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In this video, we'll create a Spring controller that has a method to intercept a URI request, and respond with a simple String message.
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Welcome back, and
congrats again on the not so
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convincing success of deploying
your first Java web app.
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Sure, we got an error when we
visited our running application, but
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that was only because our app lacks the
code to handle requests to certain URIs.
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Actually, it lacks the code to
handle requests to any URI.
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So, let's change that.
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What we're going to do is
write a Spring controller.
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In a Spring web application,
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the controller is a Java object whose job
is to handle requests to certain URIs.
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We'll apply annotations to methods
within this class that indicate
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which URI each method should handle.
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First a quick note on URIs and URLs.
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URI stands for
Uniform Resource Identifier, and
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URL stands for Uniform Resource Locator.
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You may have heard the terms
used interchangeably,
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but they're actually different.
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In general a URL is an address that
includes a method for locating a resource,
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with a protocol like HTTP or FTP.
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In addition a URL will include a host
name like teamtreehouse.com, or
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creativecommons.org.
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But a URI doesn't have to.
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Since our application will be coded
without regard to the host name,
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and without indicating the protocol,
I'll continue to use URI.
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All right, with that said, let's code
a controller to handle URI requests.
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Before creating a controller, let's
create a package where we can put them.
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To do this, I'll right click on
calm.teamtreehouse.giflib, and
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choose new package.
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I'll name this package controller.
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And to create the actual controller,
I'll right-click our new package and
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choose New > Java Class.
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Since this will be the controller
that handles all of our requests to
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gif related pages,
I'll name this one gifcontroller.
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The way that we'll indicate that
this class is a spring controller,
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is with the controller annotation
on the class itself, @Controller.
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And that comes from
the spring framework as well.
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Hit enter so that I get the auto
import statement dropped in there.
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In a spring controller you can add as many
methods as you like, though in general
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they should be related to the object the
controller refers to, in our case a gif.
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I'd like to use this controller
to handle requests to
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any page of our application that
displays a list of gifs or a single gif.
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Later on, if we'd like to add a controller
that shows a list of categories, for
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example, we might add
a category controller.
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Let's add a method that will handle
all requests to our home page.
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Since our home page would likely be a list
of the most recent animated gifs that have
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been uploaded, it makes sense to put
that method in our GifController class,
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so I'll add that method now.
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I'm going to call it
public String listGifs.
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I chose to name this method listGifs, but
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it really could be named
anything you like.
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But, how will the Spring framework
know to execute this method
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when a user requests our homepage?
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Now that is accomplished with,
you guessed it another annotation.
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This one is called request mapping, which
refers to the fact that we want to map
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a certain URI request to a Java method.
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The request mapping annotation
has several elements,
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and we'll use just one
of them in this course.
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So I'll start by adding the request
mapping annotation to the method itself
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request mapping, there it is right there,
I'll hit enter on the suggestion.
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So I get that import statement.
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The one element of the annotation
that we'll be using is called value.
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And the value we specify here will be
the URI pattern that we want to map
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to this method.
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In this case, I'll use a single forward
slash indicating that this method will
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handle requests to our application root,
or homepage.
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If you like, since the only annotation
element we use is called value,
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you can omit that name altogether.
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For now, we'll also need to add the
response body annotation to the controller
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method, to indicate that
this string we return should
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be used as the response without
any further processing.
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So, I'll add the responsebody
annotation to the method, as well.
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And, as for the return value,
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I'll choose an arbitrary string to return,
list of all the GIFs!
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All right, we're almost there.
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One more step, though.
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If we tried to use the gradle
boot run task to build and
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deploy our app, we'd find ourselves
left with a bit of disappointment.
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In fact, we'd see the same error
message as before in our browser.
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This is because the Spring framework
doesn't automatically scan your packages
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for controllers.
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You have to instruct
the framework to do so.
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Let's go back to our app config class and
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add the annotation that
accomplishes this task.
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Here we'll add a simple annotation
to tell the spring framework
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to scan the current package for
controllers.
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And this scan we'll include our
sub package name, controller.
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The annotation name is component scan.
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Excellent.
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We're ready to test this thing.
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Again, because we're using gradle,
this is pretty straight forward.
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If you don't already have your server
running, in the gradle tool window you can
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expand tasks, right click boot run and
choose run GifLib.
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Since I still have mine running,
I'm going to click the stop button first.
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After a few moments, you should see a
message in the console here that indicates
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that the process has been stopped.
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There it is, it finished running boot run.
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And in fact, since I just stopped it,
I could either go up here and right-click
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boot run and choose run again, or,
since it's the task that I just stopped,
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this gives me a convenient rerun button,
by clicking this play button right here.
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I'll click that now.
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After a few moments, you should see
some output at the bottom of the screen.
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This output is largely the same as we saw
before, with one important difference.
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If you look closely,
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you'll see a line that says the root path
has been mapped to our list gifs method.
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So if you look here,
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here is the single slash that has
been mapped onto java.lang.string.
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That's the return value
of our controller method.
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And if we scroll over, we see the fully
qualified name of the method,
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meaning package name,
class name, there it is.
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Method name.
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The slash URI that is
the root of our application,
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has been mapped to our list gifs method.
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Excellent.
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Switching over to a browser,
we can again navigate to local host.
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Including the port number 8080 and
see our message so plainly, yet
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so wonderfully on display.
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There it is.
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List of all the gifs.
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Try coding another method and
gif controller, and add a request mapping
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annotation, and the response body
annotation to capture the uri/gif.
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Returning a string message of your choice.
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When you're done writing that method,
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you'll test this in the browser by
stopping and restarting the bootrun task.
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Then coming back here to the browser and
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entering /gif at the end of the URI here.
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When you're done,
you shouldn't see this error message, but
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rather, the custom message that you've
returned in your new controller method.
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Go ahead, give it a shot.
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