Gson序列化特定类或字段的null [英] Gson serialize null for specific class or field

问题描述

我想序列化特定字段或类的空值。

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

在GSON中，选项 serializeNulls（）适用于整个JSON。

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

示例：

class MainClass {
    public String id;
    public String name;
    public Test test;
}

class Test {
    public String name;
    public String value;    
} 

MainClass mainClass = new MainClass();
mainClass.id = "101"
// mainClass has no name.
Test test = new Test();
test.name = "testName";
test.value = null;
mainClass.test = test;    

使用GSON创建JSON：

Creating JSON using GSON:

GsonBuilder builder = new GsonBuilder().serializeNulls();
Gson gson = builder.create();
System.out.println(gson.toJson(mainClass));

当前输出：

{
    "id": "101",
    "name": null,
    "test": {
        "name": "testName",
        "value": null
    }
}

所需输出：

{
    "id": "101",
    "test": {
        "name": "testName",
        "value": null
    }
}

如何实现所需的输出？

首选解决方案将具有以下属性：

Preferred solution would have the following properties:

• 默认情况下 NOT 序列化空值


• 序列化具有特定注释的字段的空值。

• Do NOT serialize nulls by default,
• Serialize nulls for fields with a specific annotation.

推荐答案

我有接口来检查何时将对象序列化为null：

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

public interface JsonNullable {
  boolean isJsonNull();
}

相应的TypeAdapter（支持只写）

And the corresponding TypeAdapter (supports write only)

public class JsonNullableAdapter extends TypeAdapter<JsonNullable> {

  final TypeAdapter<JsonElement> elementAdapter = new Gson().getAdapter(JsonElement.class);
  final TypeAdapter<Object> 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;
  }
}

按如下方式使用：

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

在Foo值应序列化为null的类中。请注意，foo值本身必须不为null，否则将忽略自定义适配器注释。

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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