How do you validate nested objects and cross-field rules in Spring?

Uses cascaded validation for nested DTOs, class-level constraints for local invariants, and domain services for state-dependent rules.

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TL;DR

I put constraints on the nested DTO and mark the containing property with @Valid so validation cascades into it. The same applies to elements of a collection when each element has its own constraints. A rule involving two fields in one request, such as an end date after a start date, can use a class-level custom constraint. The validator reports a stable field or object error without performing database or remote calls.

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Validation boundaries, collection elements, custom constraints, null semantics, and separation of structural from business validation.

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Overview
The validation mechanism depends on whether the rule belongs to one submitted object or requires application state.
I put constraints on the nested DTO and mark the containing property with @Valid so validation cascades into it. The same applies to elements of a collection when each element has its own constraints. A rule involving two fields in one request, such as an end date after a start date, can use a class-level custom constraint. The validator reports a stable field or object error without performing database or remote calls.
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Null behavior needs an explicit combination: @Valid cascades only when a value exists, so @NotNull expresses required presence. Custom validators remain stateless and reusable because validator instances may be shared. A rule such as username uniqueness depends on changing data and race conditions, so it belongs in the application or persistence boundary with a database constraint, not a network call hidden inside Bean Validation. Validation messages are client-safe contract data, not raw exception text.

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Forgetting @Valid on the containing property.
Calling a database from a constraint validator.
Assuming cascaded validation implies non-null.

How do you validate nested objects and cross-field rules in Spring?

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