一直都想过要去看一看一个activity是怎么启动起来的，但一直都没有静下心去认真看一看，现在趁着有时间好好阅读了一下源码，加上网上一些同志的分享，终于吧代码撸的比较清楚了，怕时间一久，又忘记了，赶紧记下来，如果有什么错误和遗漏的话，看官们也请指出。（本文中源码出自Android API 23）
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首先我们打开activity的startActivity方法：
@Override
public voidstartActivity(Intent intent) {
mBase.startActivity(intent);
}
Activity继承于 Context，这是个抽象类，里面并没有实现 startActivity（） 这个方法，查了一下它的子类，嗯哼，就看了 ContextWrapper实现了它，但是，这个 mBase 又是什么鬼？嗯，也是一个 Context对象，但具体是那个类呢，查了一下 Context的继承关系，找到了 ContentImpl这个类，看一下这个类的介绍：
/**
* Common implementation of Context API, which provides the base
* context object for Activity and other application components.
*/
classContextImplextendsContext {...}
这个类就是Context的Api的共同的实现类了，上面说到的 mBase 对象实际上说的就是这个类的实例了，那么这个对象是怎么来的，啥时候实例化的？不急，往下看，迟早要知道的。
好了，现在来看一下 ContentImpl中 startActivity()的实现：
@Override
public voidstartActivity(Intent intent) {
warnIfCallingFromSystemProcess();
startActivity(intent,null);
}
@Override
public voidstartActivity(Intent intent, Bundle options) {
warnIfCallingFromSystemProcess();
if((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) ==0) {
throw newAndroidRuntimeException(
"Calling startActivity() from outside of an Activity "
+" context requires the FLAG_ACTIVITY_NEW_TASK flag."
+" Is this really what you want?");
}
mMainThread.getInstrumentation().execStartActivity(
getOuterContext(),mMainThread.getApplicationThread(),null,
(Activity)null, intent, -1, options);
}
看红色代码部分，mMainThread就是 ActivityThread类的实例，整个app进程的入口main()函数也是在这个类中的，这个 mMainThread提供了一个 Instrumentation对象
/**
* Base class for implementing application instrumentation code. When running
* with instrumentation turned on, this class will be instantiated for you
* before any of the application code, allowing you to monitor all of the
* interaction the system has with the application. An Instrumentation
* implementation is described to the system through an AndroidManifest.xml's
*<instrumentation> tag.
*/
public classInstrumentation {...}
简单的说，它就是用来监控应用中各种交互行为，从注释上看，我们可以通过 Manifest来指定一个我们自己实现的 Instrumentation类，好了这个不是重点，让我们看一下一个类怎么 startActivity的吧：
publicActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent,intrequestCode, Bundle options) {
//...
//前面的不是重点
try{
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess();
intresult =ActivityManagerNative.getDefault()
.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 newRuntimeException("Failure from system", e);
}
return null;
}
好吧，怎么这家伙又把启动 activity的事情交给别人做了。。。再点进去看：咦，怎么。。怎么是 IActivityManager的接口方法，好吧，又要找它的实现类了，but 貌似找到的这个家伙 ActivityManagerProxy 并没有认真的做事情啊，它找了一个代理去 startActivity（）去了，那我们就去找这个代理去，从 IActivityManager extends IInterface 这个类的继承方式我们就能发现这其实就是Android中的Binder机制，跨进程的通信机制，ActivityManagerProxy找的代理其实就是 ActivityManagerService，ActivityManagerService属于系统进程，它对手机中所有的activity进行了统一的管理，源码中就是这么个获取的：
IBinder b = ServiceManager.getService("activity");
IActivityManager am =asInterface(b);熟悉的 aidl嘛，好了好了，感觉有点跑偏，往回扯，来看看 AMS 怎么实现 startActivity() 的：
@Override
public final intstartActivity(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho,intrequestCode,
intstartFlags, ProfilerInfo profilerInfo, Bundle options) {
returnstartActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, options,
UserHandle.getCallingUserId());
}
@Override
public final intstartActivityAsUser(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho,intrequestCode,
intstartFlags, ProfilerInfo profilerInfo, Bundle options,intuserId) {
enforceNotIsolatedCaller("startActivity");
userId = handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(), userId,
false,ALLOW_FULL_ONLY,"startActivity",null);
//TODO: Switch to user app stacks here.
returnmStackSupervisor.startActivityMayWait(caller, -1, callingPackage, intent,
resolvedType,null,null, resultTo, resultWho, requestCode, startFlags,
profilerInfo,null,null, options,false, userId,null,null);
}
找到了，一个 mStackSupervisor对象，这又是干啥的？字面上的意思就是活动栈的管理器，先不管了，让我们先看看它的实现（...啊，代码不是一般的长，咱们只划重点）：
final intstartActivityMayWait(IApplicationThread caller,intcallingUid,
String callingPackage, Intent intent, String resolvedType,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
IBinder resultTo, String resultWho,intrequestCode,intstartFlags,
ProfilerInfo profilerInfo, WaitResult outResult, Configuration config,
Bundle options,booleanignoreTargetSecurity,intuserId,
IActivityContainer iContainer, TaskRecord inTask) {
// Refuse possible leaked file descriptors
if(intent !=null&& intent.hasFileDescriptors()) {
throw newIllegalArgumentException("File descriptors passed in Intent");
}
booleancomponentSpecified = intent.getComponent() !=null;
// Don't modify the client's object!
intent =newIntent(intent);
// Collect information about the target of the Intent.
ActivityInfo aInfo =
resolveActivity(intent, resolvedType, startFlags, profilerInfo, userId);
ActivityContainer container = (ActivityContainer)iContainer;
synchronized(mService) {
if(container !=null&& container.mParentActivity!=null&&
container.mParentActivity.state!= RESUMED) {
// Cannot start a child activity if the parent is not resumed.
returnActivityManager.START_CANCELED;
}
final intrealCallingPid = Binder.getCallingPid();
final intrealCallingUid = Binder.getCallingUid();
intcallingPid;
if(callingUid >=0) {
callingPid = -1;
}else if(caller ==null) {
callingPid = realCallingPid;
callingUid = realCallingUid;
}else{
callingPid = callingUid = -1;
}
finalActivityStack stack;
if(container ==null|| container.mStack.isOnHomeDisplay()) {
stack =mFocusedStack;
}else{
stack = container.mStack;
}
stack.mConfigWillChange= config !=null&&mService.mConfiguration.diff(config) !=0;
if(DEBUG_CONFIGURATION) Slog.v(TAG_CONFIGURATION,
"Starting activity when config will change = "+ stack.mConfigWillChange);
final longorigId = Binder.clearCallingIdentity();
if(aInfo !=null&&
(aInfo.applicationInfo.privateFlags
&ApplicationInfo.PRIVATE_FLAG_CANT_SAVE_STATE) !=0) {
// This may be a heavy-weight process! Check to see if we already
// have another, different heavy-weight process running.
if(aInfo.processName.equals(aInfo.applicationInfo.packageName)) {
if(mService.mHeavyWeightProcess!=null&&
(mService.mHeavyWeightProcess.info.uid!= aInfo.applicationInfo.uid||
!mService.mHeavyWeightProcess.processName.equals(aInfo.processName))) {
intappCallingUid = callingUid;
if(caller !=null) {
Proce***ecord callerApp =mService.getRecordForAppLocked(caller);
if(callerApp !=null) {
appCallingUid = callerApp.info.uid;
}else{
Slog.w(TAG,"Unable to find app for caller "+ caller
+" (pid="+ callingPid +") when starting: "
+ intent.toString());
ActivityOptions.abort(options);
returnActivityManager.START_PERMISSION_DENIED;
}
}
IIntentSender target =mService.getIntentSenderLocked(
ActivityManager.INTENT_SENDER_ACTIVITY,"android",
appCallingUid, userId,null,null,0,newIntent[] { intent },
newString[] { resolvedType }, PendingIntent.FLAG_CANCEL_CURRENT
| PendingIntent.FLAG_ONE_SHOT,null);
Intent newIntent =newIntent();
if(requestCode >=0) {
// Caller is requesting a result.
newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_HAS_RESULT,true);
}
newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_INTENT,
newIntentSender(target));
if(mService.mHeavyWeightProcess.activities.size() >0) {
ActivityRecord hist =mService.mHeavyWeightProcess.activities.get(0);
newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_CUR_APP,
hist.packageName);
newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_CUR_TASK,
hist.task.taskId);
}
newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_NEW_APP,
aInfo.packageName);
newIntent.setFlags(intent.getFlags());
newIntent.setClassName("android",
HeavyWeightSwitcherActivity.class.getName());
intent = newIntent;
resolvedType =null;
caller =null;
callingUid = Binder.getCallingUid();
callingPid = Binder.getCallingPid();
componentSpecified =true;
try{
ResolveInfo rInfo =
AppGlobals.getPackageManager().resolveIntent(
intent,null,
PackageManager.MATCH_DEFAULT_ONLY
| ActivityManagerService.STOCK_PM_FLAGS, userId);
aInfo = rInfo !=null? rInfo.activityInfo:null;
aInfo =mService.getActivityInfoForUser(aInfo, userId);
}catch(RemoteException e) {
aInfo =null;
}
}
}
}
int res = startActivityLocked(caller, intent, resolvedType, aInfo,
voiceSession, voiceInteractor, resultTo, resultWho,
requestCode, callingPid, callingUid, callingPackage,
realCallingPid, realCallingUid, startFlags, options, ignoreTargetSecurity,
componentSpecified, null, container, inTask);
Binder.restoreCallingIdentity(origId);
if(stack.mConfigWillChange) {
// If the caller also wants to switch to a new configuration,
// do so now. This allows a clean switch, as we are waiting
// for the current activity to pause (so we will not destroy
// it), and have not yet started the next activity.
mService.enforceCallingPermission(android.Manifest.permission.CHANGE_CONFIGURATION,
"updateConfiguration()");
stack.mConfigWillChange=false;
if(DEBUG_CONFIGURATION) Slog.v(TAG_CONFIGURATION,
"Updating to new configuration after starting activity.");
mService.updateConfigurationLocked(config,null,false,false);
}
if(outResult !=null) {
outResult.result= res;
if(res == ActivityManager.START_SUCCESS) {
mWaitingActivityLaunched.add(outResult);
do{
try{
mService.wait();
}catch(InterruptedException e) {
}
}while(!outResult.timeout&& outResult.who==null);
}else if(res == ActivityManager.START_TASK_TO_FRONT) {
ActivityRecord r = stack.topRunningActivityLocked(null);
if(r.nowVisible&& r.state== RESUMED) {
outResult.timeout=false;
outResult.who=newComponentName(r.info.packageName, r.info.name);
outResult.totalTime=0;
outResult.thisTime=0;
}else{
outResult.thisTime= SystemClock.uptimeMillis();
mWaitingActivityVisible.add(outResult);
do{
try{
mService.wait();
}catch(InterruptedException e) {
}
}while(!outResult.timeout&& outResult.who==null);
}
}
}
returnres;
}
}
继续往里走~~
final intstartActivityLocked(IApplicationThread caller,
Intent intent, String resolvedType, ActivityInfo aInfo,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
IBinder resultTo, String resultWho,intrequestCode,
intcallingPid,intcallingUid, String callingPackage,
intrealCallingPid,intrealCallingUid,intstartFlags, Bundle options,
booleanignoreTargetSecurity,booleancomponentSpecified, ActivityRecord[] outActivity,
ActivityContainer container, TaskRecord inTask) {
interr = ActivityManager.START_SUCCESS;
//...
//中间各种逻辑判断，这里就不贴出来了，贼多。。。我们依旧划个重点
doPendingActivityLaunchesLocked(false);
err = startActivityUncheckedLocked(r, sourceRecord, voiceSession, voiceInteractor,
startFlags,true, options, inTask);//这里记住一下倒数第3个参数是true。
if(err <0) {
// If someone asked to have the keyguard dismissed on the next
// activity start, but we are not actually doing an activity
// switch... just dismiss the keyguard now, because we
// probably want to see whatever is behind it.
notifyActivityDrawnForKeyguard();
}
returnerr;
}
还得往里走。。。好多层：
final intstartActivityUncheckedLocked(finalActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,intstartFlags,
booleandoResume, Bundle options, TaskRecord inTask) {
//前面N个判断...
mService.grantUriPermissionFromIntentLocked(callingUid, r.packageName,
intent, r.getUriPermissionsLocked(), r.userId);
if(sourceRecord !=null&& sourceRecord.isRecentsActivity()) {
r.task.setTaskToReturnTo(RECENTS_ACTIVITY_TYPE);
}
if(newTask) {
EventLog.writeEvent(EventLogTags.AM_CREATE_TASK, r.userId, r.task.taskId);
}
ActivityStack.logStartActivity(EventLogTags.AM_CREATE_ACTIVITY, r, r.task);
targetStack.mLastPausedActivity=null;
targetStack.startActivityLocked(r, newTask, doResume, keepCurTransition, options);
if(!launchTaskBehind) {
// Don't set focus on an activity that's going to the back.
mService.setFocusedActivityLocked(r,"startedActivity");
}
returnActivityManager.START_SUCCESS;
}
这个方法就屌的不行了，好几百行，一行行看下来绝对的心力交瘁，换个角度来看，这个方法的返回值比较有特点，他们都是 ActiviyManager中的常量，让我们看一看这些常量代表的意思：
/**
* Result for IActivityManaqer.startActivity: the activity was started
* successfully as normal.
* @hide
*/
public static final intSTART_SUCCESS=0;
/**
* Result for IActivityManaqer.startActivity: the caller asked that the Intent not
* be executed if it is the recipient, and that is indeed the case.
* @hide
*/
public static final intSTART_RETURN_INTENT_TO_CALLER=1;
/**
* Result for IActivityManaqer.startActivity: activity wasn't really started, but
* a task was simply brought to the foreground.
* @hide
*/
public static final intSTART_TASK_TO_FRONT=2;
/**
* Result for IActivityManaqer.startActivity: activity wasn't really started, but
* the given Intent was given to the existing top activity.
* @hide
*/
public static final intSTART_DELIVERED_TO_TOP=3;
好了，就贴几个，可以看出这些都是我们启动以后 activity的返回的各种结果，可想而知前面的各种逻辑是干嘛的了。那么这样我们就只找成功的那一个常量值 START_SUCCESS，在 startActivityLocked（） 我们发现之后方法末尾返回了 START_SUCCESS，其余都不是，那么关键代码也就比较好找了，请看上面红色的重点~~，好了继续走你，这回就来到了 ActivityStack 这个类了：
final voidstartActivityLocked(ActivityRecord r,booleannewTask,
booleandoResume,booleankeepCurTransition, Bundle options) {
//逻辑判断。。比如Home键什么的
if(doResume) {
mStackSupervisor.resumeTopActivitiesLocked(this, r, options);
}
}
还记得前面划重点的参数吧，就是doResume，前面的一些逻辑代码咱们就不谈了，有兴趣的小盆友有时间可以慢慢看，咱们继续往下走 ActivityStackSupervisor.resumeTopActivitiesLocked()：
booleanresumeTopActivitiesLocked(ActivityStack targetStack, ActivityRecord target,
Bundle targetOptions) {
if(targetStack ==null) {
targetStack =mFocusedStack;
}
// Do targetStack first.
booleanresult =false;
if(isFrontStack(targetStack)) {
result = targetStack.resumeTopActivityLocked(target, targetOptions);
}
for(intdisplayNdx =mActivityDisplays.size() -1; displayNdx >=0; --displayNdx) {
finalArrayList stacks =mActivityDisplays.valueAt(displayNdx).mStacks;
for(intstackNdx = stacks.size() -1; stackNdx >=0; --stackNdx) {
finalActivityStack stack = stacks.get(stackNdx);
if(stack == targetStack) {
// Already started above.
continue;
}
if(isFrontStack(stack)) {
stack.resumeTopActivityLocked(null);
}
}
}
returnresult;
}
这个方法代码量比较少，关键的地方就是 resumeTopActivityLocked 了，感觉越来越接近目标了，还有点小激动。。。ActivityStack.resumeTopActivityLocked():
final booleanresumeTopActivityLocked(ActivityRecord prev, Bundle options) {
if(mStackSupervisor.inResumeTopActivity) {
// Don't even start recursing.
return false;
}
booleanresult =false;
try{
// Protect against recursion.
mStackSupervisor.inResumeTopActivity=true;
if(mService.mLockScreenShown== ActivityManagerService.LOCK_SCREEN_LEAVING) {
mService.mLockScreenShown= ActivityManagerService.LOCK_SCREEN_HIDDEN;
mService.updateSleepIfNeededLocked();
}
result = resumeTopActivityInnerLocked(prev, options);
}finally{
mStackSupervisor.inResumeTopActivity=false;
}
returnresult;
}嗯，是一个内部具体实现方法：
private booleanresumeTopActivityInnerLocked(ActivityRecord prev, Bundle options) {
//.....
ActivityStack lastStack = mStackSupervisor.getLastStack();
if (next.app != null && next.app.thread != null) {
//.........
try{
//...........
}catch(Exception e) {
// Whoops, need to restart this activity!
if(DEBUG_STATES) Slog.v(TAG_STATES,"Resume failed; resetting state to "
+ lastState +": "+ next);
next.state= lastState;
if(lastStack !=null) {
lastStack.mResumedActivity= lastResumedActivity;
}
Slog.i(TAG,"Restarting because process died: "+ next);
if(!next.hasBeenLaunched) {
next.hasBeenLaunched=true;
}else if(SHOW_APP_STARTING_PREVIEW&& lastStack !=null&&
mStackSupervisor.isFrontStack(lastStack)) {
mWindowManager.setAppStartingWindow(
next.appToken, next.packageName, next.theme,
mService.compatibilityInfoForPackageLocked(next.info.applicationInfo),
next.nonLocalizedLabel, next.labelRes, next.icon, next.logo,
next.windowFlags,null,true);
}
mStackSupervisor.startSpecificActivityLocked(next,true,false);
if(DEBUG_STACK)mStackSupervisor.validateTopActivitiesLocked();
return true;
}
// From this point on, if something goes wrong there is no way
// to recover the activity.
try{
next.visible=true;
completeResumeLocked(next);
}catch(Exception e) {
// If any exception gets thrown, toss away this
// activity and try the next one.
Slog.w(TAG,"Exception thrown during resume of "+ next, e);
requestFinishActivityLocked(next.appToken, Activity.RESULT_CANCELED,null,
"resume-exception",true);
if(DEBUG_STACK)mStackSupervisor.validateTopActivitiesLocked();
return true;
}
next.stopped=false;
}else{
// Whoops, need to restart this activity!
if(!next.hasBeenLaunched) {
next.hasBeenLaunched=true;
}else{
if(SHOW_APP_STARTING_PREVIEW) {
mWindowManager.setAppStartingWindow(
next.appToken, next.packageName, next.theme,
mService.compatibilityInfoForPackageLocked(
next.info.applicationInfo),
next.nonLocalizedLabel,
next.labelRes, next.icon, next.logo, next.windowFlags,
null,true);
}
if(DEBUG_SWITCH) Slog.v(TAG_SWITCH,"Restarting: "+ next);
}
if(DEBUG_STATES) Slog.d(TAG_STATES,"resumeTopActivityLocked: Restarting "+ next);
mStackSupervisor.startSpecificActivityLocked(next,true,true);
}
if(DEBUG_STACK)mStackSupervisor.validateTopActivitiesLocked();
return true;
}阿西吧，将近400行。。老衲差点就要圆寂在这源码上面了。。耐着性子看了好几遍，还是让老衲找出了重点，前面的判断大概都有比如从 activity 后台到前台，从暂停到恢复，回到home页，屏幕旋转,切换到不同栈 activity 等等各种情况，但是这个不是咱们要的，咱就是要一个 activity 从无到有，所以咱只留相关的代码，ActivityStackSupervisor.startSpecificActivityLocked():
voidstartSpecificActivityLocked(ActivityRecord r,
booleanandResume,booleancheckConfig) {
// Is this activity's application already running?
Proce***ecord app =mService.getProce***ecordLocked(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);
}
啊，终于找到一个比较少代码的方法了，从逻辑判断上看 realStartActivityLocked 才是开启我们应用内的方法嘛，至于 startProcessLocked，它是启动应用进程的，咱们先不管，下一篇文章再理它，看看 realStartActivityLocked 怎么个实现的：
final booleanrealStartActivityLocked(ActivityRecord r,
Proce***ecord app,booleanandResume,booleancheckConfig)
throwsRemoteException {
//...check..
app.thread.scheduleLaunchActivity(newIntent(r.intent), r.appToken,
System.identityHashCode(r), r.info,newConfiguration(mService.mConfiguration),
newConfiguration(stack.mOverrideConfig), r.compat, r.launchedFromPackage,
task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,
newIntents, !andResume,mService.isNextTransitionForward(), profilerInfo);
// do other things...
return true;
}
忽略掉其他的代码，我们只看重点，这个 thread 是谁呢？还记得我们的 ActivityThread 吧，这个thread其实就是 ActivityThread中的内部类对象 ApplicationThread，我们去看一下这个方法的实现，看了那么多代码，这里其实已经很接近了：
public final voidscheduleLaunchActivity(Intent intent, IBinder token,intident,
ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
intprocState, Bundle state, PersistableBundle persistentState,
List pendingResults, List pendingNewIntents,
booleannotResumed,booleanisForward, ProfilerInfo profilerInfo) {
updateProcessState(procState,false);
ActivityClientRecord r =newActivityClientRecord();
r.token= token;
r.ident= ident;
r.intent= intent;
r.referrer= referrer;
r.voiceInteractor= voiceInteractor;
r.activityInfo= info;
r.compatInfo= compatInfo;
r.state= state;
r.persistentState= persistentState;
r.pendingResults= pendingResults;<
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