本文以mongo-java-driver-3.5.0.jar為例

1 _id介紹：
MongoDB中儲存的文件必須有一個”_id”鍵，其對應的”_id”值是由MongoDB伺服器按照特定規律自動生成唯一的值，確保集合裡面每個文件都能被唯一標識。這個鍵的值可以是任何型別的，預設是個ObjectId物件。

2 MongoDB查詢語句：
db.user.find({ “_id” : ObjectId(“59c8cbea6157d6c78dab4ff0”) })

3 3中查詢語句實現Java語句：

public static MongoCollection<Document> connect() {  
    @SuppressWarnings("resource"
 
)  
    MongoClient client = new MongoClient("127.0.0.1", 27017);  
    MongoDatabase db = client.getDatabase("test");  
    MongoCollection<Document> collection = db.getCollection("user");  
    return collection;
} 
public static List<Document> findBy1(DBObject filter) {  
    List<Document> results = new ArrayList<Document>();  
 

    FindIterable<Document> iterables = connect().find((Bson) filter);  
    MongoCursor<Document> cursor = iterables.iterator();  
    while (cursor.hasNext()) {  
        results.add(cursor.next());  
    }  

    return results;  
}

@Test
public void findById () {
    String _id = "59c8cbea6157d6c78dab4ff0"
 
;

    // 方法1
    DBObject filter = new BasicDBObject(); 
    filter.put("_id", new ObjectId(_id)); // 該寫法在spring mvc + spring +hibernate專案中無效，具體原因不明確
    // filter.put("_id", _id); // 該寫法在spring mvc + spring +hibernate專案中有效，具體原因不明確
    List<Document> results = findBy1(filter); 
    for(Document doc : results){  
        System.out.println("方法1：" + doc.toJson());  
        // 方法1：{ "_id" : { "$oid" : "59c8cbea6157d6c78dab4ff0" }, "name" : "yy", "password" : "123", "nickname" : "ygirl2", "iid" : 2 }
    } 

    // 方法2
    Bson filter2 =  Filters.and(Filters.eq("_id", new ObjectId(_id)));
    List<Document> results2 = findBy(filter2); 
    for(Document doc : results2){  
        System.out.println("方法2：" +doc.toJson());  
        // 方法2：{ "_id" : { "$oid" : "59c8cbea6157d6c78dab4ff0" }, "name" : "yy", "password" : "123", "nickname" : "ygirl2", "iid" : 2 }
    } 

    // 方法3
    Document filter3 = new Document(); 
    filter3.append("_id", new ObjectId(_id));
    List<Document> results3 = findBy(filter3); 
    for(Document doc : results3){  
        System.out.println("方法3：" +doc.toJson()); 
        // 方法3：{ "_id" : { "$oid" : "59c8cbea6157d6c78dab4ff0" }, "name" : "yy", "password" : "123", "nickname" : "ygirl2", "iid" : 2 } 
    } 
}