Home / cs-notes / Architecture / Components / Netty / ByteBuf / methods / ensureWritable

ByteBuf

io.netty.buffer

public abstract ByteBuf ensureWritable(int minWritableBytes);

AbstractByteBuf

io.netty.buffer

    @Override
    public ByteBuf ensureWritable(int minWritableBytes) {
        ensureWritable0(checkPositiveOrZero(minWritableBytes, "minWritableBytes"));
        return this;
    }

    final void ensureWritable0(int minWritableBytes) {
        final int writerIndex = writerIndex();
        final int targetCapacity = writerIndex + minWritableBytes;
        if (targetCapacity <= capacity()) {
            ensureAccessible();
            return;
        }
        if (checkBounds && targetCapacity > maxCapacity) {
            ensureAccessible();
            throw new IndexOutOfBoundsException(String.format(
                    "writerIndex(%d) + minWritableBytes(%d) exceeds maxCapacity(%d): %s",
                    writerIndex, minWritableBytes, maxCapacity, this));
        }

        // Normalize the target capacity to the power of 2.
        final int fastWritable = maxFastWritableBytes();
        int newCapacity = fastWritable >= minWritableBytes ? writerIndex + fastWritable
                : alloc().calculateNewCapacity(targetCapacity, maxCapacity);

        // Adjust to the new capacity.
        capacity(newCapacity);
    }

    public abstract ByteBufAllocator alloc();

ByteBufAllocator

io.netty.buffer

public interface ByteBufAllocator { ... }

AbstractByteBufAllocator

package io.netty.buffer;

    @Override
    public int calculateNewCapacity(int minNewCapacity, int maxCapacity) {
        checkPositiveOrZero(minNewCapacity, "minNewCapacity");
        if (minNewCapacity > maxCapacity) {
            throw new IllegalArgumentException(String.format(
                    "minNewCapacity: %d (expected: not greater than maxCapacity(%d)",
                    minNewCapacity, maxCapacity));
        }
        final int threshold = CALCULATE_THRESHOLD; // 4 MiB page

        if (minNewCapacity == threshold) {
            return threshold;
        }

        // If over threshold, do not double but just increase by threshold.
        if (minNewCapacity > threshold) {
            int newCapacity = minNewCapacity / threshold * threshold;
            if (newCapacity > maxCapacity - threshold) {
                newCapacity = maxCapacity;
            } else {
                newCapacity += threshold;
            }
            return newCapacity;
        }

        // Not over threshold. Double up to 4 MiB, starting from 64.
        int newCapacity = 64;
        while (newCapacity < minNewCapacity) {
            newCapacity <<= 1;
        }

        return Math.min(newCapacity, maxCapacity);
    }

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