Gson補充

1.Gson支持格式化的打印

Gson默認使用toJson()的時候,打印出來的json是壓縮的,也就是節點之間沒有空格的,例如，這樣:

{"album_id":"ddd","iamge_file":"dfkdlfdf","image_title":"lfjdlfjdlfjdljfldfdf","tags":[]}

可以通過開啓格式化,如:

Gson gson = new GsonBuilder().setPrettyPrinting().create();
String jsonOutput = gson.toJson(someObject);

打印出來的結果就會變成這樣,方便在調試的時候查看json.

{
  "album_id": "ddd",
  "iamge_file": "dfkdlfdf",
  "image_title": "lfjdlfjdlfjdljfldfdf",
  "tags": []
}

2.Gson支持空對象

Gson在把Java對象解析成Json的時候是默認忽略掉null的字段，當我們需要把這些值爲null的字段也顯示出來的時候，我們可以JsonBuilder的serializeNulls()方法來解析這些空值.例如:

public class Dog {
    private String name;
    public Dog(){}
    public Dog(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
  public static void main(String[] args) {
        GsonBuilder builder=new GsonBuilder();
        //builder.serializeNulls();
        Gson gson=builder.create();
        Dog dog1=new Dog();
        String json = gson.toJson(dog1);
        System.out.println(json);
    }
}

注掉builder.serializeNulls();跟打開這行代碼的結果:
注掉:
{}
打開:
{"name":null}

3.Gson支持版本

同一個Java類可以使用@Since註解來區分不同的版本.如果沒有版本註解，默認解析所有的屬性.下面是demo:

public class VersionedClass {
  @Since(1.1) private final String newerField;
  @Since(1.0) private final String newField;
  private final String field;

  public VersionedClass() {
    this.newerField = "newer";
    this.newField = "new";
    this.field = "old";
  }
}

例子:

VersionedClass someObject = new VersionedClass();
        Gson gson = new GsonBuilder().setVersion(1.0).create();
        String jsonOutput = gson.toJson(someObject);
        System.out.println("1.0版本:"+jsonOutput);
        System.out.println();

        gson = new GsonBuilder().setVersion(1.1).create();
        System.out.println("1.1版本:"+gson.toJson(someObject));
        System.out.println();

        gson = new Gson();
        jsonOutput = gson.toJson(someObject);
        System.out.println("默認:"+jsonOutput);

打印的結果:

1.0版本:{"newField":"new","field":"old"}

1.1版本:{"newerField":"newer","newField":"new","field":"old"}

默認:{"newerField":"newer","newField":"new","field":"old"}

4.支持屬性別名

可以使用@SerializedName註解爲屬性添加別名
例子:

public  class SomeObject {
  @SerializedName("custom_naming") private final String someField;
  private final String someOtherField;

  public SomeObject(String a, String b) {
    this.someField = a;
    this.someOtherField = b;
  }
   public static void main(String[] args) {
      SomeObject someObject = new SomeObject("first", "second");
      Gson gson = new GsonBuilder().create();
      String jsonRepresentation = gson.toJson(someObject);
      System.out.println(jsonRepresentation);
}
}

打印的結果:

{"custom_naming":"first","someOtherField":"second"}

也可以使用一些Gson內置的轉換名字策略
- FieldNamingPolicy.IDENTITY
- FieldNamingPolicy.LOWER_CASE_WITH_DASHES
- FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES
- FieldNamingPolicy.UPPER_CAMEL_CASE
- FieldNamingPolicy.UPPER_CAMEL_CASE_WITH_SPACES

1.FieldNamingPolicy.IDENTITY
就是跟java類中一致

2.FieldNamingPolicy.LOWER_CASE_WITH_DASHES
例如:
- someFieldName —> some-field-name
- _someFieldName —> _some-field-name
- aStringField —> a-string-field
- aURL —> a-u-r-l

3.FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES
例如:
- someFieldName —> some_field_name
- _someFieldName —> _some_field_name
- aStringField —> a_string_field
- aURL —> a_u_r_l

4.FieldNamingPolicy.UPPER_CAMEL_CASE
例如:
- someFieldName —> SomeFieldName
- _someFieldName —> _SomeFieldName

5.FieldNamingPolicy.UPPER_CAMEL_CASE_WITH_SPACES
例如:
- someFieldName —> Some Field Name
- _someFieldName —> _Some Field Name

使用的方式:

        SomeObject someObject = new SomeObject("first", "second");
        Gson gson = new GsonBuilder().setFieldNamingPolicy(
                FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES).create();
        String jsonRepresentation = gson.toJson(someObject);
        System.out.println(jsonRepresentation);

打印結果:

{"custom_naming":"first","some_other_field":"second"}

注意前者的優先級高於後者，當使用前者的時候，按照前者的設置

參考的是Google官方的教程以及Api文檔.