Get list of keys in Complex JSON Object (Java 8)

Question

I am dealing with a JSON that looks like this:-

{
"key1": {
    "key1.1": {
        "nestedkey1": "something",
        "nestedkey2": "something",
        "nestedkey3": "Something"
    },
    "key1.2": {
        "nestedkey1": "something",
        "nestedkey2": "something",
        "nestedkey3": "Something"
    }
},
"key2": {
    "key2.1": {
        "nestedkey1": "something",
        "nestedkey2": "something",
        "nestedkey3": "Something"
    },
    "key2.2": {
        "nestedkey1": "something",
        "nestedkey2": "something",
        "nestedkey3": "Something"
    }
}...

And I don't know all the keys. I wish to obtain all the keys so that I can create a Map<String, Object> out of this. That map should look something like ("key1" -> Corresponding object )...

Is there a simple way to do this in Java?

Answer 1

Using Jackson JSON library, this json may be parsed as a Map<String, Object> using TypeReference :

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.util.Map;

public class JsonTest {

    public static void main(String[] args) throws JsonProcessingException {
        String json = "{\n"
                + "\"key1\": {\n"
                + "    \"key1.1\": {\n"
                + "        \"nestedkey1\": \"something\",\n"
                + "        \"nestedkey2\": \"something\",\n"
                + "        \"nestedkey3\": \"Something\"\n"
                + "    },\n"
                + "    \"key1.2\": {\n"
                + "        \"nestedkey1\": \"something\",\n"
                + "        \"nestedkey2\": \"something\",\n"
                + "        \"nestedkey3\": \"Something\"\n"
                + "    }\n"
                + "},\n"
                + "\"key2\": {\n"
                + "    \"key2.1\": {\n"
                + "        \"nestedkey1\": \"something\",\n"
                + "        \"nestedkey2\": \"something\",\n"
                + "        \"nestedkey3\": \"Something\"\n"
                + "    },\n"
                + "    \"key2.2\": {\n"
                + "        \"nestedkey1\": \"something\",\n"
                + "        \"nestedkey2\": \"something\",\n"
                + "        \"nestedkey3\": \"Something\"\n"
                + "    }\n"
                + "}}"; // make sure the json is valid and closing } is available

        ObjectMapper mapper = new ObjectMapper();

        Map<String, Object> map = mapper.readValue(json, new TypeReference<>() {});

        System.out.println(map);
    }
}

To get the list of all keys, a recursive method needs to be implemented to iterate over the entries of the top-level map and add keys:

public static List<String> getKeys(Map<String, Object> map) {
    List<String> keys = new ArrayList<>();
    for (Map.Entry<String, Object> entry : map.entrySet()) {
        keys.add(entry.getKey());
        if (entry.getValue() instanceof Map) {
            Map<String, Object> nested = (Map<String, Object>) entry.getValue();
            keys.addAll(getKeys(nested));
        }
    }
    return keys;
}

Similarly, a list of "prefixed" keys may be created:

public static List<String> getPrefixedKeys(String prefix, Map<String, Object> map) {
    List<String> keys = new ArrayList<>();
    for (Map.Entry<String, Object> entry : map.entrySet()) {
        String key = prefix + entry.getKey();
        keys.add(key);
        if (entry.getValue() instanceof Map) {
            Map<String, Object> nested = (Map<String, Object>) entry.getValue();
            keys.addAll(getPrefixedKeys(key + "/", nested));
        }
    }
    return keys;
}

// test
System.out.println(getPrefixedKeys("/", map));

Output :

[/key1, /key1/key1.1, /key1/key1.1/nestedkey1, /key1/key1.1/nestedkey2, /key1/key1.1/nestedkey3, 
/key1/key1.2, /key1/key1.2/nestedkey1, /key1/key1.2/nestedkey2, /key1/key1.2/nestedkey3, 
/key2, /key2/key2.1, /key2/key2.1/nestedkey1, /key2/key2.1/nestedkey2, /key2/key2.1/nestedkey3, 
/key2/key2.2, /key2/key2.2/nestedkey1, /key2/key2.2/nestedkey2, /key2/key2.2/nestedkey3]

Answer 2

String filePath ="src/main/resources/json/1.json";
FileReader reader = new FileReader(filePath);

JSONParser parser = new JSONParser();
JSONObject jsonObject = (JSONObject) parser.parse(reader);

Set<String> setKeys= jsonObject.keySet();
Map<String,Object> yourMap= new HashMap<>();
for (String key:setKeys) {
yourMap.put(key,jsonObject.get(key));
}

yourMap 准备好了！

Answer 3

The computing task is to output field names of all levels in JSON records of indefinite number of levels. The code will be lengthy if you try to handle such a scenario in Java.

It is convenient to do this in SPL, the open-source Java package. Three lines of code are enough:

	A	B
1	=i=0,json(file("records.json").read())
2	func recurse(r)	>i+=1,r.fno().run(tmp=eval("r.#"/~),B1=B1.to(:i-1)|r.fname(~),output(B1.concat("->")),if(ifr(tmp),func(recurse,tmp),(B1=B1|tmp)))
3	=func(recurse,A1)

SPL offers JDBC driver to be invoked by Java. Just store the above SPL script as jsonkeys.splx and invoke it in Java as you call a stored procedure:

…
Class.forName("com.esproc.jdbc.InternalDriver");
con= DriverManager.getConnection("jdbc:esproc:local://");
st = con.prepareCall("call jsonkeys()");
st.execute();
…