如何将这两个响应动态转换为适当的类型？
这取决于如何解释这里的“适当类型”，因为这会导致instanceof或访问者模式，以便在每次需要时尝试处理从 JSON 解析的对象时获取适当的类型。如果您无法更改 API，则可以平滑使用它的方式。这里可能的选择之一是处理此类响应，就好像一切都是列表一样。即使单个对象也可以作为仅包含一个元素的列表进行处理（许多库仅使用具有这一事实的序列/列表：Java 中的 Stream API、.NET 中的 LINQ、JavaScript 中的 jQuery 等）。
假设您有以下内容MyEntity类来处理从您需要的 API 获取的元素：
// For the testing purposes, package-visible final fields are perfect
// Gson can deal with final fields too
final class MyEntity {
final int id = Integer.valueOf(0); // not letting javac to inline 0 since it's primitive
final String name = null;
@Override
public String toString() {
return id + "=>" + name;
}
}
接下来，让我们创建一个类型适配器，它将始终对齐“true”列表和单个对象，就像它是一个列表一样：
final class AlwaysListTypeAdapter<T>
extends TypeAdapter<List<T>> {
private final TypeAdapter<T> elementTypeAdapter;
private AlwaysListTypeAdapter(final TypeAdapter<T> elementTypeAdapter) {
this.elementTypeAdapter = elementTypeAdapter;
}
static <T> TypeAdapter<List<T>> getAlwaysListTypeAdapter(final TypeAdapter<T> elementTypeAdapter) {
return new AlwaysListTypeAdapter<>(elementTypeAdapter);
}
@Override
@SuppressWarnings("resource")
public void write(final JsonWriter out, final List<T> list)
throws IOException {
if ( list == null ) {
out.nullValue();
} else {
switch ( list.size() ) {
case 0:
out.beginArray();
out.endArray();
break;
case 1:
elementTypeAdapter.write(out, list.iterator().next());
break;
default:
out.beginArray();
for ( final T element : list ) {
elementTypeAdapter.write(out, element);
}
out.endArray();
break;
}
}
}
@Override
public List<T> read(final JsonReader in)
throws IOException {
final JsonToken token = in.peek();
switch ( token ) {
case BEGIN_ARRAY:
final List<T> list = new ArrayList<>();
in.beginArray();
while ( in.peek() != END_ARRAY ) {
list.add(elementTypeAdapter.read(in));
}
in.endArray();
return unmodifiableList(list);
case BEGIN_OBJECT:
return singletonList(elementTypeAdapter.read(in));
case NULL:
return null;
case END_ARRAY:
case END_OBJECT:
case NAME:
case STRING:
case NUMBER:
case BOOLEAN:
case END_DOCUMENT:
throw new MalformedJsonException("Unexpected token: " + token);
default:
// A guard case: what if Gson would add another token someday?
throw new AssertionError("Must never happen: " + token);
}
}
}
Gson TypeAdapter设计为以流方式工作，因此从效率角度来看它们很便宜，但实施起来并不容易。这write()实现上面的方法只是为了不放throw new UnsupportedOperationException();（我假设您只阅读该 API，但不知道该 API 是否可能消耗“元素或列表”修改请求）。现在有必要创建一个类型适配器工厂，让 Gson 为每个特定类型选择正确的类型适配器：
final class AlwaysListTypeAdapterFactory
implements TypeAdapterFactory {
private static final TypeAdapterFactory alwaysListTypeAdapterFactory = new AlwaysListTypeAdapterFactory();
private AlwaysListTypeAdapterFactory() {
}
static TypeAdapterFactory getAlwaysListTypeAdapterFactory() {
return alwaysListTypeAdapterFactory;
}
@Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken)
throws IllegalArgumentException {
if ( List.class.isAssignableFrom(typeToken.getRawType()) ) {
final Type elementType = getElementType(typeToken);
// Class<T> instances can be compared with ==
final TypeAdapter<?> elementTypeAdapter = elementType == MyEntity.class ? gson.getAdapter(MyEntity.class) : null;
// Found supported element type adapter?
if ( elementTypeAdapter != null ) {
@SuppressWarnings("unchecked")
final TypeAdapter<T> castTypeAdapter = (TypeAdapter<T>) getAlwaysListTypeAdapter(elementTypeAdapter);
return castTypeAdapter;
}
}
// Not a type that can be handled? Let Gson pick a more appropriate one itself
return null;
}
// Attempt to detect the list element type
private static Type getElementType(final TypeToken<?> typeToken) {
final Type listType = typeToken.getType();
return listType instanceof ParameterizedType
? ((ParameterizedType) listType).getActualTypeArguments()[0]
: Object.class;
}
}
以及它到底是如何使用的：
private static final Type responseItemListType = new TypeToken<List<MyEntity>>() {
}.getType();
private static final Gson gson = new GsonBuilder()
.registerTypeAdapterFactory(getAlwaysListTypeAdapterFactory())
.create();
public static void main(final String... args) {
test("");
test("{\"id\":1,\"name\":\"name\"}");
test("[{\"id\":1,\"name\":\"name\"},{\"id\":1,\"name\":\"name\"}]");
test("[]");
}
private static void test(final String incomingJson) {
final List<MyEntity> list = gson.fromJson(incomingJson, responseItemListType);
System.out.print("LIST=");
System.out.println(list);
System.out.print("JSON=");
gson.toJson(list, responseItemListType, System.out); // no need to create an intermediate string, let it just stream
System.out.println();
System.out.println("-----------------------------------");
}
输出：
LIST=null
JSON=null
-----------------------------------
LIST=[1=>name]
JSON={"id":1,"name":"name"}
-----------------------------------
LIST=[1=>name, 1=>name]
JSON=[{"id":1,"name":"name"},{"id":1,"name":"name"}]
-----------------------------------
LIST=[]
JSON=[]
-----------------------------------
