuf.java

@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;
}

这里的setBytes方法有不同的实现,这里看下UnpooledUnsafeDirectByteBuf的setBytes的实现。

UnpooledUnsafeDirectByteBuf.java

@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;//当Channel 已经关闭,则返回-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,无论是PooledByteBuf、还是UnpooledXXXBuf,里面都将底层数据结构BufBuffer/array转换为ByteBuffer 来实现read操作。即无论是UnPooledXXXBuf还是PooledXXXBuf里面都有一个ByteBuffer tmpNioBuf,这个tmpNioBuf才是真正用来存储从管道Channel中读取出的内容的。到这里就完成了将channel的数据读入到了缓存Buf中。

我们具体来看看 in.read(tmpBuf); FileChannel和SocketChannel的read最后都是依赖的IOUtil来实现,代码如下

public int read(ByteBuffer dst) throws IOException {
    ensureOpen();
    if (!readable)
        throw new NonReadableChannelException();
    synchronized (positionLock) {
        int n = 0;
        int ti = -1;
        try {
            begin();
            ti = threads.add();
            if (!isOpen())
                return 0;
            do {
                n = IOUtil.read(fd, dst, -1, nd);
            } while ((n == IOStatus.INTERRUPTED) && isOpen());
            return IOStatus.normalize(n);
        } finally {
            threads.remove(ti);
            end(n > 0);
            assert IOStatus.check(n);
        }
    }
}

最后目的就是将SocketChannel中的数据读出存放到ByteBuffer dst我们看看 IOUtil.read(fd, dst, -1, nd)

static int read(FileDescriptor var0, ByteBuffer var1, long var2, NativeDispatcher var4) throws IOException {
    if (var1.isReadOnly()) {
        throw new IllegalArgumentException("Read-only buffer");
    //如果最终承载数据的buffer是DirectBuffer,则直接将数据读入到堆外内存中
    } else if (var1 instanceof DirectBuffer) {
        return readIntoNativeBuffer(var0, var1, var2, var4);
    } else {
        // 分配临时的堆外内存
        ByteBuffer var5 = Util.getTemporaryDirectBuffer(var1.remaining());

        int var7;
        try {
            // Socket I/O 操作会将数据读入到堆外内存中
            int var6 = readIntoNativeBuffer(var0, var5, var2, var4);
            var5.flip();
            if (var6 > 0) {
                // 将堆外内存的数据拷贝到堆内存中(用户定义的缓存,在jvm中分配内存)
                var1.put(var5);
            }

            var7 = var6;
        } finally {
            // 里面会调用DirectBuffer.cleaner().clean()来释放临时的堆外内存
            Util.offerFirstTemporaryDirectBuffer(var5);
        }

        return var7;
    }
}
通过上述实现可以看出,基于channel的数据读取步骤如下: 1、如果缓存内存是DirectBuffer,就直接将Channel中的数据读取到堆外内存
2、如果缓存内存是堆内存,则先申请一块和缓存同大小的临时 DirectByteBuffer var5。
3、将内核缓存中的数据读到堆外缓存var5,底层由NativeDispatcher的read实现。
4、把堆外缓存var5的数据拷贝到堆内存var1(用户定义的缓存,在jvm中分配内存)。 5、会调用DirectBuffer.cleaner().clean()来释放创建的临时的堆外内存 如果AbstractNioByteChannel.read中第一步创建的是堆外内存,则会直接将数据读入到堆外内存,并不会先创建临时堆外内存,再将数据读入到堆外内存,最后将堆外内存拷贝到堆内存 简单的说,如果使用堆外内存,则只会复制一次数据,如果使用堆内存,则会复制两次数据 我们来看看readIntoNativeBuffer
private static int readIntoNativeBuffer(FileDescriptor filedescriptor, ByteBuffer bytebuffer, long l, NativeDispatcher nativedispatcher, Object obj)  throws IOException  {  
    int i = bytebuffer.position();  
    int j = bytebuffer.limit();  
    //如果断言开启,buffer的position大于limit,则抛出断言错误  
    if(!$assertionsDisabled && i > j)  
        throw new AssertionError();  
    //获取需要读的字节数  
    int k = i > j ? 0 : j - i;  
    if(k == 0)  
        return 0;  
    int i1 = 0;  
    //从输入流读取k个字节到buffer  
    if(l != -1L)  
        i1 = nativedispatcher.pread(filedescriptor, ((DirectBuffer)bytebuffer).address() + (long)i, k, l, obj);  
    else  
        i1 = nativedispatcher.read(filedescriptor, ((DirectBuffer)bytebuffer).address() + (long)i, k);  
    //重新定位buffer的position  
    if(i1 > 0)  
        bytebuffer.position(i + i1);  
    return i1;  
}  
这个函数就是将内核缓冲区中的数据读取到堆外缓存DirectBuffer

回到AbstractNioByteChannel.read方法,继续看。

@Override
public void read() {
        //...
        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();
                    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 {
            if (!config.isAutoRead() && !isReadPending()) {
                removeReadOp();
            }
        }
    }
}

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。Pipeline的事件处理在我之前的博文中有详细的介绍。处理完成之后,再一次从Channel读取数据,直至退出循环。

4、循环次数超过maxMessagesPerRead时,即只能在管道中读取maxMessagesPerRead次数据,既是还没有读完也要退出。在上篇博文中,Boss线程接受客户端连接也用到了此变量,即当boss线程 selector检测到OP_ACCEPT事件后一次只能接受maxMessagesPerRead个客户端连接

 


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