MultiJSON

Lots of Ruby libraries parse JSON and everyone has their favorite JSON coder.
Instead of choosing a single JSON coder and forcing users of your library to be
stuck with it, you can use MultiJSON instead, which will simply choose the
fastest available JSON coder. Here's how to use it:

require "multi_json"

MultiJSON.parse('{"abc":"def"}')                        #=> {"abc" => "def"}
MultiJSON.parse('{"abc":"def"}', symbolize_names: true) #=> {abc: "def"}
MultiJSON.generate({abc: "def"})                       # convert Ruby back to JSON
MultiJSON.generate({abc: "def"}, pretty: true)         # encoded in a pretty form (if supported by the coder)

[!IMPORTANT]
1.21.0 renames the public API to match Ruby stdlib JSON. The canonical
verbs are now MultiJSON.parse / MultiJSON.generate, and the canonical
module is MultiJSON (all-caps). The legacy MultiJson constant,
MultiJSON.load / MultiJSON.dump, :symbolize_keys, and friends still
work but emit one-time deprecation warnings and will be removed in 2.0.0.
Run your app with ruby -W:deprecated to surface them; the warnings are
tagged with the :deprecated category so you can silence the whole set with
Warning[:deprecated] = false. See Deprecated in 1.21.0
for the full list.

MultiJSON.parse returns nil for nil, empty, and whitespace-only inputs
instead of raising, so a missing or blank payload is observable as a nil
return value rather than an exception. When parsing invalid JSON, MultiJSON
will throw a MultiJSON::ParseError. MultiJSON::DecodeError and
MultiJSON::LoadError are aliases for backwards compatibility.

begin
  MultiJSON.parse("{invalid json}")
rescue MultiJSON::ParseError => exception
  exception.data    #=> "{invalid json}"
  exception.cause   #=> JSON::ParserError: ...
  exception.line    #=> 1 (for adapters that report a location, e.g. Oj or the json gem)
  exception.column  #=> 2
end

Drop-in replacement for stdlib JSON

MultiJSON mirrors the surface of Ruby's stdlib JSON so
most call sites swap in with a one-line change:

- require "json"
+ require "multi_json"

- JSON.parse(text, symbolize_names: true)
+ MultiJSON.parse(text, symbolize_names: true)

- JSON.generate(object, pretty: true)
+ MultiJSON.generate(object, pretty: true)

Method names and the common options line up with stdlib so existing
pretty-print calls and option keys keep working without changes:

Deprecated in 1.21.0

The module constant and primary verbs were renamed to match Ruby
stdlib JSON.parse / JSON.generate and the JSON spec (RFC 8259).
The old names still work in 1.x but now emit a one-time deprecation
warning; they will be removed in 2.0.0.

The MultiJson constant (CamelCase) continues to work as a thin
delegator; every method call, constant lookup, and rescue clause
routes through MultiJSON transparently.

[!TIP]
The recommended upgrade path to 2.0 is: pin ~> 1.21 first, run
ruby -W:deprecated against your app or test suite to surface every
deprecation, migrate each call site to the canonical name, then bump to
~> 2.0. The 2.0 release deletes the deprecated aliases entirely, so the
warnings during 1.21.x are your map.

ParseError instance has cause reader which contains the original exception.
It also has data reader with the input that caused the problem, and line/column
readers populated for adapters whose error messages include a location (Oj and the
json gem). Adapters that don't include one (Yajl, fast_jsonparser) leave both nil.

Tuning the options cache

MultiJSON memoizes the merged option hash for each parse/generate call so
identical option hashes don't trigger repeated work. The cache is bounded —
defaulting to 1000 entries per direction — and applications that generate many
distinct option hashes can raise the ceiling at runtime:

MultiJSON::OptionsCache.max_cache_size = 5000

max_cache_size must be a positive integer; 0, negative values, and
non-integers raise ArgumentError.

Lowering the limit only takes effect for new inserts; existing cache
entries are left in place until normal eviction trims them below the
new ceiling. Call MultiJSON::OptionsCache.reset if you want to evict
immediately.

The use method, which sets the MultiJSON adapter, takes either a symbol or a
class (to allow for custom JSON parsers) that responds to both .load and .dump
at the class level.

When MultiJSON fails to load the specified adapter, it'll throw MultiJSON::AdapterError
which inherits from ArgumentError.

Writing a custom adapter

A custom adapter is any class that responds to two class methods plus
defines a ParseError constant:

class MyAdapter
  ParseError = Class.new(StandardError)

  def self.load(string, options)
    # parse string into a Ruby object, raising ParseError on failure
  end

  def self.dump(object, options)
    # serialize object to a JSON string
  end
end

MultiJSON.use(MyAdapter)

ParseError is required: MultiJSON.parse rescues MyAdapter::ParseError
to wrap parse failures in MultiJSON::ParseError, and an adapter that
omits the constant raises MultiJSON::AdapterError on the first parse
attempt instead of producing a confusing NameError.

For more, inherit from MultiJSON::Adapter to pick up shared option
merging, the defaults :load, ... / defaults :dump, ... DSL, and the
blank-input short-circuit. The built-in adapters in
lib/multi_json/adapters/ are working examples.

[!NOTE]
The adapter contract methods on the adapter class itself stay named
.load / .dump in 1.21.x (and the defaults :load, ... / defaults :dump, ... DSL keys match). The 2.0 release renames them to .parse /
.generate to align with the public API; if you ship a custom adapter,
you'll need to rename those methods (and the defaults keys) when you
upgrade.

MultiJSON tries to have intelligent defaulting. If any supported library is
already loaded, MultiJSON uses it before attempting to load others. When no
backend is preloaded, MultiJSON walks its preference list and uses the first
one that loads successfully. The list is split per platform — JRuby's
available adapter set differs from MRI's, and the bundled benchmark suite
ranks json_gem ahead of fast_jsonparser/oj/yajl on Ruby 3.4+. CI
re-runs the benchmark and fails if the observed ranking diverges from the
table below.

A dash means the adapter isn't usable on that runtime: fast_jsonparser,
oj, and yajl-ruby are MRI/TruffleRuby C extensions with no JRuby builds;
jrjackson and gson are JRuby-only. The JSON gem is a Ruby default gem,
so it's always available as a last-resort fallback on any supported Ruby.
If you have a workload where a different backend is faster, set it
explicitly with MultiJSON.use(:your_adapter).

Gem Variants

MultiJSON ships as two platform-specific gems. Bundler and RubyGems
automatically select the correct variant for your Ruby implementation:

Supported Ruby Versions

This library aims to support and is tested against the following Ruby
implementations:

• Ruby 3.2
• Ruby 3.3
• Ruby 3.4
• Ruby 4.0
• JRuby 10.0 (targets Ruby 3.4 compatibility)
• TruffleRuby 33.0 (native and JVM)

If something doesn't work in one of these implementations, it's a bug.

This library may inadvertently work (or seem to work) on other Ruby
implementations, however support will only be provided for the versions listed
above.

If you would like this library to support another Ruby version, you may
volunteer to be a maintainer. Being a maintainer entails making sure all tests
run and pass on that implementation. When something breaks on your
implementation, you will be responsible for providing patches in a timely
fashion. If critical issues for a particular implementation exist at the time
of a major release, support for that Ruby version may be dropped.

Versioning

This library aims to adhere to Semantic Versioning 2.0.0. Violations
of this scheme should be reported as bugs. Specifically, if a minor or patch
version is released that breaks backward compatibility, that version should be
immediately yanked and/or a new version should be immediately released that
restores compatibility. Breaking changes to the public API will only be
introduced with new major versions. As a result of this policy, you can (and
should) specify a dependency on this gem using the Pessimistic Version
Constraint with two digits of precision. For example:

spec.add_dependency 'multi_json', '~> 1.0'

Copyright

Copyright (c) 2010-2026 Erik Berlin, Michael Bleigh, Josh Kalderimis, and Pavel
Pravosud. See LICENSE for details.