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Inside the Android application framework - Google I/O 2009

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Viswanath J
Viswanath J

- Android applications are composed of components that can span multiple processes and tasks. Components have managed lifecycles as they transition between states like active, paused, and stopped. - Inter-process communication (IPC) allows components in different processes to share data. The Android Binder system provides efficient IPC using parcelables and bundles to pass data across processes. - Activities represent screens or views within an application. Tasks group related activities that maintain a back stack. By default each application runs in its own process but additional processes can be used to isolate components from one another.

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Inside the Android Application Framework
Managed Component Lifecycles An Android APK is a collection of components Components share a set of resources Databases, preferences, file space, etc. Also: a Linux process. Every Android component has a managed lifecycle
Basics of an Android Application Activities Tasks Processes
Activities and Tasks An Activity is a “molecule”: a discrete chunk of functionality A task is a collection of Activities A “process” is a standard Linux process
Activities and Tasks APK Package Process ContentProvider Service Activity Activity APK Package Process Activity ContentProvider Process Service Activity
Activities and Tasks APK Package Process Activity ContentProvider Process Service APK Package Process ContentProvider Service Task Activity Activity Activity
Activities Are... ...a concrete class in the API ...an encapsulation of a particular operation ...run in the process of the .APK which installed them ...optionally associated with a window (UI) ...an execution Context
Tasks Are... ...more of a notion than a concrete API entity ...a collection of related Activities ...capable of spanning multiple processes ...associated with their own UI history stack ...what users on other platforms know as “applications”
Process Basics Android process == Linux process By default, 1 process per APK By default, 1 thread per process All* components interleave events into the main thread * Most
Process Lifecycle A process is started for a given user ID when needed Binding to a Service Binding to a ContentProvider Starting an Activity Firing an IntentReceiver Remains running until killed by the system
More on Activities Activity Lifecycle Examples of Common Use Cases
The Directed Cyclic Graph of Life Activities have several states Lifecycle methods are called on transitions You typically don’t need to use them all, but they are there http://code.google.com/android/reference/android/app/Activity.html
Activity Lifecycle Three general “phases” Starting up onCreate(): first method called during lifetime, with prior state onStart()/onRestart(): signal that execution is beginning onResume(): signals that a previous pause is being undone
Activity Lifecycle Normal execution onFreeze(): save UI state (NOT intended to save persistent data) onPause: signals loss of focus and possible impending shutdown
Activity Lifecycle Shutting down onStop()/onDestroy(): final shutdown and process termination Not guaranteed to be called (and usually not, except on finish()...)
Activity Lifecycle Examples Starting a Child Activity Child Activity + Process Shutdown Returning to the Home Screen Calling finish() Explicitly Displaying a Dialog Box Semi-Transparent Windows Device Sleep
Example: Child Activity Launched Call sequence: onCreate() onStart() onResume() onFreeze() onPause() onStop() onRestart() onStart(), onResume(), ... This is the “classic” scenario.
Example: Child Activity + Process Death Call sequence: onCreate() (empty state) onStart() onResume() onFreeze() onPause() onStop() (maybe) onDestroy() (maybe) onCreate() (with state), ... Like the basic case, but onCreate() is called again, with the state saved in onFreeze().
Example: User Hits ’Home’ Call sequence: onCreate() onStart() onResume() onFreeze() onPause() onStop() (maybe) onDestroy() (maybe) Identical to the basic case -- that is, the Home key is not a special case.
Example: finish() Called Call sequence: onCreate() onStart() onResume() onPause() onStop() onDestroy() Because the Activity has been explicitly told to quit and is being removed from the task (and history stack), onFreeze() is not called, and onDestroy() is reached.
Example: Dialog Box Call sequence: onCreate() onStart() onResume() Despite appearances, dialog boxes are Views, and not Activities, so they have no effect on the owning Activity’s lifecycle.
Example: Transparent/Non-fullscreen Child Call sequence: onCreate() onStart() onResume() onFreeze() onPause() onResume() The new partial-screen window leaves a portion of the previous window visible, so onPause() is followed by onResume() (without onStop()) when the child closes.
Example: Device Goes to Sleep Call sequence: onCreate() onStart() onResume() onFreeze() onPause() onResume() The device going to sleep is identical to a non-fullscreen Activity being launched on top.
Threads on Android Overview Loopers Multi-thread Considerations
Threading Overview Each process has one thread (by default) Most components share the single thread Services and ContentProviders sometimes do not
Threads and Loopers Each thread has a Looper to handle a message queue Events from all components are interleaved into Looper e.g. View UI events, IntentReceivers firing, etc. Loopers cannot accommodate multi-threaded access They are designed to play nicely with MessageHandlers
Threads and Loopers APK Package Process Thread Looper Message Queue Thread External Service Calls Intent Receiver Activity Activity UI Events System Events Local Service Call
Threads and Views Views use Looper messages to fire events Since Loopers are 1:1 with threads, the View tree is too Threads you create cannot directly touch a View But, you can create a new Looper for your own thread
Threads in Other Contexts Services & ContentProviders sometimes run in their own threads ...but still in the same process Components can create threads, but must handle thread-safety
Service Lifecycle Started by some other Component Either explicitly, or implicitly by binding to it Explicitly-started Services run until explicitly shut down (or killed by the system during a memory crunch) Implicitly-started Services run til the last client unbinds
More on Processes Resource Management Processes & Security Controlling Processes
Process Resource Management Spawned by the special “Zygote” process Process + pre-warmed Dalvik VM == responsiveness Process runs under user ID unique to system Process + User ID == security
Processes & Security Each application is given a unique user ID No exceptions! ...except these: init, Zygote, and the main runtime Each application has direct access only to its own data Other apps’ resources are available only via defined, explicitly-exposed APIs i.e. Issuing Intents, binding to Services or ContentProviders
Inter-Process Communication Why?? Process Transparency Binder in 30 Seconds IPC using Parcelables IPC using Bundles Android IDL
Why?? All this process/Activity/task stuff is confusing... why? It’s all for the noble goal of efficiency (i.e. speed.) Serialization is slooow; memory transfers are slooow. CPU is not the bottleneck: think memory & bandwidth.
Process Transparency Process management is transparent to code. ...almost. In some cases, it’s unavoidably visible. Lifecycle is seamless, but data sometimes isn’t. Specific APIs send data across process boundaries.
IPC Overview Kernel Process Binder Bundle Intent Receiver Activity Custom Objects Service Parcel Parcelable
Binder in 30 Seconds All IPC goes through “The Binder” Binder is implemented as a kernel module + system lib Supports sophisticated cross-process data transport The framework APIs “know” how to use Binder Generally two entry points: Bundles & Parcelables
IPC - Parcelables A Parcelable is a class which can marshal its state to something Binder can handle -- namely, a “Parcel” Standard Java serialization has semantics Parcelables don’t need Supporting full serialization would mean wasting CPU cycles
IPC - Bundles Bundles are typesafe containers of primitives That is, C-like primitives: ints, strings, etc. Simple data-passing APIs use Bundles Think of onFreeze() as passing data to your future self Flat structure permits optimizations like memory-mapping
IPC - AIDL “ Android Interface Definition Language” Used to build developer-friendly APIs using Parcelables Preferred way to expose structured, complex-typed APIs Compromise between efficiency and Java usability
Wrapping Up APKs are loose collections of components Tasks (AKA apps) are bags of component instances that span processes & APKs Managed lifecycles & IPC join the “seams”

More Related Content

Inside the Android application framework - Google I/O 2009

  • 1. Inside the Android Application Framework
  • 2. Managed Component Lifecycles An Android APK is a collection of components Components share a set of resources Databases, preferences, file space, etc. Also: a Linux process. Every Android component has a managed lifecycle
  • 3. Basics of an Android Application Activities Tasks Processes
  • 4. Activities and Tasks An Activity is a “molecule”: a discrete chunk of functionality A task is a collection of Activities A “process” is a standard Linux process
  • 5. Activities and Tasks APK Package Process ContentProvider Service Activity Activity APK Package Process Activity ContentProvider Process Service Activity
  • 6. Activities and Tasks APK Package Process Activity ContentProvider Process Service APK Package Process ContentProvider Service Task Activity Activity Activity
  • 7. Activities Are... ...a concrete class in the API ...an encapsulation of a particular operation ...run in the process of the .APK which installed them ...optionally associated with a window (UI) ...an execution Context
  • 8. Tasks Are... ...more of a notion than a concrete API entity ...a collection of related Activities ...capable of spanning multiple processes ...associated with their own UI history stack ...what users on other platforms know as “applications”
  • 9. Process Basics Android process == Linux process By default, 1 process per APK By default, 1 thread per process All* components interleave events into the main thread * Most
  • 10. Process Lifecycle A process is started for a given user ID when needed Binding to a Service Binding to a ContentProvider Starting an Activity Firing an IntentReceiver Remains running until killed by the system
  • 11. More on Activities Activity Lifecycle Examples of Common Use Cases
  • 12. The Directed Cyclic Graph of Life Activities have several states Lifecycle methods are called on transitions You typically don’t need to use them all, but they are there http://code.google.com/android/reference/android/app/Activity.html
  • 13. Activity Lifecycle Three general “phases” Starting up onCreate(): first method called during lifetime, with prior state onStart()/onRestart(): signal that execution is beginning onResume(): signals that a previous pause is being undone
  • 14. Activity Lifecycle Normal execution onFreeze(): save UI state (NOT intended to save persistent data) onPause: signals loss of focus and possible impending shutdown
  • 15. Activity Lifecycle Shutting down onStop()/onDestroy(): final shutdown and process termination Not guaranteed to be called (and usually not, except on finish()...)
  • 16. Activity Lifecycle Examples Starting a Child Activity Child Activity + Process Shutdown Returning to the Home Screen Calling finish() Explicitly Displaying a Dialog Box Semi-Transparent Windows Device Sleep
  • 17. Example: Child Activity Launched Call sequence: onCreate() onStart() onResume() onFreeze() onPause() onStop() onRestart() onStart(), onResume(), ... This is the “classic” scenario.
  • 18. Example: Child Activity + Process Death Call sequence: onCreate() (empty state) onStart() onResume() onFreeze() onPause() onStop() (maybe) onDestroy() (maybe) onCreate() (with state), ... Like the basic case, but onCreate() is called again, with the state saved in onFreeze().
  • 19. Example: User Hits ’Home’ Call sequence: onCreate() onStart() onResume() onFreeze() onPause() onStop() (maybe) onDestroy() (maybe) Identical to the basic case -- that is, the Home key is not a special case.
  • 20. Example: finish() Called Call sequence: onCreate() onStart() onResume() onPause() onStop() onDestroy() Because the Activity has been explicitly told to quit and is being removed from the task (and history stack), onFreeze() is not called, and onDestroy() is reached.
  • 21. Example: Dialog Box Call sequence: onCreate() onStart() onResume() Despite appearances, dialog boxes are Views, and not Activities, so they have no effect on the owning Activity’s lifecycle.
  • 22. Example: Transparent/Non-fullscreen Child Call sequence: onCreate() onStart() onResume() onFreeze() onPause() onResume() The new partial-screen window leaves a portion of the previous window visible, so onPause() is followed by onResume() (without onStop()) when the child closes.
  • 23. Example: Device Goes to Sleep Call sequence: onCreate() onStart() onResume() onFreeze() onPause() onResume() The device going to sleep is identical to a non-fullscreen Activity being launched on top.
  • 24. Threads on Android Overview Loopers Multi-thread Considerations
  • 25. Threading Overview Each process has one thread (by default) Most components share the single thread Services and ContentProviders sometimes do not
  • 26. Threads and Loopers Each thread has a Looper to handle a message queue Events from all components are interleaved into Looper e.g. View UI events, IntentReceivers firing, etc. Loopers cannot accommodate multi-threaded access They are designed to play nicely with MessageHandlers
  • 27. Threads and Loopers APK Package Process Thread Looper Message Queue Thread External Service Calls Intent Receiver Activity Activity UI Events System Events Local Service Call
  • 28. Threads and Views Views use Looper messages to fire events Since Loopers are 1:1 with threads, the View tree is too Threads you create cannot directly touch a View But, you can create a new Looper for your own thread
  • 29. Threads in Other Contexts Services & ContentProviders sometimes run in their own threads ...but still in the same process Components can create threads, but must handle thread-safety
  • 30. Service Lifecycle Started by some other Component Either explicitly, or implicitly by binding to it Explicitly-started Services run until explicitly shut down (or killed by the system during a memory crunch) Implicitly-started Services run til the last client unbinds
  • 31. More on Processes Resource Management Processes & Security Controlling Processes
  • 32. Process Resource Management Spawned by the special “Zygote” process Process + pre-warmed Dalvik VM == responsiveness Process runs under user ID unique to system Process + User ID == security
  • 33. Processes & Security Each application is given a unique user ID No exceptions! ...except these: init, Zygote, and the main runtime Each application has direct access only to its own data Other apps’ resources are available only via defined, explicitly-exposed APIs i.e. Issuing Intents, binding to Services or ContentProviders
  • 34. Inter-Process Communication Why?? Process Transparency Binder in 30 Seconds IPC using Parcelables IPC using Bundles Android IDL
  • 35. Why?? All this process/Activity/task stuff is confusing... why? It’s all for the noble goal of efficiency (i.e. speed.) Serialization is slooow; memory transfers are slooow. CPU is not the bottleneck: think memory & bandwidth.
  • 36. Process Transparency Process management is transparent to code. ...almost. In some cases, it’s unavoidably visible. Lifecycle is seamless, but data sometimes isn’t. Specific APIs send data across process boundaries.
  • 37. IPC Overview Kernel Process Binder Bundle Intent Receiver Activity Custom Objects Service Parcel Parcelable
  • 38. Binder in 30 Seconds All IPC goes through “The Binder” Binder is implemented as a kernel module + system lib Supports sophisticated cross-process data transport The framework APIs “know” how to use Binder Generally two entry points: Bundles & Parcelables
  • 39. IPC - Parcelables A Parcelable is a class which can marshal its state to something Binder can handle -- namely, a “Parcel” Standard Java serialization has semantics Parcelables don’t need Supporting full serialization would mean wasting CPU cycles
  • 40. IPC - Bundles Bundles are typesafe containers of primitives That is, C-like primitives: ints, strings, etc. Simple data-passing APIs use Bundles Think of onFreeze() as passing data to your future self Flat structure permits optimizations like memory-mapping
  • 41. IPC - AIDL “ Android Interface Definition Language” Used to build developer-friendly APIs using Parcelables Preferred way to expose structured, complex-typed APIs Compromise between efficiency and Java usability
  • 42. Wrapping Up APKs are loose collections of components Tasks (AKA apps) are bags of component instances that span processes & APKs Managed lifecycles & IPC join the “seams”

Editor's Notes

  1. Inside Android App Framework