[英]Can't properly serialize POJO with Iterable fields using Gson
這是我要序列化的POJO:
public class Bar {
private final Foo foo;
private final Iterable<String> list;
private final Iterable<Map<String, String>> listOfMaps;
}
這是我的稱呼方式
Bar bar = new Bar();
Foo foo = new Foo();
foo.field1 = "val1";
foo.field2 = "val2";
bar.foo = foo;
bar.list = ImmutableList.<String>of("fooList");
bar.listOfMaps = ImmutableList.<Map<String,String>>of(
ImmutableMap.<String,String>of("key", "val")
);
new Gson().toJson(bar);
這是結果
{"foo":{"field1":"val1","field2":"val2"},"list":{},"listOfMaps":{}}
如您所見,POJO可以很好地序列化,但是可迭代(番石榴集合的實例)不能正確地序列化為JSON。 當我自行序列化字段時,它們顯示很好,但是當它們是Bar
字段時,它將無法正確序列化
例:
new Gson().toJson(bar.list);
["fooList"]
對於Gson來說,這似乎是一個老問題,這就是最初設計的方式。 我有一些假設,為什么可能會這樣設計,但是我認為我的假設很弱( Iterable
太基類型,默認情況下無法獲得特定實現?;它可能與Collection
和List
相交?;如果Iterable
返回無限迭代器?)。 您可以通過以下拉取請求跟蹤Iterable
支持問題: https : //github.com/google/gson/pull/854 。
首先,為什么Gson在您提到的兩種情況下都有不同的行為( .toJson(bar)
和.toJson(bar.list)
)。 對於第一種情況,Gson掃描您的bar
對象,並使用反射直接從字段中檢索類型信息,這就是它獲取Iterable
。 對於第二種情況,Gson沒有聲明類型信息,因此僅使用.getClass()
來.getClass()
實際的類(而不是類型! .getClass()
丟失類型參數化(字符串適合此測試.getClass()
是完美的),在這種情況下, Collection
具有Gson支持開箱即用。 請注意,后者也可以通過gson.toJson(..., new TypeToken<Iterable<String>>() {}.getType(), System.out)
進行復制, gson.toJson(..., new TypeToken<Iterable<String>>() {}.getType(), System.out)
明確建議給定的對象類型 (而不是類!)。
但是,您可以自己添加Iterable
支持,但是我不確定以下實施是否需要更多工作:
final class IterableMyTypeAdapterFactory
implements TypeAdapterFactory {
private static final TypeAdapterFactory iterableTypeAdapterFactory = new IterableMyTypeAdapterFactory();
private IterableMyTypeAdapterFactory() {
}
// It's an effective singleton but we do not reveal it
static TypeAdapterFactory getIterableMyTypeAdapterFactory() {
return iterableTypeAdapterFactory;
}
@Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
// Must be an iterable, subclasses should be picked up by built-in type adapters
if ( !Iterable.class.isAssignableFrom(typeToken.getRawType()) || Collection.class.isAssignableFrom(typeToken.getRawType()) ) {
// Tell Gson to pick up on its own
return null;
}
// Extract the element type. If it's raw, we assume java.lang.Object is in the play
final Type elementType = getTypeParameter0(typeToken.getType());
// Get the element type adapter
final TypeAdapter<?> elementTypeAdapter = gson.getDelegateAdapter(this, TypeToken.get(elementType));
// And get rid of wildcards
@SuppressWarnings("unchecked")
final TypeAdapter<Object> castElementTypeAdapter = (TypeAdapter<Object>) elementTypeAdapter;
// Instantiating a new iterable type adapter
final TypeAdapter<Iterable<Object>> iterableTypeAdapter = IterableTypeAdapter.get(castElementTypeAdapter);
// Cast it cheating javac
@SuppressWarnings("unchecked")
final TypeAdapter<T> castTypeAdapter = (TypeAdapter<T>) iterableTypeAdapter;
return castTypeAdapter;
}
private static Type getTypeParameter0(final Type type) {
if ( !(type instanceof ParameterizedType) ) {
return Object.class;
}
final ParameterizedType parameterizedType = (ParameterizedType) type;
return parameterizedType.getActualTypeArguments()[0];
}
private static final class IterableTypeAdapter<E>
extends TypeAdapter<Iterable<E>> {
private final TypeAdapter<E> elementTypeAdapter;
private IterableTypeAdapter(final TypeAdapter<E> elementTypeAdapter) {
this.elementTypeAdapter = elementTypeAdapter;
}
private static <E> TypeAdapter<Iterable<E>> get(final TypeAdapter<E> elementTypeAdapter) {
return new IterableTypeAdapter<>(elementTypeAdapter)
.nullSafe(); // We don't need to handle nulls ourselves anymore
}
@Override
@SuppressWarnings("resource")
public void write(final JsonWriter jsonWriter, final Iterable<E> elements)
throws IOException {
// Emit [
jsonWriter.beginArray();
for ( final E e : elements ) {
// Write each element to the downstream writer
elementTypeAdapter.write(jsonWriter, e);
}
// Emit ]
jsonWriter.endArray();
}
@Override
@SuppressWarnings("resource")
public Iterable<E> read(final JsonReader jsonReader)
throws IOException {
jsonReader.beginArray(); // Expect [
final Collection<E> elements = new ArrayList<>(); // This is probably why there is Iterable support by default
while ( jsonReader.hasNext() ) {
final E e = elementTypeAdapter.read(jsonReader); // Read each element
elements.add(e);
}
jsonReader.endArray(); // Expect ]
return elements;
}
}
}
private static final class Pack {
Iterable<String> iterable;
Collection<String> collection;
List<String> list;
Map<String, String> map;
}
private static final Gson gson = new GsonBuilder()
.registerTypeAdapterFactory(getIterableMyTypeAdapterFactory())
.create();
public static void main(final String... args) {
final List<String> fooBar = ImmutableList.of("foo", "bar");
final Pack pack = new Pack();
pack.iterable = fooBar;
pack.collection = fooBar;
pack.list = fooBar;
pack.map = ImmutableMap.of("foo", "bar");
gson.toJson(pack, System.out);
}
輸出之前:
{"iterable":{},"collection":["foo","bar"],"list":["foo","bar"],"map":{"foo":"bar"}}
輸出后:
{"iterable":["foo","bar"],"collection":["foo","bar"],"list":["foo","bar"],"map":{"foo":"bar"}}
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