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bazel mobile-install

Fast iterative development for Android

TL;DR

To install small changes to an Android app very quickly, do the following:

  1. Find the android_binary rule of the app you want to install.
  2. Disable Proguard by removing the proguard_specs attribute.
  3. Set the multidex attribute to native.
  4. Set the dex_shards attribute to 10.
  5. Connect your device running ART (not Dalvik) over USB and enable USB debugging on it.
  6. Run bazel mobile-install :your_target. App startup will be a little slower than usual.
  7. Edit the code or Android resources.
  8. Run bazel mobile-install --incremental :your_target.
  9. Enjoy not having to wait a lot.

Some command line options to Bazel that may be useful:

  • --adb tells Bazel which adb binary to use
  • --adb_arg can be used to add extra arguments to the command line of adb. One useful application of this is to select which device you want to install to if you have multiple devices connected to your workstation: bazel mobile-install --adb_arg=-s --adb_arg=<SERIAL> :your_target
  • --start_app automatically starts the app

When in doubt, look at the example or contact us.

Introduction

One of the most important attributes of a developer's toolchain is speed: there is a world of difference between changing the code and seeing it run within a second and having to wait minutes, sometimes hours, before you get any feedback on whether your changes do what you expect them to.

Unfortunately, the traditional Android toolchain for building an .apk entails many monolithic, sequential steps and all of these have to be done in order to build an Android app. At Google, waiting five minutes to build a single-line change was not unusual on larger projects like Google Maps.

bazel mobile-install makes iterative development for Android much faster by using a combination of change pruning, work sharding, and clever manipulation of Android internals, all without changing any of your app's code.

Problems with traditional app installation

We identified the following bottlenecks of building an Android app:

  • Dexing. By default, "dx" is invoked exactly once in the build and it does not know how to reuse work from previous builds: it dexes every method again, even though only one method was changed.

  • Uploading data to the device. adb does not use the full bandwidth of a USB 2.0 connection, and larger apps can take a lot of time to upload. The entire app is uploaded, even if only small parts have changed, for example, a resource or a single method, so this can be a major bottleneck.

  • Compilation to native code. Android L introduced ART, a new Android runtime, which compiles apps ahead-of-time rather than compiling them just-in-time like Dalvik. This makes apps much faster at the cost of longer installation time. This is a good tradeoff for users because they typically install an app once and use it many times, but results in slower development where an app is installed many times and each version is run at most a handful of times.

The approach of bazel mobile-install

bazel mobile-installmakes the following improvements:

  • Sharded dexing. After building the app's Java code, Bazel shards the class files into approximately equal-sized parts and invokes dx separately on them. dx is not invoked on shards that did not change since the last build.

  • Incremental file transfer. Android resources, .dex files, and native libraries are removed from the main .apk and are stored in under a separate mobile-install directory. This makes it possible to update code and Android resources independently without reinstalling the whole app. Thus, transferring the files takes less time and only the .dex files that have changed are recompiled on-device.

  • Loading parts of the app from outside the .apk. A tiny stub application is put into the .apk that loads Android resources, Java code and native code from the on-device mobile-install directory, then transfers control to the actual app. This is all transparent to the app, except in a few corner cases described below.

Sharded Dexing

Sharded dexing is reasonably straightforward: once the .jar files are built, a tool shards them into separate .jar files of approximately equal size, then invokes dx on those that were changed since the previous build. The logic that determines which shards to dex is not specific to Android: it just uses the general change pruning algorithm of Bazel.

The first version of the sharding algorithm simply ordered the .class files alphabetically, then cut the list up into equal-sized parts, but this proved to be suboptimal: if a class was added or removed (even a nested or an anonymous one), it would cause all the classes alphabetically after it to shift by one, resulting in dexing those shards again. Thus, we settled upon sharding not individual classes, but Java packages instead. Of course, this still results in dexing many shards if a new package is added or removed, but that is much less frequent than adding or removing a single class.

The number of shards is controlled by the BUILD file (using the android_binary.dex_shards attribute). In an ideal world, Bazel would automatically determine how many shards are best, but Bazel currently must know the set of actions (i.e. commands to be executed during the build) before executing any of them, so it cannot determine the optimal number of shards because it doesn't know how many Java classes there will eventually be in the app. Generally speaking, the more shards, the faster the build and the installation will be, but the slower app startup becomes, because the dynamic linker has to do more work. The sweet spot is usually between 10 and 50 shards.

Incremental File Transfer

After building the app, the next step is to install it, preferably with the least effort possible. Installation consists of the following steps:

  1. Installing the .apk (i.e. adb install)
  2. Uploading the .dex files, Android resources, and native libraries to the mobile-install directory

There is not much incrementality in the first step: the app is either installed or not. Bazel currently relies on the user to indicate if it should do this step through the --incremental command line option because it cannot determine in all cases if it is necessary.

In the second step, the app's files from the build are compared to an on-device manifest file that lists which app files are on the device and their checksums. Any new files are uploaded to the device, any files that have changed are updated, and any files that have been removed are deleted from the device. If the manifest is not present, it is assumed that every file needs to be uploaded.

Note that it is possible to fool the incremental installation algorithm by changing a file on the device, but not its checksum in the manifest. We could have safeguarded against this by computing the checksum of the files on the device, but this was deemed to be not worth the increase in installation time.

The Stub Application

The stub application is where the magic to load the dexes, native code and Android resources from the on-device mobile-install directory happens.

The actual loading is implemented by subclassing BaseDexClassLoader and is a reasonably well-documented technique. This happens before any of the app's classes are loaded, so that any application classes that are in the apk can be placed in the on-device mobile-install directory so that they can be updated without adb install.

This needs to happen before any of the classes of the app are loaded, so that no application class needs to be in the .apk which would mean that changes to those classes would require a full re-install.

This is accomplished by replacing the Application class specified in AndroidManifest.xml with the stub application. This takes control when the app is started, and tweaks the class loader and the resource manager appropriately at the earliest moment (its constructor) using Java reflection on the internals of the Android framework.

Another thing the stub application does is to copy the native libraries installed by mobile-install to another location. This is necessary because the dynamic linker needs the X bit to be set on the files, which is not possible to do for any location accessible by a non-root adb.

Once all these things are done, the stub application then instantiates the actual Application class, changing all references to itself to the actual application within the Android framework.

Results

Performance

In general, bazel mobile-install results in a 4x to 10x speedup of building and installing large apps after a small change. We computed the following numbers for a few Google products:

This, of course, depends on the nature of the change: recompilation after changing a base library takes more time.

Limitations

The tricks the stub application plays don't work in every case. We have identified the following cases where it does not work as expected:

  • When Context is cast to the Application class in ContentProvider#onCreate(). This method is called during application startup before we have a chance to replace the instance of the Application class, therefore, ContentProvider will still reference the stub application instead of the real one. Arguably, this is not a bug since you are not supposed to downcast Context like this, but this seems to happen in a few apps at Google.

  • Resources installed by bazel mobile-install are only available from within the app. If resources are accessed by other apps via PackageManager#getApplicationResources(), these resources will be from the last non-incremental install.

  • Devices that aren't running ART. While the stub application works well on Froyo and later, Dalvik has a bug that makes it think that the app is incorrect if its code is distributed over multiple .dex files in certain cases, for example, when Java annotations are used in a specific way. As long as your app doesn't tickle these bugs, it should work with Dalvik, too (note, however, that support for old Android versions isn't exactly our focus)