Why does using @GeneratedValue(strategy = GenerationType.TABLE) with an abstract superclass cause issues in a Spring application with Hibernate and MySQL?

@GeneratedValue Pitfalls with Abstract Superclass and MySQL

In an attempt to manage ID values in a Spring application with Hibernate and MySQL, using an abstract superclass BaseEntity can lead to challenges. When @GeneratedValue(strategy = GenerationType.TABLE) is employed, an error arises due to the absence of a hibernate_sequences table required by Hibernate for incrementally generating IDs.

The Underlying Issue

MySQL does not natively support sequences, which poses an obstacle for the GenerationType.TABLE approach. This strategy typically utilizes a dedicated table to track the sequence values and allocate unique IDs. Since MySQL lacks this feature, it results in the "Table 'docbd.hibernate_sequences' doesn't exist" error.

Solving the Problem

To overcome this issue and persist objects successfully, a workaround is necessary:

1. Use GenerationType.IDENTITY

This strategy relies on the database's own auto-increment mechanism, which eliminates the need for a dedicated sequence table. However, it is incompatible with the intent of using an abstract superclass to manage IDs across derived classes.

2. Create a Hibernate Sequence Table

Despite MySQL's lack of native sequence support, it allows for the creation of custom tables to emulate sequence behavior. Here's an example of creating such a table named hibernate_sequences:

CREATE TABLE IF NOT EXISTS hibernate_sequences(
  sequence_name VARCHAR(255) NOT NULL,
  sequence_next_hi_value INT UNSIGNED NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (sequence_name)
) ENGINE=InnoDB;

3. Configure Hibernate to Use the Custom Sequence

In the BaseEntity class, adjust the @GeneratedValue annotation to utilize the custom sequence table:

@GeneratedValue(strategy = GenerationType.TABLE, generator = "hilo_generator")
private Integer id;

@SequenceGenerator(name = "hilo_generator", sequenceName = "hibernate_sequences", allocationSize = 1)

4. Adjust Column Definition in Entity Table

In the CCD table, ensure that the id column definition corresponds to the custom sequence:

CREATE TABLE IF NOT EXISTS ccd(
  id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
  #other stuff
) ENGINE=InnoDB;

Fallback Method

As a last resort, if the custom sequence approach fails due to other unexpected issues, consider implementing a manual ID generation mechanism in the BaseEntity class:

private static int idSequence = 0;

public Integer getId() {
  if (id == null) {
    return idSequence++;
  }
  return id;
}

Conclusion

Utilizing an abstract superclass for ID management in a Spring application with Hibernate and MySQL requires careful consideration of the underlying database limitations. Employing the workaround of creating a custom sequence table can mitigate the issue stemming from MySQL's lack of native sequence support. However, these approaches may have performance implications or introduce additional complexity to the system design.

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