15.4. Thymeleaf Form Tools¶

15.4.2.1. Using th:field¶

We can use the th:field attribute on a form input to instruct Thymeleaf to add field-specific attributes to the form, such as name and id.

Consider our second input, which currently looks like this:

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<label>Description
   <input type="text" name="description" class="form-control" />
</label>

Above, we set the name attribute of the input element equal to description". We do this because we want this input data to be bound to the description field of the Event class when the form is submitted.

A better approach uses th:field to bind the description field when the form is rendered.

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<label>Description
   <input type="text" th:field="${event.description}" class="form-control" />
</label>

With this syntax, Thymeleaf will look for a variable named event and use its description property to set the values of the name and id attributes. The generated input looks like this:

<input type="text" id="description" name="description" class="form-control" />

We don’t need to use the id attribute in this case, but it doesn’t hurt anything by being there. Now, Thymeleaf sets name on its own, based on the field identifier.

For this to work, two more steps are necessary. First, we add constructor to Event that doesn’t require any arguments, also called a no-arg constructor.

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public Event(String name, String description, String contactEmail) {
   this();
   this.name = name;
   this.description = description;
   this.contactEmail = contactEmail;
}

public Event() {
   this.id = nextId;
   nextId++;
}

This code includes two changes:

1. A no-arg constructor has been created. It simply sets the id of the object, leaving all other fields null.
2. The previously-existing constructor now calls this(), which calls the no-arg constructor to set the id before setting the values of all other fields.

Finally, we have to pass in an empty Event created with the new no-arg constructor when rendering the form. Back in EventController, we update the handler:

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@GetMapping("create")
public String displayCreateEventForm(Model model) {
   model.addAttribute("title", "Create Event");
   model.addAttribute("event", new Event());
   return "events/create";
}

Notice line 29, which passes in an Event object created by calling the no-arg constructor.

model.addAttribute(new Event());

Using this technique on our other form fields completes the task of binding the object to the form during rendering.

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<form method="post">
   <div class="form-group">
      <label>Name
            <input type="text" th:field="${event.name}" class="form-control" />
      </label>
      <p class="error" th:errors="${event.name}"></p>
   </div>
   <div class="form-group">
      <label>Description
            <input type="text" th:field="${event.description}" class="form-control" />
      </label>
      <p class="error" th:errors="${event.description}"></p>
   </div>
   <div class="form-group">
      <label>Contact Email
            <input type="text" th:field="${event.contactEmail}" class="form-control" />
      </label>
      <p class="error" th:errors="${event.contactEmail}"></p>
   </div>
   <div class="form-group">
      <input type="submit" value="Create" class="btn btn-success" />
   </div>
</form>

One additional result of using th:field is that if the Event object has a value in any bound field, the input will be created with that value in its value attribute. For example, if the event object has a contactEmail of me@me.com, then the resulting form input would be:

<input type="text" id="contactEmail" name="contactEmail" value="[email protected]" class="form-control" />

The value is then visible in the form field when the page loads. This may not seem immediately useful, but it actually is. Recall our form submission handler:

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@PostMapping("create")
public String processCreateEventForm(@ModelAttribute @Valid Event newEvent,
                                    Errors errors, Model model) {
   if(errors.hasErrors()) {
      model.addAttribute("title", "Create Event");
      return "events/create";
   }

   EventData.add(newEvent);
   return "redirect:";
}

This method checks for validation errors and returns the user to the form if it finds any. It uses model binding to create a new event object, but this event object is also passed into the view when re-rendering the form. This means that if there are validation errors, the form will be rendered with the values that the user previously entered, preventing the user from having to re-enter all of their data.

15.4.2.2. Using th:errors¶

The Thymeleaf attribute th:errors is used similarly to th:field to display field-specific error messages. Recall that when we added our validation annotations to each model field, we also added a message argument. Setting th:errors to a field will display any validation errors for that field.

For example, let’s add a new element to the first form group:

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<div class="form-group">
   <label>Name
      <input th:field="${event.name}" class="form-control" />
   </label>
   <p class="error" th:errors="${event.name}"></p>
</div>

Setting th:errors="${event.name}" tells Thymeleaf to insert any error messages related to the name field of event into the paragraph element. We add class="error" to allow us to style this element, for example with red text. A simple rule in our styles.css file will do the trick:

.error {
  color: red;
}

Using this attribute on all of the fields gives us our final form template code:

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<form method="post">
   <div class="form-group">
      <label>Name
            <input type="text" th:field="${event.name}" class="form-control" />
      </label>
      <p class="error" th:errors="${event.name}"></p>
   </div>
   <div class="form-group">
      <label>Description
            <input type="text" th:field="${event.description}" class="form-control" />
      </label>
      <p class="error" th:errors="${event.description}"></p>
   </div>
   <div class="form-group">
      <label>Contact Email
            <input type="text" th:field="${event.contactEmail}" class="form-control" />
      </label>
      <p class="error" th:errors="${event.contactEmail}"></p>
   </div>
   <div class="form-group">
      <input type="submit" value="Create" class="btn btn-success" />
   </div>
</form>

Now, when the form is submitted with invalid data, our custom validation error messages will display just below the given inputs.

The result of submitting an empty form¶