如何将泛型类作为方法的泛型参数传递

Question

我有一个问题，传递类作为方法的通用参数，例如。我有一个简单的方法：

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

解析对指定表单的响应。我以这种方式使用它们：

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：

是否可以将泛型类作为sendRequest方法的参数传递？

我希望ItemListJSON类是通用的，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 IDE上收到了警告

类型安全：ItemListJSON类型的表达式需要未经检查的转换以符合ItemListJSON

当调用方法时，我在服务器控制台中出错：

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：

我重新编写了sendRequest方法，发现processResponse方法发生了错误，其中是ObjectMapper对对象的响应映射。

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

你可以通过传递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());
    }
}

Answer 2

使用

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

请参阅qazxsw poi的代码片段

the rest template intro