Fast JSON serializer for golang.

go get


easyjson Build Status

easyjson allows to (un-)marshal JSON golang structs without the use of reflection by generating marshaller code.

One of the aims of the library is to keep generated code simple enough so that it can be easily optimized or fixed. Another goal is to provide users with ability to customize the generated code not available in 'encoding/json', such as generating snake_case names or enabling 'omitempty' behavior by default.


go get
easyjson -all <file>.go

This will generate <file>_easyjson.go with marshaller/unmarshaller methods for structs. GOPATH variable needs to be set up correctly, since the generation invokes a go run on a temporary file (this is a really convenient approach to code generation borrowed from


Usage of .root/bin/easyjson:
        generate un-/marshallers for all structs in a file
  -build_tags string
        build tags to add to generated file
        do not delete temporary files
        don't generate MarshalJSON/UnmarshalJSON methods
        do not run 'gofmt -w' on output file
        omit empty fields by default
        use snake_case names instead of CamelCase by default
        only generate stubs for marshallers/unmarshallers methods

Using -all will generate (un-)marshallers for all structs in the file. By default, structs need to have a line beginning with easyjson:json in their docstring, e.g.:

struct A{}

-snake_case tells easyjson to generate snake_case field names by default (unless explicitly overriden by a field tag). The CamelCase to snake_case conversion algorithm should work in most cases (e.g. HTTPVersion will be converted to http_version). There can be names like JSONHTTPRPC where the conversion will return an unexpected result (jsonhttprpc without underscores), but such names require a dictionary to do the conversion and may be ambiguous.

-build_tags will add corresponding build tag line for the generated file.

marshaller/unmarshaller interfaces

easyjson generates MarshalJSON/UnmarshalJSON methods that are compatible with interfaces from 'encoding/json'. They are usable with 'json.Marshal' and 'json.Unmarshal' functions, however actually using those will result in significantly worse performance compared to custom interfaces.

MarshalEasyJSON / UnmarshalEasyJSON methods are generated for faster parsing using custom Lexer/Writer structs (jlexer.Lexer and jwriter.Writer). The method signature is defined in easyjson.Marshaler / easyjson.Unmarshaler interfaces. These interfaces allow to avoid using any unnecessary reflection or type assertions during parsing. Functions can be used manually or with easyjson.Marshal<...> and easyjson.Unmarshal<...> helper methods.

jwriter.Writer struct in addition to function for returning the data as a single slice also has methods to return the size and to send the data to an io.Writer. This is aimed at a typical HTTP use-case, when you want to know the Content-Length before actually starting to send the data.

There are helpers in the top-level package for marhsaling/unmarshaling the data using custom interfaces to and from writers, including a helper for http.ResponseWriter.

custom types

If easyjson.Marshaler / easyjson.Unmarshaler interfaces are implemented by a type involved in JSON parsing, the type will be marshaled/unmarshaled using these methods. easyjson.Optional interface allows for a custom type to integrate with 'omitempty' logic.

As an example, easyjson includes an easyjson.RawMessage analogous to json.RawMessage.

Also, there are 'optional' wrappers for primitive types in easyjson/opt package. These are useful in the case when it is necessary to distinguish between missing and default value for the type. Wrappers allow to avoid pointers and extra heap allocations in such cases.

memory pooling

The library uses a custom buffer which allocates data in increasing chunks (128-32768 bytes). Chunks of 512 bytes and larger are reused with the help of sync.Pool. The maximum size of a chunk is bounded to reduce redundancy in memory allocation and to make the chunks more reusable in the case of large buffer sizes.

The buffer code is in easyjson/buffer package the exact values can be tweaked by a buffer.Init() call before the first serialization.


  • The library is at an early stage, there are likely to be some bugs and some features of 'encoding/json' may not be supported. Please report such cases, so that they may be fixed sooner.
  • Object keys are case-sensitive (unlike encodin/json). Case-insentive behavior will be implemented as an option (case-insensitive matching is slower).
  • Unsafe package is used by the code. While a non-unsafe version of easyjson can be made in the future, using unsafe package simplifies a lot of code by allowing no-copy []byte to string conversion within the library. This is used only during parsing and all the returned values are allocated properly.
  • Floats are currently formatted with default precision for 'strconv' package. It is obvious that it is not always the correct way to handle it, but there aren't enough use-cases for floats at hand to do anything better.
  • During parsing, parts of JSON that are skipped over are not syntactically validated more than required to skip matching parentheses.
  • No true streaming support for encoding/decoding. For many use-cases and protocols, data length is typically known on input and needs to be known before sending the data.


Most benchmarks were done using a sample 13kB JSON (9k if serialized back trimming the whitespace) from The sample is very close to real-world data, quite structured and contains a variety of different types.

For small request benchmarks, an 80-byte portion of the regular sample was used.

For large request marshalling benchmarks, a struct containing 50 regular samples was used, making a ~500kB output JSON.

Benchmarks are available in the repository and are run on 'make'.

easyjson vs. encoding/json

easyjson seems to be 5-6 times faster than the default json serialization for unmarshalling, 3-4 times faster for non-concurrent marshalling. Concurrent marshalling is 6-7x faster if marshalling to a writer.

easyjson vs. ffjson

easyjson uses the same approach for code generation as ffjson, but a significantly different approach to lexing and generated code. This allows easyjson to be 2-3x faster for unmarshalling and 1.5-2x faster for non-concurrent unmarshalling.

ffjson seems to behave weird if used concurrently: for large request pooling hurts performance instead of boosting it, it also does not quite scale well. These issues are likely to be fixable and until that comparisons might vary from version to version a lot.

easyjson is similar in performance for small requests and 2-5x times faster for large ones if used with a writer.

easyjson vs. go/codec library provides compile-time helpers for JSON generation. In this case, helpers are not exactly marshallers as they are encoding-independent.

easyjson is generally ~2x faster for non-concurrent benchmarks and about 3x faster for concurrent encoding (without marshalling to a writer). Unsafe option for generated helpers was used.

As an attempt to measure marshalling performance of 'go/codec' (as opposed to allocations/memcpy/writer interface invocations), a benchmark was done with resetting lenght of a byte slice rather than resetting the whole slice to nil. However, the optimization in this exact form may not be applicable in practice, since the memory is not freed between marshalling operations.

easyjson vs 'ujson' python module

ujson is using C code for parsing, so it is interesting to see how plain golang compares to that. It is imporant to note that the resulting object for python is slower to access, since the library parses JSON object into dictionaries.

easyjson seems to be slightly faster for unmarshalling (finally!) and 2-3x faster for marshalling.

benchmark figures

The data was measured on 4 February, 2016 using current ffjson and golang 1.6. Data for go/codec was added on 4 March 2016, benchmarked on the same machine.


lib json size MB/s allocs/op B/op
standard regular 22 218 10229
standard small 9.7 14 720
-------- ----------- ------ ----------- -------
easyjson regular 125 128 9794
easyjson small 67 3 128
-------- ----------- ------ ----------- -------
ffjson regular 66 141 9985
ffjson small 17.6 10 488
-------- ----------- ------ ----------- -------
codec regular 55 434 19299
codec small 29 7 336
-------- ----------- ------ ----------- -------
ujson regular 103 N/A N/A

Marshalling, one goroutine.

lib json size MB/s allocs/op B/op
standard regular 75 9 23256
standard small 32 3 328
standard large 80 17 1.2M
---------- ----------- ------ ----------- -------
easyjson regular 213 9 10260
easyjson* regular 263 8 742
easyjson small 125 1 128
easyjson large 212 33 490k
easyjson* large 262 25 2879
---------- ----------- ------ ----------- -------
ffjson regular 122 153 21340
ffjson** regular 146 152 4897
ffjson small 36 5 384
ffjson** small 64 4 128
ffjson large 134 7317 818k
ffjson** large 125 7320 827k
---------- ----------- ------ ----------- -------
codec regular 80 17 33601
codec*** regular 108 9 1153
codec small 42 3 304
codec*** small 56 1 48
codec large 73 483 2.5M
codec*** large 103 451 66007
---------- ----------- ------ ----------- -------
ujson regular 92 N/A N/A

* marshalling to a writer, ** using ffjson.Pool(), *** reusing output slice instead of resetting it to nil

Marshalling, concurrent.

lib json size MB/s allocs/op B/op
standard regular 252 9 23257
standard small 124 3 328
standard large 289 17 1.2M
---------- ----------- ------- ----------- -------
easyjson regular 792 9 10597
easyjson* regular 1748 8 779
easyjson small 333 1 128
easyjson large 718 36 548k
easyjson* large 2134 25 4957
---------- ----------- ------ ----------- -------
ffjson regular 301 153 21629
ffjson** regular 707 152 5148
ffjson small 62 5 384
ffjson** small 282 4 128
ffjson large 438 7330 1.0M
ffjson** large 131 7319 820k
---------- ----------- ------ ----------- -------
codec regular 183 17 33603
codec*** regular 671 9 1157
codec small 147 3 304
codec*** small 299 1 48
codec large 190 483 2.5M
codec*** large 752 451 77574

* marshalling to a writer, ** using ffjson.Pool(), *** reusing output slice instead of resetting it to nil