netty 4源码分析-write

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netty 4源码分析-write

 

Netty的写操作由两个步骤组成：

1. Write:将msg存储到ChannelOutboundBuffer中
2. Flush：将msg从ChannelOutboundBuffer中flush到套接字的发送缓冲区中。

       本文介绍第一个步骤write

//DefaultChannelHandlerContext
public ChannelFuture write(Object msg) {
        return write(msg, newPromise());
} 
public ChannelFuture write(final Object msg, final ChannelPromise promise) {
        if (msg == null) {
            throw new NullPointerException("msg");
        }
        validatePromise(promise, true);
        write(msg, false, promise);
        return promise;
    }
private void write(Object msg, boolean flush, ChannelPromise promise) {
        DefaultChannelHandlerContext next = findContextOutbound();
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            next.invokeWrite(msg, promise);
            if (flush) {
                next.invokeFlush();
            }
        } else {
            int size = channel.estimatorHandle().size(msg);
            if (size > 0) {
                ChannelOutboundBuffer buffer = channel.unsafe().outboundBuffer();
                // Check for null as it may be set to null if the channel is closed already
                if (buffer != null) {
                    buffer.incrementPendingOutboundBytes(size, false);
                }
            }
            executor.execute(WriteTask.newInstance(next, msg, size, flush, promise));
        }
    }

 Write是一个Outbound事件，所以会调用outbound处理器的write方法。下面分析headHandler的write方法。

//HeadHandler        
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
            unsafe.write(msg, promise);
        }

 里面会调用AbstractUnsafe的write方法 

// AbstractUnsafe
  public void write(Object msg, ChannelPromise promise) {
            if (!isActive()) {
                // Mark the write request as failure if the channel is inactive.
                if (isOpen()) {
                    promise.tryFailure(NOT_YET_CONNECTED_EXCEPTION);
                } else {
                    promise.tryFailure(CLOSED_CHANNEL_EXCEPTION);
                }
                // release message now to prevent resource-leak
                ReferenceCountUtil.release(msg);
            } else {
                outboundBuffer.addMessage(msg, promise);
            }
        }

 outboundBuffer是AbstractUnsafe使用的一种数据结构ChannelOutboundBuffer，用来存储待发送的消息。该数据结构在实例化AbstractUnsafe的同时被初始化：

// ChannelOutboundBuffer
private static final Recycler<ChannelOutboundBuffer> RECYCLER = new Recycler<ChannelOutboundBuffer>() {
        @Override
        protected ChannelOutboundBuffer newObject(Handle handle) {
            return new ChannelOutboundBuffer(handle);
        }
    };  

static ChannelOutboundBuffer newInstance(AbstractChannel channel) {
        ChannelOutboundBuffer buffer = RECYCLER.get();
        buffer.channel = channel;
        buffer.totalPendingSize = 0;
        buffer.writable = 1;
        return buffer;
    }

 ChannelOutboundBuffer的结构如下：

 Buffer是用来存储msg的Entry结构数组，entry的结构如下：

 
ChannelOutboundBuffer实例化时，buffer数组的大小为32，nioBuffers数组的大小也为32.由于ChannelOutboundBuffer的实例化的代价实际上是很高的，看以下构造方法：

private ChannelOutboundBuffer(Handle handle) {
        this.handle = handle;
        buffer = new Entry[INITIAL_CAPACITY];
        for (int i = 0; i < buffer.length; i++) {
            buffer[i] = new Entry();
        }
        nioBuffers = new ByteBuffer[INITIAL_CAPACITY];
    }

所以netty使用基于thread-local的轻量级对象池Recycler对ChannelOutboundBuffer进行回收。当ChannelOutboundBuffer第一次被实例化且使用完毕后，会回收到Recycler中（见下面的recyle方法），下次需要用时，直接从Recycler中取（见下面的get方法），避免了再次实例化和垃圾回收的开销。

public abstract class Recycler<T> {
    private final ThreadLocal<Stack<T>> threadLocal = new ThreadLocal<Stack<T>>() {
        @Override
        protected Stack<T> initialValue() {
            return new Stack<T>(Recycler.this, Thread.currentThread());
        }
    };
    public final T get() {
        Stack<T> stack = threadLocal.get();
        T o = stack.pop();
        if (o == null) {
            o = newObject(stack);
        }
        return o;
    }
    public final boolean recycle(T o, Handle handle) {
        @SuppressWarnings("unchecked")
        Stack<T> stack = (Stack<T>) handle;
        if (stack.parent != this) {
            return false;
        }
        if (Thread.currentThread() != stack.thread) {
            return false;
        }
        stack.push(o);
        return true;
    }
    protected abstract T newObject(Handle handle);
    public interface Handle { }
    static final class Stack<T> implements Handle {
        private static final int INITIAL_CAPACITY = 256;
        final Recycler<T> parent;
        final Thread thread;
        private T[] elements;
        private int size;
        private final Map<T, Boolean> map = new IdentityHashMap<T, Boolean>(INITIAL_CAPACITY);
        @SuppressWarnings({ "unchecked", "SuspiciousArrayCast" })
        Stack(Recycler<T> parent, Thread thread) {
            this.parent = parent;
            this.thread = thread;
            elements = newArray(INITIAL_CAPACITY);
        }
        T pop() {
            int size = this.size;
            if (size == 0) {
                return null;
            }
            size --;
            T ret = elements[size];
            elements[size] = null;
            map.remove(ret);
            this.size = size;
            return ret;
        }

        void push(T o) {
            if (map.put(o, Boolean.TRUE) != null) {
                throw new IllegalStateException("recycled already");
            }

            int size = this.size;
            if (size == elements.length) {
                T[] newElements = newArray(size << 1);
                System.arraycopy(elements, 0, newElements, 0, size);
                elements = newElements;
            }

            elements[size] = o;
            this.size = size + 1;
        }

        @SuppressWarnings({ "unchecked", "SuspiciousArrayCast" })
        private static <T> T[] newArray(int length) {
            return (T[]) new Object[length];
        }
    }

 下面接着分析ChannelOutboundBuffer的addMessage方法。

// ChannelOutboundBuffer
     void addMessage(Object msg, ChannelPromise promise) {
        int size = channel.estimatorHandle().size(msg);
        if (size < 0) {
            size = 0;
        }
        Entry e = buffer[tail++];
        e.msg = msg;
        e.pendingSize = size;
        e.promise = promise;
        e.total = total(msg);

        tail &= buffer.length - 1;

        if (tail == flushed) {
            addCapacity();
        }
        // increment pending bytes after adding message to the unflushed arrays.
        // See https://github.com/netty/netty/issues/1619
        incrementPendingOutboundBytes(size, true);
    }

每次都会将msg作为一个Entry存储到buffer数组的tail位置，然后将tail自增1，自增后执行这行代码tail &= buffer.length – 1（譬如假设length为4，当已存储3个msg后，tail累加到4，和3执行与的结果得到0，因此下次的消息又重新存储到buffer的0位置）使得buffer数组可以循环存储。如果出现tail=flushed，说明空间不够，需要将数组扩容到原来大小的两倍.

incrementPendingOutboundBytes则会更新totalPendingSize，将其累加本次msg的大小。如果新的totalPendingSize超过了channel的高水位线writeBufferHighWaterMark（默认值为64 * 1024），则触发ChannelWritabilityChanged事件。（注意：如果网络很繁忙，套接字的发送缓冲区空间 不够，导致Msg不能及时从buffer中flush出去，那么不断的对channel执行write操作，会使得对数组不断地进行两倍扩容，最终导致OOM。所以最好在自己的Inbound处理器里捕获ChannelWritabilityChanged事件，然后调用channel的isWritable方法，根据结果来决定是否继续执行write操作）。

// ChannelOutboundBuffer
private static final AtomicLongFieldUpdater<ChannelOutboundBuffer> TOTAL_PENDING_SIZE_UPDATER =
            AtomicLongFieldUpdater.newUpdater(ChannelOutboundBuffer.class, "totalPendingSize");

 void incrementPendingOutboundBytes(int size, boolean fireEvent) {
        // Cache the channel and check for null to make sure we not produce a NPE in case of the Channel gets
        // recycled while process this method.
        Channel channel = this.channel;
        if (size == 0 || channel == null) {
            return;
        }

        long oldValue = totalPendingSize;
        long newWriteBufferSize = oldValue + size;
        while (!TOTAL_PENDING_SIZE_UPDATER.compareAndSet(this, oldValue, newWriteBufferSize)) {
            oldValue = totalPendingSize;
            newWriteBufferSize = oldValue + size;
        }

        int highWaterMark = channel.config().getWriteBufferHighWaterMark();

        if (newWriteBufferSize > highWaterMark) {
            if (WRITABLE_UPDATER.compareAndSet(this, 1, 0)) {
                if (fireEvent) {
                    channel.pipeline().fireChannelWritabilityChanged();
                }
            }
        }
    }

 需要注意的是，该方法是线程安全的，采用了一个技巧，使用AtomicLongFieldUpdater来对totalPendingSize进行更新，实现CAS的效果，达到并发安全读写。相对于synchronized同步，AtomicLongFieldUpdater的开销是比较小的。

 

 总结可以借鉴的几个点：

1、轻量级对象池的使用

2、buffer数组的循环存储

3、ChannelWritabilityChanged事件的触发

4、AtomicLongFieldUpdater的使用