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docs/guide-zh-CN/structure-application-components.md

+Application Components
+======================
+
+Applications are [service locators](concept-service-locators.md). They host a set of the so-called
+*application components* that provide different services for processing requests. For example,
+the `urlManager` component is responsible for routing Web requests to appropriate controllers;
+the `db` component provides DB-related services; and so on.
+
+Each application component has an ID that uniquely identifies itself among other application components
+in the same application. You can access an application component through the expression
+
+```php
+\Yii::$app->componentID
+```
+
+For example, you can use `\Yii::$app->db` to get the [[yii\db\Connection|DB connection]],
+and `\Yii::$app->cache` to get the [[yii\caching\Cache|primary cache]] registered with the application.
+
+An application component is created the first time it is accessed through the above expression. Any
+further accesses will return the same component instance.
+
+Application components can be any objects. You can register them by configuring
+the [[yii\base\Application::components]] property in [application configurations](structure-applications.md#application-configurations).
+For example,
+
+```php
+[
+    'components' => [
+        // register "cache" component using a class name
+        'cache' => 'yii\caching\ApcCache',
+
+        // register "db" component using a configuration array
+        'db' => [
+            'class' => 'yii\db\Connection',
+            'dsn' => 'mysql:host=localhost;dbname=demo',
+            'username' => 'root',
+            'password' => '',
+        ],
+
+        // register "search" component using an anonymous function
+        'search' => function () {
+            return new app\components\SolrService;
+        },
+    ],
+]
+```
+
+> Info: While you can register as many application components as you want, you should do this judiciously.
+  Application components are like global variables. Using too many application components can potentially
+  make your code harder to test and maintain. In many cases, you can simply create a local component
+  and use it when needed.
+
+
+## Bootstrapping Components <a name="bootstrapping-components"></a>
+
+As mentioned above, an application component will only be instantiated when it is being accessed the first time.
+If it is not accessed at all during a request, it will not be instantiated. Sometimes, however, you may want
+to instantiate an application component for every request, even if it is not explicitly accessed.
+To do so, you may list its ID in the [[yii\base\Application::bootstrap|bootstrap]] property of the application.
+
+For example, the following application configuration makes sure the `log` component is always loaded:
+
+```php
+[
+    'bootstrap' => [
+        'log',
+    ],
+    'components' => [
+        'log' => [
+            // configuration for "log" component
+        ],
+    ],
+]
+```
+
+
+## Core Application Components <a name="core-application-components"></a>
+
+Yii defines a set of *core* application components with fixed IDs and default configurations. For example,
+the [[yii\web\Application::request|request]] component is used to collect information about
+a user request and resolve it into a [route](runtime-routing.md); the [[yii\base\Application::db|db]]
+component represents a database connection through which you can perform database queries.
+It is with help of these core application components that Yii applications are able to handle user requests.
+
+Below is the list of the predefined core application components. You may configure and customize them
+like you do with normal application components. When you are configuring a core application component,
+if you do not specify its class, the default one will be used.
+
+* [[yii\web\AssetManager|assetManager]]: manages asset bundles and asset publishing.
+  Please refer to the [Managing Assets](output-assets.md) section for more details.
+* [[yii\db\Connection|db]]: represents a database connection through which you can perform DB queries.
+  Note that when you configure this component, you must specify the component class as well as other required
+  component properties, such as [[yii\db\Connection::dsn]].
+  Please refer to the [Data Access Objects](db-dao.md) section for more details.
+* [[yii\base\Application::errorHandler|errorHandler]]: handles PHP errors and exceptions.
+  Please refer to the [Handling Errors](tutorial-handling-errors.md) section for more details.
+* [[yii\base\Formatter|formatter]]: formats data when they are displayed to end users. For example, a number
+  may be displayed with thousand separator, a date may be formatted in long format.
+  Please refer to the [Data Formatting](output-formatting.md) section for more details.
+* [[yii\i18n\I18N|i18n]]: supports message translation and formatting.
+  Please refer to the [Internationalization](tutorial-i18n.md) section for more details.
+* [[yii\log\Dispatcher|log]]: manages log targets.
+  Please refer to the [Logging](tutorial-logging.md) section for more details.
+* [[yii\swiftmailer\Mailer|mail]]: supports mail composing and sending.
+  Please refer to the [Mailing](tutorial-mailing.md) section for more details.
+* [[yii\base\Application::response|response]]: represents the response being sent to end users.
+  Please refer to the [Responses](runtime-responses.md) section for more details.
+* [[yii\base\Application::request|request]]: represents the request received from end users.
+  Please refer to the [Requests](runtime-requests.md) section for more details.
+* [[yii\web\Session|session]]: represents the session information. This component is only available
+  in [[yii\web\Application|Web applications]].
+  Please refer to the [Sessions and Cookies](runtime-sessions-cookies.md) section for more details.
+* [[yii\web\UrlManager|urlManager]]: supports URL parsing and creation.
+  Please refer to the [URL Parsing and Generation](runtime-url-handling.md) section for more details.
+* [[yii\web\User|user]]: represents the user authentication information. This component is only available
+  in [[yii\web\Application|Web applications]]
+  Please refer to the [Authentication](security-authentication.md) section for more details.
+* [[yii\web\View|view]]: supports view rendering.
+  Please refer to the [Views](structure-views.md) section for more details.