˝reduces the amount of RAM consumed when running the application by allowing data to be mmap'd in; which causes all uncompressed data within the .apk, such as images or raw files, to be aligned on 4-byte boundaries˝
Zipalign is an archive alignment tool that provides important optimization to Android application (.apk) files. It was first introduced with 1.6 Android SDK (software development kit).
The purpose is to ensure that all uncompressed data starts with a particular alignment relative to the start of the file. Specifically, it causes all uncompressed data within the .apk, such as images or raw files, to be aligned on 4-byte boundaries. This allows all portions to be accessed directly with mmap() even if they contain binary data with alignment restrictions. The benefit is a reduction in the amount of RAM consumed when running the application.

CAN YOU PUT THIS IN MORE SIMPLE TERMS PLEASE: Zipalign is an archive. It optimizes the way an Android application package (APK) is packaged. Doing so enables the Android operating system to interact with the application more efficiently, and hence has the potential to make the application and overall the whole system much faster. Execution time is minimized for zipaligned applications, resulting in a lesser amount of RAM consumption when running the APK.

In an Android operating environment, data files stored in each application package are accessed by multiple processes, for example, the installer will read the data manifest to determine the associated permissions; the system server can read these resources for multiple reasons, like displaying notifications; the Home application, for example, will read resources to get the application’s name and icon. Since Android is based on a a true multi-tasking operating infrastructure, these files are continually and repeatedly accessed. Finally, but not least, the application itself reads the manifest data.

As Android is Linux-based, memory-mapping plays a key role in efficient handling of processes. Essentially, the optimal alignment for the Android OS’ resource-handling code is 4-byte boundaries. What this means is that, if APKs are memory-mapped to 4-byte boundaries, and aligned accordingly, the OS will not need to ˝read through˝ the whole application package to get to the desired data manifest. Every system process will know in advance where to look for it’s desired resources, and hence will execute much smoother and faster.

Summing it up, zipaligning an APK results in all uncompressed data within the package to be aligned on 4-byte boundaries, allowing all portions to be accessed directly with the memory-map. RAM consumption is lowered while execution because the querying code doesn’t have to read through the entire application package.
This tool should always be used to align your .apk file before distributing it to end-users.

Caution: zipalign must only be performed after the .apk file has been signed with your private key. If you perform zipalign before signing, then the signing procedure will undo the alignment. Also, do not make alterations to the aligned package. Alterations to the archive, such as renaming or deleting entries, will potentially disrupt the alignment of the modified entry and all later entries. And any files added to an "aligned" archive will not be aligned.

It can be found under the ˝tools˝ folder of the SDK. To use it, simply run the command:
To align infile.apk and save it as outfile.apk:

zipalign [-f] [-v] <alignment> infile.apk outfile.apk

(where infile.apk is the source file, and outfile.apk is the output file)

To confirm the alignment of existing.apk:

zipalign -c -v <alignment> existing.apk

(where existing.apk can be any application package that you need to get verified. Also, the <alignment> tag in both the commands needs to be an integral value (otherwise the command will return invalid). This value, although can be any integer, MUST always be 4, which would provide 32-bit alignment. Any other value and it will effectively do nothing)

Flags, used in these commands:
  • -f : overwrite existing
  • -v : verbose output
  • -c : confirm the alignment of the given file
Quite understandably, situation would be reserved for unaligned application packages. Resource reading would be slow and memory usage would be on the higher end of the spectrum. It would also depend on how many unaligned applications are present. For example, if less number of applications with an unaligned home application, you’d see slower application launch times. This is the best case scenario. For a worst case scenario, having a number of unaligned applications will result in the system repeatedly starting and killing processes, struggling with lags and huge battery drain.

credits: android developers, aatif
edited by arawn
Jul 21, 2012
Robbie Hood likes this.