bson/src/main/org/bson/types/ObjectId.java
public byte[] toByteArray() {
        ByteBuffer buffer = ByteBuffer.allocate(OBJECT_ID_LENGTH);
        putToByteBuffer(buffer);
        return buffer.array();  // using .allocate ensures there is a backing array that can be returned
    }
public byte[] toByteArray() {
        ByteBuffer buffer = ByteBuffer.allocate(OBJECT_ID_LENGTH);
        putToByteBuffer(buffer);
        return buffer.array();  // using .allocate ensures there is a backing array that can be returned
    }
Convert to a byte array.  Note that the numbers are stored in big-endian order.

@return the byte array