如何从JSON字符串中获取特定值

Question

//Open url and fetch JSON data
String s = "MY_URL_HERE";
URL url = new URL(s);
Scanner scan = new Scanner(url.openStream());
String str = new String();
while (scan.hasNext())
{
    str += scan.nextLine();
}
scan.close();

System.out.println(str);

str will print a string like :

{"coord":{"lon":-80.25,"lat":43.55},"weather":[{"id":800,"main":"Clear","description":"Sky is Clear","icon"...ect

I am using json_simple-1.1.jar

How do I actually use this string to extract values of my choosing? If i wanted to pull out the "description" or the "weather".

I have tried snippets from:

https://code.google.com/p/json-simple/wiki/DecodingExamples

These do not work for me, I get an error

org.json.simple.JSONObject cannot be cast to org.json.simple.JSONArray

Answer 1

Your string is not ant json array representation but json object itself . Thats what error org.json.simple.JSONObject cannot be cast to org.json.simple.JSONArray depicts. Example in your link describes an array, and each element of that array is an object:

Consider Jackson libray which is very convenient for converting Java object to / from JSON

Below represent an array, and each element of that array is an object:

[
    {
        color: "red",
        value: "#f00"
    },
    {
        color: "green",
        value: "#0f0"
    }
]

or

[
    {
        "color": "red",
        "value": "#f00"
    },
    {
        "color": "green",
        "value": "#0f0"
    }
]

Below represent the object

{
    color: "red",
    value: "#f00"
}

or

{
    "color": "red",
    "value": "#f00"
}

See here for different representation of json string

Code sample using jackson library

import java.io.IOException;

import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;


public class TestString {

    public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {

        String s="{\"coord\":{\"lon\":-80.25,\"lat\":43.55},\"weather\":[{\"id\":800,\"main\":\"Clear\",\"description\":\"Sky is Clear\"}]}";
        //String s="[{\"coord\":\"test\",\"lon\":-80.25}]";
        ObjectMapper mapper = new ObjectMapper();
        Object obj = mapper.readValue(s, Object.class );
        System.out.println("terst"+ obj );
    }

}

Answer 2

for anyone looking at this in the future,

using json_simple-1.1.jar

    String s = "YOUR_URL_HERE";
    URL url = new URL(s);
    Scanner scan = new Scanner(url.openStream());
    String str = new String();
    while (scan.hasNext())
    {
        str += scan.nextLine();
    }
    scan.close();

str now contains (in my case) an object containing more objects (**will edit when when I am sure, this is what I think)

now using this code and using http://codebeautify.org/jsonviewer

    JSONObject obj = (JSONObject) JSONValue.parse(str);

    String info = obj.get("object_name_you_wish_to_call").toString();

I see that the code above will output another Object containing all informaiton within that specific object.. In this case, all of the objects within "object_name_you_wish_to_call"

when I figure out how to retrieve a specific value, I will edit again.

Answer 3

You can do this using :

1. Pattern
2. Matcher
3. ObjectMapper
4. JsonNode

Java class :

    private Pattern pattern;
    private Matcher matcher;

    private static final String PATTERN = "description";

    private static String  StringFinal = null;

    public  void printAll(String str) {

        pattern = Pattern.compile(PATTERN);
        String finalPattern = null;  

        ObjectMapper mapper = new ObjectMapper();
        JsonNode node = mapper.readTree(str);
        Iterator<String> fieldNames = node.fieldNames();


        while(fieldNames.hasNext()){
            String fieldName = fieldNames.next();
            JsonNode fieldValue = node.get(fieldName);

            if (fieldValue.isObject()) {
                printAll(fieldValue);
            } 
            else {
                String value = fieldValue.asText();
                matcher = pattern.matcher(value);
                if(matcher.matches()){
                    StringFinal = value;
                    break;
                }
            }
        }

        System.out.println(StringFinal);
    }