我认为JsonDeserializer在这里不是一个好的选择：
- 您需要将每个类型绑定到
GsonBuilder 中的Gson 实例，这很容易出错，或者使用registerTypeHierarchyAdapter。
- 对于后者，您会遇到无限递归（如果我没记错的话：因为上下文只提供了一种反序列化 same 类型实例的机制）。
以下类型适配器工厂可以克服上述限制：
final class PolymorphicTypeAdapterFactory
implements TypeAdapterFactory {
// Let's not hard-code `Kit.class` here and let a user pick up types at a call-site
private final Predicate<? super Class<?>> predicate;
private PolymorphicTypeAdapterFactory(final Predicate<? super Class<?>> predicate) {
this.predicate = predicate;
}
static TypeAdapterFactory get(final Predicate<? super Class<?>> predicate) {
return new PolymorphicTypeAdapterFactory(predicate);
}
@Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
final Class<? super T> rawClass = typeToken.getRawType();
if ( !predicate.test(rawClass) ) {
// Something we cannot handle? Try pick the next best type adapter factory
return null;
}
// This is what JsonDeserializer fails at:
final TypeAdapter<T> writeTypeAdapter = gson.getDelegateAdapter(this, typeToken);
// Despite it's possible to use the above type adapter for both read and write, what if the `type` property points to another class?
final Function<? super Class<T>, ? extends TypeAdapter<T>> readTypeAdapterResolver = actualRawClass -> {
if ( !rawClass.isAssignableFrom(actualRawClass) ) {
throw new IllegalStateException("Cannot parse as " + actualRawClass);
}
return gson.getDelegateAdapter(this, TypeToken.get(actualRawClass));
};
return PolymorphicTypeAdapter.get(rawClass, writeTypeAdapter, readTypeAdapterResolver);
}
private static final class PolymorphicTypeAdapter<T>
extends TypeAdapter<T> {
private final Class<? super T> rawClass;
private final TypeAdapter<T> writeTypeAdapter;
private final Function<? super Class<T>, ? extends TypeAdapter<T>> readTypeAdapterResolver;
private PolymorphicTypeAdapter(final Class<? super T> rawClass, final TypeAdapter<T> writeTypeAdapter,
final Function<? super Class<T>, ? extends TypeAdapter<T>> readTypeAdapterResolver) {
this.rawClass = rawClass;
this.writeTypeAdapter = writeTypeAdapter;
this.readTypeAdapterResolver = readTypeAdapterResolver;
}
// Since constructors are meant only to assign parameters to fields, encapsulate the null-safety handling in the factory method
private static <T> TypeAdapter<T> get(final Class<? super T> rawClass, final TypeAdapter<T> writeTypeAdapter,
final Function<? super Class<T>, ? extends TypeAdapter<T>> readTypeAdapterResolver) {
return new PolymorphicTypeAdapter<>(rawClass, writeTypeAdapter, readTypeAdapterResolver)
.nullSafe();
}
@Override
@SuppressWarnings("resource")
public void write(final JsonWriter jsonWriter, final T value)
throws IOException {
jsonWriter.beginObject();
jsonWriter.name("type");
jsonWriter.value(rawClass.getName());
jsonWriter.name("properties");
writeTypeAdapter.write(jsonWriter, value);
jsonWriter.endObject();
}
@Override
public T read(final JsonReader jsonReader)
throws IOException {
jsonReader.beginObject();
// For simplicity's sake, let's assume that the class property `type` always precedes the `properties` property
final Class<? super T> actualRawClass = readActualRawClass(jsonReader);
final T value = readValue(jsonReader, actualRawClass);
jsonReader.endObject();
return value;
}
private Class<? super T> readActualRawClass(final JsonReader jsonReader)
throws IOException {
try {
requireProperty(jsonReader, "type");
final String value = jsonReader.nextString();
@SuppressWarnings("unchecked")
final Class<? super T> actualRawClass = (Class<? super T>) Class.forName(value);
return actualRawClass;
} catch ( final ClassNotFoundException ex ) {
throw new AssertionError(ex);
}
}
private T readValue(final JsonReader jsonReader, final Class<? super T> rawClass)
throws IOException {
requireProperty(jsonReader, "properties");
@SuppressWarnings("unchecked")
final Class<T> castRawClass = (Class<T>) rawClass;
final TypeAdapter<T> readTypeAdapter = readTypeAdapterResolver.apply(castRawClass);
return readTypeAdapter.read(jsonReader);
}
private static void requireProperty(final JsonReader jsonReader, final String propertyName)
throws IOException {
final String name = jsonReader.nextName();
if ( !name.equals(propertyName) ) {
throw new JsonParseException("Unexpected property: " + name);
}
}
}
}
仅针对您的Kit 类的使用示例（下面的方法参考仅检查Kit 是给定实际原始类的超类还是后者是Kit 本身）：
private static final Gson gson = new GsonBuilder()
.setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
.registerTypeAdapterFactory(PolymorphicTypeAdapterFactory.get(Kit.class::isAssignableFrom))
.create();
请注意，您的问题不是唯一的，RuntimeTypeAdapterFactory 几乎涵盖了您的情况，但 RuntimeTypeAdapterFactory 不会像您的示例那样隔离 type 和 properties。
P.S. 请注意，这种类型适配器工厂远非真正的通用：它不适用于 types（类是类型的一种特殊情况），generic 类型等。如果感兴趣，但当然不是过度设计，您可能想参考我的解决方案，使用 Type 实例 object serialization mechanism绑定到特定平台）或使用类型和泛型类型表示法parsing using JParsec（两个链接都指向俄语 StackExchange 站点）。