无法从 START_ARRAY 令牌反序列化 java.util.HashMap 的实例

Question

I am facing issue while parsing JSON using jackson-core-2.7.3.jar You can get them from here http://repo1.maven.org/maven2/com/fasterxml/jackson/core/

My JSON File is

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

and the Java Code is by which I am trying to parse this is

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);

But upon execution I am getting the error

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)

I have tried searching over stackoverflow but couldnot find a matchable solution to this type of JSON.

Any help would be appreciated.

NOTE: This JSON mentioned here is different a JSON without Key , for the first element it has value directly and inside that value it has key:value pair. I am not sure how do I access key:value pair which is inside a value.

Answer 1

This exception is raised because you're trying to deserialize a List in a Map .

The solution is create a TypeReference of List<Map<String, Object>> :

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

Answer 2

Create a simple pojo Class First

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

and use this code...

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>>(){});

or

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 will contains the List of MyClass . Now you can access this list as per your requirement.

Answer 3

It seem's that your file is representing a List of objects with Name and CreationDate fields.

So you have to just use List instead of HashMap to ObjectMapper, code is mentioned below:

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

Answer 4

Well, you are trying to convert an json string that contains array/list of objects into an object .

Ex: { "key": "value"} is the json you want to convert, but actually you are doing,

[{ "key": "value"}] .

simple fix, is remove the first and last char from your string and try. Hope it helps;)

Answer 5

Why not?

ObjectMapper mapper = new ObjectMapper();

Object obj = mapper.readValue(file, new TypeReference<Object>() {});

Answer 6

Your JSON is not a Map, but a List of Maps. So replace this:

myMap = objectMapper.readValue(mapData, HashMap.class);

with this:

myMap = objectMapper.readValue(mapData, new TypeReference<List<Map<String, String>>>(){});

Or you can change the format of your JSON data...