但问题是我有许多不同元素类型的列表，我不想为每种情况编写单独的 TypeAdapter。我也无法使用泛型 TypeAdapter>，因为在某些时候您需要知道类型。
这就是类型适配器工厂的设计目的：您可以控制Gson 实例配置中的每种类型。
final class AlwaysListTypeAdapterFactory<E>
implements TypeAdapterFactory {
// Gson can instantiate it itself
private AlwaysListTypeAdapterFactory() {
}
@Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
// If it's not a List -- just delegate the job to Gson and let it pick the best type adapter itself
if ( !List.class.isAssignableFrom(typeToken.getRawType()) ) {
return null;
}
// Resolving the list parameter type
final Type elementType = resolveTypeArgument(typeToken.getType());
@SuppressWarnings("unchecked")
final TypeAdapter<E> elementTypeAdapter = (TypeAdapter<E>) gson.getAdapter(TypeToken.get(elementType));
// Note that the always-list type adapter is made null-safe, so we don't have to check nulls ourselves
@SuppressWarnings("unchecked")
final TypeAdapter<T> alwaysListTypeAdapter = (TypeAdapter<T>) new AlwaysListTypeAdapter<>(elementTypeAdapter).nullSafe();
return alwaysListTypeAdapter;
}
private static Type resolveTypeArgument(final Type type) {
// The given type is not parameterized?
if ( !(type instanceof ParameterizedType) ) {
// No, raw
return Object.class;
}
final ParameterizedType parameterizedType = (ParameterizedType) type;
return parameterizedType.getActualTypeArguments()[0];
}
private static final class AlwaysListTypeAdapter<E>
extends TypeAdapter<List<E>> {
private final TypeAdapter<E> elementTypeAdapter;
private AlwaysListTypeAdapter(final TypeAdapter<E> elementTypeAdapter) {
this.elementTypeAdapter = elementTypeAdapter;
}
@Override
public void write(final JsonWriter out, final List<E> list) {
throw new UnsupportedOperationException();
}
@Override
public List<E> read(final JsonReader in)
throws IOException {
// This is where we detect the list "type"
final List<E> list = new ArrayList<>();
final JsonToken token = in.peek();
switch ( token ) {
case BEGIN_ARRAY:
// If it's a regular list, just consume [, <all elements>, and ]
in.beginArray();
while ( in.hasNext() ) {
list.add(elementTypeAdapter.read(in));
}
in.endArray();
break;
case BEGIN_OBJECT:
case STRING:
case NUMBER:
case BOOLEAN:
// An object or a primitive? Just add the current value to the result list
list.add(elementTypeAdapter.read(in));
break;
case NULL:
throw new AssertionError("Must never happen: check if the type adapter configured with .nullSafe()");
case NAME:
case END_ARRAY:
case END_OBJECT:
case END_DOCUMENT:
throw new MalformedJsonException("Unexpected token: " + token);
default:
throw new AssertionError("Must never happen: " + token);
}
return list;
}
}
}
现在您只需要告诉 Gson 哪些 字段格式不正确。
当然，您可以将整个 Gson 实例配置为接受此类列表，但使用 @JsonAdapter 注释让它更精确：
final class Model {
@JsonAdapter(AlwaysListTypeAdapterFactory.class)
final List<String> foo = null;
@JsonAdapter(AlwaysListTypeAdapterFactory.class)
final List<SomeObject> bleh = null;
@Override
public String toString() {
return "Model{" + "foo=" + foo + ", bleh=" + bleh + '}';
}
}
final class SomeObject {
final String some = null;
@Override
public String toString() {
return "SomeObject{" + "some='" + some + '\'' + '}';
}
}
测试数据：
single.json
{
"foo": "bar",
"bleh": {"some": "object"}
}
list.json
{
"foo": ["bar"],
"bleh": [{"some": "object"}]
}
例子：
private static final Gson gson = new Gson();
public static void main(final String... args)
throws IOException {
for ( final String resource : ImmutableList.of("single.json", "list.json") ) {
try ( final JsonReader jsonReader = getPackageResourceJsonReader(Q43412261.class, resource) ) {
final Model model = gson.fromJson(jsonReader, Model.class);
System.out.println(model);
}
}
}
还有输出：
模型{foo=[bar], bleh=[SomeObject{some='object'}]}
模型{foo=[bar], bleh=[SomeObject{some='object'}]}