feat(core): enable application config with configure/getConfig/@config

docs/site/Context.md

@@ -684,13 +684,21 @@ We also allow `@config.*` to be resolved from another binding than the current
 one:
 
 ```ts
+import {config, CoreBindings} from '@loopback/core';
+
 export class MyRestServer {
   constructor(
     // Inject the `rest.host` from the application config
-    @config({fromBinding: 'application', propertyPath: 'rest.host'})
+    @config({
+      fromBinding: CoreBinding.APPLICATION_INSTANCE,
+      propertyPath: 'rest.host',
+    })
     host: string,
     // Inject the `rest.port` from the application config
-    @config({fromBinding: 'application', propertyPath: 'rest.port'})
+    @config({
+      fromBinding: CoreBinding.APPLICATION_INSTANCE,
+      propertyPath: 'rest.port',
+    })
     port: number,
   ) {
     // ...

packages/core/src/__tests__/unit/application.unit.ts

@@ -25,6 +25,29 @@ describe('Application', () => {
 
   afterEach('clean up application', () => app.stop());
 
+  describe('app bindings', () => {
+    it('binds the application itself', () => {
+      app = new Application();
+      expect(app.getSync(CoreBindings.APPLICATION_INSTANCE)).to.equal(app);
+    });
+
+    it('binds the application config', () => {
+      const myAppConfig = {name: 'my-app', port: 3000};
+      app = new Application(myAppConfig);
+      expect(app.getSync(CoreBindings.APPLICATION_CONFIG)).to.equal(
+        myAppConfig,
+      );
+    });
+
+    it('configures the application', () => {
+      const myAppConfig = {name: 'my-app', port: 3000};
+      app = new Application(myAppConfig);
+      expect(app.getConfigSync(CoreBindings.APPLICATION_INSTANCE)).to.equal(
+        myAppConfig,
+      );
+    });
+  });
+
   describe('controller binding', () => {
     beforeEach(givenApp);
 

packages/core/src/application.ts

@@ -129,6 +129,11 @@ export class Application extends Context implements LifeCycleObserver {
     // Make options available to other modules as well.
     this.bind(CoreBindings.APPLICATION_CONFIG).to(this.options);
 
+    // Also configure the application instance to allow `@config`
+    this.configure(CoreBindings.APPLICATION_INSTANCE).toAlias(
+      CoreBindings.APPLICATION_CONFIG,
+    );
+
     this._shutdownOptions = {signals: ['SIGTERM'], ...this.options.shutdown};
   }