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Hibernate GenerationType.IDENTITY not generating sequence ids

In my springboot application, I noticed one strange issue when inserting new rows.
My ids are generated by sequence, but after I restart the application it starts from 21.
Example:
First launch, I insert 3 rows - ids generated by sequence 1,2,3
After restart second launch, I insert 3 rows ids generated from 21. So ids are 21,22 ...
Every restart It increased to 20. - This increasing pattern always 20
Refer my database table (1,2 after restart 21)
My JPA entity
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(unique = true, nullable = false)
private Long id;
I tried some stackoverflow solutions, it's not working
I tried this, not working
spring.jpa.properties.hibernate.id.new_generator_mappings=false
I want to insert rows by sequence like 1,2,3,4. Not like this 1,2,21,22, How to resolve this problem?

Although I think the question comments already provide all the information necessary to understand the problem, please, let me try explain some things and try fixing some inaccuracies.
According to your source code you are using the IDENTITY id generation strategy:
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(unique = true, nullable = false)
private Long id;
You are using an Oracle database and this is a very relevant information for the question.
Support for IDENTITY columns was introduced in Oracle 12c, probably Release 1, and in Hibernate version - I would say 5.1 although here in SO is indicated that you need at least - 5.3.
Either way, IDENTITY columns in Oracle are supported by the use of database SEQUENCEs: i.e., for every IDENTITY column a corresponding sequence is created. As you can read in the Oracle documentation this explain why, among others, all the options for creating sequences can be applied to the IDENTITY column definition, like min and max ranges, cache size, etc.
By default a sequence in Oracle has a cache size of 20 as indicated in a tiny note in the aforementioned Oracle documentation:
Note: When you create an identity column, Oracle recommends that you
specify the CACHE clause with a value higher than the default of 20 to
enhance performance.
And this default cache size is the reason that explains why you are obtaining this non consecutive numbers in your id values.
This behavior is not exclusive to Hibernate: please, just issue a simple JDBC insert statement or SQL commands with any suitable tool and you will experiment the same.
To solve the issue create your table indicating NOCACHE for your IDENTITY column:
CREATE TABLE your_table (
id NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY NOCACHE,
--...
)
Note you need to use NOCACHE and not CACHE 0 as indicated in the question comments and now in a previous version of other answers, which is an error because the value for the CACHE option should be at least 2.
Probably you could modify your column without recreating the whole table as well:
ALTER TABLE your_table MODIFY (ID GENERATED BY DEFAULT ON NULL AS IDENTITY NOCACHE);
Having said all that, please, be aware that in fact the cache mechanism is an optimization and not a drawback: in the end, and this is just my opinion, those ids are only non natural assigned IDs and, in a general use case, the cache benefits outweigh the drawbacks.
Please, consider read this great article about IDENTITY columns in Oracle.
The provided answer related to the use of the hilo optimizer could be right but it requires explicitly using the optimizer in your id field declaration which seems not to be the case.

It is related to Hi/Lo algorithm that Hibernate uses for incrementing the sequence value. Read more in this example: https://www.baeldung.com/hi-lo-algorithm-hibernate.
This is an optimization used by Hibernate, which consumes some values from the DB sequence into a pool (Java runtime) and uses them while executing appropriate INSERT statements on the table. If this optimization is turned off and set allocationSize=1, then the desired behavior (no gaps in ids) is possible (with a certain precision, not always), but for the price of making two requests to DB for each INSERT.
Examples give the idea of what is going on in the upper level of abstraction.
(Internal implementation is more complex, but here we don't care)
Scenario: user makes 21 inserts during some period of time
Example 1 (current behavior allocationSize=20)
#1 insert: // first cycle
- need next MY_SEQ value, but MY_SEQ_PREFETCH_POOL is empty
- select 20 values from MY_SEQ into MY_SEQ_PREFETCH_POOL // call DB
- take it from MY_SEQ_PREFETCH_POOL >> remaining=20-1
- execute INSERT // call DB
#2-#20 insert:
- need next MY_SEQ value,
- take it from MY_SEQ_PREFETCH_POOL >> remaining=20-i
- execute INSERT // call DB
#21 insert: // new cycle
- need next MY_SEQ value, but MY_SEQ_PREFETCH_POOL is empty
- select 20 values from MY_SEQ into MY_SEQ_PREFETCH_POOL // call DB
- take it from MY_SEQ_PREFETCH_POOL >> remaining=19
- execute INSERT // call DB
Example 2 (current behavior allocationSize=1)
#1-21 insert:
- need next MY_SEQ value, but MY_SEQ_PREFETCH_POOL is empty
- select 1 value from MY_SEQ into MY_SEQ_PREFETCH_POOL // call DB
- take it from MY_SEQ_PREFETCH_POOL >> remaining=0
- execute INSERT // call DB
Example#1: total calls to DB is 23
Example#2: total calls to DB is 42
Manual declaration of the sequence in the database will not help in this case, because, for instance in this statement\
CREATE SEQUENCE ABC START WITH 1 INCREMENT BY 1 CYCLE NOCACHE;
we control only "cache" used in the DB internal runtime, which is not visible to Hibernate. It affects sequence gaps in situations when DB stopped and started again, and this is not the case.
When Hibernate consumes values from the sequence it implies that the state of the sequence is changed on DB side. We may treat it as hotel rooms booking: a company (Hibernate) booked 20 rooms for a conference in a hotel (DB), but only 2 participants arrived. Then 18 rooms will stay empty and cannot be used by other guests. In this case the "booking period" is forever.
More details on how to configure Hibernate work with sequences is here:
https://ntsim.uk/posts/how-to-use-hibernate-identifier-sequence-generators-properly
Here is a short answer for older version of Hibernate. Still it has relevant ideas:
https://stackoverflow.com/a/5346701/2774914

Hibernate GeneratorSequence fails after 50 entries

I have an entity with the following id configuration:
public class Publication implements Serializable, Identifiable {
#Id
#GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "sequenceGenerator")
#SequenceGenerator(name = "sequenceGenerator")
private Long id;
}
with this generator (Liquibase syntax):
<createSequence incrementBy="10" sequenceName="sequence_generator" startValue="1" cacheSize="10"/>
and a Spring Data JPA Repository:
#Repository
public interface PublicationRepository extends JpaRepository<Publication, Long>, JpaSpecificationExecutor<Publication> {
// ...
}
Now I have part in my application where I create about 250 new Publication objects without an id and then do publicationRepository.saveAll(). I get the following exception:
Caused by: javax.persistence.EntityExistsException: A different object with the same identifier value was already associated with the session : [mypackage.Publication#144651]
I debugged with breakpoints and found that this always happens with the 50th object, where the assigned ID suddenly is set as an ID that is already present in the set of already saved objects – so the generator seems to return the wrong value. For collections with less than 50 objects, it works fine.
What is also strange: The objects IDs created have an increase of 1, while if if execute NEXT VALUE FOR sequence_generator on my database i get IDs in increments of 10.
Am I using the generator wrong?

You need to sync SequenceGenerator's allocationSize with your sequence's incrementBy. The default value for allocationSize is 50, which means that after every 50th insert, the hibernate will generate select nextval('sequence_name)` (or something similar depending on the dialect), to get the next starting value for IDs.
What happens in your case is that:
for the first insert Hibernate fetches next value for the sequence, which is 1. By first insert I mean first insert whenever the service/application is (re)started.
then it performs 50 inserts (default allocationSize) without asking DB what is the next value for the sequence. Generated ID will be from 1 to 50.
51st insert fetches next value for the sequence, which is 11 (startBy + incrementBy). Previously you inserted an entity with ID=11, which is why it fails to insert the new entity (PK constraint violation).
Also, every time you call select nextval on sequence, it simply does currentValue + incrementBy. For your sequence, it'll be 1, 11, 21, 31, etc.
If you enable SQL logs, you'll see following:
Calling repository.save(entity) the first time would generate
select nextval('sequence_name`);
insert into table_name(...) values (...);
Saving second entity with repository.save(entity) would generate only
insert into table_name(...) values (...);
After allocationSize number of inserts you would again see:
select nextval('sequence_name`);
insert into table_name(...) values (...);
Advantage of using sequences is to minimize the number of times Hibernate would need to talk to the DB to get the next ID. Depending on your use-case, you can adjust the allocationSize to get the best results.
Note: one of the comments suggested to use allocationSize = 1 which is very bad and will have a huge impact on performance. For Hibernate that would mean that it needs to issue select nextval every time it performs an insert. In other words, you'll have 2 SQL statements for every insert.
Note 2: Also keep in mind that you need to keep initialValue of SequenceGenerator and startValue of sequence in sync as well. allocationSize and initialValue are the two values used by the sequence generator to calculate the next value.
Note 3: It worth mentioning that depending on the algorithm used to generate sequences (hi-lo, pooled, pooled-lo, etc.), gaps may occur between service/application restarts.
Useful resources:
Hibernate pooled and pooled-lo identifier generators - in case you wish to change the algorithm used by the sequence generator to calculate the next value. There might be the case (e.g. in a concurrent environment) where the two service use the same DB sequence to generate values and their generated values might collide. For cases like that, one strategy is better that the other.

Primary keys generated by sequencegenerator are written into database incorrectly

I noticed a strange problem happening with the application I am writing.
I use Hibernate, Spring and Oracle database. There is a Orders table in the database, which has its PK generated by a sequence. I defined this in the application as follows:
#Id
#GeneratedValue(strategy = GenerationType.SEQUENCE, generator="SEQ_ORDER_ID")
#SequenceGenerator(name="SEQ_ORDER_ID", sequenceName="SEQ_ORDER_ID", allocationSize=1)
#Column(name = "ID_ORDER", length = 10, nullable = false)
For saving the Order into database, I create a new Order Order order = new Order();, set its values and use session.persist(order).
Now, the Order object has the correct primary key generated, let's say 120, while previous one was 119, but when I look into the database, the newly created order row has primary key value 121. I used debug and went through the process of creating the Order, that is how I got these values.
If I try creating a new order directly in sqldeveloper, the sequence works correctly and values increment by one.
This happens with every insert, not just Order. The primary key value written into database is consistently +2 instead of +1.
All of the Oracle sequences have the INCREMENT_BY value 1.
I tried adding sessionFactory.getCurrentSession().flush(); after the persist, but nothing changes.
Has anyone ever experienced this? Do You have any idea what I might be doing wrong here that causes this behavior?

Thanks to everyone for pointing me in the right direction!
The issue was in the triggers in my database generating new ID even though one was sent from the application. I had to add WHEN (NEW.ID_ORDER is NULL) before the trigger's BEGIN to fix this issue.
As per this answer: HIbernate issue with Oracle Trigger for generating id from a sequence

OpenJPA Sequence generator with negative values

Environment: Websphere 8.5, OpenJPA 2.0, DB2 z/OS
There are two tables: one with verified data and another with draft data (staging table) + View that display information from both tables.
To avoid Primary Key clash I've decided that staging table will have negative values as a primary key. It was working in plain SQL, but my approach failed when I tried to define a generator for staging table in Java code
Generator for negative key was defined like this:
CREATE SEQUENCE X AS INTEGER START WITH -1 INCREMENT BY -1
MINVALUE -999999 MAXVALUE 0
On entity side:
#Id
#SequenceGenerator(name="X", sequenceName="X")
#GeneratedValue(strategy=GenerationType.SEQUENCE,generator="X")
#Column(name = "ID")`
First element was created successfully (with value -1), but insertion of second element failed with
THE RANGE OF VALUES FOR THE IDENTITY COLUMN OR SEQUENCE IS EXHAUSTED. SQLCODE=-359, SQLSTATE=23522
Can you help me define #SequenceGenerator? Is it possible under Open JPA 2.0? Maybe sequence definition was wrong (MINVALUE/MAXVALUE)

First, I think your option to do a 'START WITH -99999 INCREMENT BY 1' is the best option. I'm not sure why you feel it is "not pretty". If you do this:
CREATE SEQUENCE SEQ_MYSEQ AS INTEGER START WITH -99999 INCREMENT BY 1 MINVALUE -999999 MAXVALUE 0
You are still range bound between -99999 and 0, right?
As I'll explain below, I think OpenJPA and EclipseLink like to count up. So I think you'll have better luck with this.
That said, let me answer your opening question. I have ran a test on OpenJPA and EclipseLink (since WebSphere uses Ecliplselink in WAS v9 and Liberty). I can not get your scenario to work with EclipseLink, but could get it to work with OpenJPA (but it wasn't pretty). Let me state what I did: this is the SQL I defined my sequence with (just as you listed in the description):
CREATE SEQUENCE SEQ_MYSEQ AS INTEGER START WITH -1 INCREMENT BY -1 MINVALUE -999999 MAXVALUE 0;
I defined my sequence generator in my entity as:
#Id
#SequenceGenerator(name = "IDGENERATOR", sequenceName = "SEQ_MYSEQ", allocationSize = 1, initialValue = -1)
#GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "IDGENERATOR")
private int id;
Notice that I'm telling JPA to start at -1, and to use an allocationSize of 1. The JavaDocs state the default is 50. One bummer (but not a show stopper) with allocationSize of 1 is that the JPA provider will go to the database for each sequence value (i.e. local caching will not be used). However, if this is not used, it seems both OpenJPA and EclipseLink want to count up by the allocation size. It is hard coded to count up. That is, either one will ask the DB for the next value, and then count up from there by allocationSize, NOT count down. On OpenJPA, you need to use this property:
Otherwise, by default OpenJPA executes an 'ALTER SEQUENCE' to make certain the INCREMENT BY defined in the SequenceGenerator matches what is in the database. If I don't add this property, I get the same exception about the range exhaustion. Anyway, with this, all works well on OpenJPA. On EclipseLink, I get this exception:
Exception Description: The sequence named [SEQ_MYSEQ] is setup incorrectly. Its increment does not match its pre-allocation size.
at org.eclipse.persistence.internal.jpa.EntityManagerImpl.persist(EntityManagerImpl.java:510)
at hat.test.MySeqTest.main(MySeqTest.java:28)
I didn't dig enough into EclipseLink to figure this out, but I did play around with the Sequence a bit and it seems like EclipseLink doesn't like negative values????
Thanks,
Heath Thomann

Hibernate sequence generates non-continous values

I am using a hibernate sequencegenerator to auto-generate unique values for my primary key column.The sample code is given below.
#Entity
#Table(name = "REQUEST")
#javax.persistence.SequenceGenerator(name = "REQ_SEQ", sequenceName = "REQUEST_SEQ")
public class Request {
/**
* Unique id for this request
*/
#Id
#GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "REQ_SEQ")
#Column(name = "REQ_ID")
private long requestId;
//So on
}
Everything works fine except the fact that the generated values are interleaved. For example it inserts values from 5000 to 5015(15 inserts) and then the 16th insert produces the value as 5100.Then it works fine for a few subsequent inserts and again the problem.
I dont have any problem as long as the generated values are unique but just curious to know what could be causing this. FYI, I am using Oracle.

Oracle sequences work that way. They only guarantee uniqueness but they do not guarantee consecutive values as it would hamper parallelism.
What they do internally is more or less this: when you request the next value in the sequence, Oracle precomputes a chunk of values (in your case 5000-5099) and puts it in a sequence cache, and then sets seq.nextval = 5100 on disk. But if, due to activity, the db has to discard your chunk of values from the cache, when seq.nextval is accessed the next time, it will take another chunk 5100-5199. That is, Oracle will not even try to save sequence values that have been put into cache.
The point of this is that the sequence cache is a memory structure that is faster and more parallelizable that the sequence itself, which is an on-disk structure. As we want to scale up, we want to avoid going to disk as much as possible.
You can control chunk size for a given sequence using the CACHE clause in your sequence DDL:
CREATE SEQUENCE seq2
CACHE 50;

From the Oracle doc:
CREATE SEQUENCE customers_seq
START WITH 1000
INCREMENT BY 1
NOCACHE;
The NOCACHE keyword is what you can use if you would like consequential series of IDs. Of course as explained in previous comments, that would limit the sequence throughput and may hinder your application performance.