Processes and Threads

The manifest entry for each type of component element—

<activity>

, 

<service>

, 

<receiver>

,
and 

<provider>

—supports an 

android:process

 attribute
that can specify a process in which that component should run. You can set this attribute so that each component runs in its own process or so that some components share a process while others do not. You can also set 

android:process

 so
that components of different applications run in the same process—provided that the applications share the same Linux user ID and are signed with the same certificates.

The 

<application>

 element also
supports an 

android:process

 attribute, to set a default value that applies to all components.

Foreground process
A process that is required for what the user is currently doing. A process is considered to be in the foreground if any of the following conditions are true:
It hosts an 

Activity

 that
the user is interacting with (the 

Activity

's 

onResume()

 method
has been called).
It hosts a 

Service

 that's
bound to the activity that the user is interacting with.
It hosts a 

Service

 that's
running "in the foreground"—the service has called 

startForeground()

.
It hosts a 

Service

 that's
executing one of its lifecycle callbacks (

onCreate()

, 

onStart()

,
or 

onDestroy()

).
It hosts a 

BroadcastReceiver

 that's
executing its 

onReceive()

 method.
Generally, only a few foreground processes exist at any given time. They are killed only as a last resort—if memory is so low that they cannot all continue to run. Generally, at that point, the device has reached
a memory paging state, so killing some foreground processes is required to keep the user interface responsive.

Visible process
A process that doesn't have any foreground components, but still can affect what the user sees on screen. A process is considered to be visible if either of the following conditions are true:
It hosts an 

Activity

 that
is not in the foreground, but is still visible to the user (its 

onPause()

 method
has been called). This might occur, for example, if the foreground activity started a dialog, which allows the previous activity to be seen behind it.
It hosts a 

Service

 that's
bound to a visible (or foreground) activity.
A visible process is considered extremely important and will not be killed unless doing so is required to keep all foreground processes running.

Service process
A process that is running a service that has been started with the 

startService()

 method
and does
4000
not fall into either of the two higher categories. Although service processes are not directly tied to anything the user sees, they are generally doing things that the user cares about (such as playing music in the background or downloading data on
the network), so the system keeps them running unless there's not enough memory to retain them along with all foreground and visible processes.

Background process
A process holding an activity that's not currently visible to the user (the activity's 

onStop()

 method
has been called). These processes have no direct impact on the user experience, and the system can kill them at any time to reclaim memory for a foreground, visible, or service process. Usually there are many background processes running, so they are kept
in an LRU (least recently used) list to ensure that the process with the activity that was most recently seen by the user is the last to be killed. If an activity implements its lifecycle methods correctly, and saves its current state, killing its process
will not have a visible effect on the user experience, because when the user navigates back to the activity, the activity restores all of its visible state. See the Activities document
for information about saving and restoring state.

Empty process
A process that doesn't hold any active application components. The only reason to keep this kind of process alive is for caching purposes, to improve startup time the next time a component needs to run in it. The
system often kills these processes in order to balance overall system resources between process caches and the underlying kernel caches.

Threads

When an application is launched, the system creates a thread of execution for the application, called "main." This thread is very important because it is in charge of dispatching events to the appropriate user interface widgets, including drawing events. It
is also the thread in which your application interacts with components from the Android UI toolkit (components from the 

android.widget

 and 

android.view

 packages).
As such, the main thread is also sometimes called the UI thread.

The system does not create a separate thread for each instance of a component. All components that run in the same process are instantiated in the UI thread, and system calls to each component are dispatched from that thread. Consequently, methods
that respond to system callbacks (such as 

onKeyDown()

 to
report user actions or a lifecycle callback method) always run in the UI thread of the process.