如何在mongodb中生成唯一对象id

Question

When I use Mongodb with Java, I want to generate Object id at clients. Before I insert a record, however, I have to query mongodb first to make sure that the id generated by ObjectId() method is unique. Is there any way that I can generate unique object id without accessing mongodb twice?

Answer 1

You can generate ObjectId on the client without consulting database. Such ID will be unique (you'll have to try damn hard to get two identical object ids).

ObjectId id = new ObjectId();

// or this
ObjectId id = ObjectId.get();

Answer 2

Object IDs are not like sequential ids you use in a RDMS. If they are properly generated according to the Object ID specification you will not need to worry about them being unique.

All you have to do is ensure you always create a new Object ID rather than reusing them.

Answer 3

As of MongoDB Java Driver 3.3.0, there are the following ways to create ObjectIds.

Using the constructor without parameters: Provides unique ObjectId

1. ObjectId id1 = new ObjectId(); //Generates unique id 

    1.1. ObjectId id2 = ObjectId.get(); //Calls new ObjectId();

Using the parameterized constructors: Parameters influence the uniqueness of the ObjectId

2. public ObjectId(byte[] bytes) // Receives a byte array of size 12.

3. public ObjectId(String hexString) //Receives a String that is a hexadecimal representation of 12 bytes.

4. public ObjectId(Date date) // Receives a Date object

5. public ObjectId(Date date, int counter) //Receives date and a counter

6. public ObjectId(Date date,
            int machineIdentifier,
            short processIdentifier,
            int counter) //Receives Date, MachineId, PID and counter.

7. public ObjectId(int timestamp,
            int machineIdentifier,
            short processIdentifier,
            int counter) //Receives Epoch time in sec, MachineId, PID and counter.

Understanding ObjectId:

ObjectId consists of 12 bytes, divided as follows:

               ObjectID layout

0   1   2   3   4   5   6   7   8   9   10  11

|time          |machine    |pid    |inc      |