silverstripe-framework/docs/en/02_Developer_Guides/16_Execution_Pipeline/03_App_Object_and_Kernel.md

---
title: App Object and Kernel
summary: Provides bootstrapping and entrypoint to the SilverStripe application
---

# Kernel

The [Kernel](api:SilverStripe\Core\Kernel) object provides a container for the various manifests, services, and components
which a SilverStripe application must have available in order for requests to be executed.

This can be accessed in user code via Injector


```php
use SilverStripe\Core\Kernel;
use SilverStripe\Core\Injector\Injector;

$kernel = Injector::inst()->get(Kernel::class);
echo "Current environment: " . $kernel->getEnvironment();
```

## Kernel services

Services accessible from this kernel include:

  * getContainer() -> Current [Injector](api:SilverStripe\Core\Injector\Injector) service
  * getThemeResourceLoader() -> [ThemeResourceLoader](api:SilverStripe\View\ThemeResourceLoader) Service for loading of discovered templates.
    Also used to contain nested theme sets such as the `$default` set for all root module /templates folders.
  * getEnvironment() -> String value for the current environment. One of 'dev', 'live' or 'test'

Several meta-services are also available from Kernel (which are themselves containers for
other core services) but are not commonly accessed directly:

  * getClassLoader() -> [ClassLoader](api:SilverStripe\Core\Manifest\ClassLoader) service which handles the class manifest
  * getModuleLoader() -> [Manifest](api:SilverStripe\Core\Manifest) service which handles module registration
  * getConfigLoader() -> [ConfigLoader](api:SilverStripe\Core\Config\ConfigLoader) Service which assists with nesting of [Config](api:SilverStripe\Core\Config\Config) instances
  * getInjectorLoader() -> [InjectorLoader](api:SilverStripe\Core\Injector\InjectorLoader) Service which assists with nesting of [Injector](api:SilverStripe\Core\Injector\Injector) instances

## Kernel nesting

As with Config and Injector the Kernel can be nested to safely modify global application state,
and subsequently restore state. Unlike those classes, however, there is no `::unnest()`. Instead
you should call `->activate()` on the kernel instance you would like to unnest to.


```php
$oldKernel = Injector::inst()->get(Kernel::class);
try {
    // Injector::inst() / Config::inst() are automatically updated to the new kernel
    $newKernel = $oldKernel->nest();
    Config::modify()->set(Director::class, 'alternate_base_url', '/myurl');
}
finally {
    // Any changes to config (or other application state) have now been reverted
    $oldKernel->activate();
}
```

# Application

An application represents a basic execution controller for the top level application entry point.
The role of the application is to:

 - Control bootstrapping of a provided kernel instance
 - Handle errors raised from an application
 - Direct requests to the request handler, and return a valid response

## HTTPApplication

The HTTPApplication provides a specialised application implementation for handling HTTP Requests.
This class provides basic support for HTTP Middleware, such as [ErrorControlChainMiddleware](api:SilverStripe\Core\Startup\ErrorControlChainMiddleware).

The `index.php` file in your project root contains the default application implementation.
You can customise it as required.


```php

use SilverStripe\Control\HTTPApplication;
use SilverStripe\Control\HTTPRequestBuilder;
use SilverStripe\Core\CoreKernel;
use SilverStripe\Core\Startup\ErrorControlChainMiddleware;

require __DIR__ . '/vendor/autoload.php';

// Build request and detect flush
$request = HTTPRequestBuilder::createFromEnvironment();

// Default application
$kernel = new CoreKernel(BASE_PATH);
$app = new HTTPApplication($kernel);
$app->addMiddleware(new ErrorControlChainMiddleware($app));
$response = $app->handle($request);
$response->output();
```

## Custom application actions

If it's necessary to boot a SilverStripe kernel and application, but not do any
request processing, you can use the Application::execute() method to invoke a custom
application entry point.

This may be necessary if using SilverStripe code within the context of a non-SilverStripe
application.

For example, the below will setup a request, session, and current controller,
but will leave the application in a "ready" state without performing any
routing.


```php
$request = CLIRequestBuilder::createFromEnvironment();
$kernel = new TestKernel(BASE_PATH);
$app = new HTTPApplication($kernel);
$app->execute($request, function (HTTPRequest $request) {
    // Start session and execute
    $request->getSession()->init();
    
    // Set dummy controller
    $controller = Controller::create();
    $controller->setRequest($request);
    $controller->pushCurrent();
    $controller->doInit();
}, true);
```