DynamoDB JsonMarshaller无法反序列化对象列表

Question

我有一个Java类,它是DynamoDB中表的数据模型.我想使用Dynamo中的DynamoDBMapperto save和loaditems.该班的一名成员是List<MyObject>.所以我用它JsonMarshaller<List<MyObject>>来序列化和反序列化这个字段.

该列表可以通过成功序列化JsonMarshaller.但是,当我尝试检索条目并读取列表时,它会抛出异常:java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to MyObject.看起来JsonMarshaller将数据反序列化为LinkedHashMap而不是MyObject.我该怎样摆脱这个问题？

MCVE:

// Model.java
@DynamoDBTable(tableName = "...")
public class Model {
  private String id;
  private List<MyObject> objects;

  public Model(String id, List<MyObject> objects) {
    this.id = id;
    this.objects = objects;
  }

  @DynamoDBHashKey(attributeName = "id")
  public String getId() { return this.id; }
  public void setId(String id) { this.id = id; }

  @DynamoDBMarshalling(marshallerClass = ObjectListMarshaller.class)
  public List<MyObject> getObjects() { return this.objects; }
  public void setObjects(List<MyObject> objects) { this.objects = objects; }
}

---

// MyObject.java
public class MyObject {
  private String name;
  private String property;

  public MyObject() { }
  public MyObject(String name, String property) {
    this.name = name;
    this.property = property;
  }

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

  public String getProperty() { return this.property; }
  public void setProperty(String property) { this.property = property; }
}

---

// ObjectListMarshaller.java
public class ObjectListMarshaller extends JsonMarshaller<List<MyObject>> {}

---

// Test.java
public class Test {
  private static DynamoDBMapper mapper;

  static {
    AmazonDynamoDBClient client = new AmazonDynamoDBClient(new ProfileCredentialsProvider()
    mapper = new DynamoDBMapper(client);
  }

  public static void main(String[] args) {
    MyObject obj1 = new MyObject("name1", "property1");
    MyObject obj2 = new MyObject("name2", "property2");
    List<MyObject> objs = Arrays.asList(obj1, obj2);

    Model model = new Model("id1", objs);
    mapper.save(model); // success

    Model retrieved = mapper.load(Model.class, "id1");
    for (MyObject obj : retrieved.getObjects()) { // exception
    }
  }
}

Answer 1

在较新的版本中,只需使用:

@DynamoDBAttribute(attributeName = "things")
public List<Thing> getThings() {
    return things;
}

public void setThings(final List<Thing> things) {
    this.things = things;
}

鉴于Thing的注释:

@DynamoDBDocument
public class Thing {
}

Answer 2

这里的部分问题是整个DynamoDB Mapper SDK如何处理泛型.该interface DynamoDBMarshaller<T extends Object>有一个方法T unmarshall(Class<T> clazz, String obj),其中,所述类反序列化到作为参数传递.问题是存在类型擦除,并且SDK不提供易于处理的问题.杰克逊在某些情况下更聪明(JsonMarshaller使用杰克逊),这解释了为什么serialize方法正常工作.

您需要为反序列化提供更好的实现.你可以做到这一点的一种方法是实现DynamoDBMarshaller接口而不是扩展另一个(我的意见),这样你就可以更好地控制类型的序列化方式.

这是一个基本上是复制/粘贴的示例,JsonMarshaller在反序列化中进行了小调整,List以便为您提供一个想法:

import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMarshaller;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.ObjectWriter;
import com.fasterxml.jackson.databind.type.CollectionType;

import java.util.List;

import static com.amazonaws.util.Throwables.failure;

public class MyCustomMarshaller implements DynamoDBMarshaller<List<MyObject>> {

    private static final ObjectMapper mapper = new ObjectMapper();
    private static final ObjectWriter writer = mapper.writer();

    @Override
    public String marshall(List<MyObject> obj) {

        try {
            return writer.writeValueAsString(obj);
        } catch (JsonProcessingException e) {
            throw failure(e,
                          "Unable to marshall the instance of " + obj.getClass()
                          + "into a string");
        }
    }

    @Override
    public List<MyObject> unmarshall(Class<List<MyObject>> clazz, String json) {
        final CollectionType
            type =
            mapper.getTypeFactory().constructCollectionType(List.class, MyObject.class);
        try {
            return mapper.readValue(json, type);
        } catch (Exception e) {
            throw failure(e, "Unable to unmarshall the string " + json
                             + "into " + clazz);
        }
    }
}

Answer 3

DynamoDBMarshaller 现在已弃用，但我遇到了与 DynamoDBTypeConvertedJson 完全相同的问题。如果您想将集合作为 JSON 存储在 DynamoDBMapper 类中，请使用 DynamoDBTypeConverted 并编写自定义转换器类（不要使用 DynamoDBTypeConvertedJson，它不会在未转换时返回您的集合）。

这是使用 DynamoDBTypeConverted 的解决方案

// Model.java
@DynamoDBTable(tableName = "...")
public class Model {
  private String id;
  private List<MyObject> objects;

  public Model(String id, List<MyObject> objects) {
    this.id = id;
    this.objects = objects;
  }

  @DynamoDBHashKey(attributeName = "id")
  public String getId() { return this.id; }
  public void setId(String id) { this.id = id; }

  @DynamoDBTypeConverted(converter = MyObjectConverter.class)
  public List<MyObject> getObjects() { return this.objects; }
  public void setObjects(List<MyObject> objects) { this.objects = objects; }
}

——

public class MyObjectConverter implements DynamoDBTypeConverter<String, List<MyObject>> {

    @Override
    public String convert(List<Object> objects) {
        //Jackson object mapper
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            String objectsString = objectMapper.writeValueAsString(objects);
            return objectsString;
        } catch (JsonProcessingException e) {
            //do something
        }
        return null;
    }

    @Override
    public List<Object> unconvert(String objectssString) {
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            List<Object> objects = objectMapper.readValue(objectsString, new TypeReference<List<Object>>(){});
            return objects;
        } catch (JsonParseException e) {
            //do something
        } catch (JsonMappingException e) {
            //do something
        } catch (IOException e) {
            //do something
        }
        return null;
    }
}