[英]Java: How to return a generic type
我们有一个服务调用各种 rest 端点,并将 JSON 响应转换为 object。
我们已经看到它在 java 中完成,服务可以返回泛型类型,但无法弄清楚语法。
假设我们有一堆模型用于不同的 API 响应,以及一种调用端点并返回其中一个的方法。 例如
public class MyServiceImpl implements MyService{
@Override
public <T> T doGet( String endpoint) {
:
HttpResponse<String> response = httpClient.send(request, HttpResponse.BodyHandlers.ofString());
:
Gson gson = new Gson();
T model = new T();
model = gson.fromJson(response.getBody(), model.class));
return(model);
}
并因此称为:
SomeModel model = myService.doGet("https://somesite.com/someendpoint")
显然,上面的代码是行不通的,因为你不能做“new T()”
httpClient 有一个内置的方法来做到这一点,而不是将响应作为字符串返回,但我们不能使用它有两个原因:
正如Yuliya Sheludyakova 提到的,你不能做new T();
因为javac
由于 generics 擦除而缺少类型信息。 即使此时已知类型和构造函数,也不需要创建新的 object,因为 Gson 在反序列化时返回新的 object(Yuliya Sheludyak 也提到了这一点)。
What you can do is providing the type (an instance of java.lang.reflect.Type
, java.lang.Class
is one of them with type-limited capabilities) to the fromJson
method invocation so that Gson would deserialize the payload into an object提供的类型。
public interface IService {
@Nullable
<T> T doGet(@Nonnull URL url, @Nonnull Type type)
throws IOException;
@Nullable
<T> T doGet(@Nonnull URL url, @Nonnull TypeToken<? extends T> typeToken)
throws IOException;
}
final class Service
implements IService {
// Gson instances are thread-safe and may be expensive on instantiation
private static final Gson gson = new Gson();
// Unsafe: the return type T and Type type are not bound to each other
@Nullable
@Override
public <T> T doGet(@Nonnull final URL url, @Nonnull final Type type)
throws IOException {
// Prefer not using string buffers that may be very expensive for large payloads
// Streams are much cheaper
try ( final JsonReader jsonReader = new JsonReader(new InputStreamReader(doGet(url))) ) {
return gson.fromJson(jsonReader, type);
}
}
// Safe: javac can detect if the return type and the type token are bound
@Nullable
@Override
public <T> T doGet(@Nonnull final URL url, @Nonnull final TypeToken<? extends T> typeToken)
throws IOException {
// Prefer not using string buffers that may be very expensive for large payloads
// Streams are much cheaper
try ( final JsonReader jsonReader = new JsonReader(new InputStreamReader(doGet(url))) ) {
// TypeToken.getType() is guaranteed to provide a correct bound type
return gson.fromJson(jsonReader, typeToken.getType());
}
}
private static <T> T doGet(@Nonnull final URL url) {
throw new AssertionError("Stub! " + url);
}
}
安全方法的使用示例:
private static final TypeToken<List<User>> userListTypeToken = new TypeToken<List<User>>() {};
public static void main(final String... args)
throws IOException {
final IService service = new Service();
final List<User> users = service.doGet(new URL("http://localhost:8080/users"), userListTypeToken);
for ( final User user : users ) {
System.out.println(user);
}
}
请注意,Spring RestTemplate、Retrofit 和其他库也是如此,并且可能值得在您的代码中使用这些库。
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