Netty原理：ByteBuf对Nio bytebuffer做了什么导致效率提升？(2)

ByteBuffer buffer = ByteBuffer.allocate(8);
buffer.put("yu".getBytes());
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    public final ByteBuffer put(byte[] src) {
        return put(src, 0, src.length);
    }
    // 额外接收偏移量（存储数据的起始位置）  和数据长度
    public ByteBuffer put(byte[] src, int offset, int length) {
        // 校验参数的有效性
        checkBounds(offset, length, src.length);
        // 如果要存储数据的长度 > 剩余可用空间  抛出buffer越界的异常
        if (length > remaining())
            throw new BufferOverflowException();
        // 如果剩余空间足够  计算存储的结束位置 = 偏移量 + 数据长度    
        int end = offset + length;
        for (int i = offset; i < end; i++)
            this.put(src[i]);
        return this;
    }    

    public static void main(String[] args) {
        ByteBuf buf = Unpooled.buffer(10);
        System.out.println("capacity:" + buf.capacity());
        for (int i = 0; i < 11; i++) {
            buf.writeByte(i);
        }
        System.out.println("capacity:" + buf.capacity());
        for (int i = 0; i < 65; i++) {
            buf.writeByte(i);
        }
        System.out.println("capacity:" + buf.capacity());
    }

查看源码，以AbstractByteBuf子类为依据查看，最重要的子类之一，ByteBuf的公共属性和功能都在此中实现。
ByteBuf buf = Unpooled.buffer(10);
System.out.println("capacity: " + buf.capacity());
for (int i = 0; i < 11; i++) {
    buf.writeByte(i);
}
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[ByteBuf类]
public abstract ByteBuf writeByte(int value);
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[AbstractByteBuf子类]
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    @Override
    public ByteBuf writeByte(int value) {
        // 确保可写空间足够
        ensureWritable0(1);
        // 写入数据
        _setByte(writerIndex++, value);
        return this;
    }
    // 参数为 最小写入数据的大小
    final void ensureWritable0(int minWritableBytes) {
        final int writerIndex = writerIndex();
        // 目标容量 = 当前写操作索引 + 最小写入数据大小
        final int targetCapacity = writerIndex + minWritableBytes;
        // 容量足够  不需扩容
        if (targetCapacity <= capacity()) {
            ensureAccessible();
            return;
        }
        // 容量不足时 如果目标容量 超出最大容量  抛出异常
        if (checkBounds && targetCapacity > maxCapacity) {
            ensureAccessible();
            throw new IndexOutOfBoundsException(String.format(
                    "writerIndex(%d) + minWritableBytes(%d) exceeds maxCapacity(%d): %s",
                    writerIndex, minWritableBytes, maxCapacity, this));
        }
    // 扩容逻辑
        // 获取可写空间大小
        final int fastWritable = maxFastWritableBytes();
        // 如果 可写空间 >= 所需空间   新的容量=写操作索引+可写空间大小 
        // 如果 可写空间 < 所需空间   计算要扩容的新容量大小 calculateNewCapacity方法
        int newCapacity = fastWritable >= minWritableBytes ? writerIndex + fastWritable
                : alloc().calculateNewCapacity(targetCapacity, maxCapacity);
        // Adjust to the new capacity.
        // 计算完成后 生成新的ByteBuffer
        capacity(newCapacity);
    }
    // 获取可写空间大小
    public int maxFastWritableBytes() {
        return writableBytes();
    }
[AbstractByteBufAllocator子类]
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    // 计算要扩容的新容量大小
    @Override
    public int calculateNewCapacity(int minNewCapacity, int maxCapacity) {
        // 校验参数有效性
        checkPositiveOrZero(minNewCapacity, "minNewCapacity");
        if (minNewCapacity > maxCapacity) {
            throw new IllegalArgumentException(String.format(
                    "minNewCapacity: %d (expected: not greater than maxCapacity(%d)",
                    minNewCapacity, maxCapacity));
        }
        // 扩容方式的分界点 以4M大小为界
        final int threshold = CALCULATE_THRESHOLD; // 4 MiB page
        if (minNewCapacity == threshold) {
            return threshold;
        }
        // If over threshold, do not double but just increase by threshold.、
        // 如果所需容量大于4M  按照步进的方式扩容 
        //   举例： 比如 minNewCapacity = 5M  
        if (minNewCapacity > threshold) {
            // newCapacity = 5 / 4 * 4 = 4M  确保是4的倍数
            int newCapacity = minNewCapacity / threshold * threshold;
            if (newCapacity > maxCapacity - threshold) {
                newCapacity = maxCapacity;
            } else {
                // newCapacity = 4 + 4 = 8M;
                newCapacity += threshold;
            }
            return newCapacity;
        }
        // Not over threshold. Double up to 4 MiB, starting from 64.
        // 如果所需容量大于4M  按照64的倍数扩容  找到最接近所需容量的64的倍数
        int newCapacity = 64;
        while (newCapacity < minNewCapacity) {
            newCapacity <<= 1;
        }
        // 保障在最大可接受容量范围内
        return Math.min(newCapacity, maxCapacity);
    }