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What is an Application?

In LoopBack 4, the Application class is the central class for setting up all of your module’s components, controllers, servers and bindings. The Application class extends Context, and provides the controls for starting and stopping itself and its associated servers.

When using LoopBack 4, we strongly encourage you to create your own subclass of Application to better organize your configuration and setup.

Making your own application class

By making your own application class, you can perform several additional tasks as a part of your setup:

  • Pass configuration into the base class constructor
  • Perform some asynchronous wireup before application start
  • Perform some graceful cleanup on application stop

src/widget-application.ts

import {Application} from '@loopback/core';
import {RestComponent, RestServer} from '@loopback/rest';
import {SamoflangeController, DoohickeyController} from './src/controllers';
import {WidgetApi} from './apidef/';

export class WidgetApplication extends Application {
  constructor() {
    // This is where you would pass configuration to the base constructor
    // (as well as handle your own!)
    super({
      components: [RestComponent],
    });
    const app = this; // For clarity.
    // You can bind to the Application-level context here.
    // app.bind('foo').to(bar);
    app.controller(SamoflangeController);
    app.controller(DoohickeyController);
  }

  async start() {
    // This is where you would asynchronously retrieve servers, providers and
    // other components to configure them before launch.
    const server = await app.getServer(RestServer);
    server.bind('rest.port').to(8080);
    server.api(WidgetApi);
    // The superclass start method will call start on all servers that are
    // bound to the application.
    return await super.start();
  }

  async stop() {
    // This is where you would do whatever is necessary before stopping your
    // app (graceful closing of connections, flushing buffers, etc)
    console.log('Widget application is shutting down...')
    // The superclass stop method will call stop on all servers that are
    // bound to the application.
    await super.stop();
  }
}

Configuring your application

Your application can be configured with constructor arguments, bindings, or a combination of both.

Binding configuration

Binding is the most commonly-demonstrated form of application configuration throughout our examples, and in most cases, is the recommended method for setting up your application.

In addition to the binding functions provided by Context, the Application class also provides some sugar functions for commonly used bindings, like component, server and controller:

export class MyApplication extends Application {
  constructor() {
    super();
    this.component(MagicSuite);
    this.server(RestServer, 'public');
    this.server(RestServer, 'private');

    this.controller(FooController);
    this.controller(BarController);
    this.controller(BazController);
  }
}

You can find a complete list of these functions on the Application API docs page.

Additionally, you can use more advanced forms of binding to fine-tune your application’s configuration:

export class MyApplication extends Application {
  constructor() {
    super();
  }
  this.server(RestServer);
  this.controller(FooController);
  this.bind('fooCorp.logger').toProvider(LogProvider);
  this.bind('repositories.widget')
    .toClass(WidgetRepository)
    .inScope(BindingScope.SINGLETON);
}

In the above example:

  • injection calls for fooCorp.logger will be handled by the LogProvider class.
  • injection calls for repositories.widget will be handled by a singleton instance of the WidgetRepository class.

Constructor configuration

The Application class constructor also accepts an ApplicationConfig object which contains three collections: components, controllers and servers. It will automatically create bindings for each of these collections as a part of defining the application instance.

export class MyApplication extends Application {
  constructor() {
    super({
      components: [
        MagicSuite,
      ],
      servers: {
        'public': RestServer,
        'private': RestServer,
      },
      controllers: [
        FooController,
        BarController,
        BazController,
      ]
    });
  }
}

Components

The components collection allows bulk binding of component constructors within your Application instance’s context.

For more information on how to make use of components, see Using Components.

Controllers

Much like the components collection, the controllers collection allows bulk binding of Controllers to the Application context.

Servers

The servers collection is a Map of server constructors, whose keys are used as part of the server’s binding name.

const app = new Application({
  servers: {
    'public': RestServer,
    'private': RestServer,
  },
});

In the above example, the two server instances would be bound to the Application context under the keys servers.public, and servers.private respectively.

Tips for application setup

Here are some tips to help avoid common pitfalls and mistakes.

Use unique bindings

Use binding names that are prefixed with a unique string that does not overlap with loopback’s bindings. As an example, if your application is built for your employer FooCorp, you can prefix your bindings with fooCorp.

// This is unlikely to conflict with keys used by other component developers
// or within loopback itself!
app.bind('fooCorp.widgetServer.config').to(widgetServerConfig);

Avoid use of getSync

We provide the getSync function for scenarios where you cannot asynchronously retrieve your bindings, such as in constructor bodies.

However, the number of scenarios in which you must do this are limited, and you should avoid potential race conditions and retrieve your bindings asynchronously using the get function whenever possible.

Use caution with singleton binding scopes

By default, bindings for controllers will instantiate a new instance whenever they are injected or retrieved from their binding. Your application should only set singleton binding scopes on controllers when it makes sense to do so.