netty（八）buffer源码学习3

问题 ：

• compositeByteBuf 是干什么和其他 compositeByteBuf 有何区别
• 内部实现

概述

compositeByteBuf 就像数据库中的视图，把几个表的字段组合在一起，它的应用场景比如一个自定义协议有消息头和消息体，而两者是分开到两个 ByteBuf 的，那么这时候要怎么把两个ByteBuf 放到一起管理呢？compositeByteBuf就可以解决这个问题。

源码

 compositeByteBuf 内部放着一个 Components 的数组，这个 Components 是什么呢？

    private final class Component {
        final ByteBuf buf;
        final int length;
        int offset;
        int endOffset;

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

        void freeIfNecessary() {
            // Unwrap so that we can free slices, too.
            buf.release(); // We should not get a NPE here. If so, it must be a bug.
        }
    }

可以看到实际就是 ByteBuf 的包装类，我们看一下它的使用
 addComponent 方法

    /**
     * Add the given {@link ByteBuf} on the specific index.
     *
     * Be aware that this method does not increase the {@code writerIndex} of the {@link CompositeByteBuf}.
     * If you need to have it increased you need to handle it by your own.
     *
     * @param cIndex the index on which the {@link ByteBuf} will be added
     * @param buffer the {@link ByteBuf} to add
     */
    public CompositeByteBuf addComponent(int cIndex, ByteBuf buffer) {
        addComponent0(cIndex, buffer);
        consolidateIfNeeded();
        return this;
    }

    private int addComponent0(int cIndex, ByteBuf buffer) {
        checkComponentIndex(cIndex);

        if (buffer == null) {
            throw new NullPointerException("buffer");
        }

        int readableBytes = buffer.readableBytes();
        if (readableBytes == 0) {
            return cIndex;
        }

        // No need to consolidate - just add a component to the list.
        Component c = new Component(buffer.order(ByteOrder.BIG_ENDIAN).slice());
        if (cIndex == components.size()) {
            components.add(c);
            if (cIndex == 0) {
                c.endOffset = readableBytes;
            } else {
                Component prev = components.get(cIndex - 1);
                c.offset = prev.endOffset;
                c.endOffset = c.offset + readableBytes;
            }
        } else {
            components.add(cIndex, c);
            updateComponentOffsets(cIndex);
        }
        return cIndex;
    }

removeComponent 方法同理

补充

 ByteBufUtil 这个类是可以对 ByteBuf进行操作。 

##参考资料

• 《netty权威指南》