code: protected void ensureCapacityUnsafe(int addBytes) {
  		int newCapacity = writerIndex() + addBytes;
  		if (newCapacity > capacity()) {
  			byte[] tmp = new byte[newCapacity * 3 / 2];
  			System.arraycopy(array, 0, tmp, 0, writerIndex());
  			array = tmp;
  		}
  		// int size = writableBytes();
  		// if (minWritableBytes <= size) {
  		// return;
  		// }
  		// int addSize = minWritableBytes - size;
  		// array = Arrays.copyOf(array, array.length + addSize);
  	}
docs: 保证缓冲区大小
  @param addBytes