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Intro to Java Web Development with Spark

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Messaging

6:10 with Craig Dennis

Let’s explore how to handle exceptions with grace and ease. You will write your own Flash messaging tool which you will use to inform your users one time over redirects.

Okay, we need to communicate a little bit better 0:00
with our users about the state of their experience. 0:03
And of course, because the only state that exists is what we've made 0:05
appear to be stateful, we're gonna need to jump through some hoops to get there. 0:09
A common approach to solve the problem of keeping the user informed about their 0:13
actions is to use what is known as a flash message. 0:17
You've seen them, they're usually displayed in a box on a page. 0:21
It says something like this, see the message here that says, 0:23
all right your details were updated. 0:26
Now most fully featured frameworks provide this nice to have feature, but 0:28
some micro-frameworks, ours in particular, don't quite have those yet. 0:32
But that doesn't mean we can't craft our own. 0:36
The really nice thing that our micro-framework provides is the handling 0:38
of exceptions. 0:42
Let's improve the user experience. 0:43
Okay, so first, let's take a quick look at the built-in exception handling. 0:46
So we kicked off a few of our own custom NotFoundExceptions. 0:50
Let's go to the definition here. 0:54
So we're gonna go to the NotFoundException, and I'm gonna 0:55
click in here and I'm gonna do a Cmd+B, which is gonna go to the declaration. 0:58
And then it's going to zip over to where we did it before which is in this 1:03
findBySlug method. 1:05
Okay, so anytime we findBySlug is called and 1:07
it can't find that slug it's gonna throw a NotFoundException. 1:10
But, that's what we want to have happen. 1:13
So, let's go ahead and we'll handle this NotFoundException 1:15
when it happens because right now it just kinda a gross page right. 1:19
So let's flip over to our main method. 1:22
And let's scroll down here to the bottom. 1:26
Now the name of the, its a static method and, 1:31
as you might expect it's called exception. 1:36
And then it takes the class that we're gonna try to catch. 1:40
So again, that is our NotFoundException. 1:42
And we'll do .class because we'll get the handle on the class. 1:47
And then it takes a handler here, and the handler will return an exception and then 1:51
of course our request and our response, so it's very similar to the other handlers. 1:56
Okay, when something's not found, 2:01
the proper way to tell the client is that they got a 404. 2:03
Now, we can make this page look however we want, but 2:07
we wanna make sure that we send the header, the status of 404. 2:09
It's an http status code. 2:15
Okay, now unfortunately this exception handler 2:19
isn't set up using this template style like we were doing before right? 2:23
Where there's the three parameters and we pass this new handlebars template engine, 2:26
but that doesn't mean we can't use the template engine still. 2:30
So this will help actually demystify what was happening using that so 2:34
what we can do, is we can create our own engine. 2:39
So let's do that we'll a handlebars templating engine. 2:42
We'll say engine = new HandlebarsTemplateEngine. 2:45
And really the only thing this is doing is calling render. 2:51
There's a thing on the engine called render. 2:55
And it returns html, so we'll = engine say it's String html- .render. 2:57
This is what was passed in here, right? 3:05
So it takes whatever was here, creates a new one of these and 3:07
then pushes this into the render method. 3:11
So we'll just push that in here. 3:14
So we'll say new ModelAndView. 3:15
And we don't have a model right now so we'll just say null. 3:20
But we're going to pass in a new template that we're going to build here in 3:23
a second, so not-found.hbs. 3:26
Remember, if stuff starts getting a little out of control, 3:29
you can always add it to a new line, everything will still be happy. 3:32
There we go. 3:37
Okay, so now we have html and string and 3:39
we need to just set the body of the response. 3:40
Now, you don't return from this exception handler, you set the body. 3:43
Here we go. 3:47
Okay, so, let's go make our new template. 3:51
Make a new file 3:53
Called not-found.hps. 4:01
Okay, and we will again, everything will start working like normal, 4:07
cuz we're using that engine. 4:12
So we'll say content, partial content, and 4:14
let's set the h1 to be what the You can do something clever in here. 4:19
So we couldn't find what 4:27
you were looking for. 4:32
Of course we could also override that title if we wanted to. 4:40
Say partial title and not found. 4:44
And of course we need to get our base in here to be implemented. 4:52
So we can definitely make this prettier and 5:00
push more information into the model that displays on this page. 5:02
There's some more in the teacher's notes. 5:05
Okay, so let's make sure that we caught our exception, so I'm gonna go over and 5:07
I'm gonna start my server, and say Run Main. 5:11
Okay, so he's running. 5:19
Okay, so let's go over to the page and I'm gonna go ahead and log in. 5:21
Let's go ahead and let's make a new 5:25
Idea here, again we Spark Testing 5:32
now that Spark Testing exists I'm going to click on Spark Testing. 5:36
And if I come and I change this just to be a little bit off, 5:40
we're going to change the slug to be spelled wrong and press enter. 5:45
Now I'll get our what the page. 5:48
And you'll see that there is a status code, a 404 not found, awesome. 5:49
Now, if we wanted more specific pages, we'd just make more specific exceptions. 5:57
It's pretty straightforward, right? 6:01
So now that we've explored the built in messaging, let's take a quick break right 6:03
here, and then we'll drive right back into making our own flash messages. 6:06