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Android Runtime
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Android Runtime
Android Runtime (ART) is an application runtime environment used by the Android operating system. Replacing Dalvik, the process virtual machine originally used by Android, ART performs the translation of some of the application's bytecode into native instructions that are later executed by the device's runtime environment.
Android 2.2 "Froyo" brought trace-based just-in-time (JIT) compilation into Dalvik, optimizing the execution of applications by continually profiling applications each time they run and dynamically compiling frequently executed short segments of their bytecode into native machine code. While Dalvik interprets the rest of an application's bytecode, native execution of those short bytecode segments, called "traces", provides significant performance improvements.
Unlike Dalvik, ART introduces the use of ahead-of-time (AOT) compilation by compiling the most performance-critical parts of applications (previously, the entire app) into native machine code upon their installation. This way, ART improves the overall execution efficiency and reduces power consumption, which results in improved battery autonomy on mobile devices. At the same time, ART brings faster execution of applications, improved memory allocation and garbage collection (GC) mechanisms, new applications debugging features, and more accurate high-level profiling of applications.
To maintain backward compatibility, ART uses the same input bytecode as Dalvik, supplied through standard .dex files as part of APK files, while the .odex files are replaced with Executable and Linkable Format (ELF) executables. Once an application is compiled by using ART's on-device dex2oat utility, it is run from the compiled ELF executable; as a result, ART eliminates various application execution overheads associated with Dalvik's interpretation and trace-based JIT compilation.
A disadvantage of ART is that additional time is required for compilation when an application is installed, and applications take up slightly more secondary storage (usually flash memory) to store the compiled code. Long AOT compilation became especially problematic when monthly security updates became the norm, locking users out of the OS for a long time after each update.
To improve on these, a hybrid approach was introduced in Android 7.0 (Nougat) which originally relies on JIT compilation, but later, when the device is idle and charging, compiles the most frequently used code as well as that on the UI thread to native code.
Android 4.4 "KitKat" introduced a technology preview of ART as an alternative runtime environment to Dalvik, which remained the default virtual machine. In the subsequent major Android release, Android 5.0 "Lollipop", Dalvik was entirely replaced by ART.
Android 7.0 "Nougat" turned the ART from a pure AOT to a hybrid JIT/AOT solution, while switching its Java Runtime Environment from the discontinued Apache Harmony to OpenJDK, introducing a JIT compiler with code profiling. The JIT compiler complements ART's AOT compiler, helping to improve runtime performance and save storage space by identifying "hot code" (code which is frequently used, runs on the UI thread or affects startup time), which the AOT compiler compiles to machine code while the device is idle and charging. Less-frequently used code relies on JIT compilation.
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Android Runtime
Android Runtime (ART) is an application runtime environment used by the Android operating system. Replacing Dalvik, the process virtual machine originally used by Android, ART performs the translation of some of the application's bytecode into native instructions that are later executed by the device's runtime environment.
Android 2.2 "Froyo" brought trace-based just-in-time (JIT) compilation into Dalvik, optimizing the execution of applications by continually profiling applications each time they run and dynamically compiling frequently executed short segments of their bytecode into native machine code. While Dalvik interprets the rest of an application's bytecode, native execution of those short bytecode segments, called "traces", provides significant performance improvements.
Unlike Dalvik, ART introduces the use of ahead-of-time (AOT) compilation by compiling the most performance-critical parts of applications (previously, the entire app) into native machine code upon their installation. This way, ART improves the overall execution efficiency and reduces power consumption, which results in improved battery autonomy on mobile devices. At the same time, ART brings faster execution of applications, improved memory allocation and garbage collection (GC) mechanisms, new applications debugging features, and more accurate high-level profiling of applications.
To maintain backward compatibility, ART uses the same input bytecode as Dalvik, supplied through standard .dex files as part of APK files, while the .odex files are replaced with Executable and Linkable Format (ELF) executables. Once an application is compiled by using ART's on-device dex2oat utility, it is run from the compiled ELF executable; as a result, ART eliminates various application execution overheads associated with Dalvik's interpretation and trace-based JIT compilation.
A disadvantage of ART is that additional time is required for compilation when an application is installed, and applications take up slightly more secondary storage (usually flash memory) to store the compiled code. Long AOT compilation became especially problematic when monthly security updates became the norm, locking users out of the OS for a long time after each update.
To improve on these, a hybrid approach was introduced in Android 7.0 (Nougat) which originally relies on JIT compilation, but later, when the device is idle and charging, compiles the most frequently used code as well as that on the UI thread to native code.
Android 4.4 "KitKat" introduced a technology preview of ART as an alternative runtime environment to Dalvik, which remained the default virtual machine. In the subsequent major Android release, Android 5.0 "Lollipop", Dalvik was entirely replaced by ART.
Android 7.0 "Nougat" turned the ART from a pure AOT to a hybrid JIT/AOT solution, while switching its Java Runtime Environment from the discontinued Apache Harmony to OpenJDK, introducing a JIT compiler with code profiling. The JIT compiler complements ART's AOT compiler, helping to improve runtime performance and save storage space by identifying "hot code" (code which is frequently used, runs on the UI thread or affects startup time), which the AOT compiler compiles to machine code while the device is idle and charging. Less-frequently used code relies on JIT compilation.