[英]How to convert dynamic JSON reponse with Java Gson library
我有一個可以返回JSON數組或對象的API。 JSON對象示例
{
"id": 1,
"name": "name"
}
JSON數組:
[
{
"id": 1,
"name": "name"
},
{
"id": 1,
"name": "name"
}
]
將JSON對象響應映射到POJO時,我使用:
MyEntity myEntity = new Gson().fromJson(jsonString, MyEntity.class);
當將JSON數組響應映射到POJO數組時,我使用:
MyEntity[] myEntity = new GSON().fromJson(jsonString, MyEntity[].class);
如何將這兩個響應動態轉換為適當的類型?
注意:我無法修改服務器響應,這是一個公共API。
謝謝!
編輯:
我正在嘗試實現一種自動執行此操作的方法,但是我缺少了一些東西。 方法
public <T> T convertResponseToEntity(Class<T> classOfT)
{
JsonElement jsonElement = this.gson.fromJson(getResponseAsString(), JsonElement.class);
if (jsonElement.isJsonArray()) {
Type listType = new TypeToken<T>(){}.getType();
return this.gson.fromJson(getResponseAsString(), listType);
}
return this.gson.fromJson(getResponseAsString(), (Type) classOfT);
}
它返回LinkedTreeMap
的列表。 如何修改代碼以返回與Object[]
相同的內容?
只需將其解析為JsonElement並檢查實際的元素類型:
Gson g = new Gson();
JsonParser parser = new JsonParser();
JsonElement e = parser.parse( new StringReader(jsonString) );
if(e instanceof JsonObject) {
MyEntity myEntity = g.fromJson(e, MyEntity.class);
} else {
MyEntity[] myEntity = g.fromJson(e, MyEntity[].class);
}
如何將這兩個響應動態轉換為適當的類型?
這取決於如何在此處解釋“適當的類型”,因為一旦您每次需要處理從JSON解析的對象時,它將導致instanceof
或訪客模式獲得適當的類型。 如果您無法更改API,則可以簡化其使用方式。 這里可能的選項之一是像對待所有內容一樣處理此類響應。 甚至單個對象也可以僅作為一個元素處理為列表(許多庫僅使用具有以下事實的序列/列表進行處理:Java中的Stream API,.NET中的LINQ,JavaScript中的jQuery等)。
假設您具有以下MyEntity
類來處理從所需的API獲得的元素:
// 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;
}
}
接下來,讓我們創建一個類型適配器,該適配器將始終將“ true”列表和單個對象對齊,就好像它是列表一樣:
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
設計為以流方式工作,因此從效率角度來看它們很便宜,但實現起來並不那么容易。 實現上述write()
方法只是為了不throw new UnsupportedOperationException();
在那里(我假設您只閱讀了該API,但不知道該API是否會消耗“元素或列表”修改請求)。 現在,必須創建一個類型適配器工廠,以讓Gson為每種特定類型選擇正確的類型適配器:
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;
}
}
以及如何使用它:
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("-----------------------------------");
}
輸出:
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=[]
-----------------------------------
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