源码基于：Android5.0.0
Activity作为很重要的一个组件，其内部工作过程系统也做了很多的封装，这种封装使得启动一个Activity变的异常简单：
Intent intent = new Intent(this, TestActivity.class);
startActivity(intent);
启动一个具体的Activity，然后新Activity就会被系统启动并呈现在用户眼前，这个过程对于Android的开发者来说在普通不过了，也被看做是理所当然的事情。Android作为一个优秀的基于Linux的操作系统，其内部一定有很多值得我们学习借鉴的地方。
startActivity方法有好几种重载方式，但是他们都最终会调用startActivityForResult，实现如下：
/**
* Launch an activity for which you would like a result when it finished.
* When this activity exits, your
* onActivityResult() method will be called with the given requestCode.
* Using a negative requestCode is the same as calling
* {@link #startActivity} (the activity is not launched as a sub-activity).
*
* <p>Note that this method should only be used with Intent protocols
* that are defined to return a result. In other protocols (such as
* {@link Intent#ACTION_MAIN} or {@link Intent#ACTION_VIEW}), you may
* not get the result when you expect. For example, if the activity you
* are launching uses {@link Intent#FLAG_ACTIVITY_NEW_TASK}, it will not
* run in your task and thus you will immediately receive a cancel result.
*
* <p>As a special case, if you call startActivityForResult() with a requestCode
* >= 0 during the initial onCreate(Bundle savedInstanceState)/onResume() of your
* activity, then your window will not be displayed until a result is
* returned back from the started activity. This is to avoid visible
* flickering when redirecting to another activity.
*
* <p>This method throws {@link android.content.ActivityNotFoundException}
* if there was no Activity found to run the given Intent.
*
* @param intent The intent to start.
* @param requestCode If >= 0, this code will be returned in
* onActivityResult() when the activity exits.
* @param options Additional options for how the Activity should be started.
* See {@link android.content.Context#startActivity(Intent, Bundle)}
* Context.startActivity(Intent, Bundle)} for more details.
*
* @throws android.content.ActivityNotFoundException
*
* @see #startActivity
*/
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
@Nullable Bundle options) {
if (mParent == null) {
options = transferSpringboardActivityOptions(options);
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this,
intent, requestCode, options);
if (ar != null) {
mMainThread.sendActivityResult(
mToken, mEmbeddedID, requestCode, ar.getResultCode(),
ar.getResultData());
}
if (requestCode >= 0) {
// If this start is requesting a result, we can avoid making
// the activity visible until the result is received. Setting
// this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
// activity hidden during this time, to avoid flickering.
// This can only be done when a result is requested because
// that guarantees we will get information back when the
// activity is finished, no matter what happens to it.
mStartedActivity = true;
}
cancelInputsAndStartExitTransition(options);
// TODO Consider clearing/flushing other event sources and events for child windows.
} else {
if (options != null) {
mParent.startActivityFromChild(this, intent, requestCode, options);
} else {
// Note we want to go through this method for compatibility with
// existing applications that may have overridden it.
mParent.startActivityFromChild(this, intent, requestCode);
}
}
}
在上面的代码中，我们只需要关注mParent==null这部分逻辑就可以，mParent代表的是ActivityGroup，ActivityGroup最开始被用来在一个界面中嵌入多个子Activity，但是其在API13中已经被废弃了，系统推荐采用Fragment代替ActivityGroup。上面的代码需要注意mMainThread.getApplicationThread()这个参数，它的类型是ApplicationThread，ApplicationThread是ActivityThread的一个内部类，通过后面的分析可以发现，ApplicationThread和ActivityThread在Activity的启动过程中发挥着很重要的作用。
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
IApplicationThread whoThread = (IApplicationThread) contextThread;
Uri referrer = target != null ? target.onProvideReferrer() : null;
if (referrer != null) {
intent.putExtra(Intent.EXTRA_REFERRER, referrer);
}
if (mActivityMonitors != null) {
synchronized (mSync) {
final int N = mActivityMonitors.size();
for (int i=0; i<N; i++) {
final ActivityMonitor am = mActivityMonitors.get(i);
ActivityResult result = null;
if (am.ignoreMatchingSpecificIntents()) {
result = am.onStartActivity(intent);
}
if (result != null) {
am.mHits++;
return result;
} else if (am.match(who, null, intent)) {
am.mHits++;
if (am.isBlocking()) {
return requestCode >= 0 ? am.getResult() : null;
}
break;
}
}
}
}
try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess(who);
int result = ActivityManager.getService()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target != null ? target.mEmbeddedID : null,
requestCode, 0, null, options);
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
throw new RuntimeException("Failure from system", e);
}
return null;
}
从上面的代码可以看出，启动Activity真正的实现是由ActivityManagerNative.getDefault()的startActivity方法来完成。ActivityManagerService继承自ActivityMangerNative，而ActivityMangerNative继承自Binder，因此ActivityManagerService也是Binder，它是IActivityManager的具体实现。由于ActivityMangerNative.getDefault()其实是一个IActivityManager类型的Binder对象，因此它的具体实现是AMS。可以发现，在ActivityManagerNative中，AMS这个Binder对象采用单例模式对外提供，Singleton是一个单例的封装类，第一次调用它的get方法时它会通过create方法来初始化AMS这个Binder对象，在后续的调用中则直接返回之前创建的对象，这个过程的源码如下：
public static IActivityManager getService() {
return IActivityManagerSingleton.get();
}
private static final Singleton<IActivityManager> IActivityManagerSingleton =
new Singleton<IActivityManager>() {
@Override
protected IActivityManager create() {
final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
final IActivityManager am = IActivityManager.Stub.asInterface(b);
return am;
}
};
从上面的分析可以知道，Activity由ActivityManagerNative.getDefault()来启动，而ActivityManagerNative.getDefault()是加上是AMS，因此Activity的启动过程又转移到了AMS中，为了继续分析这个过程，只需要查看AMS的startActivity方法接口。在分析AMS的startActivity之前，我们需要先看一下Instrumentation的execStartActivity方法，其中有一行代码：checkStartActivityResult(result, intent)，直观上看起来这个方法的作用是在检查Activity的结果
public static void checkStartActivityResult(int res, Object intent) {
if (!ActivityManager.isStartResultFatalError(res)) {
return;
}
switch (res) {
case ActivityManager.START_INTENT_NOT_RESOLVED:
case ActivityManager.START_CLASS_NOT_FOUND:
if (intent instanceof Intent && ((Intent)intent).getComponent() != null)
throw new ActivityNotFoundException(
"Unable to find explicit activity class "
+ ((Intent)intent).getComponent().toShortString()
+ "; have you declared this activity in your AndroidManifest.xml?");
throw new ActivityNotFoundException(
"No Activity found to handle " + intent);
case ActivityManager.START_PERMISSION_DENIED:
throw new SecurityException("Not allowed to start activity "
+ intent);
case ActivityManager.START_FORWARD_AND_REQUEST_CONFLICT:
throw new AndroidRuntimeException(
"FORWARD_RESULT_FLAG used while also requesting a result");
case ActivityManager.START_NOT_ACTIVITY:
throw new IllegalArgumentException(
"PendingIntent is not an activity");
case ActivityManager.START_NOT_VOICE_COMPATIBLE:
throw new SecurityException(
"Starting under voice control not allowed for: " + intent);
case ActivityManager.START_VOICE_NOT_ACTIVE_SESSION:
throw new IllegalStateException(
"Session calling startVoiceActivity does not match active session");
case ActivityManager.START_VOICE_HIDDEN_SESSION:
throw new IllegalStateException(
"Cannot start voice activity on a hidden session");
case ActivityManager.START_ASSISTANT_NOT_ACTIVE_SESSION:
throw new IllegalStateException(
"Session calling startAssistantActivity does not match active session");
case ActivityManager.START_ASSISTANT_HIDDEN_SESSION:
throw new IllegalStateException(
"Cannot start assistant activity on a hidden session");
case ActivityManager.START_CANCELED:
throw new AndroidRuntimeException("Activity could not be started for "
+ intent);
default:
throw new AndroidRuntimeException("Unknown error code "
+ res + " when starting " + intent);
}
}
public static final boolean isStartResultFatalError(int result) {
return FIRST_START_FATAL_ERROR_CODE <= result && result <= LAST_START_FATAL_ERROR_CODE;
}
从上面的代码可以看出，checkStartActivityResult的作用很明显，就是检查启动Activity的结果，当无法正确的启动一个Activity时，这个方法会跑出异常信息，其中最熟悉不过的就是Unable to find explicit activity class xxx; have you declared this activity in your AndroidManifest.xml?，当待启动的Activity没有在AndroidManifest中注册时，就会抛出这个异常。
接着我们看AMS的startActivity方法：
@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle options) {
return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, options,
UserHandle.getCallingUserId());
}
@Override
public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle options, int userId) {
enforceNotIsolatedCaller("startActivity");
userId = handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(), userId,
false, ALLOW_FULL_ONLY, "startActivity", null);
// TODO: Switch to user app stacks here.
return mStackSupervisor.startActivityMayWait(caller, -1, callingPackage, intent,
resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
profilerInfo, null, null, options, userId, null, null);
}
可以看出，Activity的启动过程又转移到了ActivityStackSupervisor的startActivityMayWait中，在startActivityMayWait中又调用了startActivityLocked方法，然后startActivityLocked又调用了startActivityUnchecked方法，接着startActivityUnchecked又调用了ActivityStack的resumeTopActivitiesLocked方法，这个时候启动过程已经从ActivityStackSupervisor转移到了ActivityStack。
ActivityStack的resumeTopActivitiesLocked方法实现：
final boolean resumeTopActivityLocked(ActivityRecord prev, Bundle options) {
if (inResumeTopActivity) {
// Don't even start recursing.
return false;
}
boolean result = false;
try {
// Protect against recursion.
inResumeTopActivity = true;
result = resumeTopActivityInnerLocked(prev, options);
} finally {
inResumeTopActivity = false;
}
return result;
}
从上面的代码可以看出，resumeTopActivityLocked调用了resumeTopActivityInnerLocked方法，resumeTopActivityInnerLocked又调用了ActivityStackSupervisor.startSpecificActivityLocked方法，startSpecificActivityLocked源码如下：
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);
r.task.stack.setLaunchTime(r);
if (app != null && app.thread != null) {
try {
if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
|| !"android".equals(r.info.packageName)) {
// Don't add this if it is a platform component that is marked
// to run in multiple processes, because this is actually
// part of the framework so doesn't make sense to track as a
// separate apk in the process.
app.addPackage(r.info.packageName, r.info.applicationInfo.versionCode,
mService.mProcessStats);
}
realStartActivityLocked(r, app, andResume, checkConfig);
return;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting activity "
+ r.intent.getComponent().flattenToShortString(), e);
}
// If a dead object exception was thrown -- fall through to
// restart the application.
}
mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
"activity", r.intent.getComponent(), false, false, true);
}
从上面的代码可以看出，startSpecificActivityLocked调用了realStartActivityLocked中有这样一段代码：
app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),
r.compat, r.task.voiceInteractor, app.repProcState, r.icicle, r.persistentState,
results, newIntents, !andResume, mService.isNextTransitionForward(),
profilerInfo);
其中app.thread的类型为IApplicationThread，声明如下：
/**
* System private API for communicating with the application. This is given to
* the activity manager by an application when it starts up, for the activity
* manager to tell the application about things it needs to do.
*
* {@hide}
*/
public interface IApplicationThread extends IInterface {
void schedulePauseActivity(IBinder token, boolean finished, boolean userLeaving,
int configChanges, boolean dontReport) throws RemoteException;
void scheduleStopActivity(IBinder token, boolean showWindow,
int configChanges) throws RemoteException;
void scheduleWindowVisibility(IBinder token, boolean showWindow) throws RemoteException;
void scheduleSleeping(IBinder token, boolean sleeping) throws RemoteException;
void scheduleResumeActivity(IBinder token, int procState, boolean isForward, Bundle resumeArgs)
throws RemoteException;
void scheduleSendResult(IBinder token, List<ResultInfo> results) throws RemoteException;
void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,
IVoiceInteractor voiceInteractor, int procState, Bundle state,
PersistableBundle persistentState, List<ResultInfo> pendingResults,
List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,
ProfilerInfo profilerInfo) throws RemoteException;
void scheduleRelaunchActivity(IBinder token, List<ResultInfo> pendingResults,
List<Intent> pendingNewIntents, int configChanges,
boolean notResumed, Configuration config) throws RemoteException;
void scheduleNewIntent(List<Intent> intent, IBinder token) throws RemoteException;
void scheduleDestroyActivity(IBinder token, boolean finished,
int configChanges) throws RemoteException;
void scheduleReceiver(Intent intent, ActivityInfo info, CompatibilityInfo compatInfo,
int resultCode, String data, Bundle extras, boolean sync,
int sendingUser, int processState) throws RemoteException;
static final int BACKUP_MODE_INCREMENTAL = 0;
static final int BACKUP_MODE_FULL = 1;
static final int BACKUP_MODE_RESTORE = 2;
static final int BACKUP_MODE_RESTORE_FULL = 3;
void scheduleCreateBackupAgent(ApplicationInfo app, CompatibilityInfo compatInfo,
int backupMode) throws RemoteException;
void scheduleDestroyBackupAgent(ApplicationInfo app, CompatibilityInfo compatInfo)
throws RemoteException;
void scheduleCreateService(IBinder token, ServiceInfo info,
CompatibilityInfo compatInfo, int processState) throws RemoteException;
void scheduleBindService(IBinder token,
Intent intent, boolean rebind, int processState) throws RemoteException;
void scheduleUnbindService(IBinder token,
Intent intent) throws RemoteException;
void scheduleServiceArgs(IBinder token, boolean taskRemoved, int startId,
int flags, Intent args) throws RemoteException;
void scheduleStopService(IBinder token) throws RemoteException;
static final int DEBUG_OFF = 0;
static final int DEBUG_ON = 1;
static final int DEBUG_WAIT = 2;
void bindApplication(String packageName, ApplicationInfo info, List<ProviderInfo> providers,
ComponentName testName, ProfilerInfo profilerInfo, Bundle testArguments,
IInstrumentationWatcher testWatcher, IUiAutomationConnection uiAutomationConnection,
int debugMode, boolean openGlTrace, boolean restrictedBackupMode, boolean persistent,
Configuration config, CompatibilityInfo compatInfo, Map<String, IBinder> services,
Bundle coreSettings) throws RemoteException;
void scheduleExit() throws RemoteException;
void scheduleSuicide() throws RemoteException;
void scheduleConfigurationChanged(Configuration config) throws RemoteException;
void updateTimeZone() throws RemoteException;
void clearDnsCache() throws RemoteException;
void setHttpProxy(String proxy, String port, String exclList,
Uri pacFileUrl) throws RemoteException;
void processInBackground() throws RemoteException;
void dumpService(FileDescriptor fd, IBinder servicetoken, String[] args)
throws RemoteException;
void dumpProvider(FileDescriptor fd, IBinder servicetoken, String[] args)
throws RemoteException;
void scheduleRegisteredReceiver(IIntentReceiver receiver, Intent intent,
int resultCode, String data, Bundle extras, boolean ordered,
boolean sticky, int sendingUser, int processState) throws RemoteException;
void scheduleLowMemory() throws RemoteException;
void scheduleActivityConfigurationChanged(IBinder token) throws RemoteException;
void profilerControl(boolean start, ProfilerInfo profilerInfo, int profileType)
throws RemoteException;
void dumpHeap(boolean managed, String path, ParcelFileDescriptor fd)
throws RemoteException;
void setSchedulingGroup(int group) throws RemoteException;
static final int PACKAGE_REMOVED = 0;
static final int EXTERNAL_STORAGE_UNAVAILABLE = 1;
void dispatchPackageBroadcast(int cmd, String[] packages) throws RemoteException;
void scheduleCrash(String msg) throws RemoteException;
void dumpActivity(FileDescriptor fd, IBinder servicetoken, String prefix, String[] args)
throws RemoteException;
void setCoreSettings(Bundle coreSettings) throws RemoteException;
void updatePackageCompatibilityInfo(String pkg, CompatibilityInfo info) throws RemoteException;
void scheduleTrimMemory(int level) throws RemoteException;
void dumpMemInfo(FileDescriptor fd, Debug.MemoryInfo mem, boolean checkin, boolean dumpInfo,
boolean dumpDalvik, String[] args) throws RemoteException;
void dumpGfxInfo(FileDescriptor fd, String[] args) throws RemoteException;
void dumpDbInfo(FileDescriptor fd, String[] args) throws RemoteException;
void unstableProviderDied(IBinder provider) throws RemoteException;
void requestAssistContextExtras(IBinder activityToken, IBinder requestToken, int requestType)
throws RemoteException;
void scheduleTranslucentConversionComplete(IBinder token, boolean timeout)
throws RemoteException;
void scheduleOnNewActivityOptions(IBinder token, ActivityOptions options)
throws RemoteException;
void setProcessState(int state) throws RemoteException;
void scheduleInstallProvider(ProviderInfo provider) throws RemoteException;
void updateTimePrefs(boolean is24Hour) throws RemoteException;
void scheduleCancelVisibleBehind(IBinder token) throws RemoteException;
void scheduleBackgroundVisibleBehindChanged(IBinder token, boolean enabled) throws RemoteException;
void scheduleEnterAnimationComplete(IBinder token) throws RemoteException;
String descriptor = "android.app.IApplicationThread";
int SCHEDULE_PAUSE_ACTIVITY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION;
int SCHEDULE_STOP_ACTIVITY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+2;
int SCHEDULE_WINDOW_VISIBILITY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+3;
int SCHEDULE_RESUME_ACTIVITY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+4;
int SCHEDULE_SEND_RESULT_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+5;
int SCHEDULE_LAUNCH_ACTIVITY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+6;
int SCHEDULE_NEW_INTENT_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+7;
int SCHEDULE_FINISH_ACTIVITY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+8;
int SCHEDULE_RECEIVER_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+9;
int SCHEDULE_CREATE_SERVICE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+10;
int SCHEDULE_STOP_SERVICE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+11;
int BIND_APPLICATION_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+12;
int SCHEDULE_EXIT_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+13;
int SCHEDULE_CONFIGURATION_CHANGED_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+15;
int SCHEDULE_SERVICE_ARGS_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+16;
int UPDATE_TIME_ZONE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+17;
int PROCESS_IN_BACKGROUND_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+18;
int SCHEDULE_BIND_SERVICE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+19;
int SCHEDULE_UNBIND_SERVICE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+20;
int DUMP_SERVICE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+21;
int SCHEDULE_REGISTERED_RECEIVER_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+22;
int SCHEDULE_LOW_MEMORY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+23;
int SCHEDULE_ACTIVITY_CONFIGURATION_CHANGED_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+24;
int SCHEDULE_RELAUNCH_ACTIVITY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+25;
int SCHEDULE_SLEEPING_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+26;
int PROFILER_CONTROL_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+27;
int SET_SCHEDULING_GROUP_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+28;
int SCHEDULE_CREATE_BACKUP_AGENT_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+29;
int SCHEDULE_DESTROY_BACKUP_AGENT_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+30;
int SCHEDULE_ON_NEW_ACTIVITY_OPTIONS_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+31;
int SCHEDULE_SUICIDE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+32;
int DISPATCH_PACKAGE_BROADCAST_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+33;
int SCHEDULE_CRASH_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+34;
int DUMP_HEAP_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+35;
int DUMP_ACTIVITY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+36;
int CLEAR_DNS_CACHE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+37;
int SET_HTTP_PROXY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+38;
int SET_CORE_SETTINGS_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+39;
int UPDATE_PACKAGE_COMPATIBILITY_INFO_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+40;
int SCHEDULE_TRIM_MEMORY_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+41;
int DUMP_MEM_INFO_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+42;
int DUMP_GFX_INFO_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+43;
int DUMP_PROVIDER_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+44;
int DUMP_DB_INFO_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+45;
int UNSTABLE_PROVIDER_DIED_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+46;
int REQUEST_ASSIST_CONTEXT_EXTRAS_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+47;
int SCHEDULE_TRANSLUCENT_CONVERSION_COMPLETE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+48;
int SET_PROCESS_STATE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+49;
int SCHEDULE_INSTALL_PROVIDER_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+50;
int UPDATE_TIME_PREFS_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+51;
int CANCEL_VISIBLE_BEHIND_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+52;
int BACKGROUND_VISIBLE_BEHIND_CHANGED_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+53;
int ENTER_ANIMATION_COMPLETE_TRANSACTION = IBinder.FIRST_CALL_TRANSACTION+54;
}
因为它继承自IInterface，所以它是一个Binder类型的接口，从IApplicationThread声明的接口方法可以看出，其内部包含了大量的启动、停止Activity的接口，此外还包含了启动和停止服务的接口。
private class ApplicationThread extends ApplicationThreadNative
public abstract class ApplicationThreadNative extends Binder implements IApplicationThread
由上可以看出，ApplicationThread继承了ApplicationThreadNative，而ApplicationThreadNative继承了Binder并实现了IApplicationThread，ApplicationThreadNative的作用其实和系统为AIDL文件生成的类是一样的。在ApplicationThreadNative内部，还有一个ApplicationThreadProxy类，这个类的实现其实也是系统为AIDL文件自动生成的代理类。种种迹象表明，ApplicationThreadNative就是IApplicationThread的实现者，由于ApplicationThreadNative被定义为抽象类，因此ApplicationThread就成了IApplicationThread最终的实现者。
class ApplicationThreadProxy implements IApplicationThread {
private final IBinder mRemote;
public ApplicationThreadProxy(IBinder remote) {
mRemote = remote;
}
public final IBinder asBinder() {
return mRemote;
}
public final void schedulePauseActivity(IBinder token, boolean finished,
boolean userLeaving, int configChanges, boolean dontReport) throws RemoteException {
Parcel data = Parcel.obtain();
data.writeInterfaceToken(IApplicationThread.descriptor);
data.writeStrongBinder(token);
data.writeInt(finished ? 1 : 0);
data.writeInt(userLeaving ? 1 :0);
data.writeInt(configChanges);
data.writeInt(dontReport ? 1 : 0);
mRemote.transact(SCHEDULE_PAUSE_ACTIVITY_TRANSACTION, data, null,
IBinder.FLAG_ONEWAY);
data.recycle();
}
...
}
绕了一圈最终回来了ApplicationThread，ApplicationThread最终通过scheduleLaunchActivity启动Activity：
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,
IVoiceInteractor voiceInteractor, int procState, Bundle state,
PersistableBundle persistentState, List<ResultInfo> pendingResults,
List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,
ProfilerInfo profilerInfo) {
updateProcessState(procState, false);
ActivityClientRecord r = new ActivityClientRecord();
r.token = token;
r.ident = ident;
r.intent = intent;
r.voiceInteractor = voiceInteractor;
r.activityInfo = info;
r.compatInfo = compatInfo;
r.state = state;
r.persistentState = persistentState;
r.pendingResults = pendingResults;
r.pendingIntents = pendingNewIntents;
r.startsNotResumed = notResumed;
r.isForward = isForward;
r.profilerInfo = profilerInfo;
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r);
}
在ApplicationThread中，scheduleLaunchActivity的实现很简单，就是发送 一个启动Activity的消息交由Handler处理，这个Handler有着很简单的名字：H。sendMessage的作用是发送一个消息给H处理：
private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
if (DEBUG_MESSAGES) Slog.v(
TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
+ ": " + arg1 + " / " + obj);
Message msg = Message.obtain();
msg.what = what;
msg.obj = obj;
msg.arg1 = arg1;
msg.arg2 = arg2;
if (async) {
msg.setAsynchronous(true);
}
mH.sendMessage(msg);
}
下面是Handler H对消息的处理：
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
case LAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
case RELAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
ActivityClientRecord r = (ActivityClientRecord)msg.obj;
handleRelaunchActivity(r);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
case PAUSE_ACTIVITY:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
handlePauseActivity((IBinder)msg.obj, false, (msg.arg1&1) != 0, msg.arg2,
(msg.arg1&2) != 0);
maybeSnapshot();
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
...
}
if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));
}
从Handler H对“LAUNCH_ACTIVITY”这个消息的处理可以知道，Activity的启动过程由ActivityThread的handleLaunchActivity方法来实现，部分源码如下：
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
if (localLOGV) Slog.v(TAG, "Handling launch of " + r);
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
Bundle oldState = r.state;
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed);
...
}
}
从上面的源码可以看出，performLaunchActivity方法最终完成了Activity对象的创建和启动过程，并且ActivityThread通过handleResumeActivity方法来调用被启动Activity的onResume这一生命周期方法。
performLaunchActivity这个方法主要完成了如下几件事：
- 从ActivityClientRecord中获取待启动的Activity组件信息；
- 通过Instrumentation的newActivity方法使用类加载器创建Activity对象；
- 通过LoadedApk的makeApplication方法来尝试创建Application对象；
- 创建ContextImpI对象并通过Activity的attach方法完成一些重要数据的初始化；
- 调用Activity的onCreate方法。
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
//1. 从ActivityClientRecord中获取待启动的Activity组件信息；
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);
}
ComponentName component = r.intent.getComponent();
if (component == null) {
component = r.intent.resolveActivity(
mInitialApplication.getPackageManager());
r.intent.setComponent(component);
}
if (r.activityInfo.targetActivity != null) {
component = new ComponentName(r.activityInfo.packageName,
r.activityInfo.targetActivity);
}
//2. 通过Instrumentation的newActivity方法使用类加载器创建Activity对象；
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to instantiate activity " + component
+ ": " + e.toString(), e);
}
}
try {
//3. 通过LoadedApk的makeApplication方法来尝试创建Application对象；
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
if (localLOGV) Slog.v(
TAG, r + ": app=" + app
+ ", appName=" + app.getPackageName()
+ ", pkg=" + r.packageInfo.getPackageName()
+ ", comp=" + r.intent.getComponent().toShortString()
+ ", dir=" + r.packageInfo.getAppDir());
if (activity != null) {
//4. 创建ContextImpI对象并通过Activity的attach方法完成一些重要数据的初始化；
Context appContext = createBaseContextForActivity(r, activity);
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
+ r.activityInfo.name + " with config " + config);
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.voiceInteractor);
if (customIntent != null) {
activity.mIntent = customIntent;
}
r.lastNonConfigurationInstances = null;
activity.mStartedActivity = false;
int theme = r.activityInfo.getThemeResource();
if (theme != 0) {
activity.setTheme(theme);
}
//5. 调用Activity的onCreate方法。
activity.mCalled = false;
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
if (!activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onCreate()");
}
r.activity = activity;
r.stopped = true;
if (!r.activity.mFinished) {
activity.performStart();
r.stopped = false;
}
if (!r.activity.mFinished) {
if (r.isPersistable()) {
if (r.state != null || r.persistentState != null) {
mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
r.persistentState);
}
} else if (r.state != null) {
mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
}
}
if (!r.activity.mFinished) {
activity.mCalled = false;
if (r.isPersistable()) {
mInstrumentation.callActivityOnPostCreate(activity, r.state,r.persistentState);
} else {
mInstrumentation.callActivityOnPostCreate(activity, r.state);
}
if (!activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onPostCreate()");
}
}
}
r.paused = true;
mActivities.put(r.token, r);
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to start activity " + component
+ ": " + e.toString(), e);
}
}
return activity;
}
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