fastjson - fast JSON parser and validator for Go

Features

• Fast. As usual, up to 15x faster than the standard encoding/json. See benchmarks.
• Parses arbitrary JSON without schema, reflection, struct magic and code generation contrary to easyjson.
• Provides simple API.
• Outperforms jsonparser and gjson when accessing multiple unrelated fields, since fastjson parses the input JSON only once.
• Validates the parsed JSON unlike jsonparser and gjson.
• May quickly extract a part of the original JSON with Value.Get(...).MarshalTo.
• May parse array containing values with distinct types (aka non-homogenous types). For instance, fastjson easily parses the following JSON array [123, "foo", [456], {"k": "v"}, null].
• fastjson preserves the original order of object items when calling Object.Visit.

Known limitations

• Requies extra care to work with - references to certain objects recursively returned by Parser must be released before the next call to Parse. Otherwise the program may work improperly. Adhere recommendations from docs.
• Cannot parse JSON from io.Reader. There is Scanner for parsing stream of JSON values from a string.

Usage

One-liner accessing a single field:

	s := []byte(`{"foo": [123, "bar"]}`)
	fmt.Printf("foo.0=%d\n", fastjson.GetInt(s, "foo", "0"))

	// Output:
	// foo.0=123

Accessing multiple fields with error handling:

        var p fastjson.Parser
        v, err := p.Parse(`{
                "str": "bar",
                "int": 123,
                "float": 1.23,
                "bool": true,
                "arr": [1, "foo", {}]
        }`)
        if err != nil {
                log.Fatal(err)
        }
        fmt.Printf("foo=%s\n", v.GetStringBytes("str"))
        fmt.Printf("int=%d\n", v.GetInt("int"))
        fmt.Printf("float=%f\n", v.GetFloat64("float"))
        fmt.Printf("bool=%v\n", v.GetBool("bool"))
        fmt.Printf("arr.1=%s\n", v.GetStringBytes("arr", "1"))

        // Output:
        // foo=bar
        // int=123
        // float=1.230000
        // bool=true
        // arr.1=foo

See also examples.

Security

• fastjson shouldn't crash or panic when parsing input strings specially crafted by an attacker. It must return error on invalid input JSON.
• fastjson requires up to sizeof(Value) * len(inputJSON) bytes of memory for parsing inputJSON string. Limit the maximum size of the inputJSON before parsing it in order to limit the maximum memory usage.

Performance optimization tips

• Re-use Parser and Scanner for parsing many JSONs. This reduces memory allocations overhead. ParserPool may be useful in this case.
• Prefer calling Value.Get* on the value returned from Parser instead of calling Get* one-liners when multiple fields must be obtained from JSON, since each Get* one-liner re-parses the input JSON again.
• Prefer calling once Value.Get for common prefix paths and then calling Value.Get* on the returned value for distinct suffix paths.
• Prefer iterating over array returned from Value.GetArray with a range loop instead of calling Value.Get* for each array item.

Benchmarks

Go 1.11 has been used for benchmarking.

Legend:

• small - parse small.json (190 bytes).

• medium - parse medium.json (2.3KB).

• large - parse large.json (28KB).

• canada - parse canada.json (2.2MB).

• citm - parse citm_catalog.json (1.7MB).

• twitter - parse twitter.json (617KB).

• stdjson-map - parse into a map[string]interface{} using encoding/json.

• stdjson-struct - parse into a struct containing a subset of fields of the parsed JSON, using encoding/json.

• stdjson-empty-struct - parse into an empty struct using encoding/json. This is the fastest possible solution for encoding/json, may be used for json validation. See also benchmark results for json validation.

• fastjson - parse using fastjson without fields access.

• fastjson-get - parse using fastjson with fields access similar to stdjson-struct.

$ GOMAXPROCS=1 go test github.com/valyala/fastjson -bench='Parse$'
goos: linux
goarch: amd64
pkg: github.com/valyala/fastjson
BenchmarkParse/small/stdjson-map         	  200000	      7249 ns/op	  26.21 MB/s	     960 B/op	      51 allocs/op
BenchmarkParse/small/stdjson-struct      	  500000	      3731 ns/op	  50.92 MB/s	     224 B/op	       4 allocs/op
BenchmarkParse/small/stdjson-empty-struct         	  500000	      2566 ns/op	  74.04 MB/s	     168 B/op	       2 allocs/op
BenchmarkParse/small/fastjson                     	 3000000	       411 ns/op	 461.69 MB/s	       0 B/op	       0 allocs/op
BenchmarkParse/small/fastjson-get                 	 2000000	       674 ns/op	 281.73 MB/s	       0 B/op	       0 allocs/op
BenchmarkParse/medium/stdjson-map                 	   30000	     41120 ns/op	  56.64 MB/s	   10195 B/op	     208 allocs/op
BenchmarkParse/medium/stdjson-struct              	   30000	     47793 ns/op	  48.73 MB/s	    9174 B/op	     258 allocs/op
BenchmarkParse/medium/stdjson-empty-struct        	  100000	     20962 ns/op	 111.10 MB/s	     280 B/op	       5 allocs/op
BenchmarkParse/medium/fastjson                    	  500000	      3499 ns/op	 665.50 MB/s	       0 B/op	       0 allocs/op
BenchmarkParse/medium/fastjson-get                	  300000	      3679 ns/op	 632.94 MB/s	       0 B/op	       0 allocs/op
BenchmarkParse/large/stdjson-map                  	    2000	    613724 ns/op	  45.82 MB/s	  210761 B/op	    2785 allocs/op
BenchmarkParse/large/stdjson-struct               	    5000	    284621 ns/op	  98.79 MB/s	   15616 B/op	     353 allocs/op
BenchmarkParse/large/stdjson-empty-struct         	    5000	    250047 ns/op	 112.45 MB/s	     280 B/op	       5 allocs/op
BenchmarkParse/large/fastjson                     	   30000	     48737 ns/op	 576.93 MB/s	       9 B/op	       0 allocs/op
BenchmarkParse/large/fastjson-get                 	   30000	     49540 ns/op	 567.58 MB/s	       9 B/op	       0 allocs/op
BenchmarkParse/canada/stdjson-map                 	      20	  67939960 ns/op	  33.13 MB/s	12260502 B/op	  392539 allocs/op
BenchmarkParse/canada/stdjson-struct              	      20	  67322448 ns/op	  33.44 MB/s	12260123 B/op	  392534 allocs/op
BenchmarkParse/canada/stdjson-empty-struct        	     100	  17227143 ns/op	 130.67 MB/s	     280 B/op	       5 allocs/op
BenchmarkParse/canada/fastjson                    	     200	   6082885 ns/op	 370.06 MB/s	  472007 B/op	     571 allocs/op
BenchmarkParse/canada/fastjson-get                	     200	   6038088 ns/op	 372.81 MB/s	  472007 B/op	     571 allocs/op
BenchmarkParse/citm/stdjson-map                   	      50	  27171438 ns/op	  63.57 MB/s	 5213923 B/op	   95401 allocs/op
BenchmarkParse/citm/stdjson-struct                	     100	  14270811 ns/op	 121.03 MB/s	    1989 B/op	      75 allocs/op
BenchmarkParse/citm/stdjson-empty-struct          	     100	  14335648 ns/op	 120.48 MB/s	     280 B/op	       5 allocs/op
BenchmarkParse/citm/fastjson                      	    1000	   2026047 ns/op	 852.50 MB/s	   17633 B/op	      30 allocs/op
BenchmarkParse/citm/fastjson-get                  	    1000	   2025209 ns/op	 852.85 MB/s	   17633 B/op	      30 allocs/op
BenchmarkParse/twitter/stdjson-map                	     100	  11230390 ns/op	  56.23 MB/s	 2187443 B/op	   31264 allocs/op
BenchmarkParse/twitter/stdjson-struct             	     300	   5411600 ns/op	 116.70 MB/s	     408 B/op	       6 allocs/op
BenchmarkParse/twitter/stdjson-empty-struct       	     300	   5347742 ns/op	 118.09 MB/s	     408 B/op	       6 allocs/op
BenchmarkParse/twitter/fastjson                   	    2000	    834384 ns/op	 756.86 MB/s	    2536 B/op	       2 allocs/op
BenchmarkParse/twitter/fastjson-get               	    2000	    827723 ns/op	 762.95 MB/s	    2536 B/op	       2 allocs/op

Benchmark results for json validation:

$ GOMAXPROCS=1 go test github.com/valyala/fastjson -bench='Validate$'
goos: linux
goarch: amd64
pkg: github.com/valyala/fastjson
BenchmarkValidate/small/stdjson 	 2000000	       846 ns/op	 224.51 MB/s	      72 B/op	       2 allocs/op
BenchmarkValidate/small/fastjson         	 5000000	       352 ns/op	 539.57 MB/s	       0 B/op	       0 allocs/op
BenchmarkValidate/medium/stdjson         	  200000	      9783 ns/op	 238.05 MB/s	     184 B/op	       5 allocs/op
BenchmarkValidate/medium/fastjson        	  500000	      3297 ns/op	 706.34 MB/s	       0 B/op	       0 allocs/op
BenchmarkValidate/large/stdjson          	   10000	    119264 ns/op	 235.76 MB/s	     184 B/op	       5 allocs/op
BenchmarkValidate/large/fastjson         	   30000	     48079 ns/op	 584.82 MB/s	       0 B/op	       0 allocs/op
BenchmarkValidate/canada/stdjson         	     200	   8229071 ns/op	 273.55 MB/s	     184 B/op	       5 allocs/op
BenchmarkValidate/canada/fastjson        	     500	   3662160 ns/op	 614.68 MB/s	       0 B/op	       0 allocs/op
BenchmarkValidate/citm/stdjson           	     200	   6760134 ns/op	 255.50 MB/s	     184 B/op	       5 allocs/op
BenchmarkValidate/citm/fastjson          	    1000	   1581792 ns/op	1091.93 MB/s	       0 B/op	       0 allocs/op
BenchmarkValidate/twitter/stdjson        	     500	   2596556 ns/op	 243.21 MB/s	     312 B/op	       6 allocs/op
BenchmarkValidate/twitter/fastjson       	    2000	    889861 ns/op	 709.68 MB/s	       0 B/op	       0 allocs/op

FAQ

• Q: There are a ton of other high-perf packages for JSON parsing in Go. Why creating yet another package? A: Because other packages require either rigid JSON schema via struct magic and code generation or perform poorly when multiple unrelated fields must be obtained from the parsed JSON. Additionally, fastjson provides nicer API.

• Q: What is the main purpose for fastjson? A: High-perf JSON parsing for RTB and other JSON-RPC services.

• Q: Why fastjson doesn't provide fast marshaling (serialization)? A: Because other solutions exist. I'd recommend quicktemplate for high-performance JSON marshaling :)

• Q: fastjson crashes my program! A: There is high probability of improper use.

  • Make sure you don't hold references to objects recursively returned by Parser / Scanner beyond the next Parser.Parse / Scanner.Next call if such restriction is mentioned in docs.
  • Make sure you don't access fastjson objects from concurrently running goroutines if such restriction is mentioned in docs.
  • Build and run your program with -race flag. Make sure the race detector detects zero races.
  • If your program continue crashing after fixing issues mentioned above, file a bug.