Jackson 和泛型類型引用

Question

我想將 jackson json 庫用於通用方法，如下所示：

public MyRequest<T> tester() {
    TypeReference<MyWrapper<T>> typeRef = new TypeReference<MyWrapper<T>>();  
    MyWrapper<T> requestWrapper = (MyWrapper<T>) JsonConverter.fromJson(jsonRequest, typeRef);
    return requestWrapper.getRequest();
}

public class MyWrapper<T> {

    private MyRequest<T> request;

    public MyRequest<T> getRequest() {
        return request;
    }

    public void setRequest(MyRequest<T> request) {
        this.request = request;
    }
}

public class MyRequest<T> {
     private List<T> myobjects;
        
     public void setMyObjects(List<T> ets) {
         this.myobjects = ets;
     }

     @NotNull
     @JsonIgnore
     public T getMyObject() {
         return myobjects.get(0);
     }
}

現在的問題是，當我調用請求對象內的 getMyObject() 時，Jackson 將嵌套的自定義對象作為 LinkedHashMap 返回。 有什么方法可以指定需要返回 T 對象嗎？ 例如：如果我發送了 Customer 類型的對象，那么應該從該列表返回 Customer？

Answer 1

這是 Java 類型擦除的一個眾所周知的問題：T 只是一個類型變量，您必須指明實際的類，通常作為 Class 參數。 如果沒有這些信息，最好的辦法就是使用邊界； 並且普通的 T 與“T extends Object”大致相同。 然后 Jackson 將 JSON 對象綁定為 Maps。

在這種情況下，tester方法需要訪問Class，你可以構造

JavaType type = mapper.getTypeFactory().
  constructCollectionType(List.class, Foo.class)

接着

List<Foo> list = mapper.readValue(new File("input.json"), type);

Answer 2

'JavaType' 有效！！ 我試圖將 json String 中的 List 解組（反序列化）到 ArrayList java Objects，並且幾天以來一直在努力尋找解決方案。
下面是最終給我解決方案的代碼。 代碼：

JsonMarshallerUnmarshaller<T> {
    T targetClass;

    public ArrayList<T> unmarshal(String jsonString) {
        ObjectMapper mapper = new ObjectMapper();

        AnnotationIntrospector introspector = new JacksonAnnotationIntrospector();
        mapper.getDeserializationConfig()
            .withAnnotationIntrospector(introspector);

        mapper.getSerializationConfig()
            .withAnnotationIntrospector(introspector);
        JavaType type = mapper.getTypeFactory().
            constructCollectionType(
                ArrayList.class, 
                targetclass.getClass());

        try {
            Class c1 = this.targetclass.getClass();
            Class c2 = this.targetclass1.getClass();
            ArrayList<T> temp = (ArrayList<T>) 
                mapper.readValue(jsonString,  type);
            return temp ;
        } catch (JsonParseException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return null ;
    }  
}

Answer 3

我修改了rushidesai1 的答案以包含一個工作示例。

JsonMarshaller.java

import java.io.*;
import java.util.*;

public class JsonMarshaller<T> {
    private static ClassLoader loader = JsonMarshaller.class.getClassLoader();

    public static void main(String[] args) {
        try {
            JsonMarshallerUnmarshaller<Station> marshaller = new JsonMarshallerUnmarshaller<>(Station.class);
            String jsonString = read(loader.getResourceAsStream("data.json"));
            List<Station> stations = marshaller.unmarshal(jsonString);
            stations.forEach(System.out::println);
            System.out.println(marshaller.marshal(stations));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @SuppressWarnings("resource")
    public static String read(InputStream ios) {
        return new Scanner(ios).useDelimiter("\\A").next(); // Read the entire file
    }
}

輸出

Station [id=123, title=my title, name=my name]
Station [id=456, title=my title 2, name=my name 2]
[{"id":123,"title":"my title","name":"my name"},{"id":456,"title":"my title 2","name":"my name 2"}]

JsonMarshallerUnmarshaller.java

import java.io.*;
import java.util.List;

import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.introspect.JacksonAnnotationIntrospector;

public class JsonMarshallerUnmarshaller<T> {
    private ObjectMapper mapper;
    private Class<T> targetClass;

    public JsonMarshallerUnmarshaller(Class<T> targetClass) {
        AnnotationIntrospector introspector = new JacksonAnnotationIntrospector();

        mapper = new ObjectMapper();
        mapper.getDeserializationConfig().with(introspector);
        mapper.getSerializationConfig().with(introspector);

        this.targetClass = targetClass;
    }

    public List<T> unmarshal(String jsonString) throws JsonParseException, JsonMappingException, IOException {
        return parseList(jsonString, mapper, targetClass);
    }

    public String marshal(List<T> list) throws JsonProcessingException {
        return mapper.writeValueAsString(list);
    }

    public static <E> List<E> parseList(String str, ObjectMapper mapper, Class<E> clazz)
            throws JsonParseException, JsonMappingException, IOException {
        return mapper.readValue(str, listType(mapper, clazz));
    }

    public static <E> List<E> parseList(InputStream is, ObjectMapper mapper, Class<E> clazz)
            throws JsonParseException, JsonMappingException, IOException {
        return mapper.readValue(is, listType(mapper, clazz));
    }

    public static <E> JavaType listType(ObjectMapper mapper, Class<E> clazz) {
        return mapper.getTypeFactory().constructCollectionType(List.class, clazz);
    }
}

站.java

public class Station {
    private long id;
    private String title;
    private String name;

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return String.format("Station [id=%s, title=%s, name=%s]", id, title, name);
    }
}

數據.json

[{
  "id": 123,
  "title": "my title",
  "name": "my name"
}, {
  "id": 456,
  "title": "my title 2",
  "name": "my name 2"
}]