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Gson serialize null for specific class or field

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面向向阳花
面向向阳花 2021-01-01 11:00

I want to serialize nulls for a specific field or class.

In GSON, the option serializeNulls() applies to the whole JSON.

Example:
5条回答
  • 暖寄归人
    暖寄归人 (楼主)
    2021-01-01 11:57

    I have interface to check when object should be serialized as null:

    public interface JsonNullable {
  boolean isJsonNull();
}

    And the corresponding TypeAdapter (supports write only)

    public class JsonNullableAdapter extends TypeAdapter {

  final TypeAdapter elementAdapter = new Gson().getAdapter(JsonElement.class);
  final TypeAdapter objectAdapter = new Gson().getAdapter(Object.class);

  @Override
  public void write(JsonWriter out, JsonNullable value) throws IOException {
    if (value == null || value.isJsonNull()) {
      //if the writer was not allowed to write null values
      //do it only for this field
      if (!out.getSerializeNulls()) {
        out.setSerializeNulls(true);
        out.nullValue();
        out.setSerializeNulls(false);
      } else {
        out.nullValue();
      }
    } else {
      JsonElement tree = objectAdapter.toJsonTree(value);
      elementAdapter.write(out, tree);
    }
  }

  @Override
  public JsonNullable read(JsonReader in) throws IOException {
    return null;
  }
}



    Use it as follows:
    


    public class Foo implements JsonNullable {
  @Override
  public boolean isJsonNull() {
    // You decide
  }
}

    


    In the class where Foo value should be serialized as null. Note that foo value itself must be not null, otherwise custom adapter annotation will be ignored.
    


    public class Bar {
  @JsonAdapter(JsonNullableAdapter.class)
  public Foo foo = new Foo();
}

    
    
    
             
    
                                                        
    
            
            
    
              
                
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