The process where a component runs is controlled by the manifest file. The component elements — <activity>, <service>, <receiver>, and <provider> — each have a process attribute that can specify a process where that component should run. These attributes can be set so that each component runs in its own process, or so that some components share a process while others do not. They can also be set so that components of different applications run in the same process — provided that the applications share the same Linux user ID and are signed by the same authorities. The <application> element also has a process attribute, for setting a default value that applies to all components.
All components are instantiated in the main thread of the specified process, and system calls to the component are dispatched from that thread. Separate threads are not created for each instance. Consequently, methods that respond to those calls — methods like View.onKeyDown() that report user actions and the lifecycle notifications discussed later in the Component Lifecycles section — always run in the main thread of the process. This means that no component should perform long or blocking operations (such as networking operations or computation loops) when called by the system, since this will block any other components also in the process. You can spawn separate threads for long operations, as discussed under Threads, next.
Android may decide to shut down a process at some point, when memory is low and required by other processes that are more immediately serving the user. Application components running in the process are consequently destroyed. A process is restarted for those components when there's again work for them to do.
When deciding which processes to terminate, Android weighs their relative importance to the user. For example, it more readily shuts down a process with activities that are no longer visible on screen than a process with visible activities. The decision whether to terminate a process, therefore, depends on the state of the components running in that process. Those states are the subject of a later section, Component Lifecycles.
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