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[英]Java terminology for differentiating runtime type from compile-time type
[英]Why does Gson serializes runtime type in list, not specified compile-time type?
為什么序列化時Gson似乎忽略了嵌套的泛型類型聲明?
我試圖讓Gson使用我指定的編譯時類型,而不是列表中對象的運行時類型。 我也為A.java
使用了一個抽象超類,但是下面的例子有同樣的問題。
public class A {
public String foo;
}
public class B extends A {
public String bar;
}
public static void main( String[] args ) {
Gson gson = new Gson();
B b = new B();
b.foo = "foo";
b.bar = "bar";
List<A> list = new ArrayList<A>();
list.add(b);
System.out.println(gson.toJson(b, new TypeToken<A>(){}.getType()));
System.out.println(gson.toJson(b, new TypeToken<B>(){}.getType()));
System.out.println(gson.toJson(list, new TypeToken<List<A>>(){}.getType()));
System.out.println(gson.toJson(list, new TypeToken<List<B>>(){}.getType()));
}
輸出:
{"foo":"foo"}
{"bar":"bar","foo":"foo"}
[{"bar":"bar","foo":"foo"}]
[{"bar":"bar","foo":"foo"}]
預期:
{"foo":"foo"}
{"bar":"bar","foo":"foo"}
[{"foo":"foo"}]
[{"bar":"bar","foo":"foo"}]
如果您不在乎原因,請滾動到底部,只需要修復即可。
Gson的默認CollectionTypeAdapterFactory
將其元素類型適配器包裝在稱為TypeAdapterRuntimeTypeWrapper
東西中。 選擇適當的適配器時,它使用以下優先級 :
// Order of preference for choosing type adapters
// First preference: a type adapter registered for the runtime type
// Second preference: a type adapter registered for the declared type
// Third preference: reflective type adapter for the runtime type (if it is a sub class of the declared type)
// Fourth preference: reflective type adapter for the declared type
在這種情況下,第三個優先級是B
的適配器,而第四個優先級是A的適配器。在使用默認序列化程序時,這是不可避免的,因為CollectionTypeAdapterFactory
沒有條件 :
public Adapter(Gson context, Type elementType,
TypeAdapter<E> elementTypeAdapter,
ObjectConstructor<? extends Collection<E>> constructor) {
this.elementTypeAdapter =
new TypeAdapterRuntimeTypeWrapper<E>(context, elementTypeAdapter, elementType);
this.constructor = constructor;
}
不使用CollectionTypeAdapterFactory
,不存在此包裝器,這就是為什么在前兩個示例中不發生這種包裝的原因。
解決此問題的唯一方法是注冊自定義序列化程序。 在您的用例中,為A
編寫一個可以解決問題:
public class ATypeAdapter extends TypeAdapter<A> {
public A read(JsonReader reader) throws IOException {
if (reader.peek() == JsonToken.NULL) {
reader.nextNull();
return null;
}
reader.beginObject();
String name = reader.nextName();
if(!"foo".equals(name)) throw new JsonSyntaxException("Expected field named foo");
A a = new A();
a.foo = reader.nextString();
reader.endObject();
return a;
}
public void write(JsonWriter writer, A value) throws IOException {
if (value == null) {
writer.nullValue();
return;
}
writer.beginObject();
writer.name("foo");
writer.value(value.foo);
writer.endObject();
}
}
然后,如果您這樣做:
public static void main( String[] args ) {
GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(new TypeToken<A>(){}.getType(), new ATypeAdapter());
Gson gson = builder.create();
B b = new B();
b.foo = "foo";
b.bar = "bar";
List<A> list = new ArrayList<A>();
list.add(b);
System.out.println(gson.toJson(b, new TypeToken<A>(){}.getType()));
System.out.println(gson.toJson(b, new TypeToken<B>(){}.getType()));
System.out.println(gson.toJson(list, new TypeToken<List<A>>(){}.getType()));
System.out.println(gson.toJson(list, new TypeToken<List<B>>(){}.getType()));
}
您將獲得預期的輸出:
{"foo":"foo"}
{"bar":"bar","foo":"foo"}
[{"foo":"foo"}]
[{"bar":"bar","foo":"foo"}]
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