json v2 omit zero behavior

[lpic10]

Hello,

When using json v2, we need to set OmitZeroStructFields to true during unmarshal.
Another option is to replace omitempty by omitzero in the struct tag. Note that the behavior does not change for strings, slices or maps.

See the example below:

package main

import (
        "encoding/json"
        "fmt"

        jsonv2 "encoding/json/v2"

        "github.com/oapi-codegen/nullable"
)

type InnerStruct struct {
        AnotherValue string
}

type TestStruct struct {
        Name             string                                    `json:"name"`
        ValueInt         nullable.Nullable[int]                    `json:"valueInt,omitempty"`
        ValueString      nullable.Nullable[string]                 `json:"valueString,omitempty"`
        ValueStruct      nullable.Nullable[InnerStruct]            `json:"valueStruct,omitempty"`
        ValueMap         nullable.Nullable[map[string]string]      `json:"valueMap,omitempty"`
        ValueMapStruct   nullable.Nullable[map[string]InnerStruct] `json:"valueMapStruct,omitempty"`
        ValueSliceInt    nullable.Nullable[[]int]                  `json:"valueSliceInt,omitempty"`
        ValueSliceString nullable.Nullable[[]string]               `json:"valueSliceString,omitempty"`
        ValueSliceStruct nullable.Nullable[[]InnerStruct]          `json:"valueSliceStruct,omitempty"`
}

func main() {
        var testStruct TestStruct

        testStruct.Name = "test struct"

        buf, _ := json.Marshal(&testStruct)
        buf2, _ := jsonv2.Marshal(&testStruct)
        buf3, _ := jsonv2.Marshal(&testStruct, jsonv2.OmitZeroStructFields(true))

        fmt.Println(string(buf))
        fmt.Println(string(buf2))
        fmt.Println(string(buf3))
}

Then running (requires go 1.25):

GOEXPERIMENT=jsonv2 go run jsontest.go

{"name":"test struct"}
{"name":"test struct","valueInt":0,"valueStruct":{"AnotherValue":""}}
{"name":"test struct"}

[lpic10]

Updated, we need to set OmitEmptyWithLegacySemantics, as the OmitZeroStructFields will have unintended effects, see below

package main

import (
        "encoding/json"
        "fmt"

        jsonv2 "encoding/json/v2"

        "github.com/oapi-codegen/nullable"
)

type InnerStruct struct {
        AnotherValue string
}

type TestStruct struct {
        Name             string                                    `json:"name"`
        Bool             bool                                      `json:"bool"`
        ValueInt         nullable.Nullable[int]                    `json:"valueInt,omitempty"`
        ValueString      nullable.Nullable[string]                 `json:"valueString,omitempty"`
        ValueStruct      nullable.Nullable[InnerStruct]            `json:"valueStruct,omitempty"`
        ValueMap         nullable.Nullable[map[string]string]      `json:"valueMap,omitempty"`
        ValueMapStruct   nullable.Nullable[map[string]InnerStruct] `json:"valueMapStruct,omitempty"`
        ValueSliceInt    nullable.Nullable[[]int]                  `json:"valueSliceInt,omitempty"`
        ValueSliceString nullable.Nullable[[]string]               `json:"valueSliceString,omitempty"`
        ValueSliceStruct nullable.Nullable[[]InnerStruct]          `json:"valueSliceStruct,omitempty"`
}

func main() {
        var testStruct TestStruct

        testStruct.Name = "test struct"

        buf, _ := json.Marshal(&testStruct)
        buf2, _ := jsonv2.Marshal(&testStruct)
        buf3, _ := jsonv2.Marshal(&testStruct, jsonv2.OmitZeroStructFields(true))
        buf4, _ := jsonv2.Marshal(&testStruct, json.OmitEmptyWithLegacySemantics(true))

        fmt.Println(string(buf))
        fmt.Println(string(buf2))
        fmt.Println(string(buf3))
        fmt.Println(string(buf4))
}

GOEXPERIMENT=jsonv2 go run jsontest.go

{"name":"test struct","bool":false}
{"name":"test struct","bool":false,"valueInt":0,"valueStruct":{"AnotherValue":""}}
{"name":"test struct"}
{"name":"test struct","bool":false}