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Application virtualization is a software technology that encapsulatescomputer programs from the underlyingoperating system on which they are executed. A fullyvirtualized application is not installed in the traditional sense,[1] although it is still executed as if it were. The application behaves at runtime like it is directly interfacing with the original operating system and all the resources managed by it, but can be isolated orsandboxed to varying degrees.
In this context, the term "virtualization" refers to the artifact being encapsulated (application), which is quite different from its meaning inhardware virtualization, where it refers to the artifact being abstracted (physical hardware).
Full application virtualization requires a virtualization layer.[2] Application virtualization layers replace part of theruntime environment normally provided by the operating system. The layer intercepts all disk operations of virtualized applications and transparently redirects them to a virtualized location, often a single file.[3] The application remains unaware that it accesses a virtual resource instead of a physical one. Since the application is now working with one file instead of many files spread throughout the system, it becomes easy to run the application on a different computer, and previously incompatible applications can be run side-by-side.
Application virtualization allows applications to run in environments that do not suit the native application. For example,Wine allows someMicrosoft Windows applications to run onLinux.
Application virtualization reduces system integration and administration costs by maintaining a common software baseline across multiple diverse computers in an organization. Lesser integration protects the operating system and other applications from poorly-written or buggy code. In some cases, it provides memory protection andIDE-styledebugging features, and may even run applications that are not written correctly, such as applications that try to store user data in a read-only system-owned location. (This feature assists in the implementation of theprinciple of least privilege by removing the requirement for end-users to have administrative privileges in order to run poorly written applications.) It allows incompatible applications to run side-by-side, at the same time[4] and with minimal regression testing against one another.[5] Isolating applications from the operating system has security benefits as well, as the exposure of the virtualized application does not automatically entail the exposure of the entire OS.[4]
Application virtualization also enables simplifiedoperating system migrations.[4] Applications can be transferred toremovable media or between computers without the need of installing them, becomingportable software.
Application virtualization uses fewer resources than a separatevirtual machine.
Not all computer programs can be virtualized. Some examples include applications that require adevice driver (a form of integration with the OS) and16-bit applications that need to run in shared memory space.[6]Anti-virus programs and applications that require heavy OS integration, such asWindowBlinds or StyleXP are difficult to virtualize.
Moreover, in software licensing, application virtualization bears great licensing pitfalls, mainly because both the application-virtualization software and the virtualized applications must be correctly licensed.[7]
While application virtualization can address file andRegistry-level compatibility issues between legacy applications and newer operating systems, applications that do not manage theheap correctly will not execute onWindows Vista, as they still allocate memory in the same way, regardless of whether they are virtualized.[8] For this reason, specialist application compatibility fixes (shims) may still be needed, even if the application is virtualized.[9]
Functional discrepancies within the multicompatibility model are an additional limitation, where utility-driven access points are shared within a public network. These limitations are overcome by designating a system-level share point driver.[10]
Technology categories that fall under application virtualization include: