【Netty源码分析】数据读取过程

首先客户端连接到服务端时服务端会开启一个线程，不断的监听客户端的操作。

这个线程的执行操作在NioEventLoop的run方法中，其实操作是在processSelectedKeys中，监听是否进行读操作

protected void run() {        for (;;) {            try {                switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {                    case SelectStrategy.CONTINUE:                        continue;                    case SelectStrategy.SELECT:                        select(wakenUp.getAndSet(false));                        if (wakenUp.get()) {                            selector.wakeup();                        }                    default:                        // fallthrough                }                cancelledKeys = 0;                needsToSelectAgain = false;                final int ioRatio = this.ioRatio;                if (ioRatio == 100) {                    processSelectedKeys();                    runAllTasks();                } else {                    final long ioStartTime = System.nanoTime();                    processSelectedKeys();                    final long ioTime = System.nanoTime() - ioStartTime;                    runAllTasks(ioTime * (100 - ioRatio) / ioRatio);                }                if (isShuttingDown()) {                    closeAll();                    if (confirmShutdown()) {                        break;                    }                }            } }    }

private void processSelectedKeys() {        if (selectedKeys != null) {            processSelectedKeysOptimized(selectedKeys.flip());        } else {            processSelectedKeysPlain(selector.selectedKeys());        }    }

判断selectedKeys数组中是否有值，其实就是read，write或accept事件

private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {        for (int i = 0;; i ++) {            final SelectionKey k = selectedKeys[i];            if (k == null) {                break;            }            selectedKeys[i] = null;            final Object a = k.attachment();            if (a instanceof AbstractNioChannel) {                processSelectedKey(k, (AbstractNioChannel) a);            } else {                @SuppressWarnings("unchecked")                NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;                processSelectedKey(k, task);            }        }    }

主要判断int readyOps = k.readyOps();拿到的值是否是SelectionKey.OP_READ | SelectionKey.OP_ACCEPT

private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {        final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();        if (!k.isValid()) {            final EventLoop eventLoop;            try {                eventLoop = ch.eventLoop();            } catch (Throwable ignored) {                        }                      if (eventLoop != this || eventLoop == null) {                return;            }                      unsafe.close(unsafe.voidPromise());            return;        }        try {            int readyOps = k.readyOps();                    if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {                unsafe.read();                if (!ch.isOpen()) {                    return;                }            }            if ((readyOps & SelectionKey.OP_WRITE) != 0) {                              ch.unsafe().forceFlush();            }            if ((readyOps & SelectionKey.OP_CONNECT) != 0) {                int ops = k.interestOps();                ops &= ~SelectionKey.OP_CONNECT;                k.interestOps(ops);                unsafe.finishConnect();            }        } catch (CancelledKeyException ignored) {            unsafe.close(unsafe.voidPromise());        }    }

如果int readyOps = k.readyOps();拿到的值是SelectionKey.OP_READ | SelectionKey.OP_ACCEPT就执行unsafe.read();

具体实现在AbstractNioByteChannel执行read操作

@Overridepublic final void read() {final ChannelConfig config = config();final ChannelPipeline pipeline = pipeline();final ByteBufAllocator allocator = config.getAllocator();final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();allocHandle.reset(config);ByteBuf byteBuf = null;boolean close = false;try {do {byteBuf = allocHandle.allocate(allocator);allocHandle.lastBytesRead(doReadBytes(byteBuf));if (allocHandle.lastBytesRead() <= 0) {// nothing was read. release the buffer.byteBuf.release();byteBuf = null;close = allocHandle.lastBytesRead() < 0;break;}allocHandle.incMessagesRead(1);readPending = false;pipeline.fireChannelRead(byteBuf);byteBuf = null;            } while (allocHandle.continueReading());allocHandle.readComplete();pipeline.fireChannelReadComplete();if (close) {closeOnRead(pipeline);}} catch (Throwable t) {handleReadException(pipeline, byteBuf, t, close, allocHandle);} }

读取操作是在doReadBytes(byteBuf)中，具体实现是在NioSocketChannel中

@Override    protected int doReadBytes(ByteBuf byteBuf) throws Exception {        final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();        allocHandle.attemptedBytesRead(byteBuf.writableBytes());        return byteBuf.writeBytes(javaChannel(), allocHandle.attemptedBytesRead());    }

接下来的操作是在AbstractByteBuf的writeBytes中进行。

@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;    }

读取数据操作是在setBytes函数中进行的，一般实现是在PooledUnsafeDirectByteBuf类中

 @Override    public int setBytes(int index, ScatteringByteChannel in, int length) throws IOException {        checkIndex(index, length);        ByteBuffer tmpBuf = internalNioBuffer();        index = idx(index);        tmpBuf.clear().position(index).limit(index + length);        try {            return in.read(tmpBuf);        } catch (ClosedChannelException ignored) {            return -1;        }    }

在这里我们可以看到操作in.read(tmpBuf)，这个操作就是NIO的读取数据操作了，in是SocketChannel对象，将数据读取到缓存中去，完成数据的读取过程。