我想序列化特定字段或类的空值。
在GSON中,选项serializeNulls()
适用于整个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:
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
}
}
期望的输出:
{
"id": "101",
"test": {
"name": "testName",
"value": null
}
}
如何实现所需的输出?
首选解决方案具有以下属性:
答案 0 :(得分:7)
我有一个类似于Aleksey的解决方案,但是可以应用于任何类中的一个或多个字段(例如Kotlin中的示例):
为应该序列化为null的字段创建新的注释:
@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.FIELD)
annotation class SerializeNull
创建一个TypeAdapterFactory
来检查类在编写对象时是否具有用该注释注释的字段,并从null
中删除JsonTree
且没有注释注释的字段:
class SerializableAsNullConverter : TypeAdapterFactory {
override fun <T : Any?> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T>? {
fun Field.serializedName() = declaredAnnotations
.filterIsInstance<SerializedName>()
.firstOrNull()?.value ?: name
val declaredFields = type.rawType.declaredFields
val nullableFieldNames = declaredFields
.filter { it.declaredAnnotations.filterIsInstance<SerializeNull>().isNotEmpty() }
.map { it.serializedName() }
val nonNullableFields = declaredFields.map { it.serializedName() } - nullableFieldNames
return if (nullableFieldNames.isEmpty()) {
null
} else object : TypeAdapter<T>() {
private val delegateAdapter = gson.getDelegateAdapter(this@SerializableAsNullConverter, type)
private val elementAdapter = gson.getAdapter(JsonElement::class.java)
override fun write(writer: JsonWriter, value: T?) {
val jsonObject = delegateAdapter.toJsonTree(value).asJsonObject
nonNullableFields
.filter { jsonObject.get(it) is JsonNull }
.forEach { jsonObject.remove(it) }
val originalSerializeNulls = writer.serializeNulls
writer.serializeNulls = true
elementAdapter.write(writer, jsonObject)
writer.serializeNulls = originalSerializeNulls
}
override fun read(reader: JsonReader): T {
return delegateAdapter.read(reader)
}
}
}
}
使用您的Gson实例注册适配器:
val builder = GsonBuilder().registerTypeAdapterFactory(SerializableAsNullConverter())
并注释您希望可为空的字段:
class MyClass(val id: String?, @SerializeNull val name: String?)
序列化结果:
val myClass = MyClass(null, null)
val gson = builder.create()
val json = gson.toJson(myClass)
json:
{
"name": null
}
答案 1 :(得分:4)
我有接口来检查何时将对象序列化为null:
public interface JsonNullable {
boolean isJsonNull();
}
相应的TypeAdapter(仅支持只写)
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。
public class Bar {
@JsonAdapter(JsonNullableAdapter.class)
public Foo foo = new Foo();
}
答案 2 :(得分:2)
对于那些寻求Java版本的@Joris出色答案的人,下面的代码应该可以解决问题。它基本上只是Kotlin的翻译版本,对属性的序列化名称的获取方式进行了较小的改进,以确保当序列化名称与属性名称不同时,该属性始终有效(请参阅原始答案的注释)。
这是TypeAdapterFactory
的实现:
public class NullableAdapterFactory implements TypeAdapterFactory {
@Override
public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
Field[] declaredFields = type.getRawType().getDeclaredFields();
List<String> nullableFieldNames = new ArrayList<>();
List<String> nonNullableFieldNames = new ArrayList<>();
for (Field declaredField : declaredFields) {
if (declaredField.isAnnotationPresent(JsonNullable.class)) {
if (declaredField.getAnnotation(SerializedName.class) != null) {
nullableFieldNames.add(declaredField.getAnnotation(SerializedName.class).value());
} else {
nullableFieldNames.add(declaredField.getName());
}
} else {
if (declaredField.getAnnotation(SerializedName.class) != null) {
nonNullableFieldNames.add(declaredField.getAnnotation(SerializedName.class).value());
} else {
nonNullableFieldNames.add(declaredField.getName());
}
}
}
if (nullableFieldNames.size() == 0) {
return null;
}
TypeAdapter<T> delegateAdapter = gson.getDelegateAdapter(NullableAdapterFactory.this, type);
TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
return new TypeAdapter<T>() {
@Override
public void write(JsonWriter out, T value) throws IOException {
JsonObject jsonObject = delegateAdapter.toJsonTree(value).getAsJsonObject();
for (String name: nonNullableFieldNames) {
if (jsonObject.has(name) && jsonObject.get(name) instanceof JsonNull) {
jsonObject.remove(name);
}
}
out.setSerializeNulls(true);
elementAdapter.write(out, jsonObject);
}
@Override
public T read(JsonReader in) throws IOException {
return delegateAdapter.read(in);
}
};
}
}
这是@JsonNullable
注释,用于标记目标属性:
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface JsonNullable {
}
我在对象类上将其实现为@JsonAdapter(NullableAdapterFactory.class)
注释,而不是在TypeAdapterFactory
实例上将其注册为GsonBuilder
,所以我的对象类看起来像这样:>
@JsonAdapter(NullableAdapterFactory.class)
public class Person {
public String firstName;
public String lastName;
@JsonNullable
public String someNullableInfo;
}
但是,如果需要,另一种方法也应与此代码一起工作。
答案 3 :(得分:0)
创建com.google.gson.TypeAdapter
的子类,并使用注释com.google.gson.annotations.JsonAdapter
将其注册为必填字段。或使用GsonBuilder.registerTypeAdapter
注册。在该适配器中,应实现write
(和read
)。例如:
public class JsonTestNullableAdapter extends TypeAdapter<Test> {
@Override
public void write(JsonWriter out, Test value) throws IOException {
out.beginObject();
out.name("name");
out.value(value.name);
out.name("value");
if (value.value == null) {
out.setSerializeNulls(true);
out.nullValue();
out.setSerializeNulls(false);
} else {
out.value(value.value);
}
out.endObject();
}
@Override
public Test read(JsonReader in) throws IOException {
in.beginObject();
Test result = new Test();
in.nextName();
if (in.peek() != NULL) {
result.name = in.nextString();
} else {
in.nextNull();
}
in.nextName();
if (in.peek() != NULL) {
result.value = in.nextString();
} else {
in.nextNull();
}
in.endObject();
return result;
}
}
在MainClass
中,将带有适配器的JsonAdapter
注释添加到Test
类字段中:
public static class MClass {
public String id;
public String name;
@JsonAdapter(JsonTestNullableAdapter.class)
public Test test;
}
System.out.println(new Gson.toJson(mainClass))
的输出是:
{
"id": "101",
"test": {
"name": "testName",
"value": null
}
}
答案 4 :(得分:0)
我从这里的各种答案中得到了一些想法。
此实现:
JsonNullable.isJsonNull() == true
JsonNullable.isJsonNull() == false
Parent
的{{1}}中的JsonNullable
null
将未处理的工作适当地委派给delegateAdapter
可能需要序列化为null的对象会实现此接口
TypeAdapterFactory
类型适配器工厂,将值序列化为空
/**
* [JsonNullableTypeAdapterFactory] needs to be registered with the [com.google.gson.Gson]
* serializing implementations of [JsonNullable] for [JsonNullable] to work.
*
* [JsonNullable] allows objects to choose at runtime whether they should be serialized as "null"
* serialized normally, or be omitted from the JSON output from [com.google.gson.Gson].
*
* when [isJsonNull] returns true, the subclass will be serialized to a [com.google.gson.JsonNull].
*
* when [isJsonNull] returns false, the subclass will be serialized normally.
*/
interface JsonNullable {
/**
* return true to have the entire object serialized as `null` during JSON serialization.
* return false to have this object serialized normally.
*/
fun isJsonNull(): Boolean
}
在GSON中注册thr型适配器工厂
class JsonNullableTypeAdapterFactory : TypeAdapterFactory {
override fun <T : Any?> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T>? {
return object : TypeAdapter<T>() {
private val delegateAdapter = gson.getDelegateAdapter(this@JsonNullableTypeAdapterFactory, type)
override fun read(reader: JsonReader): T = delegateAdapter.read(reader)
override fun write(writer: JsonWriter, value: T?) {
if (value is JsonNullable && value.isJsonNull()) {
val originalSerializeNulls = writer.serializeNulls
writer.serializeNulls = true
writer.nullValue()
writer.serializeNulls = originalSerializeNulls
} else {
delegateAdapter.write(writer, value)
}
}
}
}
}
示例对象被序列化为JSON
new GsonBuilder()
// ....
.registerTypeAdapterFactory(new JsonNullableTypeAdapterFactory())
// ....
.create();