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Application represents the execution context of request processing. Its main task is to resolve the user request and dispatch it to an appropriate controller for further processing. It also serves as the central place for keeping application-level configurations. For this reason, application is also called front-controller.

Application is created as a singleton by the entry script. The application singleton can be accessed at any place via Yii::app().

1. Application Configuration

By default, application is an instance of CWebApplication. To customize it, we normally provide a configuration file (or array) to initialize its property values when the application instance is being created. An alternative way of customizing application is to extend CWebApplication.

The configuration is an array of key-value pairs. Each key represents the name of a property of the application instance, and each value the corresponding property's initial value. For example, the following configuration configures the name and defaultController properties of the application.

    'name'=>'Yii Framework',

We usually store the configuration in a separate PHP script (e.g. protected/config/main.php). Inside the script, we return the configuration array as follows,

return array(...);

To apply the configuration, we pass the configuration file name as a parameter to the application's constructor, or to Yii::createWebApplication() like the following, which is usually done in the entry script:


Tip: If the application configuration is very complex, we can split it into several files, each returning a portion of the configuration array. Then, in the main configuration file, we call PHP include() to include the rest configuration files and merge them into a complete configuration array.

2. Application Base Directory

Application base directory refers to the root directory that contains all security-sensitive PHP scripts and data. By default, it is a subdirectory named protected that is located under the directory containing the entry script. It can be customized via setting the basePath property in the application configuration.

Contents under the application base directory should be protected from being accessed by Web users. With Apache HTTP server, this can be done easily by placing a .htaccess file under the base directory. The content of the .htaccess file is as follows,

deny from all

3. Application Component

Functionalities of application can be easily customized and enriched with its flexible component architecture. Application manages a set of application components, each implementing specific features. For example, application resolves a user request with the help of CUrlManager and CHttpRequest components.

By configuring the components property of application, we can customize the class and property values of any application component used in an application. For example, we can configure CMemCache component so that it can use multiple memcache servers for caching,

                array('host'=>'server1', 'port'=>11211, 'weight'=>60),
                array('host'=>'server2', 'port'=>11211, 'weight'=>40),

In the above, we add the cache element to the components array. The cache element states that the class of the component is CMemCache and its servers property should be initialized as such.

To access an application component, use Yii::app()->ComponentID, where ComponentID refers to the ID of the component (e.g. Yii::app()->cache).

An application component may be disabled by setting enabled to be false in its configuration. Null is returned when we access a disabled component.

Tip: By default, application components are created on demand. This means an application component may not be created at all if it is not accessed during a user request. As a result, the overall performance may not be degraded even if an application is configured with many components. Some application components (e.g. CLogRouter) may need to be created no matter they are accessed or not. To do so, list their IDs in the preload property of the application.

4. Core Application Components

Yii predefines a set of core application components to provide features common among Web applications. For example, the request component is used to resolve user requests and provide information such as URL, cookies. By configuring the properties of these core components, we can change the default behaviors of Yii in nearly every aspect.

Below we list the core components that are pre-declared by CWebApplication.

5. Application Lifecycles

When handling a user request, an application will undergo the following lifecycles:

  1. Pre-initializes the application with CApplication::preinit();

  2. Set up class autoloader and error handling;

  3. Register core application components;

  4. Load application configuration;

  5. Initialize the application with CApplication::init()

    • Register application behaviors;
    • Load static application components;
  6. Raise onBeginRequest event;

  7. Process the user request:

    • Resolve the user request;
    • Create controller;
    • Run controller;

    7.Raise onEndRequest event;