ART stands for Android runtime which is a new runtime like Dalvik which is introduced experimentally in the 4.4 release. This is a preview of work in progress in KitKat that can be turned on in Settings > developer option.n 4.4 both runtimes are now available, Dalvik runtime (libdvm.so) and the ART (libart.so).The user can choose it from developer options.
Difference between Dalvik and ART
Dalvik runtime uses JIT(Just-In-Time) compilation to interpret Bytecode
Now lets understand the technical info and terms in the above sentence.
A compiler is a program or set of programs that transforms source code written in a programming language (the source language) into another computer language
Bytecode also known as p-code (portable code), is a form of instruction set designed for efficient execution by a software interpreter
Interpreter is a program which translates the source code into computer/machine language[/quote]
JIT (Just-In-Time) compliation interprets the bytecode into machine language at it's runtime (at the time when the apps are executed).
Now summing up all these Dalvik runtime does everything when the apps are executed. Now this is where we must discuss odexed and deodexed ROM. usually all stock ROMs released by OEMs are only odexed only due to this reason. odex means a part of the app is already compiled and it is written into dalvik VM(VM=Virtual Machine). and it runs at boot so while execution of apps JIT has to compile only the remaining part of the code when the apps is executed, this is why odexed ROM is faster than deodexed ROM.
But the disadvantage of odexed ROM is it cannot be edited/themed easily as a part of code is seperated in a separate file, and deodexing means the odex part of the file is put back into the apk/jar itself which makes editing/theming easy since the full code is packed inside a single file but it decreases speed because JIT has to compile full code when the app is executed
ART uses AOT compilation to interpret bytecode.
I hope you can refer to dalvik paragraph to know about these technical terms.
what is AOT compilation
AOT (Ahead-Of-Time) is a method of compilation which compiles the code into machine language when the app is installed itself, means no compilation is done at the time of execution of app. This makes the apps launch speed amazingly fast.[/quote]
Advantages of ART over Dalvik
1. The apps launch speed is amazingly fast in case of ART since nothing is compiled at execution.
2. Boot speed is faster than dalvik since nothing is execued from dalvik partition as in case of odexed ROM in dalvik
3. Increases battery backup by reducing CPU work due to absence of compilation work on apps execution.
4. And finally it is a great reward to developers because most of developers are worried that odexed ROMs are faster than deodexed ROMs but they will deodex their ROMs since they heavily theme it. In case of ART whether the ROM is odexed or deodexed it doesn't matter, it compiles the full code into machine language on installation of apps. so even deodexed ROMs are as fast as odexed in ART
Some disadvantages of ART
1. Since ART precompiles apps on installation, it takes 10-20% more space upon installation than dalvik. But this can be simply solved by using apps like apps2sd/link2sd/gl2sd when your apps storage partition is full
2. One more is since it is only experimental feature now some apps are incompatible in ART which will be made compatible soon on their updates.
Here is the list of apps which are compatible and incompatible in ART.
Finally ART is one of the best feature that google brought up in android 4.4 (kitkat)