Forms

Handling user-submitted data

Subsections

File Uploads: How to upload and handle files using forms
Form Components: Enhanced input fields
Image Galleries: Upload and edit an image gallery
Multimedia Galleries: Create complex image galleries

Description

Forms are used to gather information from users. You could also use plain anchors with request attributes, but forms are way more convenient especially for more than a couple of fields and when performing validation.

The more general kind of form has some input fields that are sent to the server in a POST and received from the @Controller as method parameters. This is a standard Spring Web MVC feature and quite straightforward to implement.

For more complicated forms you’d rather have a "form backing bean" holding all the input fields. This allows the use of the Yada Form Fragments for quickly assembling the HTML of the form.

Creating a java object just to hold the form data might seem overkill when you already have a database @Entity with the same exact fields. With a simple technique you can very easily use the same @Entity classes as form backing beans and save yourself a lot of work.

Plain Forms

The standard plain forms don’t need much explanation: you have some HTML with input fields and a @Controller that receives those input fields:

<form th:action="@{/addBook}">
        <input name="bookTitle" th:value="${bookTitle}">

@Controller
public class BookController {
        @RequestMapping("/addBook")
        public String addBook(String bookTitle) {
                Book book = new Book(bookTitle);

The above code doesn’t perform any validation but in a real-world example it should check at least that the title is not empty and go back to the user with an error message. The error handling code would have to be added for any field. Writing it could be quite time-consuming if there are more than a couple of fields. This is a possible solution for error handling:

<form th:action="@{/addBook}">
        <input name="bookTitle" th:value="${bookTitle}" th:classappend="${titleError}?'formError':''">
        <div th:if="${titleError!=null}" th:text="${titleError}" class="formError">Title Error</div>

@Controller
public class BookController {
        @RequestMapping("/addBook")
        public String addBook(String bookTitle, Model model) {
                if (bookTitle.trim().length()==0) {
                        model.addAttribute("titleError", "The book title can not be empty");
                        return "bookForm";
                }
                Book book = new Book(bookTitle);

Form Backing Beans

TODO

Entity Backing Beans

When the form contains most of the fields that are defined in an existing @Entity it makes sense to use that @Entity as a backing bean. Some may argue that exposing database classes on the front-end layer would compromise layer isolation, but we never experienced anything bad by doing this.

The general idea is that Spring can create an instance of a new @Entity or read it from the database before setting form values on it. The @Controller method handling the form will receive a ready-populated @Entity where any database value has been overwritten by the form value. This is very convenient because you just have to save the object and the form values will be persisted to the database. Even more, you can write a single method to handle both addition and editing of @Entity instances, sharing the common code.

The trick is to add a @ModelAttribute to the @Controller, which is always called before Spring calls the final @RequestMapping method:

<form th:action="@{/addBook}" th:object="${book}">
        <input type="hidden" name="bookId" value="*{id}">
        <input th:field="*{bookTitle}">

@Controller
public class BookController {

        @ModelAttribute("book")
        public Book addBook(@RequestParam(value="bookId", required=false) Long id) {
                Book toEdit = null;
                Exception exception = null;
                if (id!=null) {
                        try {
                                toEdit = bookRepository.findOne(id);
                        } catch (Exception e) {
                                exception = e;
                        }
                        if (toEdit==null) {
                                log.error("Can't find Book with id={} - (creating new)", id, exception);
                        } else {
                                log.debug("Book {}-{} fetched from DB as ModelAttribute", id, toEdit.getTitle());
                        }
                }
                if (toEdit==null) {
                        toEdit = new Book();
                }
                return toEdit;
        }

        @RequestMapping("/addBook")
        public String addBook(Book book, Model model) {
                // Validation here
                ...
                bookRepository.save(book);

test that the above code works

It’s always better to use a specific name for the @Entity id, like bookId and not just id, so that you can handle different @Entity instances in the same @Controller.

Ajax Forms

To send a form via ajax you just need to add the yadaAjax class:

<form class="yadaAjax" action="/subscribe">

When a form is added dynamically via custom javascript, you also need to call

yada.enableAjaxForms();

after the change.

Any submit handler that needs to be invoked before form submission has to be listed as a SubmitHandler value:

data-yadaSubmitHandler yada:submitHandler

Both the data- attribute version and the yada: dialect version take a comma-separated list of function names to be called in order. Submission is aborted if any returns false and any following functions are skipped.

Example:

<form class="yadaAjax" action="/subscribe" yada:submitHandler="validateInput">

Submit handlers can be set on the form tag and on any submit button. The "this" object is either the form or the clicked button, depending on where the tag has been placed.

Postprocessing

There are many options to handle the server response, like replacing page content or invoking some handler. See Ajax Links for more details. For example, this form replaces a page element with the returned html:

<form class="yadaAjax" action="/subscribe" yada:updateOnSuccess="#someSection">

Form groups

Multiple forms scattered around the page can be submitted together if they belong to the same "Form Group": when any (ajax or not) form in the group is submitted, the fields from all other forms in the same group are added to the payload. If a field has the same name in more than one form, only the first found is considered.

The submission of a form group can also be triggered by an anchor or any other type of element with a href or data-href attribute (ajax or not). Any request parameter on the url is added to the payload and overwrites any form fields with the same name.

The syntax to define a form group is the following:

data attribute	yada dialect	value	description
data-yadaFormGroup	yada:formGroup	any name	Identifies the form as belonging to the given group

data attribute

yada dialect

value

description

data-yadaFormGroup

yada:formGroup

any name

Identifies the form as belonging to the given group

Example with a triggering form:

<form action="/someAction" yada:formGroup="myGroup">
	<input name="a" value="1">
	<button type="submit">Submit</button>
</form>
<form action="ignored" yada:formGroup="myGroup">
	<input name="b" value="2">
</form>
<form action="ignored" yada:formGroup="myGroup">
	<input name="c" value="3">
</form>

In the above example, submitting the first form would send "a=1&b=2&c=3" to /someAction.

A similar behavior would be obtained by using the "form" attribute on the <input> tags of child forms. The use of yada:formGroup has the following advantages:

it is quicker to type on large forms
can be used together with (ajax or not) requests from elements other than forms (e.g. ajax links)
a future improvement could allow forms to belong to multiple groups, something that can’t be done with the "form" attribute

Example with a triggering anchor in ajax:

<a href="" yada:ajax="/someUrl?a=9" yada:formGroup="myGroup">Click me</a>
<form action="/someAction" yada:formGroup="myGroup">
	<input name="a" value="1">
	<input name="c" value="3">
	<button type="submit">Submit</button>
</form>
<form action="ignored" yada:formGroup="myGroup">
	<input name="b" value="2">
</form>

In the above example, clicking on the link would send "a=9&b=2&c=3" to /someUrl.

all. Remember that button handlers receive the button itself: function editTaskFormHandler(responseText, responseHtml, form, button) {

TO BE CONTINUED