Netty分布式ByteBuf使用directArena分配缓冲区过程解析

protected ByteBuf newDirectBuffer(int initialCapacity, int maxCapacity) {
    PoolThreadCache cache = threadCache.get();
    PoolArena<ByteBuffer> directArena = cache.directArena;
    ByteBuf buf;
    if (directArena != null) { 
        buf = directArena.allocate(cache, initialCapacity, maxCapacity);
    } else {
        if (PlatformDependent.hasUnsafe()) {
            buf = UnsafeByteBufUtil.newUnsafeDirectByteBuf(this, initialCapacity, maxCapacity);
        } else {
            buf = new UnpooledDirectByteBuf(this, initialCapacity, maxCapacity);
        }
    }
    return toLeakAwareBuffer(buf);
}

PooledByteBuf<T> allocate(PoolThreadCache cache, int reqCapacity, int maxCapacity) { 
    PooledByteBuf<T> buf = newByteBuf(maxCapacity); 
    allocate(cache, buf, reqCapacity);
    return buf;
}

protected PooledByteBuf<ByteBuffer> newByteBuf(int maxCapacity) { 
    if (HAS_UNSAFE) { 
        return PooledUnsafeDirectByteBuf.newInstance(maxCapacity);
    } else {
        return PooledDirectByteBuf.newInstance(maxCapacity);
    }
}

static PooledUnsafeDirectByteBuf newInstance(int maxCapacity) {
    PooledUnsafeDirectByteBuf buf = RECYCLER.get();
    buf.reuse(maxCapacity);
    return buf;
}

final void reuse(int maxCapacity) {
    maxCapacity(maxCapacity);
    setRefCnt(1);
    setIndex0(0, 0);
    discardMarks();
}

PooledByteBuf<T> allocate(PoolThreadCache cache, int reqCapacity, int maxCapacity) { 
    PooledByteBuf<T> buf = newByteBuf(maxCapacity); 
    allocate(cache, buf, reqCapacity);
    return buf;
}

private void allocate(PoolThreadCache cache, PooledByteBuf<T> buf, final int reqCapacity) {
    //规格化
    final int normCapacity = normalizeCapacity(reqCapacity);
    if (isTinyOrSmall(normCapacity)) { 
        int tableIdx;
        PoolSubpage<T>[] table;
        //判断是不是tinty
        boolean tiny = isTiny(normCapacity);
        if (tiny) { // < 512
            //缓存分配
            if (cache.allocateTiny(this, buf, reqCapacity, normCapacity)) {
                return;
            }
            //通过tinyIdx拿到tableIdx
            tableIdx = tinyIdx(normCapacity);
            //subpage的数组
            table = tinySubpagePools;
        } else {
            if (cache.allocateSmall(this, buf, reqCapacity, normCapacity)) {
                return;
            }
            tableIdx = smallIdx(normCapacity);
            table = smallSubpagePools;
        }
        //拿到对应的节点
        final PoolSubpage<T> head = table[tableIdx];

        synchronized (head) {
            final PoolSubpage<T> s = head.next;
            //默认情况下, head的next也是自身
            if (s != head) {
                assert s.doNotDestroy && s.elemSize == normCapacity;
                long handle = s.allocate();
                assert handle >= 0;
                s.chunk.initBufWithSubpage(buf, handle, reqCapacity);

                if (tiny) {
                    allocationsTiny.increment();
                } else {
                    allocationsSmall.increment();
                }
                return;
            }
        }
        allocateNormal(buf, reqCapacity, normCapacity);
        return;
    }
    if (normCapacity <= chunkSize) {
        //首先在缓存上进行内存分配
        if (cache.allocateNormal(this, buf, reqCapacity, normCapacity)) {
            //分配成功, 返回
            return;
        }
        //分配不成功, 做实际的内存分配
        allocateNormal(buf, reqCapacity, normCapacity);
    } else {
        //大于这个值, 就不在缓存上分配
        allocateHuge(buf, reqCapacity);
    }
}