Compiling with Jack

The Jack toolchain

The Jack toolchain is deprecated, as per this announcement. You may continue to use it or try the latest preview version of Android Studio, which provides improved support for Java 8 language features built into the default toolchain.

Jack is an Android toolchain that compiles Java source into Android dex bytecode. It replaces the previous Android toolchain, which consists of multiple tools, such as javac, ProGuard, jarjar, and dx.

The Jack toolchain provides the following advantages:

  • Completely open source
    Available in AOSP; users are welcome to contribute.
  • Speeds compilation time
    Jack has specific supports to reduce compilation time: pre-dexing, incremental compilation and a Jack compilation server.
  • Handles shrinking, obfuscation, repackaging and multidex
    Using a separate package such as ProGuard is no longer necessary.

Note that beginning in Android 7.0 (N), Jack supports code coverage with JaCoCo. See Code Coverage with JaCoCo and Java 8 Language Features for details.

Jack overview

Figure 1. Jack overview

The .jack library format

Jack has its own .jack file format, which 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


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

Importing existing .jar libraries using Jill

Figure 3. Workflow to import an existing .jar library

Using Jack in your Android build

For instructions on using Jack in Android 7.0 (N) and later, see the Jack server documentation. For Android 6.0 (M), use the instructions in this section.

You don’t have to do anything differently to use Jack — just use your standard makefile commands to compile the tree or your project. Jack is the default Android build toolchain for M.

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 (e.g. in incremental mode).
  • The server is also a short-term solution to control the number of parallel Jack compilations, and so to avoid 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 so is not available externally. All these 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 settings for Jack server variables, in a full bash syntax.

Here are the available settings, with their definitions and default values:

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

Jack troubleshooting

If your computer becomes unresponsive during compilation or if you experience Jack compilations failing on “Out of memory error”

You can improve the situation by reducing the number of Jack simultaneous compilations by editing your $HOME/.jack and changing SERVER_NB_COMPILE to a lower value.

If your compilations are failing on “Cannot launch background server”

The most likely cause is TCP ports are already used on your computer. Try to change it by editing your $HOME/.jack (SERVER_PORT_SERVICE and SERVER_PORT_ADMIN variables).

If it doesn’t solve the problem, please report and attach your compilation log and the Jack server log (see ‘Finding the Jack log’ below to know where to find the server log file). To unblock the situation, disable jack compilation server by editing your $HOME/.jack and changing SERVER to false. Unfortunately this will significantly slow down your compilation and may force you to launch make -j with load control (option "-l" of make).

If your compilation gets stuck without any progress

Please report this and give us the following additional information (where possible):

  • The command line at which you are stuck.
  • The output of this command line.
  • The result of executing jack-admin server-stat.
  • The $HOME/.jack file.
  • The content of the server log with the server state dumped. To get this —
    • Find the Jack background server process by running jack-admin list-server.
    • Send a kill -3 command to this server to dump its state into the log file.
    • To locate the server log file, see ‘Finding the Jack log’ below.
  • The result of executing ls -lR $TMPDIR/jack-$USER.
  • The result of running ps j -U $USER.

You should be able to unblock yourself by killing the Jack background server (use jack-admin kill-server), and then by removing its temporary directories contained in jack-$USER of your temporary directory (/tmp or $TMPDIR).

If you have any other issues

To report bugs or request features, please use our public issue tracker, available at, with the Jack tool bug report or Jack tool feature request templates. Please attach the Jack log to the bug report.

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 it in by running jack-admin server-log

In case of reproducible Jack failures, you can get a more detailed log by setting one variable, as follows:

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

Then use your standard makefile commands to compile the tree or your project and attach its standard output and error.

To remove detailed build logs use:


Jack limitations

  • The Jack server is mono-user by default, so can be only used by one user on a computer. If it is not the case, please, choose different port numbers for each user and adjust SERVER_NB_COMPILE accordingly. You can also disable the Jack server by setting SERVER=false in your $HOME/.jack.
  • CTS compilation is slow due to current vm-tests-tf integration.
  • Bytecode manipulation tools, like JaCoCo, are not supported.

Using Jack features

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


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


Currently, Jack does not reuse the library pre-dex if shrinking/obfuscation/repackaging is used in the compilation.

Incremental compilation

Incremental compilation means that only components that were 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 only a limited set of components.


Incremental compilation is deactivated when shrinking, obfuscation, repackaging or multi-dex legacy is enabled.

Enabling incremental builds

Currently incremental compilation is not enabled by default. To enable incremental builds, add the following line to the file of the project that you want to build incrementally:

LOCAL_JACK_ENABLED := incremental

Note: The first time that you build your project with Jack if some dependencies are not built, use mma to build them, and after that you can use the standard build command.

Shrinking and Obfuscation

Jack has shrinking and obfuscation support and uses proguard configuration files to enable shrinking and obfuscation features. Here are the supported and ignored options:

Supported common options

Common options include the following:

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

Supported shrinking options

Shrinking options include the following:

  • -dontshrink

Supported obfuscation options

Obfuscation options include the following:

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

Ignored options

Ignored options include the following:

  • -dontoptimize // Jack does not optimize
  • -dontpreverify // Jack does not preverify
  • -skipnonpubliclibraryclasses
  • -dontskipnonpubliclibraryclasses
  • -dontskipnonpubliclibraryclassmembers
  • -keepdirectories
  • -target
  • -forceprocessing
  • -printusage
  • -whyareyoukeeping
  • -optimizations
  • -optimizationpasses
  • -assumenosideeffects
  • -allowaccessmodification
  • -mergeinterfacesaggressively
  • -overloadaggressively
  • -microedition
  • -verbose
  • -dontnote
  • -dontwarn
  • -ignorewarnings
  • -printconfiguration
  • -dump

Note: Other options will generate an error.


Jack uses jarjar configuration files to do the repackaging.

Note: Jack is compatible with "rule" rule types, but is not compatible with "zap" or "keep" rule types. If you need "zap" or "keep" rule types please file a feature request with a description of how you use the feature in your app.

Multidex support

Since dex files are limited to 65K methods, apps with over 65K methods must be split into multiple dex files. (See ‘Building Apps with Over 65K Methods’ for more information about multidex.)

Jack offers native and legacy multidex support.