Order of Execution in a Visualforce Page

When a user views a Visualforce page, instances of the controller, extensions, and components associated with the page are created by the server. The order in which these elements are executed can affect how the page is displayed to the user.

To fully understand the order of execution of elements on a Visualforce page, you must first understand the page's lifecycle–that is, how the page is created and destroyed during the course of a user session. The lifecycle of a page is determined not just by the content of the page, but also by how the page was requested. There are two types of Visualforce page requests:

• A get request is an initial request for a page either made when a user enters an URL or when a link or button is clicked that takes the user to a new page.
• A postback request is made when user interaction requires a page update, such as when a user clicks on a Save button and triggers a save action.

Order of Execution for Visualforce Page Get Requests

A get request is an initial request for a page either made when a user enters an URL or when a link or button is clicked that takes the user to a new page. The following diagram shows how a Visualforce page interacts with a controller extension or a custom controller class during a get request:

 

In the diagram above the user initially requests a page, either by entering a URL or clicking a link or button. This initial page request is called the get request.

1. The constructor methods on the associated custom controller or controller extension classes are called, instantiating the controller objects.
2. If the page contains any custom components, they are created and the constructor methods on any associated custom controllers or controller extensions are executed. If attributes are set on the custom component using expressions, the expressions are evaluated after the constructors are evaluated.
3. The page then executes any assignTo attributes on any custom components on the page. After the assignTo methods are executed, expressions are evaluated, the action attribute on the <apex:page> component is evaluated, and all other method calls, such as getting or setting a property value, are made.
4. If the page contains an <apex:form> component, all of the information necessary to maintain the state of the database between page requests is saved as an encrypted view state. The view state is updated whenever the page is updated.
5. The resulting HTML is sent to the browser. If there are any client-side technologies on the page, such as JavaScript, the browser executes them.

As the user interacts with the page, the page contacts the controller objects as required to execute action, getter, and setter methods.

Once a new get request is made by the user, the view state and controller objects are deleted.

Order of Execution for Visualforce Page Postback Requests

A postback request is made when user interaction requires a page update, such as when a user clicks on a Save button and triggers a save action. The following diagram shows how a Visualforce page interacts with a controller extension or a custom controller class during a postback request:

 

1. During a postback request, the view state is decoded and used as the basis for updating the values on the page.
   Note

   A component with the immediate attribute set to true bypasses this phase of the request. In other words, the action executes, but no validation is performed on the inputs and no data changes on the page.
2. After the view state is decoded, expressions are evaluated and set methods on the controller and any controller extensions, including set methods in controllers defined for custom components, are executed.
   These method calls do not update the data unless all methods are executed successfully. For example, if one of the methods updates a property and the update is not valid due to validation rules or an incorrect data type, the data is not updated and the page redisplays with the appropriate error messages.
3. The action that triggered the postback request is executed. If that action completes successfully, the data is updated. If the postback request returns the user to the same page, the view state is updated.
   Note

   The action attribute on the <apex:page> component is not evaluated during a postback request. It is only evaluated during a get request.
4. The resulting HTML is sent to the browser.

If the postback request indicates a page redirect and the redirect is to a page that uses the same controller and a proper subset of controller extensions of the originating page, a postback request is executed for that page. Otherwise, a get request is executed for the page. If the postback request contains an <apex:form> component, only the ID query parameter on a postback request is returned.