@Override
    public JsonValue getJsonValue(final int columnIndex) {
        if (this.isNull(columnIndex)) {
            return null;
        }
        int index = pageSlice.getInt(getOffset(columnIndex));
        return this.page.getJsonValueReference(index);
    }
Returns a JSON value at the specified column.

@param columnIndex  the index the column to get the JSON value
@return the JSON value
embulk-core/src/main/java/org/embulk/spi/PageReaderImpl.java