android6.0源码分析之Activity启动过程
Posted 古冥
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Activity最为android开发者最熟悉的组件，由ActivityManagerService服务进行调度管理，而ActivityManagerService的启动过程在activitymanagerservice服务源码分析一文中进行了详细分析，本文基于其对Activity的启动过程进行分析，同时也将分析AMS对Activity的调度管理。
1、 Activity的启动模式
启动模式实际上就是为了控制Activity与Task之间的关系，目前共有一下四种模式：standard、singleTop、singleTask以及singleInstance，默认的模式为standard模式。
注意：建议一般的开发者不要轻易使用后两种模式，因为不同的模式，其出栈的顺序不同，可能会导致用户按下返回键时，会有不一样的用户体验，对于启动模式的分析，推荐仔细阅读深入讲解Android中Activity launchMode,这篇文章使用图文并用的方式，很好的分析了Activity各种模式的区别，本文不再分析启动模式。
2、Activity的启动时机
Activity作为android非常重要的组件，那么我们会在哪些时机启动它呢？
通常有如下几种情形：
1. android AM命令
android系统为开发者提供了adb工具，而在adb的基础上执行adb shell就可以从PC上对手机侧执行shell命令。如启动浏览器：am start -n com.android.browser/com.android.browser.BrowserActivity。
2. 启动AMS服务时
在启动AMS服务时，在最后调用AMS的systemReady的最后阶段会启动Home界面，此时就会启动一个Activity。
3. 调用startActivity()和startActivityForResult()方法时
这是我们最常用也最熟悉的启动Activity的方式。
3、AM命令行启动Activity的过程分析
在adb中可以使用AM命令，同时在应用程序中，也可以通过AM命令来启动指定的Activity，AM是一个脚本命令，它还可以启动Service，发送广播等。
3.1 Am 框架层调用流程分析
adb执行Am命令后，最终会执行Am.java的main方法，其代码如下：
//Am.java
public static void main(String[] args)
(new Am()).run(args);
它调用的是抽象父类BaseCommand类的run方法:
//BaseCommand.java
public void run(String[] args)
if(args.length<1)
//显示命令使用说明
onShowUsage(System.out);
return;
mArgs = args;
mNextArg = 0;
mCurArgData = null;
try
onRun();
catch(...)...
跟其他命令脚本类似，如果参数为空，显示使用说明，否则调用onRun()方法，它是一个抽象方法，在Am.java中有实现:
//Am.java
@Override
public void onRun() throws Exception
//获取AMS
mAm = ActivityManagerNative.getDefault();
...
String op = nextArgRequired();
if(op.equals("start"))
//启动Activity
runStart();
else if(op.equals("startService"))
//启动服务
runStartService();
else if(op.equals("force-stop"))
//强制停止
runForceStop();
else if(op.equals("kill"))
//杀死进程
runKill();
else if
...
else
...
命令参数不同，其执行的操作不同，就本节来说，其参数为start，所以将会执行runStart()方法来启动Activity:
//Am.java
private void runStart() throws Exception
Intent intent = makeIntent(UserHandle.USER_CURRENT);
...
do
//获取包信息
...
//设置intent的标志位FLAG_ACTIVITY_NEW_TASK,即将启动一个新任务
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
ParcelFileDescriptor fd = null;
ProfilerInfo profilerInfo = null;
if(mProfileFile != null)//处理-P选项，统计性能
try
fd = openForSystemServer(new File(mProfileFile),ParcelFileDescriptior.MODE_CREATE|
ParcelFileDescriptor.MODE_TRUNCATE|ParcelFileDescriptor.MODE_READ_WRITE);
catch(...)...
profilerInfo = new ProfilerInfo(mProfileFile,fd,mSamplingInterval,mAutoStop);
...
int res;
...
if(mWaitOption)//控制是否等待启动结果，如果有-W选项，则该值为true
result = mAm.startActivityAndWait(null,null,intent,mimeType,null,null,0,mStartFlags,
profilerInfo,null,mUserId);
res = result.result;
else
res = mAm.startActivityAsUser(null,null,intent,mimeType,null,null,0,mStartFlags,
profilerInfo,null,mUserId);
//处理一些返回结果
...
while(mRepeat > 1);
startActivityAsUser()最后也是调用AMS的startActivityAndWait()方法，所以Am命令框架层的调用结束，其调用时序如下：
3.2 AMS启动Activity流程分析
Am命令将启动Activity的工作交给了AMS来进行，首先看startActivityAndWait()方法，它直接调用StackSupervisor类的startActivityMayWait()方法，其代码如下：
//ActivityStackSupervisor.java
final int startActivityMayWait(...)
...
//为了不改变用户的intent
intent = new Intent(intent);
//收集目标intent的信息,在resolveActivity方法中与PKMS交互获得
ActivityInfo aInfo = resolveActivity(intent,resolvedType,startFlags,profilerInfo,userId);
ActivityContainer container = (ActivityContainer)iContainer;
synchronized(mService)
...
final ActivityStack stack;
//取得需要启动的ActivityStack栈
if(container == null || container.mStack.isOnHomeDisplay())
stack = mFocusedStack;
else
stack = container.mStack;
...
if(aInfo!=null&&(aInfo.applicationInfo.privateFlags&
ApplicationInfo.PRIVATE_FLAG_CANT_SAVE_STATE)!=0)
//Androidmanifest.xml中的Application标签可以声明一个cantSaveState属性，若设置，其将不享受系统提
//供的状态保存/恢复功能。主要是为了保证用户体验的连续性
...
//启动Activity
int res = startActivityLocked(caller,intent,resolvedType,aInfo,voiceSession,
voiceInteractor,resultTo,resultWho,requestCode,callingPid,callingUid,callingPackage,
realCallingPid,realCallingUid,startFlags,options,ignoreTargetSecurity,
componentSpecified,null,container,inTask);
...
//如果配置Configration发生变化，则调用AMS的updateConfigurationLocked进行处理
if(stack.mConfigWillChange)
mService.enforceCallingPermission(...);
mConfigWillChange = false;
mService.updateConfigurationLocked(config,null,false);
if(outResult != null)
outResult.result = res;
if(res == IActivityManager.START_SUCCESS)
//将结果放入mWaitingActivityLaunced中保存
mWaitingActivityLaunched.add(outResult);
do
try
//等待启动结果
mService.wait();
while(!outResult.timeout&&outResult.who == null);
else if
...
...
return res;
首先通过PKMS查找匹配的ActivityInfo，然后获取调用者的pid,uid等信息，由于本文的caller为空，得到的是Am所在进程的pid和uid。接着调用startActivityLocked()方法启动Activity，最后再调用mService.wait()方法来等待启动结果，并对返回值进行一些处理，此处的等待主要是Am参数中有一个-W选项，若无此选项，则不会有这个等待，最后再将启动结果res返回。
继续分析startActivityLocked()方法:
//ActivityStackSupervisor.java
final int startActivityLocked(...)
int err = ActivityManager.START_SUCCESS;
ProcessRecord callerApp = null;
if (caller != null) //如果caller不为空，从AMS中找到它所属的ProcessRecord即进程，本文此处为null
callerApp = mService.getRecordForAppLocked(caller);
if (callerApp != null)
callingPid = callerApp.pid;
callingUid = callerApp.info.uid;
else
err = ActivityManager.START_PERMISSION_DENIED;
...
//sourceRecord用于描述启动目标Activity的那个Activity
ActivityRecord sourceRecord = null;
//resultRecord用于描述接收启动结果的Activity，即onActivityResult将被调用以通知启动结果
ActivityRecord resultRecord = null;
...
//获取Intent设置的启动标志
final int launchFlags = intent.getFlags();
//此部分与LaunchMode类似，它用于控制Activity启动结果的通知
if ((launchFlags & Intent.FLAG_ACTIVITY_FORWARD_RESULT) != 0 && sourceRecord != null)
...
...
//检查权限
final int startAnyPerm = mService.checkPermission(START_ANY_ACTIVITY, callingPid, callingUid);
...
//可为AMS设置IActivityController类型的监听者，AMS有任何动静都将回调该监听者
if (mService.mController != null)
try
Intent watchIntent = intent.cloneFilter();
//交给回调对象处理，由它判断能否继续后面的行程
abort |= !mService.mController.activityStarting(watchIntent,
aInfo.applicationInfo.packageName);
catch (RemoteException e)
mService.mController = null;
if (abort) //通知resultRecord
if (resultRecord != null)
resultStack.sendActivityResultLocked(-1, resultRecord, resultWho, requestCode,
Activity.RESULT_CANCELED, null);
ActivityOptions.abort(options);
return ActivityManager.START_SUCCESS;
//创建一个ActivityRecord对象
ActivityRecord r = new ActivityRecord(mService,this,callerApp,callingUid,...);
...
final ActivityStack stack = mFocusedStack;
if (voiceSession == null && (stack.mResumedActivity == null
|| stack.mResumedActivity.info.applicationInfo.uid != callingUid))
//判断是否有权限切换Application
if (!mService.checkAppSwitchAllowedLocked(callingPid, callingUid,realCallingPid,
realCallingUid, "Activity start"))
PendingActivityLaunch pal =new PendingActivityLaunch(r, sourceRecord,startFlags,
stack);
//将Pending的请求保存到AMS的mPendingActivityLaunches变量中
mPendingActivityLaunches.add(pal);
ActivityOptions.abort(options);
return ActivityManager.START_SWITCHES_CANCELED;
if (mService.mDidAppSwitch)
mService.mAppSwitchesAllowedTime = 0;
else
mService.mDidAppSwitch = true;
//启动处于Pending状态的Activity
doPendingActivityLaunchesLocked(false);
//调用此函数继续启动Activity
err = startActivityUncheckedLocked(r, sourceRecord, voiceSession, voiceInteractor,startFlags,
true, options, inTask);
//做一些结果处理
...
return err;
此段代码的主要工作就是处理sourceRecord和resultRecord，然后再处理app switch属性，如果当前禁止app switch，则将本次启动保存起来，等允许app switch时再处理，最后再调用startActivityUncheckedLocked()方法处理本次Activity启动请求：
//ActivityStackSupervisor.java
final int startActivityUncheckedLocked(...)
final Intent intent = r.intent;
final int callingUid = r.launchedFromUid;
...
//获取启动标志
int launchFlags = intent.getFlags();
if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_DOCUMENT) != 0 &&(launchSingleInstance
|| launchSingleTask))
//如果Intent和AndroidManifest.xml的启动模式有冲突，以AndroidManifest.xml为准
launchFlags &=~(Intent.FLAG_ACTIVITY_NEW_DOCUMENT | Intent.FLAG_ACTIVITY_MULTIPLE_TASK);
else
...
...
if (sourceRecord != null) //如果请求的发起者为空，则需要创建一个新的Task
if (sourceRecord.finishing)
if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) == 0)
launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
newTaskInfo = sourceRecord.info;
newTaskIntent = sourceRecord.task.intent;
sourceRecord = null;
sourceStack = null;
else
sourceStack = sourceRecord.task.stack;
else
sourceStack = null;
...
if(inTask==null)
if(sourceRecord==null)
//如果不是从另外的Activity启动的，则需创建新的Task
if((launchFlags&Intent.FLAG_ACTIVITY_MEW_TASK)==0&&inTask==null)
launchFlags|=Intent.FLAG_ACTIVITY_NEW_TASK;
else if(sourceRecord.launchMode==ActivityInfo.LAUNCH_SINGLE_INSTANCE)
//如果此Activity在系统中已经有实例在运行，则需要为此Activity额外创建一个Task
launchFlags|=Intent.FLAG_ACTIVITY_NEW_TASK;
else if(launchSingleInstance||launchSingleTask)
//如果是SingleInstance或者SingleTask的启动模式，同样需要创建新的Task
launchFlags|=Intent.FLAG_ACTIVITY_NEW_TASK;
...
targetStack.startActivityLocked(r,newTask,doResume,keepCurTransition,options);
...
return ActivityManager.START_SUCCESS;
首先根据Activity的启动模式以及传入的参数来确定是否需要创建新的Task以及launchFlags，然后调用startActivityLocked()方法继续启动(这里的方法参数与前面同名方法的不一样，属于重载函数)，继续分析启动过程：
//ActivityStackSupervisor.java
final void startActivityLocked(ActivityRecord r, boolean newTask,
boolean doResume, boolean keepCurTransition, Bundle options)
...
TaskRecord task = null;
if (!newTask) //如果不是已经存在的任务，则找到它在哪里
boolean startIt = true;
for (int taskNdx = mTaskHistory.size() - 1; taskNdx >= 0; --taskNdx)
task = mTaskHistory.get(taskNdx);
if (task.getTopActivity() == null)
//此task下的Activity全部执行结束
continue;
if (task == r.task)
if (!startIt) //如果它对用户是不可见的，只需将其加入未启动的
task.addActivityToTop(r);
r.putInHistory();
...
ActivityOptions.abort(options);
return;
break;
else if(task.numFullScreen > 0)
startIt = false;
...
//将Activity信息加入到history stack中并处理
task = r.task;
task.addActivityToTop(r);
task.setFrontOfTask();
r.putInHistory();
...
if(doResume)
mStackSupervisor.resumeTopActivitiesLocked(this,r,options);
此方法将根据传入的参数来找到此Activity所在的任务，以及进行相应判断将其加入到相应的history stack的最顶端(栈顶)，最后再调用ActivityStack的resumeTopActivitiesLocked方法来启动栈顶的Activity，继续分析resumeTopActivitiesLocked方法:
//ActivityStack.java
boolean resumeTopActivitiesLocked(ActivityStack targetStack, ActivityRecord target,
Bundle targetOptions)
if (targetStack == null) //如果目的任务为NULL,则启动当前聚焦的任务
targetStack = mFocusedStack;
//启动targetStack
boolean result = false;
if (isFrontStack(targetStack))
result = targetStack.resumeTopActivityLocked(target, targetOptions);
for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx)
final ArrayList<ActivityStack> stacks = mActivityDisplays.valueAt(displayNdx).mStacks;
for (int stackNdx = stacks.size() - 1; stackNdx >= 0; --stackNdx)
final ActivityStack stack = stacks.get(stackNdx);
if (stack == targetStack)
//上面已经启动了
continue;
if (isFrontStack(stack))
stack.resumeTopActivityLocked(null);
return result;
此方法很简单就是根据当前任务，调用相应的resumeTopActivityLocked()方法来启动，而resumeTopActivityLocked也只是简单的调用了resumeTopActivityInnerLocked方法，所以分析resumeTopActivityInnerLocked方法：
//ActivityStack.java
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, Bundle options)
//找到第一个还没有结束的Activity
final ActivityRecord next = topRunningActivityLocked(null);
...
//确保拥有此Activity的任务已经启动
if (mService.mStartedUsers.get(next.userId) == null)
return false;
...
//设置启动的Uid
mStackSupervisor.setLaunchSource(next.info.applicationInfo.uid);
...
ActivityStack lastStack = mStackSupervisor.getLastStack();
if (next.app != null && next.app.thread != null) //进程，线程都存在
//将此Activity置为Visible
mWindowManager.setAppVisibility(next.appToken, true);
// schedule launch ticks to collect information about slow apps.
next.startLaunchTickingLocked();
ActivityRecord lastResumedActivity =
lastStack == null ? null :lastStack.mResumedActivity;
ActivityState lastState = next.state;
next.state = ActivityState.RESUMED;
mResumedActivity = next;
next.task.touchActiveTime();
mRecentTasks.addLocked(next.task);
mService.updateLruProcessLocked(next.app, true, null);
updateLRUListLocked(next);
mService.updateOomAdjLocked();
...
try
// Deliver all pending results.
ArrayList<ResultInfo> a = next.results;
if (a != null)
final int N = a.size();
if (!next.finishing && N > 0)
next.app.thread.scheduleSendResult(next.appToken, a);
if (next.newIntents != null)
next.app.thread.scheduleNewIntent(next.newIntents, next.appToken);
...
next.sleeping = false;
mService.showAskCompatModeDialogLocked(next);
next.app.pendingUiClean = true;
next.app.forceProcessStateUpTo(mService.mTopProcessState);
next.clearOptionsLocked();
//因为进程，线程都存在，所以只需恢复此Activity
next.app.thread.scheduleResumeActivity(next.appToken, next.app.repProcState,
mService.isNextTransitionForward(), resumeAnimOptions);
catch (Exception e)
// 如果发生异常，需要重启此Activity，注意此方法最后一个参数为false
mStackSupervisor.startSpecificActivityLocked(next, true, false);
return true;
else
//因为进程或者线程为空，所以需要重新启动activity，此处最后一个参数为true
mStackSupervisor.startSpecificActivityLocked(next, true, true);
return true;
如代码可知：如果当前需要启动的Activity所在的进程和线程都已经创建，则调用ActivityThread的scheduleResumeActivity方法来恢复此Activity，此时如果发生异常，则调用startSpecificActivityLocked来重新启动此Activity，当然，我们注意到，当Activity所在的进程或者线程为启动时，它也是调用startSpecificActivityLocked方法来重新启动此Activity，但区别在此方法的最后一个参数，即进程和线程已经启动了的为flase，否则为true，下面分析startSpecificActivityLocked()方法：
//ActivityStackSupervisor.java
void startSpecificActivityLocked(ActivityRecord r,boolean andResume, boolean checkConfig)
//先获取app，据此app来判断Activity所在的Application是否已经启动
ProcessRecord app = mService.getProcessRecordLocked(r.processName,
r.info.applicationInfo.uid, true);
r.task.stack.setLaunchTime(r);
if (app != null && app.thread != null) //进程和线程不为空，即已经启动好
try
...
//真正启动Activity的函数，注意checkConfig参数
realStartActivityLocked(r, app, andResume, checkConfig);
return;
catch (RemoteException e) ...
//进程或者线程未启动
mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
"activity", r.intent.getComponent(), false, false, true);
由代码可知，它同样会先判断线程和进程是否已启动，如果已启动，调用realStartActivityLocked来启动Activity，否则调用AMS的startProcessLocked来启动进程，由于Am命令和AMS的Hone界面启动都是进程未启动的情况，所以此处先分析分两个阶段分析,此阶段的时序图如下：
3.2.1 AMS启动Activity所在进程的流程分析
由于android下的Java进程都是由Zygote进程fork出来的，所以分析此流程之前，需要对Zygote的原理有一些了解，对Zygote进程的分析请参考android源码分析之Zygote进程分析，先从AMS的startProcessLocked方法进行分析：
//ActivityManagerService.java
private final void startProcessLocked(ProcessRecord app, String hostingType,String hostingNameStr,
String abiOverride, String entryPoint, String[] entryPointArgs)
long startTime = SystemClock.elapsedRealtime();
if (app.pid > 0 && app.pid != MY_PID) //如果不是当前进程，则移除
checkTime(startTime, "startProcess: removing from pids map");
synchronized (mPidsSelfLocked)
mPidsSelfLocked.remove(app.pid);
mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
checkTime(startTime, "startProcess: done removing from pids map");
app.setPid(0);
...
//启动进程
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);
...
//通知BatteryStatsService服务此进程启动
mBatteryStatsService.noteProcessStart(app.processName, app.info.uid);
...//对进程启动结果进行处理
由代码可知，它调用了Process的start方法来启动进程，继续分析start方法：
//Process.java
public static final ProcessStartResult start(...)
try
return startViaZygote(processClass, niceName, uid, gid, gids,debugFlags, mountExternal,
targetSdkVersion, seInfo, abi, instructionSet, appDataDir, zygoteArgs);
catch (ZygoteStartFailedEx ex)
Log.e(LOG_TAG, "Starting VM process through Zygote failed");
throw new RuntimeException("Starting VM process through Zygote failed", ex);
继续看startViaZygote方法：
//Process.java
private static ProcessStartResult startViaZygote(...)
synchronized(Process.class)
ArrayList<String> argsForZygote = new ArrayList<String>();
// --runtime-args, --setuid=, --setgid=,
// and --setgroups= must go first
argsForZygote.add("--runtime-args");
argsForZygote.add("--setuid=" + uid);
//参数处理
...
return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi), argsForZygote);
同样，它首先进行一些参数处理，如runtime的参数等等，继续分析zygoteSendArgsAndGetResult方法：
//Process.java
private static ProcessStartResult zygoteSendArgsAndGetResult(ZygoteState zygoteState,
ArrayList<String> args) throws ZygoteStartFailedEx
try
//通过socket通信，此处将进程启动的启动写进socket的输出流
final BufferedWriter writer = zygoteState.writer;
final DataInputStream inputStream = zygoteState.inputStream;
writer.write(Integer.toString(args.size()));
writer.newLine();
int sz = args.size();
for (int i = 0; i < sz; i++)
String arg = args.get(i);
if (arg.indexOf('\\n') >= 0)
throw new ZygoteStartFailedEx("embedded newlines not allowed");
writer.write(arg);
writer.newLine();
writer.flush();
// Should there be a timeout on this?
ProcessStartResult result = new ProcessStartResult();
result.pid = inputStream.readInt();
if (result.pid < 0)
throw new ZygoteStartFailedEx("fork() failed");
result.usingWrapper = inputStream.readBoolean();
return result;
catch (IOException ex)
zygoteState.close();
throw new ZygoteStartFailedEx(ex);
此方法，将会与ZygoteInit中runSelectLoop()方法启动的socket进程通信，而Zygote进程将会为此Activity创建新的进程，而子进程创建好后，会调用ActivityThread的主线程的main方法，所以继续分析ActivityThread的main方法：
//ActivityThread.java
public static void main(String[] args)
...
AndroidKeyStoreProvider.install();
...
//启动Looper
Looper.prepareMainLooper();
//创建ActivityThread，即主线程
ActivityThread thread = new ActivityThread();
thread.attach(false);
//获取主Handler
if (sMainThreadHandler == null)
sMainThreadHandler = thread.getHandler();
...
Looper.loop();
此段代码将会创建一个主线程对象ActivityThread，并将启动Looper以及获取主线程的Handler来进行消息处理，并调用attach方法来进行线程处理，继续分析attach方法：
//ActivityThread.java
//此处的system参数为false，即不是系统进程，而在分析AMS的时候，其启动Home界面时，此处为true
private void attach(boolean system)
sCurrentActivityThread = this;
mSystemThread = system;
if (!system)
...
final IActivityManager mgr = ActivityManagerNative.getDefault();
try
mgr.attachApplication(mAppThread);
catch (RemoteException ex)
// Ignore
...
它调用了AMS的attachApplication方法来对线程进行包装，继续查看attachApplication方法：
//ActivityManagerService.java
@Override
public final void attachApplication(IApplicationThread thread)
synchronized (this)
int callingPid = Binder.getCallingPid();
final long origId = Binder.clearCallingIdentity();
attachApplicationLocked(thread, callingPid);
Binder.restoreCallingIdentity(origId);
只是调用了attachApplicationLocked方法：
//ActivityManagerService.java
private final boolean attachApplicationLocked(IApplicationThread thread,int pid)
//首先获取相应的app
if (pid != MY_PID && pid >= 0)
synchronized (mPidsSelfLocked)
app = mPidsSelfLocked.get(pid);
else
app = null;
...
app.makeActive(thread, mProcessStats);
app.curAdj = app.setAdj = -100;
app.curSchedGroup = app.setSchedGroup = Process.THREAD_GROUP_DEFAULT;
app.forcingToForeground = null;
updateProcessForegroundLocked(app, false, false);
...
mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
...
thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace,
isRestrictedBackupMode || !normalMode, app.persistent,
new Configuration(mConfiguration), app.compat,
getCommonServicesLocked(app.isolated),
mCoreSettingsObserver.getCoreSettingsLocked());
updateLruProcessLocked(app, false, null);
app.lastRequestedGc = app.lastLowMemory = SystemClock.uptimeMillis();
...
// 查看该进程是否有栈顶可视的Activity等待运行
if (normalMode)
try
//如果有，会调用realStartActivityLocked方法启动此Activity
if (mStackSupervisor.attachApplicationLocked(app))
didSomething = true;
catch (Exception e)
Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
badApp = true;
return true;
此处回调了ActivityThread的bindApplication方法，通过bindApplication方法来对应用进行相应的初始化，然后调用了ActivityStackSupervisor的attachApplicationLocked方法，它最后会调用realStartActivityLocked方法来真正启动Activity，本节不对Activity的调度做分析，所以先分析bindApplication方法。
//ActivityThread.java
public final void bindApplication(...)
if (services != null)
// Setup the service cache in the ServiceManager
ServiceManager.initServiceCache(services);
...
IPackageManager pm = getPackageManager();
android.content.pm.PackageInfo pi = null;
...
AppBindData data = new AppBindData();
data.processName = processName;
data.appInfo = appInfo;
...
data.persistent = persistent;
data.config = config;
data.compatInfo = compatInfo;
data.initProfilerInfo = profilerInfo;
//发送BIND_APPLICATION信号给ApplicationThread的H的handler处理
sendMessage(H.BIND_APPLICATION, data);
...
if (andResume)
app.hasShownUi = true;
app.pendingUiClean = true;
app.forceProcessStateUpTo(mService.mTopProcessState);
//通过ActivityThread调用scheduleLaunchActivity来启动Activity
app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,System.identityHashCode(r),
r.info, new Configuration(mService.mConfiguration),
new Configuration(stack.mOverrideConfig), r.compat, r.launchedFromPackage,
task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,newIntents,
...
首先通过sendMessage发送BIND_APPLICATION信号给ApplicationThread，ApplicationThread的handler会对此进行处理。至此，有关进程和线程的流程分析结束，其时序图如下：
3.2.2 AMS对Activity的调度分析
上一节分析了进程的创建和主线程的初始化过程，本节将分析AMS对Activity生命周期的管理调度，此处涉及到的主要有如下几个类：ActivityStackSupervisor，ActivityStack，ActivityThread，ApplicationThread，ApplicationThread.H，Instrumentation等。
系统通过AMS来管理应用进程的生命周期，当然包括应用的Activity、Service等的生命周期。AMS是一个独立的进程，因此它要管理应用进程，必然要进行跨进程交互。其交互如下图：
其中：
ActivityManagerProxy——AMS的代理，供应用进程调用。通过ActivityManagerNative.getDefault()获取。
ApplicationThreadProxy——应用进程的代理，供AMS进程调用。应用启动时，会将应用进程的代理传递到AMS跨进程通信，一般都会用到远程代理。ActivityManagerService继承于Binder即它实现了IBinder接口，所以它可以用于进程间通信。在app进程启动时，会建立一个Binder线程池，专门用来处理Binder IPC事物。
在3.2.1节中已经分析了：AMS在启动好了进程之后，会调用realStartActivityLocked方法来启动Activity，所以先看realStartActivityLocked方法：
//ActivityStackSupervisor.java
final boolean realStartActivityLocked(ActivityRecord r,ProcessRecord app, boolean andResume,
boolean checkConfig) throws RemoteException
...
app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,System.identityHashCode(r),
r.info, new Configuration(mService.mConfiguration),new Configuration(stack.mOverrideConfig),
r.compat, r.launchedFromPackage,task.voiceInteractor, app.repProcState, r.icicle,
r.persistentState, results,newIntents, !andResume, mService.isNextTransitionForward(),
profilerInfo);
...
由代码可知：它是通过调用ActivityThread.ApplicationThread类的scheduleLaunchActivity方法来启动Activity的：
//ActivityThread.java/ApplicationThread内部类
@Override
public final void scheduleLaunchActivity(...)
updateProcessState(procState, false);
ActivityClientRecord r = new ActivityClientRecord();
r.token = token;
r.ident = ident;
...
r.startsNotResumed = notResumed;
r.isForward = isForward;
r.profilerInfo = profilerInfo;
r.overrideConfig = overrideConfig;
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r);
此方法将发送一个LAUNCH_ACTIVITY的Message给ApplicationThread.H的handler进行处理，而此handler将会调用ApplicationThread的handleLaunchActivity进行启动处理：
//ActivityThread.java/ApplicationThread内部类
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent)
...
//确保运行的是最近的配置
handleConfigurationChanged(null, null);
...
//在create Activity之前intialize
WindowManagerGlobal.initialize();
//启动Activity，即OnCreate()
Activity a = performLaunchActivity(r, customIntent);
if (a != null)
r.createdConfig = new Configuration(mConfiguration);
Bundle oldState = r.state;
//调用OnResume()
handleResumeActivity(r.token, false, r.isForward,!r.activity.mFinished
&& !r.startsNotResumed);
...
...
如代码：首先调用performLaunchActivity来启动Activity，即调用OnCreate()生命周期，然后会调用handleResumeActivity方法来调用OnResume()生命周期。首先看performLaunchActivity():
//ActivityThread.java/ApplicationThread内部类
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent)
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null)
//如果packageInfo为空，从ActivityInfo中获取包信息
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);
//从ActivityClientRecord中获取组件名字
ComponentName component = r.intent.getComponent();
if (component == null)
//获取组件
component = r.intent.resolveActivity(mInitialApplication.getPackageManager());
r.intent.setComponent(component);
if (r.activityInfo.targetActivity != null)
//新建组件
component = new ComponentName(r.activityInfo.packageName,r.activityInfo.targetActivity);
Activity activity = null;
//创建Activity
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 (r.isPersistable())
//调用OnCreate()方法
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
else
mInstrumentation.callActivityOnCreate(activity, r.state);
...
if (!r.activity.mFinished)
activity.performStart();
r.stopped = false;
...
return activity;
此方法将会通过类加载器加载组件并新建Activity，然后通过调用Instrumentation的callActivityOnCreate方法来启动Activity，其方法定义如下：
//Instrumentation.java
public void callActivityOnCreate(Activity activity, Bundle icicle,
PersistableBundle persistentState)
prePerformCreate(activity);
activity.performCreate(icicle, persistentState);
postPerformCreate(activity);
此方法里调用了performCreate方法：
//Activity.java
final void performCreate(Bundle icicle)
onCreate(icicle);
mActivityTransitionState.readState(icicle);
performCreateCommon();
显然，它最后会调用定义的Activity的OnCreate()方法，即执行了OnCreate生命周期，同理其他生命周期的执行过程与此类似。OnCreate生命周期的时序图如下：
4、startActivity方法启动Activity的过程分析
本节将对我们熟悉的调用startActivity的启动方式进行分析，同样基于的源码版本是android 6.0，这种情况下，应用进程已经存在，不需要进行应用进程的创建启动（区别于AMS启动Home桌面等启动新应用的方式），首先看startActivity()代码：
@Override
public void startActivity(Intent intent,@Nullable Bundle options)
if(options != null)
startActivityForResult(intent,-1,options);
else
startActivityForResult(intent,-1);
由代码可知，startActivity()最终只是对startActivityForResult()方法进行简单调用，所以只需对startActivityForResult()继续分析：
public void startActivityForResult(Intent intent,int requestCode,@Nullable Bundle options)
if(mParent == null)
//执行Activity启动，并返回启动结果
Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity(this,
mMainThread.getApplicationThread(),mToken,this,intent,requestCode,options);
if(ar != null)
//向主线程发送启动结果
mMainThread.sendActivityResult(mToken,mEmbeddedID,requestCode,ar.getResultCode(),
ar.getResultData());
if(requestCode > 0)
//启动成功
mStartedActivity = true;
cancelInputsAndStartExitTransition(options);
else
//如果mParent不为空，此时最后还是会调用Instrumentation的execStartActivity()方法
if(options != null)
mParent.startActivityFromChild(this,intent,requestCode,options);
else
mParent.startActivityFromChild(this,intent,requestCode);
最后还是会调用Instrumentation的execStartActivity()方法来启动Activity，并将启动的结果返回给该应用的主线程，继续分析execStartActivity()方法：
public ActivityResult execStartActivity(Context who,IBinder contextThread,IBinder token,String
target,Intent intent,int requestCode,Bundle options)
//获取通信存根类，它派发消息给ActivityThread主线程，而主线程的mH的handler会进行相应处理
IApplicationThread whoThread = (IApplicationThread)contextThread;
...
try
intent.migrateExtraStreanToClipData();
intent.prepareToLeaveProcess();
//调用AMS的startActivity()方法，即进入AMS的调度管理
int result = ActivityManagerNative.getDefault().startActivity(whoThread,
who.getBasePackageName(),intent,intent.resolveTypeIfNeeded(
who.getContentResolver()),token,target,requestCode,0,null,options);
//检查启动结果
checkStartActivityResult(result,intent);
catch(RemoteException e)
throw new RuntimeException("Failure from system",e);
return null;
首先获得IApplicationThread对象，它用于与主线程进行通信，ActivityManagerNative.getDefault()获取的是AMS对象，所以此处将启动任务交给了AMS进行调度管理。startActivity方法会调用startActivityAsUser()方法：
@Override
public final int startActivityAsUser(...)
userId = handleIncomingUser(Binder.getCallingPid(),Binder.getCallingUid(),userId,false,
ALLOW_FULL_ONLY,"startActivity",null);
return mStackSupervisor.startActivityMayWait(caller,-1,callingPackage,intent,resolvedType,
null,null,resultTo,resultWho,requestCode,startFlags,profierInfo,null,null,options,false,
userId,null,null);
至此，它的分析同第3节中的过程一致，此处不再做分析，只是本节的启动不需要启动进程，所以它的执行分支不一致。
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