Question

我正在尝试使用java

从mongo中检索一个特定字段（"Versions.id"）

Mongo查询 - db.getCollection('SettlementInstance').find({"_id.timeSlice" : [2018,1,1], "_id.type" : "TRANSMISSION"})

这是存储在mongo集合中的数据。我想使用java

检索"id" : "3d8a3b31-aaa0-4c2e-82b3-5eaaa80d80e1"

   {
            "_id" : {
                "timeSlice" : [ 
                    2018, 
                    1, 
                    1
                ],
                "type" : "TRANSMISSION",
                "@objectName" : "SettlementInstance"
            },
            "Versions" : [ 
                {
                    "id" : "3d8a3b31-aaa0-4c2e-82b3-5eaaa80d80e1",
                    "status" : "ACTIVE",
                    "version" : NumberLong(11447)
                }
            ]
        }

Java程序我试图检索＆＃34; id＆＃34;领域＆＃34;版本＆＃34;对象，我的代码给出了异常。

String feedName = "ServicePointInputAdapter";
            Mongo mongo = new Mongo(host);
            DB db = mongo.getDB(("dbname"));
            DBCollection collection = db.getCollection("SettlementInstance");
            BasicDBObject whereQuery = new BasicDBObject();
            whereQuery.put("_id.type", "TRANSMISSION");
            whereQuery.put("_id.timeSlice", Arrays.asList(2018,1,1));
            DBCursor cursor = collection.find(whereQuery);
            try {
                while (cursor.hasNext()) {
                    DBObject Features = cursor.next();
                    BasicDBList features = (BasicDBList) Features.get("Versions");
                    BasicDBObject[] featuresArr = features.toArray(new BasicDBObject[0]);
                    for (BasicDBObject dbobj : featuresArr) {
        *****                BasicDBObject si_id = (BasicDBObject) dbobj.get("id"); ***** //Getting error in the line }
                }
            } finally {
                cursor.close();
            }
        }
    }

不幸的是我得到例外的例外 -

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to com.mongodb.BasicDBObject.

有人可以帮我解决问题并检索＆＃34; Versions.id＆＃34;来自该系列的领域。

Answer 1

由于错误清楚地表明您正试图将java.lang.String投射到com.mongodb.BasicDBObject投掷java.lang.ClassCastException

如下所示更改代码行（因为BasicDBObject实现了HashMap）：

for (BasicDBObject dbobj : featuresArr) {
    String id = (String)((HashMap)dbobj).get("id");
    System.out.println(id);
}

Answer 2

此get（）方法仅返回值。所以当你这样做时它是一个字符串

dbobj.get("id")

您应该将字段名称作为参数传递，您将获得该值。所以改变这一行如下

BasicDBObject si_id = (BasicDBObject) dbobj.get("id");

to 
String dbObjectId = dbobj.get("id");

线程＆＃34; main＆＃34;中的例外情况java.lang.ClassCastException：java.lang.String无法强制转换为com.mongodb.BasicDBObject

2 个答案: