如何将泛型 class 作为方法的泛型参数传递

Question

我在将 class 作为方法 f.e 的通用参数传递时遇到问题。我有一个简单的方法：

<T> T sendRequest(SomeRestApiRequest request, Class<T> responseClass)

哪个解析响应指定的表单。我这样使用它们：

ItemListJSON itemList = new ItemListJSON();
itemList = someRestClient.sendRequest(req, ItemListJSON.class);

对于 ItemListJSON.class 看起来像这样：

@JsonInclude(JsonInclude.Include.NON_NULL)
@JsonPropertyOrder({"totalSize","items"})
public class ItemListJSON {

    @JsonProperty("items")
    private List<SalonJSON> items;

    @JsonProperty("totalSize")
    private int totalSize;

    //...getters, setters...
}

一切都很好。但我的问题是：

是否可以将泛型 class 作为 sendRequest 方法的参数传递？

我希望 ItemListJSON class 是通用的，在我的例子中：

@JsonInclude(JsonInclude.Include.NON_NULL)
@JsonPropertyOrder({"totalSize","items"})
public class ItemListJSON<T> {

    @JsonProperty("items")
    private List<T> items;

    @JsonProperty("totalSize")
    private int totalSize;

    //...getters, setters...
}

但是当我尝试以这种方式使用 sendRequest 方法时：

ItemListJSON<SalonJSON> itemList = new ItemListJSON<SalonJSON>();
itemList = someRestClient.sendRequest(req, ItemListJSON.class);

我在 Eclipse 上收到警告 IDE

Type safety: The expression of type ItemListJSON needs unchecked conversion to conform to ItemListJSON

当调用方法时，我在服务器控制台中遇到错误：

SEVERE: Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to xxx.ServiceCategoryJSON] with root cause
java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to xxx.ServiceCategoryJSON

@编辑：

我调试了我的 sendRequest 方法，我发现错误发生在 processResponse 方法中，其中 ObjectMapper 将响应映射到对象。

private <T> T processResponse(Response response, Class<T> responseClass) throws ParseException, IOException {
        ObjectMapper om = new ObjectMapper();
        om.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);     
        return om.readValue(response.getBody(), responseClass); //throw exception
    }

Answer 1

使用

ParameterizedTypeReference<ItemListJSON<SalonJSON>> typeRef = new ParameterizedTypeReference<ItemListJSON<SalonJSON>>() {};

查看 the rest template intro

中的代码片段

RestTemplate restTemplate = new RestTemplate();
ParameterizedTypeReference<List<String>> listOfString = new ParameterizedTypeReference<List<String>>() {};
ResponseEntity<List<String>> response= restTemplate.exchange(baseUrl,HttpMethod.GET,null, listOfString);
HttpHeaders headers = response.getHeaders();
MediaType contentType = headers.getContentType();
long date = headers.getDate();
List<String> getOrDefault = headers.getOrDefault("X-Forwarded", Collections.singletonList("Does not exists"));

Answer 2

您可以通过传递 com.fasterxml.jackson.core.type.TypeReference 而不是 Class<T>

来做到这一点

public class GenericSerializationTest {

    @Data //lombok
    public static class ItemListJSON<T> {
        private List<T> items;
    }

    @Data //lombok
    public static class StructureExample {
        private final String name;
        private final Double price;
    }

    public static class Sender {
        private final ObjectMapper objectMapper = new ObjectMapper();

        public <T> T sendRequest(String json, TypeReference typeReference) throws IOException {
            //sender logic - in this case I assume that json is API response
            return objectMapper.readValue(json, typeReference);
        }
    }

    @Test
    public void testMethod() throws IOException {
        Sender sender = new Sender();
        ItemListJSON<StructureExample> test = sender.sendRequest("{\"items\": [{\"name\":\"MacBook Pro\",\"price\":101.345}, {\"name\":\"MacMini\",\"price\":102.345}]}", new TypeReference<ItemListJSON<StructureExample>>() {});

        assertEquals("Should contain only 2 items", 2, test.getItems().size());
        assertEquals("Name of first item is not correct", "MacBook Pro", test.getItems().get(0).getName());
        assertEquals("Name of second item is not correct", "MacMini", test.getItems().get(1).getName());
    }
}

如何将泛型 class 作为方法的泛型参数传递

How to pass generic class as a generic parameter of method

java

spring

generic-collections