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Optimized JSON
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Latest commit 5852d94 Jun 29, 2016 Peter Ohler Updated notes

Oj gem

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A fast JSON parser and Object marshaller as a Ruby gem.


require 'oj'

h = { 'one' => 1, 'array' => [ true, false ] }
json = Oj.dump(h)

# json =
# {
#   "one":1,
#   "array":[
#     true,
#     false
#   ]
# }

h2 = Oj.load(json)
puts "Same? #{h == h2}"
# true

By default Oj uses the :object mode which is used to marshal and unmarshal Ruby objects. Deserialize arbitrary JSON in object mode may lead to unexpected results. :compat mode is a better choice for rails and :strict mode is a better choice for general JSON parsing. See the options section below for details.


gem install oj

or in Bundler:

gem 'oj'


Optimized JSON (Oj), as the name implies, was written to provide speed optimized JSON handling. It was designed as a faster alternative to Yajl and other common Ruby JSON parsers. So far it has achieved that, and is about 2 times faster than any other Ruby JSON parser, and 3 or more times faster at serializing JSON.

Oj has several dump or serialization modes which control how Ruby Objects are converted to JSON. These modes are set with the :mode option in either the default options or as one of the options to the dump method. In addition to the various options there are also alternative APIs for parsing JSON.

The fastest alternative parser API is the Oj::Doc API. The Oj::Doc API takes a completely different approach by opening a JSON document and providing calls to navigate around the JSON while it is open. With this approach, JSON access can be well over 20 times faster than conventional JSON parsing.

The Oj::Saj and Oj::ScHandler APIs are callback parsers that walk the JSON document depth first and makes callbacks for each element. Both callback parser are useful when only portions of the JSON are of interest. Performance up to 20 times faster than conventional JSON is possible if only a few elements of the JSON are of interest.


To change default serialization mode use the following form. Attempting to modify the Oj.default_options Hash directly will not set the changes on the actual default options but on a copy of the Hash:

Oj.default_options = {:mode => :compat }
  • :mode [Symbol] mode for dumping and loading JSON. Important format details here.

    • :object mode will dump any Object as a JSON Object with keys that match the Ruby Object's variable names without the '@' prefix character. This mode has the best performance and is the default.

    • :strict mode will only allow the 7 basic JSON types to be serialized. Any other Object will raise an Exception.

    • :null mode replaces any Object that is not one of the JSON types with a JSON null.

    • :compat mode attempts to be compatible with other systems. It will serialize any Object, but will check to see if the Object implements an to_hash or to_json method. If either exists, that method is used for serializing the Object. Since as_json is more flexible and produces more consistent output, it is preferred over the to_json method. If neither the to_json or to_hash methods exists, then the Oj internal Object variable encoding is used.

  • :indent [Fixnum] number of spaces to indent each element in an JSON document, zero is no newline between JSON elements, negative indicates no newline between top level JSON elements in a stream

  • :circular [Boolean] support circular references while dumping.

  • :auto_define [Boolean] automatically define classes if they do not exist.

  • :symbol_keys [Boolean] use symbols instead of strings for hash keys.

  • :escape_mode [Symbol] determines the characters to escape.

    • :newline allows unescaped newlines in the output.

    • :json follows the JSON specification. This is the default mode.

    • :xss_safe escapes HTML and XML characters such as & and <.

    • :ascii escapes all non-ascii or characters with the hi-bit set.

  • :class_cache [Boolean] cache classes for faster parsing (if dynamically modifying classes or reloading classes then don't use this)

  • :time_format [Symbol] time format when dumping in :compat and :object mode

    • :unix time is output as a decimal number in seconds since epoch including fractions of a second.

    • :unix_zone similar to the :unix format but with the timezone encoded in the exponent of the decimal number of seconds since epoch.

    • :xmlschema time is output as a string that follows the XML schema definition.

    • :ruby time is output as a string formatted using the Ruby to_s conversion.

  • :bigdecimal_as_decimal [Boolean] dump BigDecimal as a decimal number or as a String

  • :bigdecimal_load [Symbol] load decimals as BigDecimal instead of as a Float.

    • :bigdecimal convert all decimal numbers to BigDecimal.

    • :float convert all decimal numbers to Float.

    • :auto the most precise for the number of digits is used.

  • :create_id [String] create id for json compatible object encoding, default is json_create.

  • :second_precision [Fixnum] number of digits after the decimal when dumping the seconds portion of time

  • :float_precision [Fixnum] number of digits of precision when dumping floats, 0 indicates use Ruby

  • :use_to_json [Boolean] call to_json() methods on dump, default is false

  • :nilnil [Boolean] if true a nil input to load will return nil and not raise an Exception

  • :allow_gc [Boolean] allow or prohibit GC during parsing, default is true (allow).

  • :quirks_mode [Boolean] Allow single JSON values instead of documents, default is true (allow).

  • :nan [:null|:huge|:word|:raise|:auto] How to dump Infinity, -Infinity, and NaN in null, strict, and compat mode. :null places a null, :huge places a huge number, :word places Infinity or NaN, :raise raises and exception, :auto uses default for each mode which are :raise for :strict, :null for :null, and :word for :compat. Default is :auto.


Release 2.17.0

  • Added an option provide an alternative Hash class for loading.

  • Added the Oj::EasyHash class.

Release 2.16.1

  • Thanks to hsbt for fixing a compile issue with Ruby 2.4.0-preview1.

Older release notes.



Oj is compatible with Ruby 1.8.7, 1.9.2, 1.9.3, 2.0.0, 2.1, 2.2, 2.3 and RBX. Support for JRuby has been removed as JRuby no longer supports C extensions and there are bugs in the older versions that are not being fixed.


Although up until 4.1 Rails uses multi_json, an issue in Rails causes ActiveSupport to fail to make use Oj for JSON handling. There is a gem to patch this for Rails 3.2 and 4.0. As of the Oj 2.6.0 release the default behavior is to not use the to_json() method unless the :use_to_json option is set. This provides another work around to the rails older and newer behavior.

The latest ActiveRecord is able to work with Oj by simply using the line:

serialize :metadata, Oj

In version Rails 4.1, multi_json has been removed, and this patch is unnecessary and will no longer work. Instead, use the oj_mimic_json gem along with oj in your Gemfile to have Oj mimic the JSON gem and be used in its place by ActiveSupport JSON handling:

gem 'oj'
gem 'oj_mimic_json'

Security and Optimization

Two settings in Oj are useful for parsing but do expose a vulnerability if used from an untrusted source. Symbolized keys can cause memory to be filled with previous versions of ruby. Ruby 2.1 and below does not garbage collect Symbols. The same is true for auto defining classes in all versions of ruby; memory will also be exhausted if too many classes are automatically defined. Auto defining is a useful feature during development and from trusted sources but it allows too many classes to be created in the object load mode and auto defined is used with an untrusted source. The Oj.strict_load() method sets and uses the most strict and safest options. It should be used by developers who find it difficult to understand the options available in Oj.

The options in Oj are designed to provide flexibility to the developer. This flexibility allows Objects to be serialized and deserialized. No methods are ever called on these created Objects but that does not stop the developer from calling methods on them. As in any system, check your inputs before working with them. Taking an arbitrary String from a user and evaluating it is never a good idea from an unsecure source. The same is true for Object attributes as they are not more than Strings. Always check inputs from untrusted sources.



GitHub repo:

RubyGems repo:

Follow @peterohler on Twitter for announcements and news about the Oj gem.

Performance Comparisons

Oj Strict Mode Performance compares Oj strict mode parser performance to other JSON parsers.

Oj Compat Mode Performance compares Oj compat mode parser performance to other JSON parsers.

Oj Object Mode Performance compares Oj object mode parser performance to other marshallers.

Oj Callback Performance compares Oj callback parser performance to other JSON parsers.

Links of Interest

Fast XML parser and marshaller on RubyGems:

Fast XML parser and marshaller on GitHub:

Oj Object Encoding Format describes the OJ Object JSON encoding format.

Need for Speed for an overview of how Oj::Doc was designed.

OjC, a C JSON parser: also at

Piper Push Cache, push JSON to browsers:

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