On top of the Linux kernel, there are the middleware, libraries and APIs written in C, and application software running on an application framework which includes Java-compatible libraries. Development of the Linux kernel continues independently of other Android's source code bases.
Until version 5.0, Android used Dalvik as a process virtual machine with trace-based just-in-time (JIT) compilation to run Dalvik "dex-code" (Dalvik Executable), which is usually translated from the Java bytecode. Following the trace-based JIT principle, in addition to interpreting the majority of application code, Dalvik performs the compilation andnative execution of select frequently executed code segments ("traces") each time an application is launched.[138][139][140] Android 4.4 introduced Android Runtime (ART) as a new runtime environment, which usesahead-of-time (AOT) compilation to entirely compile the application bytecode into machine code upon the installation of an application. In Android 4.4, ART was an experimental feature and not enabled by default; it became the only runtime option in the next major version of Android, 5.0.[141]
For its Java library, the Android platform uses a subset of the now discontinued Apache Harmony project.[142] In December 2015, Google announced that the next version of Android would switch to a Java implementation based onOpenJDK.[143]
Android's standard C library, Bionic, was developed by Google specifically for Android, as a derivation of the BSD's standard C library code. Bionic itself has been designed with several major features specific to the Linux kernel. The main benefits of using Bionic instead of the GNU C Library (glibc) or uClibc are its smaller runtime footprint, and optimization for low-frequency CPUs. At the same time, Bionic is licensed under the terms of the BSD licence, which Google finds more suitable for the Android's overall licensing model.[140]
Aiming for a different licensing model, toward the end of 2012 Google switched the Bluetooth stack in Android from the GPL-licensed BlueZ to the Apache-licensed BlueDroid.[144]
Android does not have a native X Window System by default, nor does it support the full set of standard GNU libraries. This made it difficult to port existing Linux applications or libraries to Android,[137] until version r5 of the Android Native Development Kit brought support for applications written completely in C or C++.[145] Libraries written in C may also be used in applications by injection of a small shim and usage of the JNI.[146]
Since Marshmallow, "Toybox", a collection of command line utilities (mostly for use by apps, as Android doesn't provide a command line interface by default), replaced similar "Toolbox" collection.[147]
Open-source community
Android has an active community of developers and enthusiasts who use the Android Open Source Project (AOSP) source code to develop and distribute their own modified versions of the operating system.[148] These community-developed releases often bring new features and updates to devices faster than through the official manufacturer/carrier channels, with a comparable level of quality;[23] provide continued support for older devices that no longer receive official updates; or bring Android to devices that were officially released running other operating systems, such as the HP TouchPad. Community releases often come pre-rooted and contain modifications not provided by the original vendor, such as the ability to overclock or over/undervolt the device's processor.[149] CyanogenMod is the most widely used community firmware,[150] and acts as a foundation for numerous others. There have also been attempts with varying degrees of success to port Android to iPhones, notably the iDroid Project.[151]
Historically, device manufacturers and mobile carriers have typically been unsupportive of third-party firmware development. Manufacturers express concern about improper functioning of devices running unofficial software and the support costs resulting from this.[152] Moreover, modified firmwares such as CyanogenMod sometimes offer features, such as tethering, for which carriers would otherwise charge a premium. As a result, technical obstacles including locked bootloaders and restricted access to root permissions are common in many devices. However, as community-developed software has grown more popular, and following a statement by the Librarian of Congress in the United States that permits the "jailbreaking" of mobile devices,[153] manufacturers and carriers have softened their position regarding third party development, with some, including HTC,[152]Motorola,[154] Samsung[155][156] and Sony,[157] providing support and encouraging development. As a result of this, over time the need to circumvent hardware restrictions to install unofficial firmware has lessened as an increasing number of devices are shipped with unlocked or unlockable bootloaders, similar to Nexus series of phones, although usually requiring that users waive their devices' warranties to do so.[152] However, despite manufacturer acceptance, some carriers in the US still require that phones are locked down, frustrating developers and customers.[158]
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