[编织消息框架][netty源码分析]13 ByteBuf 实现类CompositeByteBuf职责与实现

public class CompositeByteBuf extends AbstractReferenceCountedByteBuf implements Iterable<ByteBuf> {
    
    private final ByteBufAllocator alloc;
    private final boolean direct;
    //组合内容
    private final List<Component> components;
    
    //内部类Component，指针记录
    private static final class Component {
        final ByteBuf buf;
        final int length;
        int offset;
        int endOffset;

        Component(ByteBuf buf) {
            this.buf = buf;
            length = buf.readableBytes();
        }

        void freeIfNecessary() {
            buf.release(); // We should not get a NPE here. If so, it must be a bug.
        }
    }
    //Iterator 实现，只要实现hasNext跟next，维护nextIndex即可
    private final class CompositeByteBufIterator implements Iterator<ByteBuf> {
        private final int size = components.size();
        private int index;

        @Override
        public boolean hasNext() {
            return size > index;
        }

        @Override
        public ByteBuf next() {
            if (size != components.size()) {
                throw new ConcurrentModificationException();
            }
            if (!hasNext()) {
                throw new NoSuchElementException();
            }
            try {
                return components.get(index++).buf;
            } catch (IndexOutOfBoundsException e) {
                throw new ConcurrentModificationException();
            }
        }
    }
    
    //由于readBytes跟writeBytes逻辑差不多，r/w时先定位那个component之后再进行操作
    //这里用到二分查找法
    public int toComponentIndex(int offset) {
        checkIndex(offset);

        for (int low = 0, high = components.size(); low <= high;) {
            int mid = low + high >>> 1;  //>>>1 无符号右移1位 相当于high/2 
            Component c = components.get(mid);
            //在右半边
            if (offset >= c.endOffset) {
                low = mid + 1;
            } else if (offset < c.offset) { //在左半边
                high = mid - 1;
            } else {
                return mid;
            }
        }

        throw new Error("should not reach here");
    }
}

小结：

由于CompositeByteBuf太部份逻辑处理是对区块处理，不具有分析价值，本人认为有价值部分为二分查找算法实践同Iterable实现