9. A system, comprising: a data processing apparatus; and software stored in non-transitory computer readable storage medium storing instructions executable by the data processing apparatus and that upon such execution cause the data processing apparatus to perform operations comprising: for each application page of a native application that generates application pages for display on a user device within the native application, the native application operating independent of a browser application that can operate on the user device: extracting content from the application page, the content including content of a plurality of different content types, the content for each content type being extracted by a corresponding extractor specific to that content type; generating application page data describing content of the application page, the content described by the application page data including text that a user device displays on the application page when the user device displays the application page; and indexing the application page data for the native application in an index that is searchable by a search engine.
9. A system, comprising: a data processing apparatus; and software stored in non-transitory computer readable storage medium storing instructions executable by the data processing apparatus and that upon such execution cause the data processing apparatus to perform operations comprising: for each application page of a native application that generates application pages for display on a user device within the native application, the native application operating independent of a browser application that can operate on the user device: extracting content from the application page, the content including content of a plurality of different content types, the content for each content type being extracted by a corresponding extractor specific to that content type; generating application page data describing content of the application page, the content described by the application page data including text that a user device displays on the application page when the user device displays the application page; and indexing the application page data for the native application in an index that is searchable by a search engine. 11. The system of claim 9 , the operations further comprising: receiving, from a publisher of the native application, data specifying application pages to be indexed for the native application; and accessing only the application pages that are specified to be indexed for the native application.