diff --git a/framework/yii/base/Application.php b/framework/yii/base/Application.php
index da2ad6b..07e8428 100644
--- a/framework/yii/base/Application.php
+++ b/framework/yii/base/Application.php
@@ -144,18 +144,8 @@ abstract class Application extends Module
 	public function __construct($config = [])
 	{
 		Yii::$app = $this;
-		if (!isset($config['id'])) {
-			throw new InvalidConfigException('The "id" configuration is required.');
-		}
-		if (isset($config['basePath'])) {
-			$this->setBasePath($config['basePath']);
-			unset($config['basePath']);
-		} else {
-			throw new InvalidConfigException('The "basePath" configuration is required.');
-		}
 
 		$this->preInit($config);
-
 		$this->registerErrorHandlers();
 		$this->registerCoreComponents();
 
@@ -165,10 +155,23 @@ abstract class Application extends Module
 	/**
 	 * Pre-initializes the application.
 	 * This method is called at the beginning of the application constructor.
+	 * It initializes several important application properties.
+	 * If you override this method, please make sure you call the parent implementation.
 	 * @param array $config the application configuration
+	 * @throws InvalidConfigException if either [[id]] or [[basePath]] configuration is missing.
 	 */
 	public function preInit(&$config)
 	{
+		if (!isset($config['id'])) {
+			throw new InvalidConfigException('The "id" configuration is required.');
+		}
+		if (isset($config['basePath'])) {
+			$this->setBasePath($config['basePath']);
+			unset($config['basePath']);
+		} else {
+			throw new InvalidConfigException('The "basePath" configuration is required.');
+		}
+
 		if (isset($config['vendorPath'])) {
 			$this->setVendorPath($config['vendorPath']);
 			unset($config['vendorPath']);
@@ -183,6 +186,7 @@ abstract class Application extends Module
 			// set "@runtime"
 			$this->getRuntimePath();
 		}
+
 		if (isset($config['timeZone'])) {
 			$this->setTimeZone($config['timeZone']);
 			unset($config['timeZone']);
@@ -192,6 +196,31 @@ abstract class Application extends Module
 	}
 
 	/**
+	 * @inheritdoc
+	 */
+	public function init()
+	{
+		parent::init();
+		$this->initExtensions($this->extensions);
+	}
+
+	/**
+	 * Initializes the extensions.
+	 * @param array $extensions the extensions to be initialized. Please refer to [[extensions]]
+	 * for the structure of the extension array.
+	 */
+	protected function initExtensions($extensions)
+	{
+		foreach ($extensions as $extension) {
+			if (isset($extension['bootstrap'])) {
+				/** @var Extension $class */
+				$class = $extension['bootstrap'];
+				$class::init();
+			}
+		}
+	}
+
+	/**
 	 * Loads components that are declared in [[preload]].
 	 * @throws InvalidConfigException if a component or module to be preloaded is unknown
 	 */
diff --git a/framework/yii/base/Extension.php b/framework/yii/base/Extension.php
index dac9552..c25a043 100644
--- a/framework/yii/base/Extension.php
+++ b/framework/yii/base/Extension.php
@@ -8,11 +8,21 @@
 namespace yii\base;
 
 /**
+ * Extension is the base class that may be extended by individual extensions.
+ *
+ * Extension serves as the bootstrap class for extensions. When an extension
+ * is installed via composer, the [[init()]] method of its Extension class (if any)
+ * will be invoked during the application initialization stage.
+ *
  * @author Qiang Xue <qiang.xue@gmail.com>
  * @since 2.0
  */
 class Extension
 {
+	/**
+	 * Initializes the extension.
+	 * This method is invoked at the end of [[Application::init()]].
+	 */
 	public static function init()
 	{
 	}