使用“key”和“value”属性反序列化 JSON 映射不适用于 Jackson

Posted 2023-04-12

【中文标题】使用“key”和“value”属性反序列化 JSON 映射不适用于 Jackson

【英文标题】：Deserializing JSON map with "key" & "value" properties does not work with Jackson

【发布时间】：2014-01-09 03:58:46

【问题描述】：

问题

首先，下面的序列化 JSON 序列化有意义吗？ 如果是这样，为什么我没有取回地图？ 在反序列化方面我能做些什么？

Map 属性的 JSON 序列化（摘录）：

  "attributes": 
    "entry": [
      
        "key": "operating system",
        "value": "GNU/Linux"
      ,
      
        "key": "allergies",
        "value": "weed"
      
    ]
  

用于反序列化的 POJO：

class Contact implements Comparable<Contact>, Serializable 
    @JsonProperty("attributes")
    private Map<String, String> attributes;
    ...

导致此异常：

Thread-4889 An exception occurred during request network execution :Could not read JSON: Can not deserialize instance of java.lang.String out of START_ARRAY token
            at [Source: libcore.net.http.FixedLengthInputStream@43822760; line: 1, column: 17] (through reference chain: com.example.model.Contact["attributes"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_ARRAY token
            at [Source: libcore.net.http.FixedLengthInputStream@43822760; line: 1, column: 17] (through reference chain: com.example.model.Contact["attributes"])
    org.springframework.http.converter.HttpMessageNotReadableException: Could not read JSON: Can not deserialize instance of java.lang.String out of START_ARRAY token
            at [Source: libcore.net.http.FixedLengthInputStream@43822760; line: 1, column: 17] (through reference chain: com.example.model.Contact["attributes"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_ARRAY token
            at [Source: libcore.net.http.FixedLengthInputStream@43822760; line: 1, column: 17] (through reference chain: com.example.model.Contact["attributes"])
            at org.springframework.http.converter.json.MappingJackson2HttpMessageConverter.readInternal(MappingJackson2HttpMessageConverter.java:126)
            at org.springframework.http.converter.AbstractHttpMessageConverter.read(AbstractHttpMessageConverter.java:147)
            at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:76)
            at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:484)
            at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:439)
            at org.springframework.web.client.RestTemplate.getForObject(RestTemplate.java:237)
            at com.example.providers.Query$1.loadDataFromNetwork(Query.java:99)
            at com.octo.android.robospice.request.CachedSpiceRequest.loadDataFromNetwork(CachedSpiceRequest.java:45)
            at com.octo.android.robospice.request.RequestRunner.processRequest(RequestRunner.java:130)
            at com.octo.android.robospice.request.RequestRunner$1.run(RequestRunner.java:197)
            at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:390)
            at java.util.concurrent.FutureTask.run(FutureTask.java:234)
            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
            at java.lang.Thread.run(Thread.java:841)
     Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_ARRAY token
            at [Source: libcore.net.http.FixedLengthInputStream@43822760; line: 1, column: 17] (through reference chain: com.example.model.Contact["attributes"])
            at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:691)
            at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:46)
            at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:11)
            at com.fasterxml.jackson.databind.deser.std.MapDeserializer._readAndBindStringMap(MapDeserializer.java:430)
            at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:312)
            at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:26)
            at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:525)
            at com.fasterxml.jackson.databind.deser.impl.FieldProperty.deserializeAndSet(FieldProperty.java:106)
            at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:242)
            at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:118)
            at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:227)
            at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.des

反序列化后在调试器中检查时的属性对象：

改成后进一步检查：

@JsonProperty("attributes")
private Map<String, List<Map<String, String>>> attributes;

依赖关系：

com.fasterxml.jackson.core:jackson-core:2.3.0 com.fasterxml.jackson.core:jackson-databind:2.3.0 com.fasterxml.jackson.core:jackson-annotations:2.3.0

【参考方案1】：

如果我们将Map 转换为json：

    Map<String, String> map = new HashMap<String, String>();
    map.put("operating system", "GNU/Linux");
    map.put("allergies", "weed");

输出将是：

"operating system":"GNU/Linux","allergies":"weed"

我们可以看到没有key/value。

解决方案

WrapperObject

@JsonIgnoreProperties(ignoreUnknown = true)
public class WrapperObject  // we can give any name to class, its only external     

    private Attributes attributes;

    public WrapperObject() 

    public Attributes getAttributes() 
        return attributes;
        

属性

@JsonIgnoreProperties(ignoreUnknown = true)  
public class Attributes 

    public Attributes() 

    private ArrayList<Entry> entry;  

    public ArrayList<Entry> getEntry() 
        return entry;
        

参赛作品

@JsonIgnoreProperties(ignoreUnknown = true)    
public  class Entry 

    private String key;
    private String value;
       
    
    public Entry() 


    public String getKey() 
        return key;
    


    public String getValue() 
        return value;
    

启动器

public static void main(String[] args) throws JsonParseException, JsonMappingException,     IOException 
    String str = "" + 
            "  \"attributes\": " + 
            "    \"entry\": [" + 
            "      " + 
            "        \"key\": \"operating system\"," + 
            "        \"value\": \"GNU/Linux\"" + 
            "      ," + 
            "      " + 
            "        \"key\": \"allergies\"," + 
            "        \"value\": \"weed\"" + 
            "      " + 
            "    ]" + 
            "  " + 
            "";

    
    
    ObjectMapper mapper = new ObjectMapper();
    WrapperObject mj = mapper.readValue(str, WrapperObject.class);

    if(mj == null)
        System.err.println("null");
    
    // dummy check
    System.out.println(mj.getAttributes().getEntry().get(0).getKey());

输出：

operating system