Creazione di un'applicazione Web con Spring Boot e Angular

1. Panoramica

Spring Boot e Angular formano un potente tandem che funziona alla grande per lo sviluppo di applicazioni web con un ingombro minimo.

In questo tutorial, useremo Spring Boot per implementare un backend RESTful e Angular per creare un frontend basato su JavaScript.

2. L'applicazione Spring Boot

La funzionalità della nostra applicazione web demo sarà davvero piuttosto semplicistica. Sarà solo limitato al recupero e alla visualizzazione di un elenco di entità JPA da un database H2 in memoria e alla persistenza di nuove entità tramite un semplice modulo HTML.

2.1. Le dipendenze di Maven

Ecco le dipendenze del nostro progetto Spring Boot:

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

Si noti che abbiamo incluso spring-boot-starter-web perché lo useremo per creare il servizio REST e spring-boot-starter-jpa per implementare il livello di persistenza.

La versione del database H2 è gestita anche dal genitore Spring Boot.

2.2. La classe di entità JPA

Per prototipare rapidamente il livello di dominio della nostra applicazione, definiamo una semplice classe di entità JPA, che sarà responsabile della modellazione degli utenti:

@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' interfaccia UserRepository

Poiché avremo bisogno della funzionalità CRUD di base sulle entità utente , dobbiamo anche definire un'interfaccia UserRepository :

@Repository public interface UserRepository extends CrudRepository{} 

2.4. Il controller REST

Ora implementiamo l'API REST. In questo caso, è solo un semplice controller 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); } } 

Non c'è nulla di intrinsecamente complesso nella definizione della classe UserController .

Naturalmente, l'unica implementazione dettaglio da notare qui è l'uso del @CrossOrigin nota . Come suggerisce il nome, l'annotazione abilita Cross-Origin Resource Sharing (CORS) sul server.

Questo passaggio non è sempre necessario. Poiché stiamo distribuendo il nostro frontend angolare su // localhost: 4200 e il nostro backend di avvio su // localhost: 8080 , il browser negherebbe altrimenti le richieste dall'uno all'altro.

Per quanto riguarda i metodi del controller, getUser () recupera tutte le entità utente dal database. Allo stesso modo, il metodo addUser () mantiene una nuova entità nel database, che viene passata nel corpo della richiesta.

Per semplificare le cose, abbiamo deliberatamente lasciato fuori dall'implementazione del controller l'attivazione della convalida di Spring Boot prima di rendere persistente un'entità. Nella produzione, tuttavia, non possiamo fidarci dell'input dell'utente, quindi la convalida lato server dovrebbe essere una funzionalità obbligatoria.

2.5. Avvio dell'applicazione Spring Boot

Infine, creiamo una classe di bootstrap standard di Spring Boot e popoliamo il database con alcune entità utente :

@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); }; } }

Ora eseguiamo l'applicazione. Come previsto, dovremmo vedere un elenco di entità utente stampato sulla console all'avvio:

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'applicazione angolare

Con la nostra applicazione demo Spring Boot attiva e funzionante, creiamo ora una semplice applicazione Angular, in grado di utilizzare l'API del controller REST.

3.1. Installazione della CLI angolare

Useremo Angular CLI, una potente utility da riga di comando, per creare la nostra applicazione Angular.

Angular CLI è uno strumento estremamente prezioso poiché ci consente di creare un intero progetto Angular da zero, generando componenti, servizi, classi e interfacce con pochi comandi .

Una volta installato npm (Node Package Manager), apriremo una console di comando e digiteremo il comando:

npm install -g @angular/[email protected]

Questo è tutto. Il comando precedente installerà l'ultima versione di Angular CLI.

3.2. Impalcature di progetto con CLI angolare

È un dato di fatto, possiamo generare la nostra struttura di applicazione angolare da zero. Ma onestamente, questo è un compito soggetto a errori e che richiede tempo che dovremmo evitare in tutti i casi.

Invece, lasceremo che Angular CLI faccia il duro lavoro per noi. Quindi, apriamo una console di comando, quindi spostiamoci nella cartella in cui vogliamo che venga creata la nostra applicazione e digitiamo il comando:

ng new angularclient

Il nuovo comando genererà l'intera struttura dell'applicazione all'interno della directory angularclient .

3.3. Punto di ingresso dell'applicazione angolare

Se guardiamo all'interno della cartella angularclient , vedremo che Angular CLI ha effettivamente creato un intero progetto per noi.

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.

Una volta avviata l'applicazione Spring Boot, apriamo una console di comando e digita il seguente comando:

ng serve --open

Questo avvierà il server di sviluppo live di Angular e aprirà anche il browser in // localhost: 4200 .

Dovremmo vedere la barra di navigazione con i pulsanti per elencare le entità esistenti e per aggiungerne di nuove. Se clicchiamo sul primo pulsante, dovremmo vedere sotto la barra di navigazione una tabella con l'elenco delle entità persistenti nel database:

Allo stesso modo, facendo clic sul secondo pulsante verrà visualizzato il modulo HTML per la persistenza di una nuova entità:

5. conclusione

In questo tutorial, abbiamo imparato come creare un'applicazione web di base con Spring Boot e Angular .

Come al solito, tutti gli esempi di codice mostrati in questo tutorial sono disponibili su GitHub.