Evolving the Android Core with Aspects

Carlo Pescio @CarloPescio http://aspectroid.com
Carlo Pescio
Evolving the Android Core
with Aspects

Android on custom devices
New hardware:
- Ethernet
- TV tuner
- Modbus
Different target / product line / market:
- Not a “personal device”
- Keeping “user data” not an option
- USB devices should autostart apps w/out asking user
- Foreground app can’t win
- Change low memory policies

Android architecture
Looks familiar ? 

AudioManager
private void onSetWiredDeviceConnectionState(int device, int state,
String name)
{
synchronized (mConnectedDevices) {
if((state == 0) && ((device == AudioSystem.DEVICE_OUT_WIRED_HEADSET) ||
(device == AudioSystem.DEVICE_OUT_WIRED_HEADPHONE)||
(device == AudioSystem.DEVICE_OUT_LINE))) {
setBluetoothA2dpOnInt(true);
}
boolean isUsb = ((device & ~AudioSystem.DEVICE_OUT_ALL_USB) == 0) ||
(((device & AudioSystem.DEVICE_BIT_IN) != 0) &&
((device & ~AudioSystem.DEVICE_IN_ALL_USB) == 0));
handleDeviceConnection((state == 1), device, (isUsb ? name : ""));
if (state != 0) {
if ((device == AudioSystem.DEVICE_OUT_WIRED_HEADSET) ||
(device == AudioSystem.DEVICE_OUT_WIRED_HEADPHONE) ||
(device == AudioSystem.DEVICE_OUT_LINE)) {
setBluetoothA2dpOnInt(false);
}

Evolving open source code
Plugin architecture: lucky you
Or: just fork & change the damn code
pull + 3 way compare
• Feature / Artifact symmetry is broken
• Features are scattered in small patches
• Product families => multiple forks

aspectroid
exploring the
aspect-oriented
design space

Aspect Orientation
Born to address Cross-Cutting, Scattered Concerns
Error handling, Transaction handling
Not very successful so far
No design discipline evolved
Core features:
Customize behavior w/out changing code
Avoid scattering a single concern around

AspectJ crash course
Inter-type declaration:
add data members, member functions,
implement interfaces, …
aspect ScreenStateSettingsAspect
{
private CheckBoxPreference
DevelopmentSettings.stayOnPreference;
}

Pointcut (the “where”)
identify matching parts of code
(sophisticated syntax)
pointcut settingsUpdated() :
execution( private void
PowerManagerService.updateStayOnLocked(int));
pointcut adbSet() :
within(UsbDeviceManager) &&
set(boolean *.mAdbEnabled);

Advice (the when and how)
execute code before / after / around code
matched by a pointcut (join point)
after(boolean newVal) :
adbSet() && args(newVal)
{
// do something here
}

Aspect
scoping mechanism; also: artifact
privileged aspect
ScreenStatePowerAspect {
… inter-type declarations
… pointcuts
… advices
… functions
… data
}

AspectJ & Android
Apps: easy
download plugin, add AspectJ nature to project
Core: not so easy
Shell calling Makefiles calling Python calling make
Strong expectations (class files etc.)
AJC can compile regular Java files
aspects don’t need .aj extension, .java is ok

AspectJ in the core
- Create a compiler wrapper around ajc in C
- Standardizes options and behavior
- Like: files in a file list must have absolute path
- Minimal changes to the build scripts
- If you link with the aspectjrt library, I call the compiler wrapper
- Applicable to other JVM languages

# C o m m o n d e f i n i t i o n t o i n v o k e j a v a c o n t h e h o s t a n d t a r g e t . # # S o m e h i s t o r i c a l n o t e s : # - b e l o w w e w r i t e t h e l i s t o f j a v a
f i l e s t o j a v a - s o u r c e - l i s t t o a v o i d a r g u m e n t # l i s t l e n g t h p r o b l e m s w i t h C y g w i n # - w e f i l t e r o u t d u p l i c a t e j a v a f i l e n a m e s
b e c a u s e e c l i p s e ' s c o m p i l e r # d o e s n ' t l i k e t h e m . # # $ ( 1 ) : j a v a c # $ ( 2 ) : b o o t c l a s s p a t h d e f i n e c o m p i l e - j a v a $ ( h i d e ) r m - f
$ @ $ ( h i d e ) r m - r f $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) $ ( h i d e ) m k d i r - p $ ( d i r $ @ ) $ ( h i d e ) m k d i r - p
$ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) $ ( c a l l u n z i p - j a r - f i l e s , $ ( P R I V A T E _ S T A T I C _ J A V A _ L I B R A R I E S ) ,
$ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) ) $ ( c a l l d u m p - w o r d s - t o - f i l e , $ ( P R I V A T E _ J A V A _ S O U R C E S ) , $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R )
/ j a v a - s o u r c e - l i s t ) $ ( h i d e ) i f [ - d " $ ( P R I V A T E _ S O U R C E _ I N T E R M E D I A T E S _ D I R ) " ] ; t h e n f i n d
$ ( P R I V A T E _ S O U R C E _ I N T E R M E D I A T E S _ D I R ) - n a m e ' * . j a v a ' > > $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) / j a v a - s o u r c e - l i s t ; f i $ ( h i d e ) t r '
' ' n ' < $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) / j a v a - s o u r c e - l i s t | s o r t - u > $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) / j a v a -
s o u r c e - l i s t - u n i q $ ( h i d e ) i f [ - s $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) / j a v a - s o u r c e - l i s t - u n i q ] ; t h e n $ ( i f $ ( f i n d s t r i n g
a s p e c t j r t , $ ( P R I V A T E _ S T A T I C _ J A V A _ L I B R A R I E S ) ) , $ ( s u b s t j a v a c , a j c w , $ ( 1 ) ) , $ ( 1 ) ) - e n c o d i n g U T F - 8 $ ( s t r i p
$ ( P R I V A T E _ J A V A C _ D E B U G _ F L A G S ) ) $ ( i f $ ( f i n d s t r i n g t r u e , $ ( P R I V A T E _ W A R N I N G S _ E N A B L E ) ) , $ ( x l i n t _ u n c h e c k e d ) , ) $ ( 2 )
$ ( a d d p r e f i x - c l a s s p a t h , $ ( s t r i p $ ( c a l l n o r m a l i z e - p a t h - l i s t , $ ( P R I V A T E _ A L L _ J A V A _ L I B R A R I E S ) ) ) ) $ ( i f
$ ( f i n d s t r i n g t r u e , $ ( P R I V A T E _ W A R N I N G S _ E N A B L E ) ) , $ ( x l i n t _ u n c h e c k e d ) , ) - e x t d i r s " " - d $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R )
$ ( P R I V A T E _ J A V A C F L A G S ) @ $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) / j a v a - s o u r c e - l i s t - u n i q | | ( r m - r f
$ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) ; e x i t 4 1 ) f i $ ( i f $ ( P R I V A T E _ J A V A _ L A Y E R S _ F I L E ) , $ ( h i d e ) b u i l d / t o o l s / j a v a - l a y e r s . p y
$ ( P R I V A T E _ J A V A _ L A Y E R S _ F I L E ) @ $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) / j a v a - s o u r c e - l i s t - u n i q , ) $ ( h i d e ) r m - f
$ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) / j a v a - s o u r c e - l i s t $ ( h i d e ) r m - f $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) / j a v a - s o u r c e - l i s t -
u n i q $ ( i f $ ( P R I V A T E _ J A R _ E X C L U D E _ F I L E S ) , $ ( h i d e ) f i n d $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) - n a m e $ ( w o r d 1 ,
$ ( P R I V A T E _ J A R _ E X C L U D E _ F I L E S ) ) $ ( a d d p r e f i x - o - n a m e , $ ( w o r d l i s t 2 , 9 9 9 , $ ( P R I V A T E _ J A R _ E X C L U D E _ F I L E S ) ) ) | x a r g s
r m - r f ) $ ( i f $ ( P R I V A T E _ J A R _ P A C K A G E S ) , $ ( h i d e ) f i n d $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) - m i n d e p t h 1 - t y p e f
$ ( f o r e a c h p k g , $ ( P R I V A T E _ J A R _ P A C K A G E S ) , - n o t - p a t h $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) / $ ( s u b s t . , / , $ ( p k g ) ) / * )
- d e l e t e ; f i n d $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) - e m p t y - d e l e t e ) $ ( i f $ ( P R I V A T E _ J A R _ E X C L U D E _ P A C K A G E S ) , $ ( h i d e ) r m
- r f $ ( f o r e a c h p k g , $ ( P R I V A T E _ J A R _ E X C L U D E _ P A C K A G E S ) , $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) / $ ( s u b s t
. , / , $ ( p k g ) ) ) ) $ ( i f $ ( P R I V A T E _ R M T Y P E D E F S ) , $ ( h i d e ) $ ( R M T Y P E D E F S ) - v $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) ) $ ( i f
$ ( P R I V A T E _ J A R _ M A N I F E S T ) , $ ( h i d e ) s e d - e ' s / % B U I L D _ N U M B E R % / $ ( B U I L D _ N U M B E R ) / ' $ ( P R I V A T E _ J A R _ M A N I F E S T ) >
$ ( d i r $ @ ) / m a n i f e s t . m f & & j a r - c f m $ @ $ ( d i r $ @ ) / m a n i f e s t . m f - C $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) . ,
$ ( h i d e ) j a r - c f $ @ - C $ ( P R I V A T E _ C L A S S _ I N T E R M E D I A T E S _ D I R ) . ) e n d e f
AspectJ in the core
Definitions.mk -> define compile-java
$(if $(findstring aspectjrt,
$(PRIVATE_STATIC_JAVA_LIBRARIES)),
$(subst javac,ajcw,$(1)), $(1))

Does it actually work?
Add a useful feature without changing sources

How do we start?
• Explore code
• “Stay on while charging”
• UsbDeviceManager, PowerManagerService
• See ebook for details on the quest
• Tested as much as possible outside the core
• See ebook again
• Add aspects
• Make aspects robust !

Extend the Settings app
- Add a new checkbox
- Persist the checkbox state somewhere
- Show the persisted value on resume

Settings Aspect (1/3)
privileged aspect ScreenStateSettingsAspect
{
private CheckBoxPreference
DevelopmentSettings.stayOnPreference;
pointcut onCreateExecuted() :
execution(public void DevelopmentSettings.onCreate(..)) ;
pointcut onResumeExecuted() :
execution(public void DevelopmentSettings.onResume(..)) ;

after(DevelopmentSettings self):onCreateExecuted()&&target(self)
{
final Context ctx = self.getActivity();
PreferenceCategory targetCategory = (PreferenceCategory)self.
findPreference(DevelopmentSettings.DEBUG_DEBUGGING_CATEGORY_KEY);
self.stayOnPreference = new CheckBoxPreference(ctx);
self.stayOnPreference.setKey("aspectroidScreenState");
self.stayOnPreference.setTitle("Prevent sleeping while debugging");
self.stayOnPreference.setSummary("only for usb debugging");
targetCategory.addPreference(self.stayOnPreference);
self.stayOnPreference.setOnPreferenceChangeListener(
new OnPreferenceChangeListener() {
public boolean onPreferenceChange(
Preference preference, Object newValue) {
API.persistPreference(ctx, (Boolean) newValue);
return true;
} }); }

after(DevelopmentSettings self) : onResumeExecuted() &&
target(self)
{
final Context ctx = self.getActivity();
boolean on = API.retrievePreference(ctx);
self.stayOnPreference.setChecked(on);
}
}


Am I debugging?
- USB debugging enabled
- USB connected to a computer
- Not just charging
- Not in OTG / host mode

UsbDeviceManager
See ebook for details

USB Aspect (1/3)
aspect ScreenStateUsbAspect {
boolean adbEnabled = false;
boolean adbConnected = false;
Context context;
before(Context ctx):
execution(UsbDeviceManager.new(Context)) && args( ctx ) {
context = ctx;
}

USB Aspect (2/3)
pointcut adbSet() : within(UsbDeviceManager) &&
set(boolean *.mAdbEnabled);
after(boolean newVal) : adbSet() && args(newVal) {
adbEnabled = newVal;
checkAdbOn();
}

USB Aspect (3/3)
pointcut connectedSet() : within(UsbDeviceManager) &&
set(boolean *.mConnected);
after(boolean newVal) : connectedSet() && args(newVal) {
adbConnected = newVal;
checkAdbOn();
}
void checkAdbOn() {
boolean isOn = adbEnabled && adbConnected;
API.storeAdbState(context, isOn);
}
} 

PowerManagerService (1)
public void systemReady(IAppOpsService appOps) {
// …
final ContentResolver resolver =
mContext.getContentResolver();
// …
resolver.registerContentObserver(
Settings.Global.getUriFor(
Settings.Global.STAY_ON_WHILE_PLUGGED_IN),
false, mSettingsObserver, UserHandle.USER_ALL);
//…
updateSettingsLocked();
//…
}

PowerManagerService (2)
private void updateStayOnLocked(int dirty) {
if( (dirty &
(DIRTY_BATTERY_STATE | DIRTY_SETTINGS)) != 0 ) {
final boolean wasStayOn = mStayOn;
if( mStayOnWhilePluggedInSetting != 0 && !
isMaximumScreenOffTimeoutFromDeviceAdminEnforcedLocked()){
mStayOn = mBatteryManagerInternal.isPowered(
mStayOnWhilePluggedInSetting);
}
else {
mStayOn = false;
}
if( mStayOn != wasStayOn ) {
mDirty |= DIRTY_STAY_ON;
}
}
}

Power Aspect (1/2)
privileged aspect ScreenStatePowerAspect {
pointcut systemReadyExecuted() : execution(
public void PowerManagerService.systemReady(..));
after(PowerManagerService self) :
systemReadyExecuted() && target(self) {
API.registerScreenOnObserver(self.mContext,
self.mSettingsObserver);
}

Power Aspect (2/2)
pointcut settingsUpdated() : execution(
private void PowerManagerService.updateStayOnLocked(int));
void around(PowerManagerService self, int dirty) :
settingsUpdated() && args(dirty) && target(self) {
proceed(self, dirty);
boolean keepOn = API.shouldKeepScrenOn(self.mContext);
if( keepOn && !self.mStayOn ) {
self.mStayOn = true;
self.mDirty |= PowerManagerService.DIRTY_STAY_ON;
}
}
} 

Is it worth doing?
Core of science: experiments & falsifiability
port the aspects
from 5.0 to 6.0
Hardest candidate: PowerManagerService
Power mgmt changed in 6.0 to include Doze
Also the most fragile of my aspects

File compare map
5.0 vs 6.0

Still working?
Class: PowerManagerService still there
Member functions:
systemReady
updateStayOnLocked
Data members:
mContext, mSettingsObserver,
mStayOn, mDirty
still there, same
purpose & signature
still there,
same purpose


Conclusions: core benefits
- Feature traceability and selection
- Features as artifacts, not changesets
- Simplified maintenance / porting

Challenges
Aspectj is still “one project at a time”
Core is not aspect-friendly
very long methods, hard to pointcut inside
Core itself would actually benefit A LOT from AOP
ok google? 
Rejection of the unfamiliar
3-way compare is a known pain
branch X product in a product family is a known pain

Why learning this stuff?
- New ways to solve problems
Customize FOSS by adding code
- New ways to structure your code
aspect-friendly
mix OOP and AOP
- In the end:
you maintain LESS code
[good] aspects can be quite resilient

Get in touch
carlo.pescio@gmail.com
@CarloPescio
http://eptacom.net
http://aspectroid.com

Evolving the Android Core with Aspects

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