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Optimized JSON
C Ruby Logos Shell

Oj gem

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


gem install oj

or in Bundler:

gem 'oj'




GitHub repo:

RubyGems repo:

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

Build Status

Build Status

Release 2.13.1

  • Thanks to Comboy for the fix to a large number rounding bug.

Release 2.13.0

  • Oj no longer raises an exception if the to_hash method of an object does not return a Hash. ActiveRecord has decided that to_hash should return an Array instead so Oj now encodes what ever is returned.

  • Added a register_odd_raw function that allows odd marshal functions to return raw JSON as a string to be included in the dumped JSON.

  • The register_odd function now supports modules in additions to classes.

Older release notes.


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. The default mode is the :object mode.

  • :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.

  • :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 is the highest performance mode.

  • :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.

To change default serialization mode:

Oj.default_options = {:mode => :compat }



Oj is compatible with Ruby 1.8.7, 1.9.2, 1.9.3, 2.0.0, 2.1, 2.2 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 :my_attr, 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'

Proper Use

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.

Simple JSON Writing and Parsing Example

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

Alternative JSON Processing APIs

Oj offers a few alternative APIs for processing JSON. The fastest one 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. The API is simple to use but does require a different approach than the conventional parse followed by access approach used by conventional JSON parsing.


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

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