MongoDb的collection中，每个文档都有一个唯一标识：_id
ObjectId存储结构：
ObjectId是_id的默认类型，它占用12个字节的存储空间，以24位16进制数形式存储(一个字节用2个16进制数代表)
1-4字节：从标准纪元开始的时间，单位是秒
5-7字节：所在主机的唯一标识符，通常是机器主机名的hash值，可以确保在分布式环境中，不同主机生成不同的hash值，不产生冲突；
8-9字节：产生ObjectId的进程的标识符，可以确保一个机器上并发的多个线程产生的ObjectId的唯一性
有了前9个字节，1秒内不同机器上不同进程产生的ObjectId可以保证唯一性,一个机器一个进程，最后3个字节是自增计数器。一秒钟最多允许每个进程拥有256^3（16 777 216）个不同的ObjectId。
ObjectId可以在服务器端生成，也可以有驱动生成，我们看一下mongo-java-driver中是怎么生成的……
我们在程序里往数据库里增加数据时，会调驱动提供的insertOne方法，我们看一下insertOne的实现
@Override
public void insertOne(final TDocument document, final InsertOneOptions options) {
notNull("document", document);
TDocument insertDocument = document;
if (getCodec() instanceof CollectibleCodec) {
insertDocument = ((CollectibleCodec) getCodec()).generateIdIfAbsentFromDocument(document);
}
executeSingleWriteRequest(new InsertRequest(documentToBsonDocument(insertDocument)), options.getBypassDocumentValidation());
}
这个方法里，调用generateIdIfAbsentFromDocument生成文档的唯一标识_id，
private static final String ID_FIELD_NAME = "_id";
@Override
public BsonDocument generateIdIfAbsentFromDocument(final BsonDocument document) {
if (!documentHasId(document)) {
document.put(ID_FIELD_NAME, new BsonObjectId(new ObjectId()));
}
return document;
}
很传统，如果文档中没有_id字段，调用new ObjectId()生成一个，放入文档，接下来看看ObjectId的构造方法，
**
* Create a new object id.
*/
public ObjectId() {
this(new Date());//获取当前时间 (time.getTime() / 1000)
}
/**
* Constructs a new instance using the given date.
*
* @param date the date
*/
public ObjectId(final Date date) {
this(dateToTimestampSeconds(date), MACHINE_IDENTIFIER, PROCESS_IDENTIFIER, NEXT_COUNTER.getAndIncrement(), false);
}
构造方法里，我们可以看到ObjectId的四个基本要素，时间&机器标识&进程号&计数器，时间的生成方式比较简单，从注释里可以看出来。接下来，我们看看剩下三个要素在代码里是怎么生成的。
MACHINE_IDENTIFIER是int类型，PROCESS_IDENTIFIER是short类型，在static块中初始化：
static {
try {
MACHINE_IDENTIFIER = createMachineIdentifier();
PROCESS_IDENTIFIER = createProcessIdentifier();
} catch (Exception e) {
throw new RuntimeException(e);
}
}
1）MACHINE_IDENTIFIER初始化
private static int createMachineIdentifier() {
// build a 2-byte machine piece based on NICs info
int machinePiece;
try {
StringBuilder sb = new StringBuilder();
Enumeration e = NetworkInterface.getNetworkInterfaces();
while (e.hasMoreElements()) {
NetworkInterface ni = e.nextElement();
sb.append(ni.toString());
byte[] mac = ni.getHardwareAddress();
if (mac != null) {
ByteBuffer bb = ByteBuffer.wrap(mac);
try {
sb.append(bb.getChar());
sb.append(bb.getChar());
sb.append(bb.getChar());
} catch (BufferUnderflowException shortHardwareAddressException) { //NOPMD
// mac with less than 6 bytes. continue
}
}
}
machinePiece = sb.toString().hashCode();
} catch (Throwable t) {
// exception sometimes happens with IBM JVM, use random
machinePiece = (new SecureRandom().nextInt());
LOGGER.warn("Failed to get machine identifier from network interface, using random number instead", t);
}
machinePiece = machinePiece & LOW_ORDER_THREE_BYTES;
return machinePiece;
}
2）PROCESS_IDENTIFIER
// Creates the process identifier. This does not have to be unique per class loader because
// NEXT_COUNTER will provide the uniqueness.
private static short createProcessIdentifier() {
short processId;
try {
String processName = java.lang.management.ManagementFactory.getRuntimeMXBean().getName();
if (processName.contains("@")) {
processId = (short) Integer.parseInt(processName.substring(0, processName.indexOf('@')));
} else {
processId = (short) java.lang.management.ManagementFactory.getRuntimeMXBean().getName().hashCode();
}
} catch (Throwable t) {
processId = (short) new SecureRandom().nextInt();
LOGGER.warn("Failed to get process identifier from JMX, using random number instead", t);
}
return processId;
}
3）NEXT_COUNTER
private static final AtomicInteger NEXT_COUNTER = new AtomicInteger(new SecureRandom().nextInt());
最后将四个要素组合，转换成24位的16进制数
private ObjectId(final int timestamp, final int machineIdentifier, final short processIdentifier, final int counter,
final boolean checkCounter) {
if ((machineIdentifier & 0xff000000) != 0) {
throw new IllegalArgumentException("The machine identifier must be between 0 and 16777215 (it must fit in three bytes).");
}
if (checkCounter && ((counter & 0xff000000) != 0)) {
throw new IllegalArgumentException("The counter must be between 0 and 16777215 (it must fit in three bytes).");
}
this.timestamp = timestamp;
this.machineIdentifier = machineIdentifier;
this.processIdentifier = processIdentifier;
this.counter = counter & LOW_ORDER_THREE_BYTES;
}
参考：
1.MongoDB权威指南（第2版）
2.mongo-java-driver 3.5.0