MongoDB Java API NoTouchElement

我正在为MongoDB构建一个JavaAPI。更具体地说，现在我正在尝试将连接到MongoDB的过程封装起来，并在一个方法中选择一个数据库和一个从该数据库到它自己的类的集合

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;

 }
}

每当我尝试执行此操作时，我都会发现（我删除了数据库和集合名称，并且main方法的行号不合适，因为我针对问题简化了它）：

第二次调用该方法时，扫描仪没有等待我的输入。有什么建议吗

谢谢。 干杯 Magnus

根据文档，如果实现Closeable，它将关闭底层读卡器/流

由于您的代码关闭了扫描仪，因此您无法在第一次之后读取System.in（这正是您看到的）

为什么不向MongoDBConnector添加一个close方法，如：

然后在主线中执行以下操作：

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();
        }
    }
}

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:)

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 
}

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();
        }
    }
}