Mobile Lucene: A Lucene Fork for Android and iOS

This is an experimental fork of Lucene. The aim is to make Lucene run on Android, and then use Google's j2objc to translate the Java code into Objective-C, thus porting Lucene to iOS and OS X.

There have been various Lucene ports, but most of them are several releases behind the mainline (5.x as of August 2015). This project strives to make its porting strategy repeated applicable to subsequent versions of Lucene. Since I started working on this project, Lucene 5.2.1 and Lucene 5.3 have been released, and I was able to track Lucene 5.3 by first rebasing the current tree and then tackling the changes between the two releases.

This is still an experiment and it requires a lot of work ahead. Below is a high-level overview.

Quick Start

To build the JARs, use the commands:

cd lucene
ant -Dmobilejars=../build/<destination> clean mobile-build-modules-without-test

Where <destination> is a directory of your choice. The prefix ../build/ is a trick to gather JARs to a unified folder under lucene/build/

Currently, the original Lucene tests don't run with this fork. Many ant tasks are also potentially broken.

Porting Lucene to Android

Lucene 5.x makes use of many JDK 1.7-only APIs, most notably NIO.2. Android does not have NIO.2, and therefore I've made a library to "backport" those NIO.2 classes. The library, org.lukhnos.portmobile, also comes with some other classes missing in Android. This is a very crude backport and there is currently no test.

Because it's cumbersome to load Java resources in Android, and combining metadata in JAR files when DEXing is also a complicated task, I've also made changes to the SPI loading mechanism in Lucene so that it uses a hard-coded list of "loaded" classes.

There are a few more changes needed. I'll try to document them in detail when I find the time.

Use Source Code Transformation to Port

A transform.py script is provided to transform the Lucene source code. This is to make sure that our changes are systematic. This also enforces us to keep Android-specific changes to a minimum. More importantly, you may still want to develop your Lucene app in the latest Java, NIO.2 and all, then create an automated fork using this transformation tool that imports the mobile version of Lucene.

This script transforms the java code to make it compatible with j2objc.

Porting to Objective-C

Once we have made the code Android-compatible, it is then possible to use j2objc to translate Java to Objective-C. Since Objective-C does not have garbage collection, we need to break a few cyclic references using j2objc's weak reference annotations (Java WeakReference is not used to minimize source code changes).

First, set up the J2ObjC dependencies. If you haven't installed it, download version 2.8 from its GitHub releases page. After you have made sure that the command j2objc exists in your PATH, run within the lucene directory:

cd lucene
./setup-j2objc.sh

Then run the translate.py script to generate new obj-c classes. The classes are generated within the folder: ./build/objc/.

./translate.py

Ported Packages

• lucene-analyzers-common (but without those that need BreakIterator, and org.apache.lucene.analysis.hunspell.ISO8859_14Decoder is not available in the Objective-C translation)
• lucene-core
• lucene-highlighter
• lucene-join
• lucene-memory
• lucene-misc (but no org.apache.lucene.store.*)
• lucene-queries
• lucene-queryparser
• lucene-sandbox
• lucene-suggest

To Do

• Currently portmobile only ports the absolute minimum of NIO.2 that Lucene uses. Since it does not have everything, Lucene test framework cannot be built. There are a number of things in NIO.2 that may not be backportable using pre-JDK 7 APIs, and so whether we can make Lucene tests run is an open question.
• Only a very small portion of Lucene is used and tested in sample mobile apps.
• The transform and translate scripts need a lot of clean-up.
• ant tasks need working.
• The project needs a lot of documentation.