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An Application Is a Bundle

An application in Mac OS X is packaged as a type of bundle. A bundle, to echo the definition in the chapter “Bundles”, is a directory containing executable code and the resources to support that code. Application bundles as well as loadable bundles (such as plug-ins) are file packages, directories that the Finder presents to users as a single file. The major distinguishing characteristic between the types of bundles—applications, frameworks, plug-ins, and other dynamically loadable packages of code and resources—is the nature of the executable. Application executables are generally self-sufficient binaries that users can launch from the Finder, usually by double-clicking. Applications may or may not contain secondary bundles, such as plug-ins, but they always contain their main bundle.

Bundles bring a number of benefits that are either specific to applications or that apply mostly to them:

• The same (Carbon) application bundle can run, without modification, in Mac OS 9 and Mac OS X.


• Applications can include different localizations. Applications can automatically display the set of localized resources (including the application name itself) that matches a user’s language preference. Moreover, you can add a new localization to the application package, and it displays those resources (if they are for a preferred language) after the user relaunches the application.


• Client computers can run applications on a server.


• Customers can easily download applications from a website or obtain them through email.


• Applications are easy to install and uninstall; all the user must do is drag the application package onto a volume or, for uninstall, drag it to the Trash. (This feature does not preclude more complicated installations from taking place.)


• Because applications are file packages, users cannot “break” them by removing or changing essential parts of them. Users can, however, change the names of applications without affecting them.


• Applications can support multiple architectures as well as multiple volume formats.

What makes these features possible is the hierarchical internal organization of bundles. The different pieces of an application go in specific named locations within the application package. This standard internal organization of the pieces of an application enables related parts of the operating system—such as the Finder and the resource-finding and code-loading mechanisms of the system—to perform their functions. For example, executable files for multiple platforms (Mac OS 9 and Mac OS X) are put in separate subdirectories with standard names. The same goes for localized resources, plug-ins, and private and versioned frameworks.

The Finder, Sherlock, Navigation Services, and other Apple-provided applications and services that browse or examine the file system do not descend into application packages. The Finder responds to double-clicks on an application package by launching the application.

As they do with other bundles, Apple’s development tools support the creation of application packages.

For additional general information about bundles, see the chapter “Bundles”. For further information about the Finder and its role in relation to applications, see the chapter “The Finder”.

Last updated: 2003-08-21

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