包含字符串映射的Java解析Json对象，以映射到字符串和字符串数组

Question

I am trying to parse a json string that is an object that contains a map of string keys to a map of string keys to a string array. I have simplified the json string to just the object that I am struggling to parse but is actually an object embedded within other objects.

Json:

{"restrictions": {
    "master": "bsu.school",
    "conditions": {
        "HCI": [
            "CMP",
            "HRM",
            "PUBL",
            "CRA",
            "FTV",
            "AGS",
            "MAS",
            "MCOM",
            "BMPP",
            "BUS",
            "CUS",
            "FSS",
            "PHET",
            "HIST",
            "ENL",
            "STR",
            "CWR",
            "STAF"
        ],
        "MPA": [
            "MAPE",
            "MAPS",
            "BACC",
            "MASW",
            "BAMU",
            "BAPA",
            "MAPA",
            "MATY",
            "MACM",
            "BADR",
            "KIST",
            "BADA",
            "PARM",
            "BAP",
            "BACM",
            "BATP",
            "MACO",
            "BACMT",
            "CSMT",
            "BAAM"
        ],
        "BSAD": [
            "BSAD3",
            "BSAD1",
            "BSAD2"
        ]
    }
}}

I have created a simple test class to just test parsing this particular object:

import java.io.IOException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class JsonParseTest {

    public static void main(String[] args) {
        String json = "{\"restrictions\":{\"master\":\"bsu.school\",\"conditions\":{\"HCI\":[\"CMP\",\"HRM\",\"PUBL\",\"CRA\",\"FTV\",\"AGS\",\"MAS\",\"MCOM\",\"BMPP\",\"BUS\",\"CUS\",\"FSS\",\"PHET\",\"HIST\",\"ENL\",\"STR\",\"CWR\",\"STAF\"],\"MPA\":[\"MAPE\",\"MAPS\",\"BACC\",\"MASW\",\"BAMU\",\"BAPA\",\"MAPA\",\"MATY\",\"MACM\",\"BADR\",\"KIST\",\"BADA\",\"PARM\",\"BAP\",\"BACM\",\"BATP\",\"MACO\",\"BACMT\",\"CSMT\",\"BAAM\"],\"BSAD\":[\"BSAD3\",\"BSAD1\",\"BSAD2\"]}}}";
        ObjectMapper objectMapper = new ObjectMapper();
        // Convert the json string to Restrictions object
        try {
            RestrictionsContainer restrictionsContainer = objectMapper.readValue(json, RestrictionsContainer.class);
            System.out.println("restrictionsContainer " + restrictionsContainer);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

RestrictionsContainer class:

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown = true)
public class RestrictionsContainer {
    private Restrictions restrictions = null;

    /**
     * @return the restrictions
     */
    public Restrictions getRestrictions() {
        return restrictions;
    }

    /**
     * @param restrictions the restrictions to set
     */
    public void setRestrictions(Restrictions restrictions) {
        this.restrictions = restrictions;
    }

    /* (non-Javadoc)
     * @see java.lang.Object#toString()
     */
    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("RestrictionsContainer [restrictions=").append(restrictions).append("]");
        return builder.toString();
    }
}

Restrictions class:

import java.util.HashMap;
import java.util.Map;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown = true)
public class Restrictions {
    private String master = null;

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

    /**
     * @return the master
     */
    public String getMaster() {
        return master;
    }

    /**
     * @param master the master to set
     */
    public void setMaster(String master) {
        this.master = master;
    }

    /**
     * @return the conditions
     */
    public Map<String, Object> getConditions() {
        return conditions;
    }

    /**
     * @param conditions the conditions to set
     */
    public void setConditions(Map<String, Object> conditions) {
        this.conditions = conditions;
    }

    /* (non-Javadoc)
     * @see java.lang.Object#toString()
     */
    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("Restrictions [master=").append(master).append(", conditions=").append(conditions).append("]");
        return builder.toString();
    }
}

The code above works when I set the the Conditions to a map of String to Objects but what I really want is to specify what type of objects it is which I think should be:

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

However, when I use this instead I get the following error message:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.util.LinkedHashMap` out of START_ARRAY token
 at [Source: (String)"{"restrictions":{"master":"bsu.school","conditions":{"HCI":["CMP","HRM","PUBL","CRA","FTV","AGS","MAS","MCOM","BMPP","BUS","CUS","FSS","PHET","HIST","ENL","STR","CWR","STAF"],"MPA":["MAPE","MAPS","BACC","MASW","BAMU","BAPA","MAPA","MATY","MACM","BADR","KIST","BADA","PARM","BAP","BACM","BATP","MACO","BACMT","CSMT","BAAM"],"BSAD":["BSAD3","BSAD1","BSAD2"]}}}"; line: 1, column: 60] (through reference chain: RestrictionsContainer["restrictions"]->Restrictions["conditions"]->java.util.LinkedHashMap["HCI"])
    at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
    at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1342)
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1138)
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1092)
    at com.fasterxml.jackson.databind.deser.std.StdDeserializer._deserializeFromEmpty(StdDeserializer.java:599)....

The string keys for the conditions object are dynamic so I can't use an object.

What is the correct way to define the conditions object?

Answer 1

You are defining the type of conditions wrong:

private Map<String, Map<String,List<String>>> conditions = new HashMap<>(); // WRONG

Actually the type of conditions in your json is:

private Map<String, List<String>> conditions;

Change this and your code works fine.

Answer 2

There is nothing wrong with your code, but rather Jackson's inability to "directly reference types" .Type reference Returns the Type representing the direct superclass of the entity (class, interface, primitive type or void) represented by this Class.

If the superclass is a parameterized type, the Type object returned must accurately reflect the actual type parameters used in the source code.

Your fix should be something like this:

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