Difference between @GeneratedValue and @GenericGenerator

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When using an ORM it is often necessary to generate a primary key value.

The @GeneratedValue annotation denotes that a value for a column, which must be annotated with @Id is generated. The elements strategy and generator on the annotation describe how the generated value is obtained.

There are four possible values for the strategy element on the @GeneratedValue annotation: IDENTITY, AUTO, TABLE and SEQUENCE. See more.

So to answer Part 2 of your question, the code snippet is indicating that the value of userId will be obtained through a sequence in the database.

The generator element of the @GeneratedValue annotation denotes the name of the primary key generator. In Part1 of your question, the code snippet indicates that a generator named increment will be used to obtain the primary key value. increment is then defined in the next annotation @GenericGenerator. @GenericGenerator is a hibernate annotation used to denote a custom generator, which can be a class or shortcut to a generator supplied by Hibernate. increment is a shortcut to a Hibernate generator that:

generates identifiers of type long, short or int that are unique only
when no other process is inserting data into the same table. Do not
use in a cluster.

In the Third Part of your question, the code uses a hilo Hibernate generator that:

uses a hi/lo algorithm to efficiently generate identifiers of type
long, short or int, given a table and column (by default
hibernate_unique_key and next_hi respectively) as a source of hi
values. The hi/lo algorithm generates identifiers that are unique only
for a particular database.

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