Application virtualization aims to improve application compatibility and manageability by encapsulating applications from the underlying operating system on which they are executed. A virtualized application is not installed in the traditional sense, although it is still executed as if it is.

When a virtualised application is launched, the virtualization layer intercepts and transparently redirects any operations such as file I/O by that application. This application believes it has direct access to the underlying operating system resources, although in effect it does not.

Overview

Application virtualization is used in modern operating systems such as Microsoft Windows and Linux. For example, Windows Vista implements limited file and Registry virtualization so that legacy applications that try to save user data in a system location can still work on the platform.

Full application virtualization requires a virtualization layer to be installed on a machine. For example Microsoft Application Virtualization is an example of this technology for the Windows platform.

A common misconception is that a runtime environment is application virtualization, although it may appear to be so.

A similar, but different principle is Operating system virtualizations which encapsulate the entire operating system from the hardware, as opposed to applications from the operating system.

Pros

Allows applications to run in environments that do not suit the native application (e.g. Wine allows Microsoft Windows applications to run on Linux).

May protect the operating system and other applications from poorly written or buggy code.

Uses fewer resources than a separate virtual machine.

Run applications that are not written correctly, for example applications that try to store user data in a read-only system-owned location.

Run incompatible applications side-by-side, at the same time.

Maintain a standard configuration in the underlying operating system across multiple computers in an organization, regardless of the applications being used, thereby keeping costs down.

Implement the security principle of least privilege by removing the requirement for end-users to have Administrator privileges in order to run poorly written applications.

Cons

Applications may need to be packaged for the virtualisation environment.

Client virtualization software is required.

Increased resource requirements (memory and disk storage).