Json:使用接口泛型类型列表反序列化接口类

Question

>我根据用户类型序列化用户界面类型的子类。这些子类具有列表。角色也是一个接口。我已经编写了自定义 Gson 适配器：

public class InterfaceAdapter<T> implements JsonSerializer<T>, JsonDeserializer<T> {
public JsonElement serialize(T object, Type interfaceType, JsonSerializationContext context) {
final JsonObject wrapper = new JsonObject();
wrapper.addProperty("type", object.getClass().getName());
wrapper.add("data", context.serialize(object));
return wrapper;
}
public T deserialize(JsonElement elem, Type interfaceType, JsonDeserializationContext context) throws JsonParseException {
final JsonObject wrapper = (JsonObject) elem;
final JsonElement typeName = get(wrapper, "type");
final JsonElement data = get(wrapper, "data");
final Type actualType = typeForName(typeName);
return context.deserialize(data, actualType);
}
private Type typeForName(final JsonElement typeElem) {
try {
return Class.forName(typeElem.getAsString());
} catch (ClassNotFoundException e) {
throw new JsonParseException(e);
}
}
private JsonElement get(final JsonObject wrapper, String memberName) {
final JsonElement elem = wrapper.get(memberName);
if (elem == null) throw new JsonParseException("no '" + memberName + "' member found in what was expected to be an interface wrapper");
return elem;
}
}

但是，问题是我得到错误：Caused by: java.lang.UnsupportedOperationException: Interface can't be instantiated! Interface name: Domain.Users.role.Role。我知道有什么问题。Gson 不知道选择什么具体的实现进行实例化。如何告诉 Gson 要实例化哪个类？

以下是完整的堆栈跟踪：

FATAL EXCEPTION: main
Process: org.ucomplex.ucomplex, PID: 346
java.lang.RuntimeException: Unable to create application org.ucomplex.ucomplex.Common.base.UCApplication: java.lang.RuntimeException: Unable to invoke no-args constructor for ? extends org.ucomplex.ucomplex.Domain.Users.role.Role. Register an InstanceCreator with Gson for this type may fix this problem.
at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4729)
at android.app.ActivityThread.access$1600(ActivityThread.java:153)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1412)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5442)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628)
Caused by: java.lang.RuntimeException: Unable to invoke no-args constructor for ? extends org.ucomplex.ucomplex.Domain.Users.role.Role. Register an InstanceCreator with Gson for this type may fix this problem.
at com.google.gson.internal.ConstructorConstructor$14.construct(ConstructorConstructor.java:226)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:210)
at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:41)
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:82)
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:61)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:129)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:220)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:129)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:220)
at com.google.gson.Gson.fromJson(Gson.java:887)
at com.google.gson.Gson.fromJson(Gson.java:952)
at com.google.gson.internal.bind.TreeTypeAdapter$GsonContextImpl.deserialize(TreeTypeAdapter.java:162)
at org.ucomplex.ucomplex.Domain.Users.InterfaceAdapter.deserialize(InterfaceAdapter.java:36)
at com.google.gson.internal.bind.TreeTypeAdapter.read(TreeTypeAdapter.java:69)
at com.google.gson.Gson.fromJson(Gson.java:887)
at com.google.gson.Gson.fromJson(Gson.java:852)
at com.google.gson.Gson.fromJson(Gson.java:801)
at com.google.gson.Gson.fromJson(Gson.java:773)
at org.ucomplex.ucomplex.Common.FacadePreferences.getUserDataFromPref(FacadePreferences.java:49)
at org.ucomplex.ucomplex.Common.base.UCApplication.onCreate(UCApplication.java:52)
at android.app.Instrumentation.callApplicationOnCreate(Instrumentation.java:1014)
at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4726)
at android.app.ActivityThread.access$1600(ActivityThread.java:153) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1412) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:148) 
at android.app.ActivityThread.main(ActivityThread.java:5442) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628) 
Caused by: java.lang.UnsupportedOperationException: Interface can't be instantiated! Interface name: org.ucomplex.ucomplex.Domain.Users.role.Role
at com.google.gson.internal.UnsafeAllocator.assertInstantiable(UnsafeAllocator.java:117)
at com.google.gson.internal.UnsafeAllocator.access$000(UnsafeAllocator.java:31)
at com.google.gson.internal.UnsafeAllocator$1.newInstance(UnsafeAllocator.java:49)
at com.google.gson.internal.ConstructorConstructor$14.construct(ConstructorConstructor.java:223)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:210) 
at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:41) 
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:82) 
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:61) 
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:129) 
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:220) 
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:129) 
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:220) 
at com.google.gson.Gson.fromJson(Gson.java:887) 
at com.google.gson.Gson.fromJson(Gson.java:952) 
at com.google.gson.internal.bind.TreeTypeAdapter$GsonContextImpl.deserialize(TreeTypeAdapter.java:162) 
at org.ucomplex.ucomplex.Domain.Users.InterfaceAdapter.deserialize(InterfaceAdapter.java:36) 
at com.google.gson.internal.bind.TreeTypeAdapter.read(TreeTypeAdapter.java:69) 
at com.google.gson.Gson.fromJson(Gson.java:887) 
at com.google.gson.Gson.fromJson(Gson.java:852) 
at com.google.gson.Gson.fromJson(Gson.java:801) 
at com.google.gson.Gson.fromJson(Gson.java:773) 
at org.ucomplex.ucomplex.Common.FacadePreferences.getUserDataFromPref(FacadePreferences.java:49) 
at org.ucomplex.ucomplex.Common.base.UCApplication.onCreate(UCApplication.java:52) 
at android.app.Instrumentation.callApplicationOnCreate(Instrumentation.java:1014) 
at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4726) 
at android.app.ActivityThread.access$1600(ActivityThread.java:153) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1412) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:148) 
at android.app.ActivityThread.main(ActivityThread.java:5442) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628) 

编辑：这个问题被标记为重复。但是，标记的问题回答了如何反序列化接口类型(而不是以最聪明的方式)。我的问题不在于此。我知道如何反序列化顶级接口类型。就我而言，我有具有另一种接口类型列表的接口的基本实现。此类用于组合。 想象一下：

interface IA {}
interface IR {}
class R1 implements IR {}
class R2 implements IR {}
class A1 implements IA {
private List<IR> list;
}
class A2 implements IA {
private A1 member;
}
class A3 implements IA {
private A1 member;
}

脱private List<IR> list;时出现问题。

Answer 1

我不确定您是否正确注册了类型适配器(您是否为每个类都使用了registerTypeHierarchyAdapter或registerTypeAdapter？)，但是您面临着一个经典的问题，您必须存储正确的类名(甚至是包括泛型的类型名称)才能通过其具体类型检索对象。不幸的是，InstanceCreator不会帮助你，因为它只能接受某种类型。RuntimeTypeAdapterFactory，作为 Google Gson 附加功能的一部分实现，旨在使用类型别名，因此您必须映射每个已知接口。

无论如何，您可以通过实现适当的类型适配器工厂来使其完全与接口无关。例如：

final class InterfaceTypeAdapterFactory
implements TypeAdapterFactory {
// Effectively a singleton totally holding no state
private static final TypeAdapterFactory interfaceTypeAdapterFactory = new InterfaceTypeAdapterFactory();
private InterfaceTypeAdapterFactory() {
}
// However, let's encapsulate the instantiation
static TypeAdapterFactory getInterfaceTypeAdapterFactory() {
return interfaceTypeAdapterFactory;
}
@Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
// Checking if it's an interface
return !typeToken.getRawType().isInterface()
// If it's not, then just let Gson pick up a proper type adapter
? null
// Otherwise return a null-safe custom type adapter
: new InterfaceTypeAdapter<T>(gson).nullSafe();
}
private static final class InterfaceTypeAdapter<T>
extends TypeAdapter<T> {
private static final String TYPE_PROPERTY = "type";
private static final String DATA_PROPERTY = "data";
private final Gson gson;
private InterfaceTypeAdapter(final Gson gson) {
this.gson = gson;
}
@Override
@SuppressWarnings("resource")
public void write(final JsonWriter out, final T value)
throws IOException {
// Here we're just writing a property value similar to one you did
out.beginObject();
out.name(TYPE_PROPERTY);
out.value(value.getClass().getName());
out.name(DATA_PROPERTY);
gson.toJson(value, value.getClass(), out);
out.endObject();
}
@Override
public T read(final JsonReader in)
throws IOException {
try {
// Deserialization is more complex
// Make sure that the current value is an object
in.beginObject();
final String name = in.nextName();
final Object value;
switch ( name ) {
// If the first property in the stream was type...
case TYPE_PROPERTY:
final String type = in.nextString();
// Then require the next property to be data
if ( !in.nextName().equals(DATA_PROPERTY) ) {
throw new MalformedJsonException("Expected " + DATA_PROPERTY + " at " + in);
}
// And delegate the deserialization to Gson
value = gson.fromJson(in, Class.forName(type));
break;
// If some some reason, the order of data and type was messed...
case DATA_PROPERTY:
// Then store the current value as a JSON tree to deserialize it later
// It consumes more memory than the `case TYPE_PROPERTY` case, and you can consider this one as the worst case
final JsonElement jsonElement = gson.fromJson(in, JsonElement.class);
if ( !in.nextName().equals(TYPE_PROPERTY) ) {
throw new MalformedJsonException("Expected " + TYPE_PROPERTY + " at " + in);
}
// Restore the value from the tree
value = gson.fromJson(jsonElement, Class.forName(in.nextString()));
break;
default:
throw new MalformedJsonException("Unrecognized " + name + " at " + in);
}
if ( in.hasNext() ) {
throw new IllegalStateException("Unexpected " + in.nextName() + " at " + in);
}
in.endObject();
@SuppressWarnings("unchecked")
final T castValue = (T) value;
return castValue;
} catch ( final ClassNotFoundException ex ) {
throw new IOException(ex);
}
}
}
}

使用示例：

interface IWhatever {
}

final class Wrapper {
final IWhatever whatever;
Wrapper(final IWhatever whatever) {
this.whatever = whatever;
}
}

final class Foo
implements IWhatever {
}

final class Bar
implements IWhatever {
}

private static final Gson gson = new GsonBuilder()
.registerTypeAdapterFactory(getInterfaceTypeAdapterFactory())
.create();
public static void main(final String... args) {
final Wrapper before = new Wrapper(new Foo());
final String json = gson.toJson(before);
System.out.println(json);
final Wrapper after = gson.fromJson(json, Wrapper.class);
System.out.println(after.whatever.getClass());
}

输出：

{">

whatever"：{"type"："q44005695.Foo"，"data"：{}}}
class q44005695.傅

请注意，此示例仅适用于类，因此会丢失类型参数化。如果需要更高级的方法来保留泛型的类型参数化，可以参考以下内容：

• https://ru.stackoverflow.com/a/605380/13379
• https://ru.stackoverflow.com/a/605388/13379