MongoDB Java-API NoSuchElement

Question

I'm builing on a Java API for MongoDB. More specific, right now I'm trying to encapsulate the process of connecting to MongoDB and choosing a database and a collection from that database to it's own class in a single method.

package mongoDB;

import java.net.UnknownHostException;
import java.util.*;
import com.mongodb.*;

public class MongoDBConnector {
 public DBCollection mongoEasy (String hostname, int port) {

    try {

        Mongo m = new Mongo(hostname, port);

        //choose database
        List<String> databaseNames = m.getDatabaseNames();
        System.out.println("Choose database:  " + databaseNames.toString());
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        if(!databaseNames.contains(input.toString())){
            System.out.println("Database does not exist.");
            System.exit(1);
        }
        DB db = m.getDB(input.toString());

        //choose collection
        Set<String> collectionNames = db.getCollectionNames();
        System.out.println("Choose collection " + collectionNames.toString());
        input = scanner.nextLine();
        if(!collectionNames.contains(input.toString())){
            System.out.println("collection does not exist");
            System.exit(1);
        }
        DBCollection dbCollection = db.getCollection(input.toString());
        scanner.close();
        return dbCollection;

        } catch (UnknownHostException e) {
            System.out.println("Connection failed.");
            System.exit(1);
        } catch(MongoException e){
            System.out.println("Fail.");
            System.exit(1);
        } finally{
        }
    return null;

 }
}

If i create an instance of MongoDBConnector and call the mongoEasy a single time within a main method everything is fine. However if i call the method a second time it pumps out a NoSuchElementException. Take this example: package mongoDB;

import com.mongodb.*;
import mongoDB.MongoDBConnector;

public class MongoDBOperations2 {
 public static void main(String[] args) {

    MongoDBConnector mongoDBConnector = new MongoDBConnector();
    DBCollection collection = mongoDBConnector.mongoEasy("localhost", 27017);

    collection = mongoDBConnector.mongoEasy("localhost", 27017);
 }
}

Any time I'm trying to execute this I get (I deleted the database and collection names and line number for main method doesn't fit because i simplefied it for the question):

Choose database [...]
test
Choose collection [...]
test
Choose database [local, StudiumUlm, test]
Exception in thread "main" java.util.NoSuchElementException: No line found
    at java.util.Scanner.nextLine(Unknown Source)
    at mongoDB.MongoDBConnector.mongoEasy(MongoDBConnector.java:18)
    at mongoDB.MongoDBOperations2.main(MongoDBOperations2.java:)

The second time I call the method the scanner is not waiting for my input. Any suggestions?

Thanks. Cheers, Magnus

Answer 1

According to the Scanner.close() documentation, it will close the underlying Reader/Stream if it implements Closeable .

Since your code closes the Scanner, you can't read from System.in after the first time(which is exactly what you are seeing).

Why not add a close method to your MongoDBConnector like :

public class MongoDBConnector {

    Scanner scanner;
    public MongoDBConnector() {
       this.scanner = new Scanner(System.in);
    }
    public void close(){
        this.scanner.close();
    }
    // the rest of your code but don't create a new scanner or close it 
}

Then in your mainline do :

public class MongoDBOperations2 {
    public static void main(String[] args) {

        MongoDBConnector mongoDBConnector = new MongoDBConnector();
        try {
            DBCollection collection = mongoDBConnector.mongoEasy("localhost", 27017);

            collection = mongoDBConnector.mongoEasy("localhost", 27017);
        }finally {
            mongoDBConnector.close();
        }
    }
}