spring-form-handling

HTML forms submit data that must be extracted, validated, bound to objects, and re-displayed with error messages on failure. Manual form handling requires tedious request parameter extraction, field-by-field validation, and error tracking — all duplicated across every form in the application.

Spring MVC provides comprehensive form handling: @ModelAttribute binds form fields to Java objects, @Valid triggers Bean Validation annotations, and BindingResult captures validation errors for display in the view. Spring form tags (<form:form>, <form:input>, <form:errors>) integrate with the model for seamless two-way binding.

1. Form backing object: A Java bean (POJO) holds form data; added to model via @ModelAttribute
2. GET request: Controller adds empty/ pre-populated form object to model, returns form view
3. Form submission: POST request with form fields → Spring binds fields to @ModelAttribute parameters
4. Validation: @Valid on the parameter triggers annotation validation; errors go to BindingResult (must be immediately after the validated parameter)
5. Error handling: If BindingResult.hasErrors(), return to form view — errors display via <form:errors>
6. Success: If no errors, process data (save to DB) and redirect (POST-Redirect-GET pattern)

• @ModelAttribute: Binds form fields to Java object fields by name match
• @Valid / @Validated: Triggers Bean Validation (JSR-380) annotations like @NotBlank, @Email, @Size
• BindingResult: Must immediately follow @Valid parameter; holds field and global errors
• Spring form tags: <form:form>, <form:input>, <form:errors> — automatically populate and display errors
• POST-Redirect-GET: Prevents duplicate form submission on refresh by redirecting after successful POST
• Type conversion: Automatic conversion from String (form field) to int, long, date, etc.

• Built from: Spring MVC — Form handling is part of Spring MVC’s web functionality
• Built from: Spring Controller — Controller methods handle form GET and POST
• Related: Spring MVC Exception Handling — Validation errors are distinct from exceptions
• Related: Try-Catch-Finally — BindingResult tracks validation errors similarly to structured error collection

• BindingResult position: Must immediately follow the @Valid parameter — any fields between them cause binding failure
• Errors on GET: First GET request has no BindingResult — use separate method or check if it’s a rebind
• Nested properties: @Valid on nested objects requires cascading validation (@Valid on the nested field)
• Conversion errors: Type mismatch (e.g., “abc” for int field) goes to BindingResult as a FieldError
• Custom validators: Implement Validator interface and register; or use @Pattern for simple regex validation