json

重写MarshalJSON

修改结构内某一属性

直接修改结构的值会引发无穷递归导致 panic：

  1. type User struct {
  2.     Name string `json:"name"`
  3. }
  5. func (u User) MarshalJSON() ([]byte, error) {
  6.     u.Name = "li"
  8.     return json.Marshal(u) // panic
  9. }
  11. func TestPanic(t *testing.T) {
  12.     res,err:= json.Marshal(User{Name: "wang"})
  13.     fmt.Println(string(res), err)
  14. }

🌈 通过重命名一个 Alias 解决：

  1. type User struct {
  2.     Name   string   `json:"name"`
  3.     Gender string   `json:"gender"`
  4.     Role   []string `json:"roles"`
  5. }
  7. func (u User) MarshalJSON() ([]byte, error) {
  8.     type Alias User
  9.     v := struct {
  10.         Alias
  11.         Age int `json:"age"`
  12.     }{
  13.         Alias: Alias(u),
  14.         Age:   18,
  15.     }
  16.     if v.Role == nil {
  17.         v.Role = []string{}
  18.     }
  20.     return json.Marshal(v)
  21. }
  23. func TestAlias(t *testing.T) {
  24.     res, err := json.Marshal(User{Name: "wang", Gender: "man"})
  25.     fmt.Println(string(res), err)
  26.     // {"name":"wang","gender":"man","roles":[],"age":18} <nil>
  27. }

匿名字段重写MarshalJSON，顶层结构的MarshalJSON失效

常规 Alias 会导致最终调用匿名字段的 MarshalJSON：

  1. type User struct {
  2.     Name string `json:"name"`
  3. }
  4. func (u User) MarshalJSON() ([]byte, error) {
  5.     type Alias User
  6.     v := struct {
  7.         Alias
  8.         Age int `json:"age"`
  9.     }{
  10.         Alias: Alias(u), Age:   18,
  11.     }
  12.     return json.Marshal(v) 
  13. }
  15. type UserPermissions struct {
  16.     User
  17.     Permissions []string `json:"permissions"`
  18. }
  20. func (u UserPermissions) MarshalJSON() ([]byte, error) {
  21.     type Alias UserPermissions
  22.     v := struct {Alias}{Alias: Alias(u)}
  23.     return json.Marshal(v)
  24. }
  26. func TestUserPermissions(t *testing.T) {
  27.     res, err := json.Marshal(UserPermissions{User: User{Name: "wang"}, Permissions: []string{"one", "two"}})
  28.     fmt.Println(string(res), err)
  29.     // {"name":"wang","age":18} <nil>
  30. }

🌈 通过一个中间map来解决：
感谢：
Struct含有匿名字段，该匿名字段有MarshalJSON方法，导致Struct不能正确转换成json字符串！！
Idiomatic way to embed struct with custom MarshalJSON() method

  1. type User struct {
  2.     Name string `json:"name"`
  3. }
  4. func (u User) MarshalJSON() ([]byte, error) {
  5.     type Alias User
  6.     v := struct {
  7.         Alias
  8.         Age int `json:"age"`
  9.     }{
  10.         Alias: Alias(u),
  11.         Age:   18,
  12.     }
  13.     return json.Marshal(v) 
  14. }
  16. type UserPermissions struct {
  17.     User
  18.     Permissions []string `json:"permissions"`
  19. }
  21. func (u UserPermissions) MarshalJSON() ([]byte, error) {
  22.     pJSON, err := u.User.MarshalJSON()
  23.     if err != nil {
  24.         return nil, err
  25.     }
  26.     eJSON, err := json.Marshal(map[string]interface{}{
  27.         "permissions": u.Permissions,
  28.     })
  29.     if err != nil {
  30.         return nil, err
  31.     }
  32.     eJSON[0] = ','
  33.     return append(pJSON[:len(pJSON)-1], eJSON...), nil
  34. }
  36. func TestUserPermissions(t *testing.T) {
  37.     res, err := json.Marshal(UserPermissions{User: User{Name: "wang"}, Permissions: []string{"one", "two"}})
  38.     fmt.Println(string(res), err)
  39.     // {"name":"wang","age":18,"permissions":["one","two"]} <nil>
  40. }