Create a Data Repository Component

So now you've had a chance to see the full flow of data from the storage of GIF 0:01 details in a GIF object to the adding of a GIF object to a model map and 0:04 finally the access of that object from a timely template for display. 0:09 There's one problem with our GIF detail page at this point and 0:13 that is it's only able to display the details of one hard coded GIF. 0:16 The compiler bot. 0:20 But what if we want the controller method and 0:22 template to control the display of any GIF object according to the URL requested? 0:24 For that, we'll need to step up our game. 0:29 In particular, we'll need to store a collection of GIF objects in memory 0:33 that our application can choose from. 0:36 Next, we'll be adding what's called a repository of GIF objects. 0:39 A repository is essentially a collection that we've stored somewhere in memory or 0:44 in a database. 0:48 For this simpler application, we'll store it in memory using a standard Java list. 0:49 Let's get started. 0:54 Before we make our GIF repository, I'm going to create a package that would hold 0:56 any data repositories we might want to include in our application. 1:00 So I'm going to right-click on the giflib package, and 1:03 choose New, Package, and call it data. 1:07 In this package is where I'll put the GIF repository, 1:12 which will be a new Java class named, not surprisingly, GifRepository. 1:15 This class will act as both our storage device for 1:24 GIF objects as well as methods for interacting with those GIF objects. 1:26 In a future course we'll show you how to shift your storage mechanism to a database 1:31 but that's a bit outside the scope of the current course. 1:35 So for now we'll stick with storing our GIF objects into a static Java list. 1:37 To begin, I'll create that list as a field. 1:42 Make sure to import java.util.List and 1:47 com.teamtreehouse.giflib.model.gif. 1:51 And I'll call this ALL_GIFS. 1:56 Notice I'm using the all caps naming convention because this 1:59 is a class constant. 2:02 In this course, we're going to predefine our GIFs. 2:04 Though in a production application, again, this would likely come from a database. 2:07 For this I'm going to use a convenience method offered by the arrays class called 2:12 asList. 2:16 In to it I can pass any number of GIF objects. 2:17 So arrays, it comes from the java.util package, .asList. 2:24 This is, as opposed to constructing a new array list, 2:32 then using the add method a bunch of times to add individual objects. 2:35 I'll include six GIF objects here, one for 2:40 each image I've given you in the GIFS directory. 2:43 In the teacher's notes I've given you this block of code 2:46 in case you wanna save yourself some typing. 2:48 I'll go ahead and copy and paste that now so 2:51 that you don't have to watch me type all that code. 2:53 Give myself some space there. 2:56 Copy that I will get back in here and I will paste that right inside there. 2:58 Now, there's one import I need to resolve and that's of the local date class. 3:03 So I'll hit option enter in Inteli J its automatically take its first suggestion. 3:07 Might move this back one tab as well. 3:13 Cool, and that's our storage device. 3:16 I simple JAVA list. 3:18 But I also mentioned that the beyond storing our data, 3:20 this class would offer us methods for retrieving that data. 3:23 Because this list is private, we'll need to add some public 3:27 methods in order to access this list outside of the class. 3:30 Let's add one of those methods now. 3:35 We'll make a method that allows us to retrieve a GIF object with a given name. 3:37 So it will return a GIF object. 3:42 And I'm gonna call it findByName. 3:45 I'll give it a String parameter, so that the collar of this method can 3:49 pass at the name to retrieve the GIF object associated with that name. 3:54 So inside this method, I'm going to iterate over 3:59 all of my GIF objects in the list, so I'll use this enhanced for loop. 4:02 Oops, I forgot the name of my variable here. 4:07 Gif in ALL_GIFS, so it's a for 4:10 each loop or enhanced for loop for every GIF object. 4:15 In all GIFS I want to do the following. 4:18 If the particular GIF object I'm looking at has a name that equals the name 4:22 that was passed in, well then what I wanna do is return exactly that GIF object. 4:29 Now, if I get through the entire list without ever finding a GIF 4:34 matching this name, then what I want to do is return null. 4:38 We'll leverage this method for 4:45 our GIF controller class, which I'll switch to now. 4:46 We'll want to leverage that method we just wrote from the GIF details method here so 4:52 that we can feed the GIF details template with any GIF object instead of 4:56 this one that we've hard coded. 5:00 In order to have access to the GIF repository we'll need a reference to a GIF 5:02 repository object, since we'll likely access this from any controller method 5:07 I'll add it as an instance field. 5:11 And we'll do that up top. 5:13 Private GifRepository and 5:14 I'll just call it GifRepository. 5:18 Of course, without assigning anything to GifRepository, 5:23 if we try to call any of its methods like the find my name 5:26 method in any of our controller methods, we'll fall on our faces. 5:30 Because the field will be null and 5:33 we'll encounter a null pointer exception as I'm sure you've seen many times before. 5:35 There are a couple of ways we could go about initializing this GifRepository 5:41 field. 5:45 One way would be to use a constructor in this class. 5:46 And when the constructor is called we'd assign the GifRepository 5:49 field a new instance of a GifRepository class. 5:53 But there's actually another feature in spring that we can leverage here to 5:57 reduce our coding. 6:00 It turns out that Spring can initialize fields for 6:02 us as long as it can find a spring component of the same class for the field. 6:04 In our case it needs a spring component for GifRepository. 6:10 To tell spring that we want it to auto assign our field, 6:13 we use the autowired annotation. 6:17 This tells Spring to construct and 6:20 assign a GifRepository object to this instance field as soon as it's needed. 6:22 The next question is, how will Spring know that 6:28 the GifRepository class is a valid Spring component, as I mentioned? 6:30 Here's a surprise, we'll use an annotation. 6:36 Remember how we annotated our AppConfig class with ComponentScan? 6:39 Because of this, Spring will scan that same package and all of it's sub packages. 6:44 And consider all classes annotated with component as valid 6:49 options depending on the class name. 6:53 So all we need to do is switch to the GifRepository 6:57 class and add the component annotation. 7:02 So that's applied at the class level and it also comes from the Spring framework. 7:05 There we go. 7:11 Now, Spring will pick this up during the initial component scan as a result of our 7:12 app config being loaded. 7:17 And when the GIF controller needs a GifRepository class, 7:19 Spring will create one for us or use one that's already been created. 7:22 The process of this runtime creation of objects that is the injection 7:27 using the Autowired annotation. 7:32 This process without explicitly writing code to construct and 7:34 assign objects is called dependency injection or DI for short. 7:38 That is, we didn't ever write code that calls the GifRepository constructor and 7:44 assign the resulting object to our GifRepository field. 7:49 Instead, we let Spring wire our objects together with Autowire. 7:53 As you continue your study of Java development, and 7:58 especially with Spring, you'll be seeing much more DI. 8:01 It's one of Spring's most powerful and core features. 8:05 Now, back to our gifDetail's controller method. 8:08 What we can do at this point is replace the creation of our GIF object with a call 8:11 to the findByName method of the GifRepository field. 8:15 So instead of creating a new object here, I will call on a gifRepository object, 8:19 the findByName method. 8:24 And to it we'll pass the name of the GIF that we'd like 8:28 to grab from our gifRepository. 8:31 Here, I'll use android-explosion. 8:34 Forget my semi colon there. 8:38 While this replacement will produce the same result, it represents 8:41 a giant step in the direction of creating a dynamic detail page. 8:44 Let's reboot and redeploy our app to make sure that we haven't broken everything. 8:49 So I'll start my boot run task, and 8:53 remember it could take a moment, you start it too soon you might see an error. 8:55 Sometimes it's definitely safe to wait for 9:00 that message in your console saying that the task has been complete. 9:02 So I'll click bootRun again, And 9:08 after I see all my output come through and see that Tomcat has started, I will switch 9:13 back to my browser, and I'll refresh this to make sure nothing is yet broken. 9:18 Excellent. 9:24 All is well. 9:25 It looks like our web app is still intact from the user's standpoint, so 9:27 we're in great shape! 9:30 Well, that was quite a bit of information in coding. 9:32 It may seem like a lot to digest right now, so 9:35 if you're feeling a bit overwhelmed don't panic. 9:37 You can always go back in the video to re-watch the parts that are still unclear. 9:40 Or you can come back to them after you finish the course. 9:45 Also, as you're coding the application on your own machine, 9:48 if you run into any problems, please use the forum to post your questions, and 9:52 include your code as necessary. 9:56 Let's take a break now before finishing the task of making our detail page, 9:59 present the GIF details given by a specific GIF's URI. 10:03