将LinkedHashMap转换为复杂对象

问题描述

我有一个应用程序使用Jackson将一些数据存储在DynamoDB中，以将我的复杂对象转换为JSON。

例如，我编组的对象可能如下所示：

  private String aString; 
 private List< SomeObject> someObjectList; 
  

WhereObject可能如下所示：

  private int anInteger; 
 private SomeOtherObject; 
  

，SomeOtherObject可能如下所示：

  private long aLong; 
 private float aFloat; 
  

这是一个很好的对象获取编组没有问题，并作为JSON字符串存储在DB。 / p>

当到达从DynamoDB检索数据的时间Jackson自动检索JSON并将其转换回来... EXCEPT'someObjectList'作为 List< LinkedHashMap> 不是 List< SomeObject> ！这是杰克逊的标准行为，它不是一个错误，这是发生。

现在这导致一个问题。我的代码库认为它处理一个 List< SomeObject> 但现实是它处理一个 List< LinkedHashMap> ！我的问题是如何让我的LinkedHashMap回一个SomeObject。显然这是一个手动过程，但是我的意思是我不能提取值。

如果我这样做：

  for（LinkedHashMap lhm：someObjectList）{
 //将值转换回
} 
  pre> 
 
 
我收到一个编译错误，告诉我someObjectList的类型是SomeObject，不是LinkedHashMap。
 
 
 
我这样做：
  for（someObject lhm：someObjectList）{
 //将值转换回
 } 
  
 
 
我得到一个运行时错误告诉我LinkedHashMap不能转换为SomeObject。
解决方案
您可以使用 ObjectMapper.convertValue（）为整个列表。但你需要知道类型转换为：
  POJO pojo = mapper.convertValue（singleObject，POJO.class）; 
 //或：
 List< POJO> pojos = mapper.convertValue（listOfObjects，new TypeReference< List< POJO>（）{}）; 
  
这与功能相同：
  byte [] json = mapper.writeValueAsBytes（singleObject）; 
 POJO pojo = mapper.readValue（json，POJO.class）; 
  
但是避免将数据实际序列化为JSON，而是使用内存中事件序列作为中间步骤。
 
I've got an application that stores some data in DynamoDB using Jackson to marshall my complex object into a JSON. 
For example the object I'm marshalling might look like this:
private String aString;
private List<SomeObject> someObjectList;
Where SomeObject might look like this:
private int anInteger;
private SomeOtherObject;
and SomeOtherObject might look like this:
private long aLong;
private float aFloat;
This is fine an the object gets marshalled no problem and stored in the DB as a JSON string.
When it comes time to retrieve the data from DynamoDB Jackson automatically retrieves the JSON and converts it back... EXCEPT that 'someObjectList' is returned as a List<LinkedHashMap> not as a List<SomeObject>! This is standard behaviour for Jackson, its not a mistake that this is happening.
So now this leads to a problem. My code base thinks its dealing with a List<SomeObject> but the reality is that its handling a List<LinkedHashMap>! My question is how do I get my LinkedHashMap back into a 'SomeObject'. Obviously this is a manual process but what I mean is I can't even extract the values.
If I do this:
for (LinkedHashMap lhm : someObjectList) {
    // Convert the values back
}
I get a compile error telling me that someObjectList is of type 'SomeObject' not LinkedHashMap.
If I do this:
for (SomeObject lhm : someObjectList) {
    // Convert the values back
}
I get a runtime error telling me that LinkedHashMap cannot be cast to 'SomeObject'.
 解决方案 
You can use ObjectMapper.convertValue(), either value by value or even for the whole list. But you need to know the type to convert to:
POJO pojo = mapper.convertValue(singleObject, POJO.class);
// or:
List<POJO> pojos = mapper.convertValue(listOfObjects, new TypeReference<List<POJO>>() { });
this is functionally same as if you did:
byte[] json = mapper.writeValueAsBytes(singleObject);
POJO pojo = mapper.readValue(json, POJO.class);
but avoids actual serialization of data as JSON, instead using an in-memory event sequence as the intermediate step.
                        
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