如何使用GSON只返回一个特定的空字段

Question

如果我有一个简单的对象，如下所示：

String name;
String email;
int age;
boolean isDeveloper;

假设接收到的JSON对象具有值：

{"name":null,"email":null,"age":26,"isDeveloper":true}

当使用GSON作为默认值反序列化这个JSON时，我有：

{"age":26,"isDeveloper":true}

但是电子邮件字段丢失会导致我的应用程序失败，所以我想添加

email = null;

序列化回JSON，并且只有此字段值为null。也不要忽略任何非空字段。

不应将其他null值添加到生成的JSON中。

我尝试使用默认的GSON构建器进行反序列化，然后使用允许空值的GSON进行序列化：

Gson gson = new GsonBuilder().serializeNulls().create();

问题是：这将查找对象类中的所有空/空值，并将它们设置为所有

{"name":null,"email":null,"age":26,"isDeveloper":true}

如何将电子邮件属性设置为null，然后序列化回只包含此字段为null的JSON，而不忽略任何其他非null值？

我使用的是gson v2.2.4

以下是示例代码：

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
public class App
{
private class UserSimple {
public String name;
public String email;
public int age;
public boolean isDeveloper;

UserSimple(String name, String email, int age, boolean isDeveloper) {
this.name = name;
this.email = email;
this.age = age;
this.isDeveloper = isDeveloper;
}
}
public static void main( String[] args )
{
String userJson = "{'age':26,'email':'[e-mail]','isDeveloper':true,'name':null}";
Gson gson = new GsonBuilder()
.setPrettyPrinting()
.disableHtmlEscaping()
.create();

Gson gsonBuilder = new GsonBuilder().serializeNulls().create();
UserSimple deserializedObj = gson.fromJson(userJson, UserSimple.class);
System.out.println("n"+gson.toJson(deserializedObj));
deserializedObj.email = null;

String serializedObj = gsonBuilder.toJson(deserializedObj, UserSimple.class);
System.out.println("n"+serializedObj);

}
}

Answer 1

您可以创建自己的自定义适配器来解决眼前的问题：

class MyCustomTypeAdapter extends TypeAdapter<UserSimple> {
@Override
public void write(JsonWriter writer, UserSimple userSimple) throws IOException {
writer.beginObject();
if(userSimple.getName() != null){
writer.name("name");
writer.value(userSimple.getName());
}
// you want to include email even if it's null
writer.name("email");
writer.value(userSimple.getEmail());
if(userSimple.getAge() != null){
writer.name("age");
writer.value(userSimple.getAge());
}
if(userSimple.getDeveloper() != null){
writer.name("isDeveloper");
writer.value(userSimple.getDeveloper());
}
writer.endObject();
}
public UserSimple read(JsonReader reader) throws IOException {
// you could create your own
return null;
}
} 

输入：

String userJson = "{'age':null,'email':null,'isDeveloper':true,'name':'somename'}";

输出：

serializedObj :{"name":"somename","email":null,"isDeveloper":true}

输入：

String userJson = "{'age':20,'email':null,'isDeveloper':true,'name':'somename'}";

输出：

serializedObj :{"name":"somename","email":null,"age":20,"isDeveloper":true}

看看https://google.github.io/gson/apidocs/com/google/gson/TypeAdapter.html

Answer 2

你考虑过使用jackson吗？我认为它比Gson更强大、更快。但这只是我的看法，我无意冒犯任何人。我也遇到过这样的情况，杰克逊可以做一些gson做不到的事情。

这是为你做这项工作的代码：

import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.ObjectWriter;
public class Main {
static class Person {
@JsonInclude(JsonInclude.Include.ALWAYS)
private String name;
@JsonInclude(JsonInclude.Include.NON_NULL)
private String email;
@JsonInclude(JsonInclude.Include.NON_NULL)
private Integer age;
public Person(String name, String email, Integer age) {
this.name = name;
this.email = email;
this.age = age;
}
public String getName() {
return name;
}
public String getEmail() {
return email;
}
public Integer getAge() {
return age;
}
}
public static void main(String[] args) throws Exception {
ObjectWriter ow = new ObjectMapper().writer().withDefaultPrettyPrinter(); // pretty printing only enabled for demonstration purposes
String test = ow.writeValueAsString(new Person(null, null, 2));
System.out.println(test);
}
}

输出为：

{
"name" : null,
"age" : 2
}

CCD_ 1也被输出，因为它的包含策略是CCD_。

在maven依赖包括：

<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-core</artifactId>
<version>2.9.6</version>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-annotations</artifactId>
<version>2.9.6</version>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.9.6</version>
</dependency>

还有这样的答案：Gson为特定的类或字段序列化null，但看起来不是很好。