HB Blog 128: Android Jack And Jill Toolchain.

Jack is a new 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.
For information related to Android Build System visit post HB Blog 51: Android Build System - How Android Apk Is Built.

The Jack toolchain provides the following advantages:

• Completely open source
  Available in AOSP; partners 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.

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

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

How to use Jack in Android build?
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.

Jack and Jill are available in Build Tools version 21.1.1, and greater, via the SDK Manager. Complementary Gradle and Android Studio support is also already available in the Android 1.0.0+ Gradle plugin.'
Using Gradle, add the following to your module-level build config file:

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android { ... buildToolsVersion '21.1.2' defaultConfig { // Enable the experimental Jack build tools. jackOptions { enabled true } } ... }

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.