Jakarta Bean Validation is a specification which
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lets you express constraints on object models via annotations
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lets you write custom constraints in an extensible way
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provides the APIs to validate objects and object graphs
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provides the APIs to validate parameters and return values of methods and constructors
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reports the set of violations (localized)
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runs on Java SE but is integrated in Java EE 6 and later; Bean Validation 2.0 is part of Java EE 8
E.g. like so:
public class User {
private String email;
@NotNull @Email
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
}
public class UserService {
public void createUser(@Email String email, @NotNull String name) {
// ...
}
}
While you can run validation manually, it is more natural to let other specifications and frameworks validate data at the right time (user input in presentation frameworks, business service execution by CDI, entity insert or update by JPA).
In other words, run once, constrain anywhere.