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Compiling with Jack for Android 6.0–8.1

Jack is an Android toolchain that compiled Java source into Android dex bytecode. You don’t have to do anything differently to use Jack—just use your standard makefile commands to compile the tree or your project. Android 8.1 is the last release that uses Jack.

About Jack

Jack works as shown in Figure 1.

Jack overview
Figure 1. Jack overview

Jack library format

Jack has its own .jack file format that contains the pre-compiled dex code for the library, allowing for faster compilation (pre-dex).

Jack library file contents
Figure 2. Jack library file contents

Jill

The Jill tool translates the existing .jar libraries into the new library format, as shown below.

Importing .jar libraries with Jill
Figure 3. Workflow to import an existing .jar library

Jack compilation server

The first time Jack is used, it launches a local Jack compilation server on your computer. This server:

  • Brings an intrinsic speedup because it avoids launching a new host JRE JVM, loading Jack code, initializing Jack, and warming up the JIT at each compilation. It also provides very good compilation times during small compilations (for example, in incremental mode).
  • Is a short-term solution to control the number of parallel Jack compilations. The server avoids overloading your computer (memory or disk issue) because it limits the number of parallel compilations.

The Jack server shuts itself down after an idle time without any compilation. It uses two TCP ports on the localhost interface and isn't available externally. All parameters (number of parallel compilations, timeout, ports number, etc.) can be modified by editing the $HOME/.jack file.

$HOME/.jack file

The $HOME/.jack file contains the following settings for Jack server variables in a full bash syntax:

  • SERVER=true enables the server feature of Jack.
  • SERVER_PORT_SERVICE=8072 sets the TCP port number of the server for compilation purposes.
  • SERVER_PORT_ADMIN=8073 sets the TCP port number of the server for admin purposes.
  • SERVER_COUNT=1 is unused.
  • SERVER_NB_COMPILE=4 sets the maximum number of allowed parallel compilations.
  • SERVER_TIMEOUT=60 sets the number of idle seconds the server must wait without any compilation before shutting itself down.
  • SERVER_LOG=${SERVER_LOG:=$SERVER_DIR/jack-$SERVER_PORT_SERVICE.log} sets the file where server logs are written. By default, this variable can be overloaded by an environment variable.
  • JACK_VM_COMMAND=${JACK_VM_COMMAND:=java} sets the default command used to launch a JVM on the host. By default, this variable can be overloaded by environment variable.

Troubleshooting Jack compilations

Problem Action
Your computer becomes unresponsive during compilation or you experience Jack compilations failing on Out of memory error Reduce the number of simultaneous Jack compilations by editing $HOME/.jack and changing SERVER_NB_COMPILE to a lower value.
Compilations are failing on Cannot launch background server The most likely cause is TCP ports are already used on your computer. Change ports by editing $HOME/.jack (SERVER_PORT_SERVICE and SERVER_PORT_ADMIN variables). To unblock the situation, disable the Jack compilation server by editing $HOME/.jack and changing SERVER to false. Unfortunately this significantly slows down your compilation and may force you to launch make -j with load control (option -l of make).
Compilation gets stuck without any progress To unblock the situation, kill the Jack background server using jack-admin kill-server) then remove the temporary directories contained in jack-$USER of your temporary directory (/tmp or $TMPDIR).

Finding the Jack log

If you ran a make command with a dist target, the Jack log is located at $ANDROID_BUILD_TOP/out/dist/logs/jack-server.log. Otherwise, you can find the log by running jack-admin server-log. In case of reproducible Jack failures, you can get a more detailed log by setting the following variable:

export ANDROID_JACK_EXTRA_ARGS="--verbose debug --sanity-checks on -D sched.runner=single-threaded"

Use standard makefile commands to compile the tree (or your project) and attach standard output and error. To remove detailed build logs, run:

unset ANDROID_JACK_EXTRA_ARGS

Jack limitations

  • By default, the Jack server can be used by only one user on a computer. To support additional users, select different port numbers for each user and adjust SERVER_NB_COMPILE accordingly. You can also disable the Jack server by setting SERVER=false in $HOME/.jack.
  • CTS compilation is slow due to current vm-tests-tf integration.
  • Bytecode manipulation tools (such as JaCoCo) aren't supported.

Using Jack

Jack supports Java programming language 1.7 and integrates the additional features described below.

Pre-dexing

When generating a Jack library file, the .dex of the library is generated and stored inside the .jack library file as a pre-dex. When compiling, Jack reuses the pre-dex from each library. All libraries are pre-dexed.

Jack libraries with pre-dex
Figure 4. Jack libraries with pre-dex

Jack doesn't reuse the library pre-dex if shrinking, obfuscation, or repackaging is used in the compilation.

Incremental compilation

Incremental compilation means that only the components touched since the last compilation (and their dependencies) are recompiled. Incremental compilation can be significantly faster than a full compilation when changes are limited to a set of components.

Incremental compilation is disabled by default (and is automatically deactivated when shrinking, obfuscation, repackaging or multi-dex legacy is enabled). To enable incremental builds, add the following line to the Android.mk file of the project that you want to build incrementally:

LOCAL_JACK_ENABLED := incremental

Shrinking and obfuscation

Jack uses ProGuard configuration files to enable shrinking and obfuscation.

Common options include the following:

  • @
  • -include
  • -basedirectory
  • -injars
  • -outjars (only 1 output jar supported)
  • -libraryjars
  • -keep
  • -keepclassmembers
  • -keepclasseswithmembers
  • -keepnames
  • -keepclassmembernames
  • -keepclasseswithmembernames
  • -printseeds

Shrinking options include the following:

  • -dontshrink

Obfuscation options include the following:

  • -dontobfuscate
  • -printmapping
  • -applymapping
  • -obfuscationdictionary
  • -classobfuscationdictionary
  • -packageobfuscationdictionary
  • -useuniqueclassmembernames
  • -dontusemixedcaseclassnames
  • -keeppackagenames
  • -flattenpackagehierarchy
  • -repackageclasses
  • -keepattributes
  • -adaptclassstrings

Ignored options include the following:

  • -dontoptimize (Jack doesn't optimize)
  • -dontpreverify (Jack doesn't preverify)
  • -skipnonpubliclibraryclasses
  • -dontskipnonpubliclibraryclasses
  • -dontskipnonpubliclibraryclassmembers
  • -keepdirectories
  • -target
  • -forceprocessing
  • -printusage
  • -whyareyoukeeping
  • -optimizations
  • -optimizationpasses
  • -assumenosideeffects
  • -allowaccessmodification
  • -mergeinterfacesaggressively
  • -overloadaggressively
  • -microedition
  • -verbose
  • -dontnote
  • -dontwarn
  • -ignorewarnings
  • -printconfiguration
  • -dump

Repackaging

Jack uses jarjar configuration files to do repackaging. While Jack is compatible with "rule" rule types, it is not compatible with "zap" or "keep" rule types.

Multidex support

Jack offers native and legacy multidex support. Because dex files are limited to 65K methods, apps with over 65K methods must be split into multiple dex files. For more details, refer to Enable multidex for apps with over 64K methods.