How Efficient Are Hibernate's Interceptors Compared To Database Triggers?

Since I can't make MySQL to update a column with ON UPDATE, I'm thinking of using Hibernate's interceptors, to update an updated table each time the row is updated.

My only concern is, how much of a performance penalty does it imply, compared to the ideal case of having MySQL updating it?

There's no significant performance penalty and you don't need an interceptor either.

I created an example on GitHub for this.

You need to create a JPA callback listener:

public class UpdatableListener {

    @PrePersist
    @PreUpdate
    private void setCurrentTimestamp(Object entity) {
        if(entity instanceof Updatable) {
            Updatable updatable = (Updatable) entity;
            updatable.setTimestamp(new Date());
        }
    }

}

which uses an interface that's defined like this:

public interface Updatable {

    void setTimestamp(Date timestamp);

    Date getTimestamp();
}

You can then define a base class for all your entities:

@MappedSuperclass
@EntityListeners(UpdatableListener.class)
public class BaseEntity implements Updatable {

    @Column(name = "update_timestamp")
    private Date timestamp;

    @Override
    public void setTimestamp(Date timestamp) {
        this.timestamp = timestamp;
    }

    @Override
    public Date getTimestamp() {
        return timestamp;
    }
}

And then simply have your entities extend it:

@Entity(name = "Post")
@Table(name = "post")
public class Post extends BaseEntity {
    ...  
}

@Entity(name = "PostComment")
@Table(name = "post_comment")
public static class PostComment extends BaseEntity {
    ...  
}

When you modify these entities:

doInJPA(entityManager -> {
    Post post = entityManager.find(Post.class, 1L);
    post.setTitle("Post");
    for(PostComment comment : post.getComments()) {
        comment.setReview("Review");
    }
});

Hibernate will take care of setting the timestamp column:

UPDATE  post
SET     update_timestamp = '2016-02-06 17:03:26.759' ,
        title = 'Post'
WHERE   id = 1
UPDATE  post_comment
SET     update_timestamp = '2016-02-06 17:03:26.76' ,
        post_id = 1 ,
        review = 'Review'
WHERE   id = 1
UPDATE  post_comment
SET     update_timestamp = '2016-02-06 17:03:26.76' ,
        post_id = 1 ,
        review = 'Review'
WHERE   id = 2