`json.Marshal()`将结构体的数组字段转换为映射，这是不应该的。

huangapple
go评论117阅读模式
英文:

`json.Marshal()` converts an array field of a struct to map which it shouldn't

问题

我想将来自github.com/compose-spec的types.Project对象序列化为JSON。

然而，这个结构体中应该是一个数组的字段总是被序列化为一个映射。

  1. package main
  3. import (
  4. 	"encoding/json"
  6. 	types "github.com/compose-spec/compose-go/types"
  7. )
  9. func main() {
  10. 	out := types.Project{
  11. 		Name: "foo",
  12. 		Services: []types.ServiceConfig{ // <- 注意这个字段是一个数组
  13. 			{
  14. 				Name:  "bar",
  15. 				Image: "hello-world",
  16. 			},
  17. 		},
  18. 	}
  20. 	buf, err := json.Marshal(out)
  21. 	if err != nil {
  22. 		panic(err)
  23. 	}
  25. 	println(string(buf)) // <- 注意Services字段现在是一个映射，这是错误的！
  27. 	var in types.Project
  28. 	if err := json.Unmarshal(buf, &in); err != nil {
  29. 		panic(err)
  30. 	}
  31. }

代码运行失败：

  1. {"name":"foo","services":{"bar":{"command":null,"entrypoint":null,"image":"hello-world"}}}
  2. panic: json: cannot unmarshal object into Go struct field Project.services of type types.Services
  4. goroutine 1 [running]:
  5. main.main()
  6. 	/tmp/sandbox1081064727/prog.go:29 +0x168

out对象被序列化为：

  1. {
  2.     "name": "foo",
  3.     "services": {
  4.         "bar": {
  5.             "command": null,
  6.             "entrypoint": null,
  7.             "image": "hello-world"
  8.         }
  9.     }
  10. }

实际上应该是这样的：

  1. {
  2.     "name": "foo",
  3.     "services": [
  4.         {
  5.             "name": "bar",
  6.             "command": null,
  7.             "entrypoint": null,
  8.             "image": "hello-world"
  9.         }
  10.     ]
  11. }
英文:

I want to serialize object of types.Project from github.com/compose-spec into JSON

However, a field of ths struct which is supposed to be array, is always serialized into map.

  1. package main
  3. import (
  4. 	&quot;encoding/json&quot;
  6. 	types &quot;github.com/compose-spec/compose-go/types&quot;
  7. )
  9. func main() {
  10. 	out := types.Project{
  11. 		Name: &quot;foo&quot;,
  12. 		Services: []types.ServiceConfig{ // &lt;- notice this field is an array
  13. 			{
  14. 				Name:  &quot;bar&quot;,
  15. 				Image: &quot;hello-world&quot;,
  16. 			},
  17. 		},
  18. 	}
  20. 	buf, err := json.Marshal(out)
  21. 	if err != nil {
  22. 		panic(err)
  23. 	}
  25. 	println(string(buf)) // &lt;- notice the Services field now a map, which is incorrect!
  27. 	var in types.Project
  28. 	if err := json.Unmarshal(buf, &amp;in); err != nil {
  29. 		panic(err)
  30. 	}
  31. }

The code fails to run:

  1. {&quot;name&quot;:&quot;foo&quot;,&quot;services&quot;:{&quot;bar&quot;:{&quot;command&quot;:null,&quot;entrypoint&quot;:null,&quot;image&quot;:&quot;hello-world&quot;}}}
  2. panic: json: cannot unmarshal object into Go struct field Project.services of type types.Services
  4. goroutine 1 [running]:
  5. main.main()
  6. 	/tmp/sandbox1081064727/prog.go:29 +0x168

The out object is serialized as

  1. {
  2.     &quot;name&quot;: &quot;foo&quot;,
  3.     &quot;services&quot;: {
  4.         &quot;bar&quot;: {
  5.             &quot;command&quot;: null,
  6.             &quot;entrypoint&quot;: null,
  7.             &quot;image&quot;: &quot;hello-world&quot;
  8.         }
  9.     }
  10. }

which should really be something like

  1. {
  2.     &quot;name&quot;: &quot;foo&quot;,
  3.     &quot;services&quot;: [
  4.         {
  5.             &quot;name&quot;: &quot;bar&quot;,
  6.             &quot;command&quot;: null,
  7.             &quot;entrypoint&quot;: null,
  8.             &quot;image&quot;: &quot;hello-world&quot;
  9.         }
  10.     ]
  11. }

答案1

得分: 2

你看到的行为是正确的，符合compose文件规范，规范中指出：

Compose文件必须声明一个services根元素，其键是服务名称的字符串表示，值是服务定义。

将列表转换为映射是通过compose-go/types.go中的自定义编组器实现的：

  1. // MarshalYAML使Services实现yaml.Marshaller
  2. func (s Services) MarshalYAML() (interface{}, error) {
  3.     services := map[string]ServiceConfig{}
  4.     for _, service := range s {
  5.         services[service.Name] = service
  6.     }
  7.     return services, nil
  8. }
  10. // MarshalJSON使Services实现json.Marshaler
  11. func (s Services) MarshalJSON() ([]byte, error) {
  12.     data, err := s.MarshalYAML()
  13.     if err != nil {
  14.         return nil, err
  15.     }
  16.     return json.MarshalIndent(data, "", "  ")
  17. }
英文:

The behavior you're seeing is correct with respect to the compose file specification, which says:

> A Compose file MUST declare a services root element as a map whose keys are string representations of service names, and whose values are service definitions.

The transformation of the list to a map is implemented by the custom marshalers in compose-go/types.go:

  1. // MarshalYAML makes Services implement yaml.Marshaller
  2. func (s Services) MarshalYAML() (interface{}, error) {
  3. 	services := map[string]ServiceConfig{}
  4. 	for _, service := range s {
  5. 		services[service.Name] = service
  6. 	}
  7. 	return services, nil
  8. }
  10. // MarshalJSON makes Services implement json.Marshaler
  11. func (s Services) MarshalJSON() ([]byte, error) {
  12. 	data, err := s.MarshalYAML()
  13. 	if err != nil {
  14. 		return nil, err
  15. 	}
  16. 	return json.MarshalIndent(data, &quot;&quot;, &quot;  &quot;)
  17. }

huangapple
  • 本文由 发表于 2023年2月16日 12:27:26
  • 转载请务必保留本文链接：https://go.coder-hub.com/75467868.html
  • docker-compose
  • go
  • json
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