How to treat ConversionFailedException as a validation error? And why is th:object null for unrelated fields if ConversionFailedException occurs? - spring

Question

Say in my entity I have a var yearOfBirth: Year?:
#Entity
#Table(name = "employee")
class Employee(
#Id #GeneratedValue(strategy = GenerationType.IDENTITY)
val id: Long = 0,
#field:NotEmpty
var lastName: String?,
#field:NotEmpty
var email: String?,
#field:Enumerated(value = EnumType.STRING)
var sex: Sex?,
#field:Past
#field:NotNull
var yearOfBirth: Year?,
)
The controller just saves the update. When a client makes a call, an integer is expected for yearOfBirth, but of course, anything could be posted:
#PostMapping("/pages/profile")
fun profileUpdate(#ModelAttribute("employee") #Validated employee: Employee, result: BindingResult, model: Model, principal: Principal) : String {
println(result.hasErrors())
if(result.hasErrors()){
result
.getFieldErrors()
.stream()
.forEach{f -> println(f.getField() + ": " + f.getDefaultMessage())};
println(employee.yearOfBirth)
println(employee.lastName)
return "profile"
}
employeeRepository.save(employee)
return "profile"
}
And this is the part of the template relating to my questions:
<form action="#" th:action="#{/pages/profile}" th:object="${employee}" method="post">
<input type="hidden" th:field="${employee.id}"/>
<input class="input" th:classappend="${#fields.hasErrors('lastName') ? 'is-danger' : ''}" type="text"
th:field="${employee.lastName}"
placeholder="Last name">
<div class="help is-danger" th:if="${#fields.hasErrors('lastName')}" th:errors="*{lastName}"></div>
<input class="input" type="text"
th:field="${employee.yearOfBirth}"
placeholder="1234">
</div>
<div class="help is-danger" th:if="${#fields.hasErrors('yearOfBirth')}" th:errors="*{yearOfBirth}"></div>
<select th:field="${employee.sex}">
<option th:each="sex : ${T(org.example.validation.domain.Sex).values()}"
th:value="${sex}"
th:text="${sex.toString()}"
th:selected="${employee != null && sex.toString() == employee.sex}"></option>
</select>
<div class="help is-danger" th:if="${#fields.hasErrors('sex')}" th:errors="*{sex}"></div>
</form>
With this, if someone enters an invalid year, say asdf, the backend encounters an exception because it can't convert the String to a Year.
yearOfBirth: Failed to convert value of type 'java.lang.String' to required type 'java.time.Year'; nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [#javax.validation.constraints.Past #javax.validation.constraints.NotNull java.time.Year] for value 'asdf'; nested exception is java.lang.IllegalArgumentException: Parse attempt failed for value [asdf]
The error is passed along as a whole to the frontend.
My first question: how do I treat any marshalling/conversion error as a validation error in my application with a reasonable default message like "invalid type of value" or somesuch? I see I could implement a method tagged with #ExceptionHandler(ConversionFailedException.class) but I don't see how I can access the original object there to pass it back to the frontend.
But what I don't understand at all, is that for another field in the rendered Thymeleaf template, the employee somehow is null if such an exception occurs. If I remove the employee != null check from th:selected="${employee != null && sex.toString() == employee.sex}", then the page cannot be rendered and this exception is thrown:
2022-06-18 15:14:39.794 ERROR 40079 --- [nio-8080-exec-7] org.thymeleaf.TemplateEngine : [THYMELEAF][http-nio-8080-exec-7] Exception processing template "profile": Exception evaluating SpringEL expression: "sex.toString() == employee.sex" (template: "profile" - line 63, col 49)
org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "sex.toString() == employee.sex" (template: "profile" - line 63, col 49)
at org.thymeleaf.spring5.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:292) ~[thymeleaf-spring5-3.0.14.RELEASE.jar:3.0.14.RELEASE]
(...)
Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'sex' cannot be found on null
at org.springframework.expression.spel.ast.PropertyOrFieldReference.readProperty(PropertyOrFieldReference.java:213) ~[spring-expression-5.3.15.jar:5.3.15]
(...)
2022-06-18 15:14:39.796 ERROR 40079 --- [nio-8080-exec-7] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "sex.toString() == employee.sex" (template: "profile" - line 63, col 49)] with root cause
org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'sex' cannot be found on null
at org.springframework.expression.spel.ast.PropertyOrFieldReference.readProperty(PropertyOrFieldReference.java:213) ~[spring-expression-5.3.15.jar:5.3.15]
But this only happens if the validation error is from a ConversionFailedException. For other validation errors (e.g. NotEmpty from the lastName field), employee remains not null. It also only affects this th:selected expression, but in other places (e.g. th:field="${employee.lastName}" above), the object is still not null. How can that even be? Why is employee null in one expression of the template but not another?
I have a complete but minimal working example but I'm not sure where to upload it.