The JSR 303 default message interpolation algorithm allows you to customize messages by supplying a resource bundle named ValidationMessages. Create a ValidationMessages.properties file in the classpath containing:
javax.validation.constraints.NotNull.message=CUSTOM NOT NULL MESSAGE
javax.validation.constraints.Size.message=CUSTOM SIZE MESSAGE
This changes the default message for the @Size constraint, so you should use the @Size constraint instead of the Hibernate-specific @Length constraint.
Instead of changing the default message for all constraints, you can change the message for a specific constraint instance. Set the message attribute on the constraint:
@NotNull(message = "{email.notnull}")
private String email;
And add the message to the ValidationMessages.properties file:
email.notnull=E-mail address is required