Implement an entity Callbacks with SpringBoot and MongoDB

Bassem 27/10

Some times we need to carry out certain actions before sending a record to the database, as we know for sql databases Spring data provides some life-cycle callback methods like @PostPersist and @PreUpdate, but if your appplication relay on a MongoDb, there are not such annotations in the Spring Data MongoDb module, in spite of that, the mongo module provides some Entity Callbacks. In this article we are going to see how to implement them.

I developed a classic application to shorten URLs with the use of "Entity Callbacks". The application is based on spring boot and MongoDB as a database. I decided to make the short URL equal to the last six characters of the document/entity Id. For knowledge, the MongoDB Id object is composed of the current 4-byte timestamp value, a 5-byte random value and a 3-byte incrementing counter, which is initialized to a random value. More info can be found on the official docs.
The document to be saved into the database, is the following:

@Document
public class Url {
    @Id
    private ObjectId id;
    private String fullUrl;
    private String shortUrl;
    private long count;
..
}

My main objective is to modify the "shortUrl" property before hitting the database, using a callback.
Spring data mongodb module provides different callbacks, such as BeforeConvertCallback, BeforeSaveCallback, and others. The complete list can be found here. Therefore to accomplish our objective, we are going to create a component which will implement the BeforeConvertCallback interface as follow:

@Component
public class UrlListener implements BeforeConvertCallback<Url> {

   @Override
   public Url onBeforeConvert(Url entity, String collection) {
       if (entity.getId() == null) {
           ObjectId id = new ObjectId();
           entity.setId(id);
           entity.setShortUrl(id.toHexString().substring(18, 24));
       }
       return entity;
   }

This class will be invoked before a Url object is converted to org.bson.Document (binary json, the MongoDB way to represent data structure). I am using the "beforeConver" because for some reason with "beforeSave", the modified data with the callback class get replaced with a new ObjectId before going into the database.
Examining this class first, we make sure that the entity is a new one, by inspecting the Id property. After that, we assign a new id by the ObjectId constructor finally, we set the shortUrl with the last six characters of the id's hexstring representation. Now every time we save a Url the "shortUrl" property will be set automatically.

That's it; all the application's code can be found on GitHub. The same application is developed also with QUARKUS to get advantage of theGraalVM, reduce the memory footprint and start-up time, you can find it here.

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