Bluetooth Blues

I spent the better part of two days working on Bluetooth connectivity for an Android app I’m developing. Going into it, I had virtually no experience working with Bluetooth, especially on Android. I quickly discovered some of the peculiarities of the platform’s Bluetooth API.

In addition to connecting to Bluetooth devices, the client wanted to pair and unpair from the app. The easy way out, and probably The Android Way™, would be to pass that responsibility off to the OS, à la an Intent:

This will bring up the Bluetooth settings menu, from which you can pair/unpair devices, but the problem is that it’s a complete context switch for the user—they are no longer in your application. I was looking to provide a more seamless experience so that the user didn’t have to leave the app at all to pair a device.

Device Discovery

The entry point for Bluetooth interaction in Android is through the BluetoothAdapter, which is used to orchestrate the device discovery process and fetch paired devices. Calling startDiscovery() will tell the adapter to start scanning for devices, and when one is found, an Intent will be fired off which can then be intercepted by a BroadcastReceiver.

The above code shows how the device discovery process is kicked off and how a BroadcastReceiver is registered to listen for discovery Intents. Note that the BroadcastReceiver is unregistered and discovery is canceled in onDestroy.

In order to react to discovery events, we must implement a BroadcastReceiver.

Device Pairing

Once you have a handle on the BluetoothDevice received in the BroadcastHandler, how do you actually pair with it? Looking at the documentation for the class, you’ll see that there are no methods for doing this. This is where things start to get a little strange.

Diving into the source code for BluetoothDevice, you’ll actually find that there is functionality for doing pairing and unpairing, but the methods are hidden from the API using the @hide annotation. What’s more interesting is that the methods are, in fact, public.

Evidently, device pairing is intended to be performed only by platform applications, which is a little curious considering the permission needed to perform pairing, android.permission.BLUETOOTH_ADMIN, is accessible by third-party applications. Nonetheless, this means we actually can pair a BluetoothDevice, just not in the way the Android engineers intended.

To access the BluetoothDevice methods needed, createBond and removeBond, we can use reflection.

The pairDevice method will prompt the user to enter a PIN for the discovered device, circumventing the need to open the Bluetooth settings. As such, the pairing does not actually complete until the correct PIN is entered. The boolean value returned from the method indicates whether the pairing process was successfully kicked off or not.

It goes without saying that this code, while functional, is volatile because these methods are technically not part of the public API, so they could change or disappear in future platform releases.

We can add an Intent filter to our BroadcastReceiver to listen for pairing events using the action BluetoothDevice.ACTION_BOND_STATE_CHANGED.

There are a few other hidden methods in BluetoothDevice, like cancelPairingUserInput, setPairingConfirmation, convertPinToBytes, and setPin, that you could potentially use to customize the pairing process or perform it programmatically, but use them at your own risk.

Once the devices are paired, they can be connected using one of BluetoothDevice’s createRfcommSocketToServiceRecord or createInsecureRfcommSocketToServiceRecord methods after determining the UUID to use, either with getUuids or fetchUuidsWithSdp (or, in most cases, using the well-known UUID 00001101-0000-1000-8000-00805F9B34FB).

It’s very likely that Android’s Bluetooth API is subject to change soon. It already has changed in some of the more recent releases, although I’m not entirely sure why Google isn’t providing a stable API for pairing. Jelly Bean 4.2 introduces a new Bluetooth stack, moving from BlueZ to a Broadcom solution, so my guess is that it’s related to this.

Implementing Spring-like Classpath Scanning in Android

One of the things that Spring 2.5 introduced back in 2007 was component scanning, a feature which removed the need for XML bean configuration and instead allowed developers to declare their beans using Java annotations. Rather than this:

We can do this:

It’s a pretty simple idea since Java makes it very easy to introspectively check a class’s annotations at runtime through its reflection API. Spring’s component scan feature also allows you to specify the base package(s) to scan for beans.

The big question is how do we get access to the classes in the classpath, specifically, those in the desired package? Java SE doesn’t provide an API for doing it, but there are ways to accomplish this. The most common (if not the only) approach is to load classes by relying on the file system. We know that we can use the ClassLoader to load a class by its package-qualified name, so it becomes a matter of retrieving the file names.

Getting the classpath itself in Java SE is easy:

This will yield something that looks like “/Users/Tyler/Workspace/Test/bin:/Users/Tyler/Workspace/Test/lib/gson-2.1.jar”. Loading the files from here is pretty straightforward, as is filtering on the package name since it maps to a directory one-to-one.

Another similar approach is to use the ClassLoader to load the resources directly:

Transition to Android

Unfortunately, these solutions don’t lend themselves to Android, which made implementing classpath scanning a little more difficult for Infinitum. The reason for this is, more or less, because of the way Android’s Dalvik VM is designed. When an Android application is compiled, the Dalvik bytecode is packaged into a file called “classes.dex” inside the APK. The good news is that the Android SDK provides an API for interacting with DEX files through the DexFile class.

In order to access classes.dex, we need a handle on the APK itself, which is actually quite easy to do:

The above code opens a DexFile for the running APK. Of course, this can have some performance implications. Opening the DexFile will potentially cause the VM to pass classes.dex through a process known as “dexopt”, which is a program that performs bytecode verification and optimization. This is an expensive process, but since we’re opening a DexFile for the APK itself, classes.dex should have already undergone this process, meaning dexopt won’t be run again.

The DexFile gives us access to the classes contained in classes.dex as an enumeration of strings representing the package-qualified class names. With this, we can iterate over the class names and load any which match the desired package.

This gets the job done, and it’s essentially how Infinitum accomplishes component scanning. However, it’s a very expensive operation. DexFile.entries() yields every class in the classpath — that is, every class in classes.dex — which includes not just application binaries, but also those of any libraries included.

It’s great that we can introspect every class in the classpath, but if we’re only interested in classes of a particular package, we’re out of luck. Every class is compiled into classes.dex and, short of decompiling it1,  there’s no way to pull out the classes we want without iterating over the entire classpath.

So, for now we settle with this somewhat inefficient solution. Nonetheless, it accomplishes what it needs to at the cost of maybe a few hundred milliseconds2, so maybe it’s not such a bad approach in the grand scheme of things.

  1. Tools for decompiling DEX files exist, such as Baksmali, but doing such a thing at runtime — if it’s even possible — would arguably not gain you any performance benefits. Still, this is something worth exploring. []
  2. On the emulator running on my MacBook Pro, the classpath scanning takes about 600 milliseconds, while on my Galaxy Nexus, it takes about 200 milliseconds. []

Introducing InfinitumFramework.com

rendering

Here’s a dose of shameless self-promotion. It’s coming up on a year since I started development on Infinitum, and I’m targeting its first full release on its birthday, February 11. Shortly before I moved the project to GitHub, they deprecated the downloads service, so I needed to fine a home for distributing the binaries as well as the Javadoc.

GitHub offers its pages service, but I figured I’d just host it myself. I threw together a website in a couple days and the result is www.infinitumframework.com. This website will be used to host the latest (and previous) releases of the framework, its documentation, and, in the future, announcements and updates for it.

Stay tuned as Infinitum approaches its first official release!

Modularizing Infinitum: A Postmortem

In addition to getting the code migrated from Google Code to GitHub, one of my projects over the holidays was to modularize the Infinitum Android framework I’ve been working on for the past year.

Infinitum began as a SQLite ORM and quickly grew to include a REST ORM implementation,  REST client, logging wrapper, DI framework, AOP module, and, of course, all of the framework tools needed to support these various functionalities. It evolved as I added more and more features in a semi-haphazard way. In my defense, the code was organized. It was logical. It made sense. There was no method, but there also was no madness. Everything was in an appropriately named package. Everything was coded to an interface. There was no duplicated code. However, modularity — in terms of minimizing framework dependencies — wasn’t really in mind at the time, and the code was all in a single project.

The Wild, Wild West

The issue wasn’t how the code was organized, it was how the code was integrated. The project was cowboy coding at its finest. I was the only stakeholder, the only tester, the only developer — judge, jury, and executioner. I was building it for my own personal use after all. Consequently, there was no planning involved, unit testing was somewhere between minimal and non-existent, and what got done was at my complete discretion. Ultimately, what was completed any given day, more or less, came down to what I felt like working on.

What started as an ORM framework became a REST framework, which became a logging framework, which became an IOC framework, which became an AOP framework. All of these features, built from the ground up, were tied together through a context, which provided framework configuration data. More important, the Infinitum context stored the bean factory used for storing and retrieving bean definitions used by both the framework and the client. The different modules themselves were not tightly coupled, but they were connected to the context like feathers on a bird.

infinitum-arch

The framework began to grow large. It was only about 300KB of actual code (JARed without ProGuard compression), but it had a number of library dependencies, namely Dexmaker, Simple XML, and GSON, which is over 1MB combined in size. Since it’s an Android framework, I wanted to keep the footprint as small as possible. Additionally, it’s likely that someone wouldn’t be using all of the features in the framework. Maybe they just need the SQLite ORM, or just the REST client, or just dependency injection. The way the framework was structured, they had to take it all or none.

A Painter Looking for a Brush

I began to investigate ways to modularize it. As I illustrated, the central problem lay in the fact that the Infinitum context had knowledge of all of the different modules and was responsible for calling and configuring their APIs. If the ORM is an optional dependency, the context should not need to have knowledge of it. How can the modules be decoupled from the context?

Obviously, there is a core dependency, Infinitum Core, which consists of the framework essentials. These are things used throughout the framework in all of the modules — logging, DI1, exceptions, and miscellaneous utilities. The goal was to pull off ORM, REST, and AOP modules.

My initial approach was to try and use the decorator pattern to “decorate” the Infinitum context with additional functionality. The OrmContextDecorator would implement the ORM-specific methods, the AopContextDecorator would implement the AOP-specific methods, and so on. The problem with this was that it would still require the module-specific methods to be declared in the Infinitum context interface. Not only would they need to be stubbed out in the context implementation, a lot of module interfaces would need to be shuffled and placed in Infinitum Core  in order to satisfy the compiler. The problem remained; the context still had knowledge of all the modules.

I had another idea in mind. Maybe I could turn the Infinitum context from a single point of configuration to a hierarchical structure where each module has its own context as a “child” of the root context. The OrmContext interface could extend the InfinitumContext interface, providing ORM-specific functionality while still inheriting the core context methods. The implementation would then contain a reference to the parent context, so if it was unable to perform a certain piece of functionality, it could delegate to the parent. This could work. The Infinitum context has no notion of module X, Y, or Z, and, in effect, the control has been inverted. You could call it the Hollywood Principle — “Don’t call us, we’ll call you.”

infinitum-context-hierarchy

There’s still one remaining question: how do we identify the “child” contexts and subsequently initialize them? The solution is to maintain a module registry. This registry will keep track of the optional framework dependencies and is responsible for initializing them if they are available. We use a marker class from each module, a class we know exists if the dependency is included in the classpath, to check its availability.

Lastly, we use reflection to instantiate an instance of the module context. I used an enum to maintain a registry of Infinitum modules. I then extended the enum to add an initialize method which loads a context instance.

The modules get picked up during a post-processing step in the ContextFactory. It’s this step that also adds them as child contexts to the parent.

New modules can be added to the registry without any changes elsewhere. As long as the context has been implemented, they will be picked up and processed automatically.

Once this architecture was in place, separating the framework into different projects was simple. Now Infinitum Core can be used by itself if only dependency injection is needed, the ORM can be included if needed for SQLite, AOP included for aspect-oriented programming, and Web for the RESTful web service client and various HTTP utilities.

We Shape Our Buildings, and Afterwards, Our Buildings Shape Us

I think this solution has helped to minimize some of the complexity a bit. As with any modular design, not only is it more extensible, it’s more maintainable. Each module context is responsible for its own configuration, so this certainly helped to reduce complexity in the InfinitumContext implementation as before it was handling the initialization for the ORM, AOP, and REST pieces. It also worked out in that I made the switch to GitHub2 by setting up four discrete repositories, one for each module.

In retrospect, I would have made things a lot easier on myself if I had taken a more modular approach from the beginning. I ended up having to reengineer quite a bit, although once I had a viable solution, it actually wasn’t all that much work. I was fortunate in that I had things fairly well designed (perhaps not at a very high level, but in general) and extremely organized. It’s difficult to anticipate change, but chances are you’ll be kicking yourself if you don’t. I started the framework almost a year ago, and I never imagined it would grow to what it is today.

  1. I was originally hoping to pull out dependency injection as a separate module, but the framework relies heavily on it to wire up components. []
  2. Now that the code’s pushed to GitHub, I begin the laborious task of migrating the documentation over from Google Code. []

Dalvik Bytecode Generation

Earlier, I discussed the use of dynamic proxies and how they can be implemented in Java. As we saw, a necessary part of proxying classes is bytecode generation. From its onset, something I wanted to include in Infinitum was lazy loading. I also wanted to provide support for AOP down the road. Consequently, it was essential to include some way to generate bytecode at runtime.

The obvious choice would be to use a library like Cglib or Javassist, but sadly neither of those would work. That’s because Android does not use a Java VM, it uses its own virtual machine called Dalvik. As a result, Java source code isn’t compiled into Java bytecode (.class files), but rather Dalvik bytecode (.dex files). Since Cglib and Javassist are designed for Java bytecode manipulation, they do not work on the Android platform.1

What’s a programmer to do? Fortunately, some Googlers developed a new library for runtime code generation targeting the Dalvik VM called Dexmaker.

It has a small, close-to-the-metal API. This API mirrors the Dalvik bytecode specification giving you tight control over the bytecode emitted. Code is generated instruction-by-instruction; you bring your own abstract syntax tree if you need one. And since it uses Dalvik’s dx tool as a backend, you get efficient register allocation and regular/wide instruction selection for free.

Even better, Dexmaker provides an API for directly creating proxies called ProxyBuilder. If you followed my previous post on generating proxies, then using ProxyBuilder is a piece of cake. Similar to Java’s Proxy class, ProxyBuilder relies on an InvocationHandler to specify a proxy’s behavior.

Dexmaker enabled me to implement lazy loading and AOP within the Infinitum framework. It also opens up the possibility of using Mockito for unit testing in an Android environment because Mockito relies on proxies for generating mocks.2

  1. ASMDEX, a Dalvik-compatible bytecode-manipulation library was released in March 2012, meaning Cglib could, in theory, be ported to Android since it relies on ASM. []
  2. Infinitum is actually unit tested using Robolectric, which allows for testing Android code in a standard JVM. []