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 的DynamoDBMapperto
save和loaditem。班上一名成员是一名List<MyObject>。因此，我使用JsonMarshaller<List<MyObject>>来对该字段进行序列化和反序列化。

该列表可以通过成功地序列化JsonMarshaller。但是，当我尝试取回条目并阅读列表时，它会引发异常：java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to MyObject。好像JsonMarshaller将数据反序列化为LinkedHashMap而不是MyObject。我如何摆脱这个问题？

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

问题的一部分是整个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，所述类反序列化到被作为参数传递。问题在于存在类型擦除，而SDK并不能轻松解决该问题。杰克逊在某些情况下会更聪明（JsonMarshaller使用杰克逊），这说明了该serialize方法正确工作的原因。

您需要为反序列化提供更好的实现。您可以执行此操作的一种方法是实现DynamoDBMarshaller接口，而不是扩展另一个接口（我认为），以便您可以更好地控制类型的序列化方式。

这是一个本质上是复制/粘贴的示例，JsonMarshaller对反序列化进行了细微调整，List以使您有一个想法：

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);
        }
    }
}