.. Licensed to the Apache Software Foundation (ASF) under one .. or more contributor license agreements. See the NOTICE file .. distributed with this work for additional information .. regarding copyright ownership. The ASF licenses this file .. to you under the Apache License, Version 2.0 (the .. "License"); you may not use this file except in compliance .. with the License. You may obtain a copy of the License at

.. http://www.apache.org/licenses/LICENSE-2.0

.. Unless required by applicable law or agreed to in writing, .. software distributed under the License is distributed on an .. "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY .. KIND, either express or implied. See the License for the .. specific language governing permissions and limitations .. under the License.

.. default-domain:: cpp .. highlight:: cpp

.. cpp:namespace:: arrow::json

================== Reading JSON files

Line-separated JSON files can either be read as a single Arrow Table with a :class:~TableReader or streamed as RecordBatches with a :class:~StreamingReader.

Both of these readers require an :class:arrow::io::InputStream instance representing the input file. Their behavior can be customized using a combination of :class:~ReadOptions, :class:~ParseOptions, and other parameters.

.. seealso:: :ref:JSON reader API reference <cpp-api-json>.

TableReader

:class:~TableReader reads an entire file in one shot as a :class:~arrow::Table. Each independent JSON object in the input file is converted to a row in the output table.

.. code-block:: cpp

#include "arrow/json/api.h"

{ // ... arrow::MemoryPool* pool = default_memory_pool(); std::shared_ptrarrow::io::InputStream input = ...;

  auto read_options = arrow::json::ReadOptions::Defaults();
  auto parse_options = arrow::json::ParseOptions::Defaults();

  // Instantiate TableReader from input stream and options
  auto maybe_reader = arrow::json::TableReader::Make(pool, input, read_options, parse_options);
  if (!maybe_reader.ok()) {
     // Handle TableReader instantiation error...
  }
  auto reader = *maybe_reader;

  // Read table from JSON file
  auto maybe_table = reader->Read();
  if (!maybe_table.ok()) {
     // Handle JSON read error
     // (for example a JSON syntax error or failed type conversion)
  }
  auto table = *maybe_table;

}

StreamingReader

:class:~StreamingReader reads a file incrementally from blocks of a roughly equal byte size, each yielding a :class:~arrow::RecordBatch. Each independent JSON object in a block is converted to a row in the output batch.

All batches adhere to a consistent :class:~arrow::Schema, which is derived from the first loaded batch. Alternatively, an explicit schema may be passed via :class:~ParseOptions.

.. code-block:: cpp

#include "arrow/json/api.h"

{ // ... auto read_options = arrow::json::ReadOptions::Defaults(); auto parse_options = arrow::json::ParseOptions::Defaults();

  std::shared_ptr<arrow::io::InputStream> stream;
  auto result = arrow::json::StreamingReader::Make(stream,
                                                   read_options,
                                                   parse_options);
  if (!result.ok()) {
     // Handle instantiation error
  }
  std::shared_ptr<arrow::json::StreamingReader> reader = *result;

  for (arrow::Result<std::shared_ptr<arrow::RecordBatch>> maybe_batch : *reader) {
     if (!maybe_batch.ok()) {
        // Handle read/parse error
     }
     std::shared_ptr<arrow::RecordBatch> batch = *maybe_batch;
     // Operate on each batch...
  }

}

Data types

Since JSON values are typed, the possible Arrow data types on output depend on the input value types. Top-level JSON values should always be objects. The fields of top-level objects are taken to represent columns in the Arrow data. For each name/value pair in a JSON object, there are two possible modes of deciding the output data type:

if the name is in :member:ParseOptions::explicit_schema, conversion of the JSON value to the corresponding Arrow data type is attempted;
otherwise, the Arrow data type is determined via type inference on the JSON value, trying out a number of Arrow data types in order.

The following tables show the possible combinations for each of those two modes.

.. table:: Explicit conversions from JSON to Arrow :align: center

.. table:: Implicit type inference from JSON to Arrow :align: center