无法从START_ARRAY标记中反序列化java.util.HashMap的实例

Question

我在使用jackson-core-2.7.3.jar解析JSON时遇到问题你可以从这里得到它们http://repo1.maven.org/maven2/com/fasterxml/jackson/core/

我的JSON文件是

[
    {
        "Name":  "System Idle Process",
        "CreationDate":  "20160409121836.675345+330"
    },
    {
        "Name":  "System",
        "CreationDate":  "20160409121836.675345+330"
    },
    {
        "Name":  "smss.exe",
        "CreationDate":  "20160409121836.684966+330"
    }
]

而Java代码是我试图解析的

byte[] mapData = Files.readAllBytes(Paths.get("process.txt"));
Map<String,String> myMap = new HashMap<String, String>();
ObjectMapper objectMapper=new ObjectMapper();
myMap = objectMapper.readValue(mapData, HashMap.class);
System.out.println("Map is: "+myMap);

但在执行时我收到了错误

com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.util.HashMap out of START_ARRAY token
 at [Source: [B@34ce8af7; line: 1, column: 1]
at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:216)
at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:873)
at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:869)
at com.fasterxml.jackson.databind.deser.std.StdDeserializer._deserializeFromEmpty(StdDeserializer.java:874)
at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:337)
at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:26)
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3789)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2872)

我已经尝试搜索stackoverflow但无法找到这种类型的JSON的匹配解决方案.

任何帮助,将不胜感激.

注意:这里JSON提到的是不同的,JSON没有Key,对于第一个元素,它直接具有值,并且在该值内部具有key:value对.我不知道如何访问key:value值内的对.

Answer 1

创建一个简单的pojoClass First

class MyClass
{
@JsonProperty
private String Name;
@JsonProperty
private String CreationDate;
}

并使用此代码...

byte[] mapData = Files.readAllBytes(Paths.get("process.txt"));

ObjectMapper objectMapper=new ObjectMapper();
//add this line  
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);    
List<MyClass> myObjects = mapper.readValue(mapData , new TypeReference<List<MyClass>>(){});

要么

byte[] mapData = Files.readAllBytes(Paths.get("process.txt"));

ObjectMapper objectMapper=new ObjectMapper();
 //add this line  
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);    

List<MyClass> myObjects = mapper.readValue(mapData , mapper.getTypeFactory().constructCollectionType(List.class, MyClass.class));

myObjects将包含List的MyClass.现在您可以根据您的要求访问此列表.

Answer 2

看起来您的文件表示具有Name和CreationDate字段的对象列表.

所以你必须使用List代替HashMap到ObjectMapper,代码如下:

List<HashMap> dataAsMap = objectMapper.readValue(mapData, List.class);

Answer 3

好吧，您正在尝试将包含对象数组/列表的json 字符串转换为object。

例如：{ "key": "value"}是你想要转换的json，但实际上你正在做，

[{ "key": "value"}]。

简单的修复方法是从字符串中删除第一个和最后一个字符并尝试。希望能帮助到你;）

Answer 4

因为你想反序列化，引发此异常List的Map。

解决方案是创建一个TypeReference List<Map<String, Object>>：

List<Map<String, Object>> myObjects = 
          mapper.readValue(mapData , new TypeReference<List<Map<String, Object>>>(){});