Understanding AAPT2: Android Resource Compilation and the FLAT File Format

AAPT2 is the default Android resource compiler introduced in Android Studio 3.0, replacing the older AAPT. It splits the resource processing into two stages: compile and link. During the compile phase, individual resource files (XML, PNG, etc.) are converted into binary FLAT files that contain both the original resource data and metadata such as file path, type, and configuration. This enables incremental builds because only changed resources need recompilation.

The compile command is invoked via aapt2 compile path-to-input-files [options] -o output-directory/ . It accepts options like --dir for scanning a directory, --zip for a ZIP archive, -o for output path, and switches such as --no-crunch (disable PNG processing), --legacy (treat old errors as warnings), and -v (verbose logging).

Internally, the compile workflow loads input resources via a file collection (directory or ZIP), determines the output archive type (directory or ZIP), and then processes each resource through one of four compile functions: CompileFile (raw copy), CompileTable (values XML → ARSC), CompileXml (general XML → protobuf), and CompilePng (PNG optimization and 9‑patch handling). Each function builds a ResourceFile or XmlResource object, serializes it to protobuf, and writes the header and data using WriteHeaderAndDataToWriter , which wraps an IArchiveWriter with a ContainerWriter to produce the FLAT container format.

The FLAT file format begins with a magic number ( 0x54504141 ), version, and entry count, followed for each entry by a header (resource type, sizes) and the protobuf‑serialized resource metadata plus the raw resource data. For XML resources, the DOM tree is flattened into XmlNode protobuf; for values resources, a ResourceTable protobuf is used; for PNGs, optional crunching and nine‑patch border removal are applied before writing.

By encoding both metadata and raw content, FLAT files allow the subsequent link step to merge them into the final APK’s resources.arsc and R.java, while supporting fast incremental builds and smaller binary resource blobs.