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Android Internals
for Developers
Effie Barak
(@CodingChick)
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Android is based on Linux
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Basic Android Architecture
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Kernel
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a process is an instance of a
computer program that is
being executed
-Wikipedia
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Kernel
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Kernel
1. OOM Killer
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Kernel
1. OOM Killer
2. Wakelocks
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Init
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Init
4 First user space process
4 The root of all other processes
4 Spawns everything else
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Zygote
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Zygote
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fork() and exec()
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Processes and UIDs
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Binder IPC
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ConnectivityManager connectivityManager = (ConnectivityManager)
MyApplication.getInstance().getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo info = connectivityManager.getActiveNetworkInfo();
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public class ConnectivityManager {
private final IConnectivityManager mService;
...
public NetworkInfo getActiveNetworkInfo() {
try {
return mService.getActiveNetworkInfo();
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
...
}
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interface IConnectivityManager
{
Network getActiveNetwork();
Network getActiveNetworkForUid(int uid, boolean ignoreBlocked);
NetworkInfo getActiveNetworkInfo();
NetworkInfo getActiveNetworkInfoForUid(int uid, boolean ignoreBlocked);
NetworkInfo getNetworkInfo(int networkType);
NetworkInfo getNetworkInfoForUid(in Network network, int uid, boolean ignoreBlocked);
NetworkInfo[] getAllNetworkInfo();
Network getNetworkForType(int networkType);
Network[] getAllNetworks();
NetworkCapabilities[] getDefaultNetworkCapabilitiesForUser(int userId);
...
}
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!
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Binder
public boolean transact(int code, Parcel data, Parcel reply, int flags)
protected boolean onTransact(int code, Parcel data, Parcel reply,
int flags) throws RemoteException {
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AIDL tool
4 A bridge between the actual method and the
transaction
4 Generates two parts- Proxy and Stub
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IConnectivityManager.java- Proxy
@Override public android.net.Network getActiveNetwork() throws android.os.RemoteException
{
android.os.Parcel _data = android.os.Parcel.obtain();
android.os.Parcel _reply = android.os.Parcel.obtain();
android.net.Network _result;
try {
_data.writeInterfaceToken(DESCRIPTOR);
mRemote.transact(Stub.TRANSACTION_getActiveNetwork, _data, _reply, 0);
_reply.readException();
if ((0!=_reply.readInt())) {
_result = android.net.Network.CREATOR.createFromParcel(_reply);
}
else {
_result = null;
}
}
finally {
_reply.recycle();
_data.recycle();
}
return _result;
}
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IConnectivityManager.java- Stub
public static abstract class Stub extends android.os.Binder implements android.net.IConnectivityManager
{
...
@Override public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException
{
switch (code)
{
...
case TRANSACTION_getActiveNetwork:
{
data.enforceInterface(DESCRIPTOR);
android.net.Network _result = this.getActiveNetwork();
reply.writeNoException();
if ((_result!=null)) {
reply.writeInt(1);
_result.writeToParcel(reply, android.os.Parcelable.PARCELABLE_WRITE_RETURN_VALUE);
}
else {
reply.writeInt(0);
}
return true;
}
...
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public class ConnectivityService extends IConnectivityManager.Stub
implements PendingIntent.OnFinished {
...
@Override
public NetworkInfo getActiveNetworkInfo() {
enforceAccessPermission();
final int uid = Binder.getCallingUid();
final NetworkState state = getUnfilteredActiveNetworkState(uid);
filterNetworkStateForUid(state, uid, false);
maybeLogBlockedNetworkInfo(state.networkInfo, uid);
return state.networkInfo;
}
private Network getActiveNetworkForUidInternal(final int uid, boolean ignoreBlocked) {
final int user = UserHandle.getUserId(uid);
int vpnNetId = NETID_UNSET;
synchronized (mVpns) {
final Vpn vpn = mVpns.get(user);
if (vpn != null && vpn.appliesToUid(uid)) vpnNetId = vpn.getNetId();
}
NetworkAgentInfo nai;
if (vpnNetId != NETID_UNSET) {
synchronized (mNetworkForNetId) {
nai = mNetworkForNetId.get(vpnNetId);
}
if (nai != null) return nai.network;
}
nai = getDefaultNetwork();
if (nai != null
&& isNetworkWithLinkPropertiesBlocked(nai.linkProperties, uid, ignoreBlocked)) {
nai = null;
}
return nai != null ? nai.network : null;
}
}
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NetworkInfo info = connectivityManager.getActiveNetworkInfo();
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System server
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Managers in system server
4 Activity Manager
4 Window manager
4 Package manager
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Window Manager
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Responsiblities
4 Keep track of window activities
4 Visibilty
4 Transitions and animations
4 Good UI experience
4 System level gestures
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Package manager
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Responsiblities
4 Installation of new applications
4 Resolving the correct activity
4 Creating a list of possible activites who can match a
certain filter
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Activities manager
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Responsiblities
4 Managing applications (activities, services, content
providers)
4 Managing lifecycles
4 Managing the actual user
4 Memory trimming, managing excessive power usage
4 Handling configuration changes
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Managing applications
4 Stack
4 Task (process >= 1)
4 Activity
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Processes and Activities
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Managing Lifecycle
4 One Resumed app
4 Has surface/ visible
4 Thread priority/ cpu affinity
4 Gain or lose certain permissions
4 OOM adjustments
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OOM Adjustments
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oom_adj score:
4 applyOomAdjLocked and computeOomAdjLocked
4 AMS gets notified back and removes the Activities
related
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Starting a new activity
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public final class ActivityManagerService ... {
@Override
public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
...
return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent,
resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
profilerInfo, null, null, bOptions, false, userId, null, null);
}
}
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class ActivityStarter {
final int startActivityMayWait(IApplicationThread caller, int callingUid,
String callingPackage, Intent intent, String resolvedType,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
IBinder resultTo, String resultWho, int requestCode, int startFlags,
ProfilerInfo profilerInfo, IActivityManager.WaitResult outResult, Configuration config,
Bundle bOptions, boolean ignoreTargetSecurity, int userId,
IActivityContainer iContainer, TaskRecord inTask) {
...
ResolveInfo rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId);
}
}
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1. Find the correct activity
public class PackageManagerService extends IPackageManager.Stub {
// Explicit activity
@Override
public ActivityInfo getActivityInfo(ComponentName component, int flags, int userId) {
...
}
// Implicit activity
@Override
public ResolveInfo resolveIntent(Intent intent, String resolvedType,
int flags, int userId) {
...
}
}
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2. Create a new ActivityRecord
final int startActivityLocked(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,
String callingPackage, int realCallingPid, int realCallingUid, int startFlags,
ActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
ActivityRecord[] outActivity, ActivityStackSupervisor.ActivityContainer container,
TaskRecord inTask) {
ActivityRecord r = new ActivityRecord(mService, callerApp, callingUid, callingPackage,
intent, resolvedType, aInfo, mService.mConfiguration, resultRecord, resultWho,
requestCode, componentSpecified, voiceSession != null, mSupervisor, container,
options, sourceRecord);
}
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3. Talk to Window Manager to do
things
mWindowManager.prepareAppTransition(newTask
? r.mLaunchTaskBehind
? TRANSIT_TASK_OPEN_BEHIND
: TRANSIT_TASK_OPEN
: TRANSIT_ACTIVITY_OPEN, keepCurTransition);
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4. Resume the top activity
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
if (mStackSupervisor.inResumeTopActivity) {
...
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5. Decide if we already have a process
to use
void startSpecificActivityLocked(ActivityRecord r,
boolean andResume, boolean checkConfig) {
// Is this activity's application already running?
ProcessRecord app = mService.getProcessRecordLocked(r.processName,
r.info.applicationInfo.uid, true);
...
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6. Create a new process if we don't
private final void startProcessLocked(ProcessRecord app, String hostingType,
String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
...
Process.ProcessStartResult startResult = Process.start(entryPoint,
app.processName, uid, uid, gids, debugFlags, mountExternal,
app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet,
app.info.dataDir, entryPointArgs);
...
}
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7. Start a new process with Zygote
private static ProcessStartResult startViaZygote(final String processClass,
final String niceName,
final int uid, final int gid,
final int[] gids,
int debugFlags, int mountExternal,
int targetSdkVersion,
String seInfo,
String abi,
String instructionSet,
String appDataDir,
String[] extraArgs)
throws ZygoteStartFailedEx {
...
return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi), argsForZygote);
}
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8. When Zygote "calls back"- attach
the correct activity
private final boolean attachApplicationLocked(IApplicationThread thread,
int pid) {
...
thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
app.instrumentationUiAutomationConnection, testMode,
mBinderTransactionTrackingEnabled, enableTrackAllocation,
isRestrictedBackupMode || !normalMode, app.persistent,
new Configuration(mConfiguration), app.compat,
getCommonServicesLocked(app.isolated),
mCoreSettingsObserver.getCoreSettingsLocked());
...
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Thanks and credits
4 Dave Smith
4 Tim Murray
4 Dianne Hackborn
4 Rob Carr