Capturing Form Validation Errors

The process of adding form validation in Spring is threefold. 0:00 First, we'll annotate the entity class' fields with JPA validation annotations, 0:04 which come from the javax.validation.constraints package. 0:10 Then, we'll capture validation in what's called 0:14 a BindingResult in the controller final. 0:16 If there are validation errors, we'll redirect back to the form and 0:20 display those validation errors. 0:23 Let's get started by adding those validation annotations and by redirecting 0:26 back to the form without adding the category when errors are encountered. 0:30 The first step I mentioned was adding validation annotations and 0:36 we'll do that in the entity classes. 0:39 Let's head to the category class. 0:41 So under the model package, I will open the category class. 0:43 Here, I'll demonstrate just a few of the annotations we can use. 0:47 Let's start with the name field. 0:51 Let's make a decision that the value has to be present and 0:54 that its length must be between, say, three and twelve characters. 0:57 You can change this, if you'd like. 1:01 To do this, we'll add the NotNull annotation, 1:03 which will ensure that it's not blank, as well as the Size annotation. 1:06 And here, we can specify a minimum and maximum value. 1:12 So I'll say min = 3 and max = 12. 1:18 Next, let's add validation annotations for the colorCode field. 1:22 So I'll do that right here. 1:26 This value should be a valid hexadecimal color code as used in CSS, so 1:29 let's mark it as NotNull, so we make sure it's provided, and to ensure 1:33 that the value is a valid hexadecimal value, let's use the Pattern annotation. 1:40 Now, I'm going to use a regular expression here on which we could have an entire 1:46 course. 1:50 Check the teacher's notes for more regular expression info, and just note for 1:51 our purposes here, that regular expressions can be used to 1:56 intelligently detect patterns in strings. 2:00 So, I will start with the regular expression element in the annotation here. 2:04 And as far as the pattern I'm checking for, I'll enclose that in quotes. 2:10 I'd like it to start with a hash sign cuz a hex code in CSS starts with a hash sign. 2:14 And have each subsequent character come from the character class of either zero 2:19 through nine, lowercase a through f, uppercase A through 2:24 F, and I'd like that to be repeated six times. 2:30 And it looks like my right bracket snuck in there, as well. 2:36 There was IntelliJ doing all the work for me. 2:40 Okay. 2:44 That's all I'd like to validate in this entity. 2:44 Next, we'll go to the category controller to trigger validation and 2:47 capture the results. 2:51 Here, we'll focus on the add category method, which is down a bit, there we go. 2:53 What I'd like to do here is first trigger validation according to the constraints we 3:00 just added to the category entity. 3:03 And it turns out, this is really simple. 3:05 We add a valid annotation to the category parameter. 3:08 So this category parameter, before, we add the Valid annotation. 3:12 And just like that, it will trigger validation, 3:17 according to the constraints that we put on the category class. 3:19 Next, we'll need a way to capture errors, and in Spring, 3:24 this is most easily accomplished by including a binding result parameter, so 3:27 I’ll add that after the category parameter, BindingResult, 3:31 and I’ll just call it, result. 3:35 Now what we need to do is actually take alternate action if errors are found, 3:38 such as redirecting back to the form. 3:42 To detect errors, we can use an if statement. 3:45 I'll do that before our categories save. 3:48 So we can say if the result has errors, 3:50 what we can do here is redirect back to the form. 3:55 And to do that, we'll return a redirect back to the you URI that's used 4:03 to render the form, remember, this is not the name of the Thymeleaf view, but rather 4:09 the URI that will need to be requested in order for the form to be rendered. 4:15 Okay. 4:21 We're ready to test this. 4:22 Let's reboot and try to add a category with invalid data. 4:23 So I’m going to stop my current instance of my app, and 4:27 then I can reboot that thing, and 4:32 we'll wait for the app to both compile and fully boot. 4:36 There we go, application started, I'll switch back to Chrome. 4:41 Let me first try to add a category without supplying any data whatsoever. 4:45 So I'll add a category, and there we are, right back at the form, it may look like 4:50 nothing happened, but I certainly clicked add and got right back to the form. 4:55 Let me add a two character category name, and a color. 5:00 Remember, we said that the category name had to be between three and 5:04 twelve characters in length. 5:08 So I'll say Sp, and then I'll choose Blue, and click Add. 5:10 And once again, we're right back at the form, and 5:16 finally, let's make sure that we can successfully add a category if we do, 5:18 indeed, provide valid data for all fields. 5:24 So, I will type Spring here. 5:27 I'll choose Blue, why not, and click Add. 5:30 Great, there it is. 5:35 Now we still have more work to do, but 5:37 you're well on your way to fully implemented form validation. 5:38 So we've taken the critical step of introducing server side validation 5:42 into the flow of our application data, but we left out another critical piece. 5:46 When our form was re-displayed, we offered no indication that there was an error. 5:51 This could certainly leave users confused as to what just happened. 5:56 So next, we'll add form validation messages that tell users 5:59 exactly what went wrong when something does go wrong. 6:03