Do not include the validation starter in web starters by default

[bclozel]

We've noticed that many applications are not using the validation features in web applications, while validator libraries on the classpath come with a cost.

We could do the following:

• remove the spring-boot-starter-validation dependency from spring-boot-starter-web and spring-boot-starter-webflux
• create a new spring-boot-starter-validation-web that brings in the validation dependencies without the tomcat-embed-el which is currently excluded by the web starters
• properly document that change in the release notes and the reference docs

[bclozel]

We'll do the following:

• remove the spring-boot-starter-validation from spring-boot-starter-web by default
• change spring-boot-starter-validation to use org.glassfish:jakarta.el instead of the current choice of default implementation
• update all the web servers starters to use that same el implementation for all

With this change, we don't need to add a new starter and we can consistently use the same el implementation.

[ofabricio]

I'm surprised to figure out not most projects are not using validations, because all my web apps use them. I had to include this new starter.

I'm wondering what is people using instead?

[bclozel]

The rationale behind this decision is not that most applications aren’t using validation, but rather that there is a significant amount of them and that the (startup and runtime) cost associated with the validation starter is far from negligible.

We usually try to strike the right balance between doing as much as possible with the developer’s intent and keeping setups efficient and lightweight. « Not all web applications should use validation and we want to optimize efficiency » is all that you should read into this decision.

[springframeworkguru]

This change caught me by surprise a little bit. In upgrading to 2.3.0, I added the validation API, but not the hibernate impl. My unit tests started covering validation started to fail. Without test coverage, might not have caught this.

Maybe consider failing startup if the API is present, and impl is not?