easyjson

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.

usage

go get github.com/mailru/easyjson/...
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 https://github.com/pquerna/ffjson).

options

Usage of easyjson:
  -all
        generate un-/marshallers for all structs in a file
  -build_tags string
        build tags to add to generated file
  -leave_temps
        do not delete temporary files
  -omit_empty
        omit empty fields by default
  -snake_case
        use snake_case names instead of CamelCase by default
  -stubs
        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.:

//easyjson:json
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.

limitations

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.
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 to use 'switch' for filling out structs and working around limitations of standard functions like 'strconv.ParseInt'.
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.

benchmarks

Most benchmarks were done using a sample 13kB JSON (9k if serialized back trimming the whitespace) from https://dev.twitter.com/rest/reference/get/search/tweets. 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

github.com/ugorji/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.

Unmarshalling

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