Construyendo una aplicación web con Spring Boot y Angular

1. Información general

Spring Boot y Angular forman un poderoso tándem que funciona muy bien para desarrollar aplicaciones web con una huella mínima.

En este tutorial, usaremos Spring Boot para implementar un backend RESTful y Angular para crear un frontend basado en JavaScript.

2. La aplicación Spring Boot

La funcionalidad de nuestra aplicación web de demostración será bastante simplista. Se limitará a obtener y mostrar una lista de entidades JPA de una base de datos H2 en memoria y conservar otras nuevas a través de un formulario HTML simple.

2.1. Las dependencias de Maven

Aquí están las dependencias de nuestro proyecto Spring Boot:

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

Tenga en cuenta que incluimos spring-boot-starter-web porque lo usaremos para crear el servicio REST y spring-boot-starter-jpa para implementar la capa de persistencia.

La versión de la base de datos H2 también es administrada por el padre Spring Boot.

2.2. La clase de entidad JPA

Para crear un prototipo rápido de la capa de dominio de nuestra aplicación, definamos una clase de entidad JPA simple, que será responsable de modelar a los usuarios:

@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. La interfaz UserRepository

Dado que necesitaremos la funcionalidad CRUD básica en las entidades de usuario , también debemos definir una interfaz de UserRepository :

@Repository public interface UserRepository extends CrudRepository{}

2.4. El controlador REST

Ahora, implementemos la API REST. En este caso, es solo un simple controlador 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); } }

No hay nada intrínsecamente complejo en la definición de la clase UserController .

Por supuesto, el único detalle de implementación que vale la pena mencionar aquí es el uso de la anotación @CrossOrigin . Como su nombre lo indica, la anotación habilita el uso compartido de recursos entre orígenes (CORS) en el servidor.

Este paso no siempre es necesario. Dado que estamos implementando nuestra interfaz angular en // localhost: 4200 y nuestro backend de arranque en // localhost: 8080 , el navegador denegaría las solicitudes de una a otra.

Con respecto a los métodos del controlador, getUser () recupera todas las entidades de usuario de la base de datos. De manera similar, el método addUser () conserva una nueva entidad en la base de datos, que se pasa en el cuerpo de la solicitud.

Para simplificar las cosas, deliberadamente dejamos fuera de la implementación del controlador que activa la validación de Spring Boot antes de conservar una entidad. En producción, sin embargo, simplemente no podemos confiar en la entrada del usuario, por lo que la validación del lado del servidor debería ser una característica obligatoria.

2.5. Bootstrapping de la aplicación Spring Boot

Finalmente, creemos una clase de arranque estándar de Spring Boot y completemos la base de datos con algunas entidades de usuario :

@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); }; } }

Ahora, ejecutemos la aplicación. Como era de esperar, deberíamos ver una lista de entidades de usuario impresas en la consola al inicio:

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. La aplicación angular

Con nuestra aplicación Spring Boot de demostración en funcionamiento, ahora creemos una aplicación Angular simple, capaz de consumir la API del controlador REST.

3.1. Instalación de CLI angular

Usaremos Angular CLI, una poderosa utilidad de línea de comandos, para crear nuestra aplicación Angular.

Angular CLI es una herramienta extremadamente valiosa ya que nos permite crear un proyecto Angular completo desde cero, generando componentes, servicios, clases e interfaces con solo unos pocos comandos .

Una vez que hayamos instalado npm (Node Package Manager), abriremos una consola de comandos y escribiremos el comando:

npm install -g @angular/[email protected]

Eso es. El comando anterior instalará la última versión de Angular CLI.

3.2. Proyecto de andamios con CLI angular

De hecho, podemos generar nuestra estructura de aplicación Angular desde cero. Pero honestamente, esta es una tarea propensa a errores y que requiere mucho tiempo y que debemos evitar en todos los casos.

En cambio, dejaremos que Angular CLI haga el trabajo duro por nosotros. Entonces, abramos una consola de comandos, luego naveguemos a la carpeta donde queremos que se cree nuestra aplicación y escriba el comando:

ng new angularclient

El nuevo comando generará toda la estructura de la aplicación dentro del directorio angularclient .

3.3. El punto de entrada de la aplicación angular

Si miramos dentro de la carpeta angularclient , veremos que Angular CLI ha creado efectivamente un proyecto completo para nosotros.

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:

selector – the HTML selector used to bind the component to the HTML template file
templateUrl – the HTML template file associated with the component
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 }}

#	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:

Path – a string that matches the URL in the browser address bar
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 vez que se haya iniciado la aplicación Spring Boot, abramos una consola de comandos y escriba el siguiente comando:

ng serve --open

Esto iniciará el servidor de desarrollo en vivo de Angular y también abrirá el navegador en // localhost: 4200 .

Deberíamos ver la barra de navegación con los botones para listar entidades existentes y para agregar nuevas. Si hacemos clic en el primer botón, deberíamos ver debajo de la barra de navegación una tabla con la lista de entidades persistentes en la base de datos:

De manera similar, al hacer clic en el segundo botón, se mostrará el formulario HTML para conservar una nueva entidad:

5. Conclusión

En este tutorial, aprendimos cómo construir una aplicación web básica con Spring Boot y Angular .

Como de costumbre, todos los ejemplos de código que se muestran en este tutorial están disponibles en GitHub.