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In the preceding few chapters we have learned that Android applications run within virtual machine processes and that they are comprised of multiple components in the form of activities, Services and Broadcast Receivers. The goal of this chapter is to expand on this knowledge by looking at the lifecycle of applications and activities within the Android runtime system.
Regardless of the fanfare about how much memory and computing power resides in the mobile devices of today compared to the desktop systems of yesterday, it is important to keep in mind that these devices are still considered to be “resource constrained” by the standards of modern desktop and laptop based systems, particularly in terms of memory. As such, a key responsibility of the Android system is to ensure that these limited resources are managed effectively and that both the operating system and the applications running on it remain responsive to the user at all times. In order to achieve this, Android is given full control over the lifecycle and state of both the processes in which the applications run, and the individual components that comprise those applications.
== Android Applications and Resource Management ==
Each running Android applications, as previously established, run within instances of the Dalvik virtual machine with each virtual machine being application is viewed by the operating system as a separate process. If the system identifies that resources on the device are reaching capacity it will take steps to terminate processes to free up memory.
When making a determination as to which process to terminate in order to free up memory, the system takes into consideration both the priority and state of all currently running processes, combining these factors to create what is referred to by Google as an importance hierarchy. Processes are then terminated starting with the lowest priority and working up the hierarchy until sufficient resources have been liberated for the system to function.
Mobile devices are typically considered to be resource constrained, particularly in terms of onboard memory capacity. Consequently, a prime responsibility of the Android operating system is to ensure that applications, and the operating system in general, remain responsive to the user.
Applications are hosted on Android within processes (essentially instances of the Dalvik virtual machine). Each application, in turn, is made up of components in the form of activities and Services.
The Android runtime system has the power to terminate both processes and individual activities in order to free up memory. Process state is taken into consideration by the runtime system when deciding whether a process is a suitable candidate for termination. The state of a process is largely dependent upon the status of the activities hosted by that process.
