Could not read JSON: Cannot construct instance of `java.util.LinkedHashMap`

Question

If I got will get an unsure rows JSON data. How should I set of my class?

This is my current class

public class ChatMessage {

private Map<String, String> message = new HashMap<>();

@JsonAnyGetter
public Map<String, String> any(){
    return this.message;
}

public Map<String, String> getMessage() {
    return this.message;
}

@JsonAnySetter
public void setMessage(String key, String value) {
    message.put(key, value);
}

@Override
public String toString() {
    return "Message [message=" + message + "]";
}
}

this is my json send from js

{"type":"message","user":"james","to":"","message":"Hi every"}

I got error now

org.springframework.messaging.converter.MessageConversionException: Could not 
read JSON: Cannot construct instance of `java.util.LinkedHashMap` (although 
at least one Creator exists): no String-argument constructor/factory method 
to deserialize from String value ('Welcome james join the room')
at [Source: (byte[])" . 
{"type":"message","user":"james","to":"","message":"Welcome james join the 
room"}"; line: 1, column: 52] (through reference chain: 
chat.model.ChatMessage["message"]); nested exception is         
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct         
instance of `java.util.LinkedHashMap` (although at least one Creator exists): 
no String-argument constructor/factory method to deserialize from String 
value ('Welcome james join the room')
at [Source: (byte[])" . 
{"type":"message","user":"james","to":"","message":"Welcome james join the 
room"}"; line: 1, column: 52] (through reference chain: 
chat.model.ChatMessage["message"])

because my json would be like

{"type":"message",
 "user":"james",
 "to":"",
 "message":"Welcome james join the room",
 "xxx":"xxxxxxxxxxxxx"}

or

{"type":"message",
 "user":"james",
 "to":"",
 "message":"Welcome james join the room",
 "yyy":"xxxxxxxxxxxxx"}

how should I set my class? Thx

how is this working? did you tried to check after converting to object? — Rence Abishek
– Rence Abishek, Commented May 30, 2019 at 6:07

S.K. · Accepted Answer · 2018-11-04 19:43:12Z

2

Rename either the message field in the json or message field in the POJO. As per your POJO structure, jackson is trying to deserialize "message":"Hi every" to the message field in your POJO and is not able to create a Map out of it (Since it's just a String).

Your code will work with following json:

{"type":"message","user":"james","to":"","json-message":"Hi every"}

OR

following POJO:

public class ChatMessage {

private Map<String, String> map = new HashMap<>();

@JsonAnyGetter
public Map<String, String> any(){
    return this.map;
}

public Map<String, String> getMap() {
    return this.map;
}

@JsonAnySetter
public void setMap(String key, String value) {
    map.put(key, value);
}

@Override
public String toString() {
    return "Map [map=" + map + "]";
}
}

answered Nov 4, 2018 at 19:43

S.K.

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1 Comment

Rence Abishek Over a year ago

if we try to change the objectname from message to json-message, how jackson-object mapper would identify and convert to object?

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