ART, which stands for Android Runtime, handles app execution in a fundamentally different way from Dalvik. The current runtime relies on a Just-In-Time (JIT) compiler to interpret bytecode, a generic version of the original application code. In a manner of speaking, apps are only partially compiled by developers, then the resulting code must go through an interpreter on a user's device each and every time it is run. The process involves a lot of overhead and isn't particularly efficient, but the mechanism makes it easy for apps to run on a variety of hardware and architectures. ART is set to change this process by pre-compiling that bytecode into machine language when apps are first installed, turning them into truly native apps. This process is called Ahead-Of-Time (AOT) compilation. By removing the need to spin up a new virtual machine or run interpreted code, startup times can be cut down immensely and ongoing execution will become faster, as well.
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fully compiled machine code will usually consume more storage space than that of bytecode. This is because each symbol in bytecode is representative of several instructions in machine code. Of course, the increase in size isn't going to be particularly significant, not usually more than 10%-20% larger. That might sound like a lot when APKs can get pretty large, but the executable code only makes up a fraction of the size in most apps.
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