我承认 mlk 的回答，但鉴于我已经（并且仍然需要）JSON 对象的 POJO 表示，我觉得自动映射仍然比手动查找更好。
挑战在于，正如我所说，在gson.fromJson(...) 将填充的相应 POJO 中，缺失的和显式的 null 值都设置为 null。 （与 R 的 NULL 和 NA 不同，Java 只有一种表示“不存在”。）
但是，通过使用Java 8's Optionals 对我的数据结构进行建模，我可以做到这一点：区分未设置的内容和设置为null 的内容。这是我最终得到的结果：
1) 我用Optional<T> 替换了我的数据对象中的所有字段。
public class BasicObjectOptional {
private Optional<String> someKey;
private Optional<Integer> someNumber;
private Optional<String> mayBeNull;
public BasicObjectOptional() {
}
public BasicObjectOptional(boolean initialize) {
if (initialize) {
someKey = Optional.ofNullable("someValue");
someNumber = Optional.ofNullable(42);
mayBeNull = Optional.ofNullable(null);
}
}
@Override
public String toString() {
return String.format("someKey = %s, someNumber = %s, mayBeNull = %s",
someKey, someNumber, mayBeNull);
}
}
或嵌套的：
public class ComplexObjectOptional {
Optional<String> theTitle;
Optional<List<Optional<String>>> stringArray;
Optional<BasicObjectOptional> theObject;
public ComplexObjectOptional() {
}
public ComplexObjectOptional(boolean initialize) {
if (initialize) {
theTitle = Optional.ofNullable("Complex Object");
stringArray = Optional.ofNullable(Arrays.asList(Optional.ofNullable("Hello"),Optional.ofNullable("World")));
theObject = Optional.ofNullable(new BasicObjectOptional(true));
}
}
@Override
public String toString() {
return String.format("theTitle = %s, stringArray = %s, theObject = (%s)", theTitle, stringArray, theObject);
}
}
2) 实现了序列化器和反序列化器based on this useful SO answer。
public class OptionalTypeAdapter<E> extends TypeAdapter<Optional<E>> {
public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() {
//@Override
public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
Class<T> rawType = (Class<T>) type.getRawType();
if (rawType != Optional.class) {
return null;
}
final ParameterizedType parameterizedType = (ParameterizedType) type.getType();
final Type actualType = parameterizedType.getActualTypeArguments()[0];
final TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(actualType));
return new OptionalTypeAdapter(adapter);
}
};
private final TypeAdapter<E> adapter;
public OptionalTypeAdapter(TypeAdapter<E> adapter) {
this.adapter = adapter;
}
@Override
public void write(JsonWriter out, Optional<E> value) throws IOException {
if(value == null || !value.isPresent()){
out.nullValue();
} else {
adapter.write(out, value.get());
}
}
@Override
public Optional<E> read(JsonReader in) throws IOException {
final JsonToken peek = in.peek();
if(peek != JsonToken.NULL){
return Optional.ofNullable(adapter.read(in));
}
in.nextNull();
return Optional.empty();
}
}
3) 在初始化 Gson 时注册了这个适配器。
Gson gsonOptFact = new GsonBuilder()
.serializeNulls() // matter of taste, just for output anyway
.registerTypeAdapterFactory(OptionalTypeAdapter.FACTORY)
.create();
这使我可以编写 JSON，以便将 null 和空 Optional 序列化为 null（或简单地从输出中删除），同时将 JSON 读入 Optional 字段，这样 如果字段是 null 我知道它在 JSON 输入中丢失了，如果字段是 Optional.empty 我知道它在输入中设置为 null。
例子：
System.out.println(gsonOptFact.toJson(new BasicObjectOptional(true)));
// {"someKey":"someValue","someNumber":42,"mayBeNull":null}
System.out.println(gsonOptFact.toJson(new ComplexObjectOptional(true)));
// {"theTitle":"Complex Object","stringArray":["Hello","World"],"theObject":{"someKey":"someValue","someNumber":42,"mayBeNull":null}}
// Now read back in:
String basic = "{\"someKey\":\"someValue\",\"someNumber\":42,\"mayBeNull\":null}";
String complex = "{\"theTitle\":\"Complex Object\",\"stringArray\":[\"Hello\",\"world\"],\"theObject\":{\"someKey\":\"someValue\",\"someNumber\":42,\"mayBeNull\":null}}";
String complexMissing = "{\"theTitle\":\"Complex Object\",\"theObject\":{\"someKey\":\"someValue\",\"mayBeNull\":null}}";
BasicObjectOptional boo = gsonOptFact.fromJson(basic, BasicObjectOptional.class);
System.out.println(boo);
// someKey = Optional[someValue], someNumber = Optional[42], mayBeNull = Optional.empty
ComplexObjectOptional coo = gsonOptFact.fromJson(complex, ComplexObjectOptional.class);
System.out.println(coo);
// theTitle = Optional[Complex Object], stringArray = Optional[[Optional[Hello], Optional[world]]], theObject = (Optional[someKey = Optional[someValue], someNumber = Optional[42], mayBeNull = Optional.empty])
ComplexObjectOptional coom = gsonOptFact.fromJson(complexMissing, ComplexObjectOptional.class);
System.out.println(coom);
// theTitle = Optional[Complex Object], stringArray = null, theObject = (Optional[someKey = Optional[someValue], someNumber = null, mayBeNull = Optional.empty])
我认为这将使我能够很好地将 JSON Merge Patch 与我现有的数据对象集成。