mongo2中只有find，findOne是mongo3中才有的，所以旧的写法想达到和findOne一样的效果，就需要在find后在加first()
In MongoDB 2 Java driver there was a method on the DBCollection class named findOne().
In the MongoDB 3 Java driver API, the findOne() method isn't there. So your new code for finding exactly one document becomes similar too this one:
collection.find(eq("_id", 3)).first()