Création d'une application Web avec Spring Boot et Angular

1. Vue d'ensemble

Spring Boot et Angular forment un tandem puissant qui fonctionne parfaitement pour développer des applications Web avec un encombrement minimal.

Dans ce didacticiel, nous utiliserons Spring Boot pour implémenter un backend RESTful et Angular pour créer un frontend basé sur JavaScript.

2. L'application Spring Boot

La fonctionnalité de notre application Web de démonstration sera en effet assez simpliste. Il sera simplement limité à l'extraction et à l'affichage d'une liste d'entités JPA à partir d'une base de données H2 en mémoire, et à la persistance de nouvelles entités via un formulaire HTML simple.

2.1. Les dépendances de Maven

Voici les dépendances de notre projet Spring Boot:

 org.springframework.boot spring-boot-starter-web   org.springframework.boot spring-boot-starter-data-jpa   com.h2database h2 runtime 

Notez que nous avons inclus spring-boot-starter-web car nous l'utiliserons pour créer le service REST et spring-boot-starter-jpa pour implémenter la couche de persistance.

La version de la base de données H2 est également gérée par le parent Spring Boot.

2.2. La classe d'entité JPA

Pour prototyper rapidement la couche de domaine de notre application, définissons une classe d'entité JPA simple, qui sera responsable de la modélisation des utilisateurs:

@Entity public class User { @Id @GeneratedValue(strategy = GenerationType.AUTO) private long id; private final String name; private final String email; // standard constructors / setters / getters / toString } 

2.3. L' interface UserRepository

Comme nous aurons besoin de la fonctionnalité CRUD de base sur les entités User , nous devons également définir une interface UserRepository :

@Repository public interface UserRepository extends CrudRepository{} 

2.4. Le contrôleur REST

Maintenant, implémentons l'API REST. Dans ce cas, il ne s'agit que d'un simple contrôleur REST.

@RestController @CrossOrigin(origins = "//localhost:4200") public class UserController { // standard constructors private final UserRepository userRepository; @GetMapping("/users") public List getUsers() { return (List) userRepository.findAll(); } @PostMapping("/users") void addUser(@RequestBody User user) { userRepository.save(user); } } 

Il n'y a rien de complexe en soi dans la définition de la classe UserController .

Bien sûr, le seul détail d'implémentation à noter ici est l'utilisation de l' annotation @CrossOrigin . Comme son nom l'indique, l'annotation active le partage de ressources cross-origin (CORS) sur le serveur.

Cette étape n'est pas toujours nécessaire. Puisque nous déployons notre frontend angulaire sur // localhost: 4200 et notre backend de démarrage sur // localhost: 8080 , le navigateur refuserait autrement les demandes de l'un à l'autre.

Concernant les méthodes du contrôleur, getUser () récupère toutes les entités User de la base de données. De même, la méthode addUser () conserve une nouvelle entité dans la base de données, qui est passée dans le corps de la requête.

Pour simplifier les choses, nous avons délibérément laissé de côté l'implémentation du contrôleur déclenchant la validation Spring Boot avant de conserver une entité. En production, cependant, nous ne pouvons tout simplement pas faire confiance aux entrées de l'utilisateur, donc la validation côté serveur devrait être une fonctionnalité obligatoire.

2.5. Démarrage de l'application Spring Boot

Enfin, créons une classe d'amorçage Spring Boot standard et remplissons la base de données avec quelques entités utilisateur :

@SpringBootApplication public class Application { public static void main(String[] args) { SpringApplication.run(Application.class, args); } @Bean CommandLineRunner init(UserRepository userRepository) { return args -> { Stream.of("John", "Julie", "Jennifer", "Helen", "Rachel").forEach(name -> { User user = new User(name, name.toLowerCase() + "@domain.com"); userRepository.save(user); }); userRepository.findAll().forEach(System.out::println); }; } }

Maintenant, exécutons l'application. Comme prévu, nous devrions voir une liste des entités utilisateur imprimées sur la console au démarrage:

User{id=1, name=John, [email protected]} User{id=2, name=Julie, [email protected]} User{id=3, name=Jennifer, [email protected]} User{id=4, name=Helen, [email protected]} User{id=5, name=Rachel, [email protected]}

3. L'application angulaire

Avec notre application de démonstration Spring Boot opérationnelle, créons maintenant une application Angular simple, capable de consommer l'API du contrôleur REST.

3.1. Installation CLI angulaire

Nous utiliserons Angular CLI, un puissant utilitaire de ligne de commande, pour créer notre application Angular.

Angular CLI est un outil extrêmement précieux car il nous permet de créer un projet Angular complet à partir de zéro, en générant des composants, des services, des classes et des interfaces avec seulement quelques commandes .

Une fois que nous avons installé npm (Node Package Manager), nous allons ouvrir une console de commande et taper la commande:

npm install -g @angular/[email protected]

That's it. The above command will install the latest version of Angular CLI.

3.2. Project Scaffolding with Angular CLI

As a matter of fact, we can generate our Angular application structure from the ground up. But honestly, this is an error-prone and time-consuming task that we should avoid in all cases.

Instead, we'll let Angular CLI do the hard work for us. So, let's open a command console, then navigate to the folder where we want our application to be created and type the command:

ng new angularclient

The new command will generate the entire application structure within the angularclient directory.

3.3. The Angular Application's Entry Point

If we look inside the angularclient folder, we'll see that Angular CLI has effectively created an entire project for us.

Angular's application files use TypeScript, a typed superset of JavaScript that compiles to plain JavaScript. However, the entry point of any Angular application is a plain old index.html file.

Let's edit this file, as follows:

    Spring Boot - Angular Application         

As we can see above, we included Bootstrap 4, so we can give our application UI components a more fancy look. Of course, it's possible to pick up another UI kit from the bunch available out there.

Please notice the custom tags inside the section. At first sight, they look rather weird, as is not a standard HTML 5 element.

Let's keep them right there, as is the root selector that Angular uses for rendering the application's root component.

3.4. The app.component.ts Root Component

To better understand how Angular binds an HTML template to a component, let's go to the src/app directory and edit the app.component.ts TypeScript file – the root component:

import { Component } from '@angular/core'; @Component({ selector: 'app-root', templateUrl: './app.component.html', styleUrls: ['./app.component.css'] }) export class AppComponent { title: string; constructor() { this.title = 'Spring Boot - Angular Application'; } }

For obvious reasons, we'll not dive deep into learning TypeScript. Even so, let's notice that the file defines an AppComponent class, which declares a field title of type string (lower-cased). Definitively, it's typed JavaScript.

Additionally, the constructor initializes the field with a string value, which is pretty similar to what we do in Java.

The most relevant part is the @Component metadata marker or decorator, which defines three elements:

  1. selector – the HTML selector used to bind the component to the HTML template file
  2. templateUrl – the HTML template file associated with the component
  3. styleUrls – one or more CSS files associated with the component

As expected, we can use the app.component.html and app.component.css files to define the HTML template and the CSS styles of the root component.

Finally, the selector element binds the whole component to the selector included in the index.html file.

3.5. The app.component.html File

Since the app.component.html file allows us to define the root component's HTML template — the AppComponent class — we'll use it for creating a basic navigation bar with two buttons.

If we click the first button, Angular will display a table containing the list of User entities stored in the database. Similarly, if we click the second one, it will render an HTML form, which we can use for adding new entities to the database:

{{ title }}

  • List Users
  • Add User

The bulk of the file is standard HTML, with a few caveats worth noting.

The first one is the {{ title }} expression. The double curly braces {{ variable-name }} is the placeholder that Angular uses for performing variable interpolation.

Let's keep in mind that the AppComponent class initialized the title field with the value Spring Boot – Angular Application. Thus, Angular will display the value of this field in the template. Likewise, changing the value in the constructor will be reflected in the template.

The second thing to note is the routerLink attribute.

Angular uses this attribute for routing requests through its routing module (more on this later). For now, it's sufficient to know that the module will dispatch a request to the /users path to a specific component and a request to /adduser to another component.

In each case, the HTML template associated with the matching component will be rendered within the placeholder.

3.6. The User Class

Since our Angular application will fetch from and persist User entities in the database, let's implement a simple domain model with TypeScript.

Let's open a terminal console and create a model directory:

ng generate class user

Angular CLI will generate an empty User class. Let's populate it with a few fields:

export class User { id: string; name: string; email: string; }

3.7. The UserService Service

With our client-side domain User class already set, let's now implement a service class that performs GET and POST requests to the //localhost:8080/users endpoint.

This will allow us to encapsulate access to the REST controller in a single class, which we can reuse throughout the entire application.

Let's open a console terminal, then create a service directory, and within that directory, issue the following command:

ng generate service user-service

Now, let's open the user.service.ts file that Angular CLI just created and refactor it:

import { Injectable } from '@angular/core'; import { HttpClient, HttpHeaders } from '@angular/common/http'; import { User } from '../model/user'; import { Observable } from 'rxjs/Observable'; @Injectable() export class UserService { private usersUrl: string; constructor(private http: HttpClient) { this.usersUrl = '//localhost:8080/users'; } public findAll(): Observable { return this.http.get(this.usersUrl); } public save(user: User) { return this.http.post(this.usersUrl, user); } }

We don't need a solid background on TypeScript to understand how the UserService class works. Simply put, it encapsulates within a reusable component all the functionality required to consume the REST controller API that we implemented before in Spring Boot.

The findAll() method performs a GET HTTP request to the //localhost:8080/users endpoint via Angular's HttpClient. The method returns an Observable instance that holds an array of User objects.

Likewise, the save() method performs a POST HTTP request to the //localhost:8080/users endpoint.

By specifying the type User in the HttpClient‘s request methods, we can consume back-end responses in an easier and more effective way.

Lastly, let's notice the use of the @Injectable() metadata marker. This signals that the service should be created and injected via Angular's dependency injectors.

3.8. The UserListComponent Component

In this case, the UserService class is the thin middle-tier between the REST service and the application's presentation layer. Therefore, we need to define a component responsible for rendering the list of User entities persisted in the database.

Let's open a terminal console, then create a user-list directory and generate a user list component:

ng generate component user-list

Angular CLI will generate an empty component class that implements the ngOnInit interface. The interface declares a hook ngOnInit() method, which Angular calls after it has finished instantiating the implementing class, and after calling its constructor, too.

Let's refactor the class so that it can take a UserService instance in the constructor:

import { Component, OnInit } from '@angular/core'; import { User } from '../model/user'; import { UserService } from '../service/user.service'; @Component({ selector: 'app-user-list', templateUrl: './user-list.component.html', styleUrls: ['./user-list.component.css'] }) export class UserListComponent implements OnInit { users: User[]; constructor(private userService: UserService) { } ngOnInit() { this.userService.findAll().subscribe(data => { this.users = data; }); } } 

The implementation of the UserListComponent class is pretty self-explanatory. It simply uses the UserService's findAll() method to fetch all the entities persisted in the database and stores them in the users field.

Additionally, we need to edit the component's HTML file, user-list.component.html, to create the table that displays the list of entities:


    
# Name Email
{{ user.id }} {{ user.name }} {{ user.email }}

Notice the use of the *ngFor directive. The directive is called a repeater, and we can use it for iterating over the contents of a variable and iteratively rendering HTML elements. In this case, we used it for dynamically rendering the table's rows.

In addition, we used variable interpolation for showing the id,name, and email of each user.

3.9. The UserFormComponent Component

Similarly, we need to create a component that allows us to persist a new User object in the database.

Let's create a user-form directory and type the following:

ng generate component user-form 

Next, let's open the user-form.component.ts file and add to the UserFormComponent class a method for saving a User object:

import { Component } from '@angular/core'; import { ActivatedRoute, Router } from '@angular/router'; import { UserService } from '../service/user.service'; import { User } from '../model/user'; @Component({ selector: 'app-user-form', templateUrl: './user-form.component.html', styleUrls: ['./user-form.component.css'] }) export class UserFormComponent { user: User; constructor( private route: ActivatedRoute, private router: Router, private userService: UserService) { this.user = new User(); } onSubmit() { this.userService.save(this.user).subscribe(result => this.gotoUserList()); } gotoUserList() { this.router.navigate(['/users']); } }

In this case, UserFormComponent also takes a UserService instance in the constructor, which the onSubmit() method uses for saving the supplied User object.

Since we need to redisplay the updated list of entities once we have persisted a new one, we call the gotoUserList() method after the insertion, which redirects the user to the /users path.

In addition, we need to edit the user-form.component.html file and create the HTML form for persisting a new user in the database:

 Name Name is required Email Email is required Submit 

At a glance, the form looks pretty standard. But it encapsulates a lot of Angular's functionality behind the scenes.

Let's notice the use of the ngSubmit directive, which calls the onSubmit() method when the form is submitted.

Next, we have defined the template variable #userForm, so Angular adds automatically an NgForm directive, which allows us to keep track of the form as a whole.

The NgForm directive holds the controls that we created for the form elements with an ngModel directive and a name attribute and also monitors their properties, including their state.

The ngModel directive gives us two-way data binding functionality between the form controls and the client-side domain model – the User class.

This means that data entered in the form input fields will flow to the model – and the other way around. Changes in both elements will be reflected immediately via DOM manipulation.

Additionally, ngModel allows us to keep track of the state of each form control and perform client-side validation, by adding to each control different CSS classes and DOM properties.

In the above HTML file, we used the properties applied to the form controls only to display an alert box when the values in the form have been changed.

3.10. The app-routing.module.ts File

Although the components are functional in isolation, we still need to use a mechanism for calling them when the user clicks the buttons in the navigation bar.

This is where the RouterModule comes into play. So, let's open the app-routing.module.ts file, and configure the module, so it can dispatch requests to the matching components:

import { NgModule } from '@angular/core'; import { Routes, RouterModule } from '@angular/router'; import { UserListComponent } from './user-list/user-list.component'; import { UserFormComponent } from './user-form/user-form.component'; const routes: Routes = [ { path: 'users', component: UserListComponent }, { path: 'adduser', component: UserFormComponent } ]; @NgModule({ imports: [RouterModule.forRoot(routes)], exports: [RouterModule] }) export class AppRoutingModule { } 

As we can see above, the Routes array instructs the router which component to display when a user clicks a link or specifies a URL into the browser address bar.

A route is composed of two parts:

  1. Path – a string that matches the URL in the browser address bar
  2. Component – the component to create when the route is active (navigated)

If the user clicks the List Users button, which links to the /users path, or enters the URL in the browser address bar, the router will render the UserListComponent component's template file in the placeholder.

Likewise, if they click the Add User button, it will render the UserFormComponent component.

3.11. The app.module.ts File

Next, we need to edit the app.module.ts file, so Angular can import all the required modules, components, and services.

Additionally, we need to specify which provider we'll use for creating and injecting the UserService class. Otherwise, Angular won't be able to inject it into the component classes:

import { BrowserModule } from '@angular/platform-browser'; import { NgModule } from '@angular/core'; import { AppRoutingModule } from './app-routing.module'; import { FormsModule } from '@angular/forms'; import { HttpClientModule } from '@angular/common/http'; import { AppComponent } from './app.component'; import { UserListComponent } from './user-list/user-list.component'; import { UserFormComponent } from './user-form/user-form.component'; import { UserService } from './service/user.service'; @NgModule({ declarations: [ AppComponent, UserListComponent, UserFormComponent ], imports: [ BrowserModule, AppRoutingModule, HttpClientModule, FormsModule ], providers: [UserService], bootstrap: [AppComponent] }) export class AppModule { }

4. Running the Application

Finally, we're ready to run our application.

To accomplish this, let's first run the Spring Boot application, so the REST service is alive and listening for requests.

Une fois l'application Spring Boot démarrée, ouvrons une console de commande et tapez la commande suivante:

ng serve --open

Cela démarrera le serveur de développement en direct d'Angular et ouvrira également le navigateur à // localhost: 4200 .

Nous devrions voir la barre de navigation avec les boutons pour lister les entités existantes et pour en ajouter de nouvelles. Si nous cliquons sur le premier bouton, nous devrions voir en dessous de la barre de navigation un tableau avec la liste des entités persistantes dans la base de données:

De même, cliquer sur le deuxième bouton affichera le formulaire HTML permettant de conserver une nouvelle entité:

5. Conclusion

Dans ce didacticiel, nous avons appris à créer une application Web de base avec Spring Boot et Angular .

Comme d'habitude, tous les exemples de code présentés dans ce didacticiel sont disponibles à l'adresse over sur GitHub.