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easyjson Build Status Go Report Card

Package easyjson provides a fast and easy way to marshal/unmarshal Go structs to/from JSON without the use of reflection. In performance tests, easyjson outperforms the standard encoding/json package by a factor of 4-5x, and other JSON encoding packages by a factor of 2-3x.

easyjson aims to keep generated Go code simple enough so that it can be easily optimized or fixed. Another goal is to provide users with the ability to customize the generated code by providing options not available with the standard encoding/json package, such as generating “snake_case” names or enabling omitempty behavior by default.



# for Go < 1.17
go get -u


# for Go >= 1.17
go get && go install


easyjson -all <file>.go

The above will generate <file>_easyjson.go containing the appropriate marshaler and unmarshaler funcs for all structs contained in <file>.go.

Please note that easyjson requires a full Go build environment and the GOPATH environment variable to be set. This is because easyjson code generation invokes go run on a temporary file (an approach to code generation borrowed from ffjson).


someStruct := &SomeStruct{Field1: "val1", Field2: "val2"}
rawBytes, err := easyjson.Marshal(someStruct)


someStruct := &SomeStruct{}
err := easyjson.Unmarshal(rawBytes, someStruct)

Please see the GoDoc for more information and features.


Usage of easyjson:
    	generate marshaler/unmarshalers for all structs in a file
  -build_tags string
        build tags to add to generated file
  -gen_build_flags string
        build flags when running the generator while bootstrapping
        use simple bytes instead of Base64Bytes for slice of bytes
    	do not delete temporary files
    	don't generate MarshalJSON/UnmarshalJSON funcs
    	do not run 'gofmt -w' on output file
    	omit empty fields by default
  -output_filename string
    	specify the filename of the output
    	process the whole package instead of just the given file
    	use snake_case names instead of CamelCase by default
        use lowerCamelCase instead of CamelCase by default
    	only generate stubs for marshaler/unmarshaler funcs
        return error if some unknown field in json appeared
        disable unescaping of \uXXXX string sequences in member names

Using -all will generate marshalers/unmarshalers for all Go structs in the file excluding those structs whose preceding comment starts with easyjson:skip. For example:

type A struct {}

If -all is not provided, then only those structs whose preceding comment starts with easyjson:json will have marshalers/unmarshalers generated. For example:

type A struct {}

Additional option notes:

  • -snake_case tells easyjson to generate snake_case field names by default (unless overridden by a field tag). The CamelCase to snake_case conversion algorithm should work in most cases (ie, HTTPVersion will be converted to “http_version”).

  • -build_tags will add the specified build tags to generated Go sources.

  • -gen_build_flags will execute the easyjson bootstapping code to launch the actual generator command with provided flags. Multiple arguments should be separated by space e.g. -gen_build_flags="-mod=mod -x".

Structure json tag options

Besides standard json tag options like ‘omitempty’ the following are supported:

  • ‘nocopy’ - disables allocation and copying of string values, making them refer to original json buffer memory. This works great for short lived objects which are not hold in memory after decoding and immediate usage. Note if string requires unescaping it will be processed as normally.
  • ‘intern’ - string “interning” (deduplication) to save memory when the very same string dictionary values are often met all over the structure. See below for more details.

Generated Marshaler/Unmarshaler Funcs

For Go struct types, easyjson generates the funcs MarshalEasyJSON / UnmarshalEasyJSON for marshaling/unmarshaling JSON. In turn, these satisfy the easyjson.Marshaler and easyjson.Unmarshaler interfaces and when used in conjunction with easyjson.Marshal / easyjson.Unmarshal avoid unnecessary reflection / type assertions during marshaling/unmarshaling to/from JSON for Go structs.

easyjson also generates MarshalJSON and UnmarshalJSON funcs for Go struct types compatible with the standard json.Marshaler and json.Unmarshaler interfaces. Please be aware that using the standard json.Marshal / json.Unmarshal for marshaling/unmarshaling will incur a significant performance penalty when compared to using easyjson.Marshal / easyjson.Unmarshal.

Additionally, easyjson exposes utility funcs that use the MarshalEasyJSON and UnmarshalEasyJSON for marshaling/unmarshaling to and from standard readers and writers. For example, easyjson provides easyjson.MarshalToHTTPResponseWriter which marshals to the standard http.ResponseWriter. Please see the GoDoc listing for the full listing of utility funcs that are available.

Controlling easyjson Marshaling and Unmarshaling Behavior

Go types can provide their own MarshalEasyJSON and UnmarshalEasyJSON funcs that satisfy the easyjson.Marshaler / easyjson.Unmarshaler interfaces. These will be used by easyjson.Marshal and easyjson.Unmarshal when defined for a Go type.

Go types can also satisfy the easyjson.Optional interface, which allows the type to define its own omitempty logic.

Type Wrappers

easyjson provides additional type wrappers defined in the easyjson/opt package. These wrap the standard Go primitives and in turn satisfy the easyjson interfaces.

The easyjson/opt type wrappers are useful when needing to distinguish between a missing value and/or when needing to specifying a default value. Type wrappers allow easyjson to avoid additional pointers and heap allocations and can significantly increase performance when used properly.

Memory Pooling

easyjson uses a buffer pool that allocates data in increasing chunks from 128 to 32768 bytes. Chunks of 512 bytes and larger will be reused with the help of sync.Pool. The maximum size of a chunk is bounded to reduce redundant memory allocation and to allow larger reusable buffers.

easyjson's custom allocation buffer pool is defined in the easyjson/buffer package, and the default behavior pool behavior can be modified (if necessary) through a call to buffer.Init() prior to any marshaling or unmarshaling. Please see the GoDoc listing for more information.

String interning

During unmarshaling, string field values can be optionally interned to reduce memory allocations and usage by deduplicating strings in memory, at the expense of slightly increased CPU usage.

This will work effectively only for string fields being decoded that have frequently the same value (e.g. if you have a string field that can only assume a small number of possible values).

To enable string interning, add the intern keyword tag to your json tag on string fields, e.g.:

type Foo struct {
  UUID  string `json:"uuid"`         // will not be interned during unmarshaling
  State string `json:"state,intern"` // will be interned during unmarshaling

Issues, Notes, and Limitations

  • easyjson is still early in its development. As such, there are likely to be bugs and missing features when compared to encoding/json. In the case of a missing feature or bug, please create a GitHub issue. Pull requests are welcome!

  • Unlike encoding/json, object keys are case-sensitive. Case-insensitive matching is not currently provided due to the significant performance hit when doing case-insensitive key matching. In the future, case-insensitive object key matching may be provided via an option to the generator.

  • easyjson makes use of unsafe, which simplifies the code and provides significant performance benefits by allowing no-copy conversion from []byte to string. That said, unsafe is used only when unmarshaling and parsing JSON, and any unsafe operations / memory allocations done will be safely deallocated by easyjson. Set the build tag easyjson_nounsafe to compile it without unsafe.

  • easyjson is compatible with Google App Engine. The appengine build tag (set by App Engine‘s environment) will automatically disable the use of unsafe, which is not allowed in App Engine’s Standard Environment. Note that the use with App Engine is still experimental.

  • Floats are formatted using the default precision from Go's strconv package. As such, easyjson will not correctly handle high precision floats when marshaling/unmarshaling JSON. Note, however, that there are very few/limited uses where this behavior is not sufficient for general use. That said, a different package may be needed if precise marshaling/unmarshaling of high precision floats to/from JSON is required.

  • While unmarshaling, the JSON parser does the minimal amount of work needed to skip over unmatching parens, and as such full validation is not done for the entire JSON value being unmarshaled/parsed.

  • Currently there is no true streaming support for encoding/decoding as typically for many uses/protocols the final, marshaled length of the JSON needs to be known prior to sending the data. Currently this is not possible with easyjson's architecture.

  • easyjson parser and codegen based on reflection, so it won't work on package main files, because they cant be imported by parser.


Most benchmarks were done using the example 13kB example JSON (9k after eliminating whitespace). This example is similar to real-world data, is well-structured, and contains a healthy variety of different types, making it ideal for JSON serialization benchmarks.


  • For small request benchmarks, an 80 byte portion of the above example was used.

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

  • Benchmarks are showing the results of easyjson's default behaviour, which makes use of unsafe.

Benchmarks are available in the repository and can be run by invoking make.

easyjson vs. encoding/json

easyjson is roughly 5-6 times faster than the standard encoding/json for unmarshaling, and 3-4 times faster for non-concurrent marshaling. Concurrent marshaling is 6-7x faster if marshaling to a writer.

easyjson vs. ffjson

easyjson uses the same approach for JSON marshaling as ffjson, but takes a significantly different approach to lexing and parsing JSON during unmarshaling. This means easyjson is roughly 2-3x faster for unmarshaling and 1.5-2x faster for non-concurrent unmarshaling.

As of this writing, ffjson seems to have issues when used concurrently: specifically, large request pooling hurts ffjson's performance and causes scalability issues. These issues with ffjson can likely be fixed, but as of writing remain outstanding/known issues with ffjson.

easyjson and ffjson have similar performance for small requests, however easyjson outperforms ffjson by roughly 2-5x times for large requests when used with a writer.

easyjson vs. go/codec

go/codec provides compile-time helpers for JSON generation. In this case, helpers do not work like marshalers as they are encoding-independent.

easyjson is generally 2x faster than go/codec for non-concurrent benchmarks and about 3x faster for concurrent encoding (without marshaling to a writer).

In an attempt to measure marshaling performance of go/codec (as opposed to allocations/memcpy/writer interface invocations), a benchmark was done with resetting length 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 marshaling 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 important to note that the resulting object for python is slower to access, since the library parses JSON object into dictionaries.

easyjson is slightly faster for unmarshaling and 2-3x faster than ujson for marshaling.

Benchmark Results

ffjson results are from February 4th, 2016, using the latest ffjson and go1.6. go/codec results are from March 4th, 2016, using the latest go/codec and go1.6.


libjson sizeMB/sallocs/opB/op

Marshaling, one goroutine.

libjson sizeMB/sallocs/opB/op

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

Marshaling, concurrent.

libjson sizeMB/sallocs/opB/op

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