自定义 JSON 解析 (Gson)

Question

How can I customize serialization/deserialization of a specific field?

Here's my json:

[{
"Short Name": "X",
"Address": "X Address",
"Latitude": 40.739982,
"Longitude": -73.978053,
"Type": "A, B"
},
{
"Short Name": "Y",
"Address": "Y Address",
"Latitude": 40.852856,
"Longitude": -73.938098,
"Type": "A, B, C, D"
}]

Using Gson, now I can parse the json and get a JAVA object with String type; but I want to split the Type string and get a type String[] .

public class MyClass {
    @SerializedName("Short Name")
    @Expose
    private String ShortName;

    @Expose
    private String Address;

    @Expose
    private Double Latitude;

    @Expose
    private Double Longitude; 

    @Expose
    private String Type; 
    // how can I split the string 
    // and get private String[] Type;
    // when deserializing
}

Answer 1

You need to implement a custom Type Adapter.

You can do in this way:

JsonDeserializer<String[]> deserializer = new JsonDeserializer<String[]>() {

    @Override
    public String[] deserialize(JsonElement jsonElem, Type type,
            JsonDeserializationContext context) throws JsonParseException {

        if(jsonElem == null) {
            return null;
        }

        return jsonElem.getAsString().split(", ");
    }

};

Gson gson = new GsonBuilder()
                    .registerTypeAdapter(String[].class, deserializer)
                    .create();

Type listType = new TypeToken<ArrayList<MyClass>>() {}.getType();
List<MyClass> t = gson.fromJson(json, listType);

And modify your MyClass bean from:

@Expose
private String Type;

to:

@Expose
private String[] Type;

In the same way you can add a serializer if you need to serialize MyClass object in order to convert the "Type" array to String comma separed!

Answer 2

Easy. Json allows you to use Numbers(Int/Long/Double), Strings, Lists or Objects. What you need is a list.

JSON:

"Type" : { "type": "array", "items" : {"type": "string"} }

eg "Type" : ["A","B","C","D"]

JAVA:

List<String> Type;

Later on you can use any Java List implementation such as ArrayList.

eg Type = new ArrayList();

Answer 3

You can use JsonAdapter for that. Here is example from ducumentation.

Here is an example of how this annotation is used:
   @JsonAdapter(UserJsonAdapter.class)
   public class User {
     public final String firstName, lastName;
     private User(String firstName, String lastName) {
       this.firstName = firstName;
       this.lastName = lastName;
     }
   }
   public class UserJsonAdapter extends TypeAdapter<User> {
     @Override public void write(JsonWriter out, User user) throws IOException {
       // implement write: combine firstName and lastName into name
       out.beginObject();
       out.name("name");
       out.value(user.firstName + " " + user.lastName);
       out.endObject();
       // implement the write method
     }
     @Override public User read(JsonReader in) throws IOException {
       // implement read: split name into firstName and lastName
       in.beginObject();
       in.nextName();
       String[] nameParts = in.nextString().split(" ");
       in.endObject();
       return new User(nameParts[0], nameParts[1]);
     }
   }
   

Since User class specified UserJsonAdapter.class in @JsonAdapter annotation, it will automatically be invoked to serialize/deserialize User instances. Here is an example of how to apply this annotation to a field.

   private static final class Gadget {
     @JsonAdapter(UserJsonAdapter2.class)
     final User user;
     Gadget(User user) {
       this.user = user;
     }
   }
   

It's possible to specify different type adapters on a field, that field's type, and in the com.google.gson.GsonBuilder. Field annotations take precedence over GsonBuilder-registered type adapters, which in turn take precedence over annotated types.

The class referenced by this annotation must be either a TypeAdapter or a TypeAdapterFactory, or must implement one or both of JsonDeserializer or JsonSerializer. Using TypeAdapterFactory makes it possible to delegate to the enclosing Gson instance.