Question

我正在用java编写一个程序来读取MongoDB的查询结果。

下面是代码（给出例外）：

if(output != null){
                BasicDBObject whereQuery = new BasicDBObject();
                DBCursor cursor = null;
                for (DBObject obj : output.results()) {
                    String UserID = obj.get("UserID").toString();
                    whereQuery.put("User_ID", UserID);                                                              

                    cursor = users.find(whereQuery);
                    System.out.println(cursor.toString());
                    System.out.println("This user name who tweeted "+ cursor.next());


                }
            }

当我运行此代码时，只使用最后一行中的sysout语句打印游标的第一个值，第二个值来自＆＃39; for＆＃39;循环导致异常。不知道为什么？

在cursor.next（）之前的sysout语句一直打印游标中的当前值，将其放在那里以验证游标是否实际具有值。

我已经为cursor.next（）尝试了hasNext（）while循环，它只是第一次进入循环，其余的检查只是跳出来。

Error stack trace:
   Exception in thread "main" java.util.NoSuchElementException
   at com.mongodb.QueryResultIterator.next(QueryResultIterator.java:111)
   at com.mongodb.DBCursor._next(DBCursor.java:510)
   at com.mongodb.DBCursor.next(DBCursor.java:581)
   at ReadData.referencedDataModel(ReadData.java:97)
   at ReadData.main(ReadData.java:131)

Answer 1

我不确定你的答案，但值得一试：

String UserID = obj.get("UserID").toString();
cursor = users.find(whereQuery);
if(cursor.hasNext()) {
    // the object will be your object, its basically a json document
    BasicDBObject obj = cursor.next(); 

    // print obj to console as json file
    System.out.println(obj);

    // you can get any property with obj.get...(); method such as .getString()
    // The below code will extract the User_ID from the object you received via find query 
    string id = obj.getString("User_ID");
}

如果您确定结果只有一个文档，则可以使用此代码：

BasicDBObject obj = users.findOne(whereQuery);

关于java和Mongo非常有用的资源：

http://docs.mongodb.org/ecosystem/tutorial/getting-started-with-java-driver/

Answer 2

if(output != null){
                BasicDBObject whereQuery = new BasicDBObject();
                DBCursor cursor;

                for (DBObject obj : output.results()) {

                    String UserID = obj.get("UserID").toString();
                    System.out.println("The real results "+ UserID);

                    whereQuery.put("User_ID", UserID);                                                  
                    DBObject value = users.findOne(whereQuery);
                    if(value != null)
                        System.out.println("The user who used the word in the tweet: " +value.get("User_Name").toString());
                    else
                        System.out.println("The user profile is not available");
                }

            }

螺丝游标！

谢谢@Disposer！

cursor.next（）总是返回“java.util.NoSuchElementException”

2 个答案: