学习笔记：
在前面SystemUI启动流程中说到，在SystemServer中会去启动各种系统服务，这里的launcher也是启动的其中一个服务ActivityManagerService去启动的。在android10之前，系统四大组件的启动都是在ActivityManagerService中，在android10中，单独抽出了一个ActivityTaskManagerService，主要负责Activity的管理和调度。这里先来看下ActivityManagerService服务的启动：
// SystemServer.java
private void startBootstrapServices() {
... ...
// Activity manager runs the show.
traceBeginAndSlog("StartActivityManager");
// TODO: Might need to move after migration to WM.
ActivityTaskManagerService atm = mSystemServiceManager.startService(
ActivityTaskManagerService.Lifecycle.class).getService();
mActivityManagerService = ActivityManagerService.Lifecycle.startService(
mSystemServiceManager, atm);
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
mActivityManagerService.setInstaller(installer);
mWindowManagerGlobalLock = atm.getGlobalLock();
traceEnd();
... ...
}
当SystemServiceManager调用 startService() 时，就会通过反射去创建传进去的class，然后在调用创建对象的 onStart() 方法，这里就是去初始化ActivityTaskManagerService和ActivityManagerService对象，并不会去启动launcher，而且系统服务启动分为三块：
// SystemServer.java
startBootstrapServices(); // 启动引导服务
startCoreServices(); // 启动核心服务
startOtherServices(); // 启动其他服务
在startOtherServices（）中可以看到：
// SystemServer.java
private void startOtherServices() {
... ...
// We now tell the activity manager it is okay to run third party
// code. It will call back into us once it has gotten to the state
// where third party code can really run (but before it has actually
// started launching the initial applications), for us to complete our
// initialization.
mActivityManagerService.systemReady(() -> {
... ...
});
}
这就说明在所有服务初始化完成后，在这里会通知ActivityManagerService的systemReady()启动 launcher 的进程。
下面一起来看 launcher 是如何被启动起来的：
// ActivityManagerService.java
public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) {
... ...
mAtmInternal.startHomeOnAllDisplays(currentUserId, "systemReady");
... ...
mAtmInternal.resumeTopActivities(false /* scheduleIdle */);
... ...
}
mAtmInternal类型是ActivityTaskManagerInternal，这是一个抽象类，其实现是ActivityTaskManagerService的内部类LocalService：
// ActivityTaskManagerService.java
final class LocalService extends ActivityTaskManagerInternal {
... ...
@Override
public boolean startHomeOnAllDisplays(int userId, String reason) {
synchronized (mGlobalLock) {
return mRootActivityContainer.startHomeOnAllDisplays(userId, reason);
}
}
... ...
@Override
public void resumeTopActivities(boolean scheduleIdle){
synchronized (mGlobalLock){
mRootActivityContainer.resumeFocusedStacksTopActivities();
if(scheduleIdle){
mStackSupervisor.scheduleIdleLocked();
}
}
}
... ...
}
这里的mRootActivityContainer类型是RootActivityContainer，按照文档上的说明，这是一个临时类，主要是将ActivityStackSupervisor.java中的一部分逻辑分离出来，在android11中，这个类就已经找不到了。替换成了RootWindowContainer，这里我们跟进 mRootActivityContainer.startHomeOnAllDisplays() 方法看，最终会调用到：startHomeOnDisplay() :
// RootActivityContainer.java
boolean startHomeOnDisplay(int userId,String reason,int displayId,boolean allowInstrumenting,boolean fromHomeKey){
// Fallback to top focused display if the displayId is invalid.
if(displayId==INVALID_DISPLAY){
displayId=getTopDisplayFocusedStack().mDisplayId;
}
Intent homeIntent=null;
ActivityInfo aInfo=null;
if(displayId==DEFAULT_DISPLAY){
//拿到需要启动launcher的intent，通过resolveHomeActivity解析出需要启动的Activity
homeIntent=mService.getHomeIntent();
aInfo=resolveHomeActivity(userId,homeIntent);
}else if(shouldPlaceSecondaryHomeOnDisplay(displayId)){
Pair<ActivityInfo, Intent> info=resolveSecondaryHomeActivity(userId,displayId);
aInfo=info.first;
homeIntent=info.second;
}
if(aInfo==null||homeIntent==null){
return false;
}
if(!canStartHomeOnDisplay(aInfo,displayId,allowInstrumenting)){
return false;
}
// Updates the home component of the intent.
homeIntent.setComponent(new ComponentName(aInfo.applicationInfo.packageName,aInfo.name));
homeIntent.setFlags(homeIntent.getFlags()|FLAG_ACTIVITY_NEW_TASK);
// Updates the extra information of the intent.
if(fromHomeKey){
homeIntent.putExtra(WindowManagerPolicy.EXTRA_FROM_HOME_KEY,true);
}
// Update the reason for ANR debugging to verify if the user activity is the one that
// actually launched.
final String myReason=reason+":"+userId+":"+UserHandle.getUserId(
aInfo.applicationInfo.uid)+":"+displayId;
mService.getActivityStartController().startHomeActivity(homeIntent,aInfo,myReason,
displayId);
return true;
}
这里可以分为两步看：
1、通过ActivityTaskManagerService的getHomeIntent（）获取到需要启动的intent，在通过resolveHomeActivity（）解析出需要启动Activity的信息，
2、mService.getActivityStartController()获取到的是ActivityStartController，这个类的主要作用是用于控制委派启动的Activity。
先来看下ActivityTaskManagerService的getHomeIntent() :
// ActivityTaskManagerService.java
Intent getHomeIntent() {
Intent intent = new Intent(mTopAction, mTopData != null ? Uri.parse(mTopData) : null);
intent.setComponent(mTopComponent);
intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) {
intent.addCategory(Intent.CATEGORY_HOME);
}
return intent;
}
注意看 intent.addCategory(Intent.CATEGORY_HOME) ，这个代表的就是要启动Activity的意图，通常来说，整个系统的只会有一个应用会在清单文件中配置CATEGORY_HOME，如果配置了多个，系统在启动的时候就会要求用户手动去选择哪个作为启动应用，如果在系统设置应用中进行配置了，就会选择配置的那个应用启动。（这个 CATEGORY_HOME 配置，说白了就只有 launcher 程序才会有）。
回到主线，接着看mService.getActivityStartController().startHomeActivity() :
// ActivityStartController.java
void startHomeActivity(Intent intent, ActivityInfo aInfo, String reason,
TaskDisplayArea taskDisplayArea) {
final ActivityOptions options = ActivityOptions.makeBasic();
options.setLaunchWindowingMode(WINDOWING_MODE_FULLSCREEN);
if (!ActivityRecord.isResolverActivity(aInfo.name)) {
// The resolver activity shouldn't be put in home stack because when the foreground is
// standard type activity, the resolver activity should be put on the top of current
// foreground instead of bring home stack to front.
options.setLaunchActivityType(ACTIVITY_TYPE_HOME);
}
final int displayId = taskDisplayArea.getDisplayId();
options.setLaunchDisplayId(displayId);
options.setLaunchTaskDisplayArea(taskDisplayArea.mRemoteToken
.toWindowContainerToken());
// The home activity will be started later, defer resuming to avoid unneccerary operations
// (e.g. start home recursively) when creating home stack.
mSupervisor.beginDeferResume();
final ActivityStack homeStack;
try {
// Make sure home stack exists on display area.
homeStack = taskDisplayArea.getOrCreateRootHomeTask(ON_TOP);
} finally {
mSupervisor.endDeferResume();
}
mLastHomeActivityStartResult = obtainStarter(intent, "startHomeActivity: " + reason)
.setOutActivity(tmpOutRecord)
.setCallingUid(0)
.setActivityInfo(aInfo)
.setActivityOptions(options.toBundle())
.execute();
mLastHomeActivityStartRecord = tmpOutRecord[0];
if (homeStack.mInResumeTopActivity) {
// If we are in resume section already, home activity will be initialized, but not
// resumed (to avoid recursive resume) and will stay that way until something pokes it
// again. We need to schedule another resume.
mSupervisor.scheduleResumeTopActivities();
}
}
/**
* @return A starter to configure and execute starting an activity. It is valid until after
* {@link ActivityStarter#execute} is invoked. At that point, the starter should be
* considered invalid and no longer modified or used.
*/
ActivityStarter obtainStarter(Intent intent, String reason) {
return mFactory.obtain().setIntent(intent).setReason(reason);
}
这里主要是先获取一个ActivityStarter（主要用于启动Activity），然后把需要的参数设置进去，最后再调用它的ActivityStarter.execute（）方法：
// ActivityStarter.java
int execute() {
try {
... ...
final LaunchingState launchingState;
synchronized (mService.mGlobalLock) {
final ActivityRecord caller = ActivityRecord.forTokenLocked(mRequest.resultTo);
// 通过ActivityMetricsLogger.notifyActivityLaunching创建LaunchingState并记录创建Activity开始的时间
launchingState = mSupervisor.getActivityMetricsLogger().notifyActivityLaunching(
mRequest.intent, caller);
}
... ...
// 执行启动请求
res = executeRequest(mRequest);
... ...
// Notify ActivityMetricsLogger that the activity has launched.
// ActivityMetricsLogger will then wait for the windows to be drawn and populate
// WaitResult.
// 通过ActivityMetricsLogger.notifyActivityLaunched记录Activity启动完成的时间
mSupervisor.getActivityMetricsLogger().notifyActivityLaunched(launchingState, res,
mLastStartActivityRecord);
// 同时将Request.waitResult添加到ActivityStackSupervisor的mWaitingActivityLaunched中，等待窗口绘制完成
return getExternalResult(mRequest.waitResult == null ? res
: waitForResult(res, mLastStartActivityRecord));
} finally {
onExecutionComplete();
}
}
通过上面代码我们知道，启动是在 executeRequest() 方法里：
// ActivityStarter.java
private int executeRequest(Request request) {
... ...
//调用 startActivityUnchecked
mLastStartActivityResult = startActivityUnchecked(r, sourceRecord, voiceSession,
request.voiceInteractor, startFlags, true /* doResume */, checkedOptions, inTask,
restrictedBgActivity, intentGrants);
return mLastStartActivityResult;
}
ActivityStarter.startActivityUnchecked()
// ActivityStarter.java
private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, Task inTask,
boolean restrictedBgActivity, NeededUriGrants intentGrants) {
int result = START_CANCELED;
... ...
try {
//延时布局
mService.deferWindowLayout();
//执行startActivityInner
result = startActivityInner(r, sourceRecord, voiceSession, voiceInteractor,
startFlags, doResume, options, inTask, restrictedBgActivity, intentGrants);
} finally {
//恢复布局
mService.continueWindowLayout();
}
... ...
return result;
}
ActivityStarter.startActivityInner()
int startActivityInner() {
... ...
if (mDoResume) {
//ActivityRecord 记录着 Activity 信息
final ActivityRecord topTaskActivity =
mStartActivity.getTask().topRunningActivityLocked();
if (!mTargetStack.isTopActivityFocusable()
|| (topTaskActivity != null && topTaskActivity.isTaskOverlay()
&& mStartActivity != topTaskActivity)) {
mTargetStack.getDisplay().mDisplayContent.executeAppTransition();
} else {
//执行resumeFocusedStacksTopActivities
mRootWindowContainer.resumeFocusedStacksTopActivities(
mTargetStack, mStartActivity, mOptions);
}
}
... ...
return START_SUCCESS;
}
RootWindowContainer.resumeFocusedStacksTopActivities()
// RootWindowContainer.java
boolean resumeFocusedStacksTopActivities(
ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
... ...
//如果是栈顶Activity，启动resumeTopActivityUncheckedLocked
if (targetStack != null && (targetStack.isTopStackInDisplayArea()
|| getTopDisplayFocusedStack() == targetStack)) {
result = targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}
... ...
if (!resumedOnDisplay) {
//获取栈顶的 ActivityRecord
final ActivityStack focusedStack = display.getFocusedStack();
if (focusedStack != null) {
//执行resumeTopActivityUncheckedLocked
result |= focusedStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}else if (targetStack == null) {
result |= resumeHomeActivity(null /* prev */, "no-focusable-task",
display.getDefaultTaskDisplayArea());
}
}
return result;
}
ActivityStack.resumeTopActivityUncheckedLocked()
// ActivityStack.java
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
if (mInResumeTopActivity) {
// Don't even start recursing.
return false;
}
boolean result = false;
try {
mInResumeTopActivity = true;
// 执行 resumeTopActivityInnerLocked,
// 最终调用到 ActivityStackSupervisor.startSpecificActivity()
result = resumeTopActivityInnerLocked(prev, options);
final ActivityRecord next = topRunningActivity(true /* focusableOnly */);
if (next == null || !next.canTurnScreenOn()) {
checkReadyForSleep();
}
} finally {
mInResumeTopActivity = false;
}
return result;
}
ActivityStackSupervisor.startSpecificActivity()
// ActivityStackSupervisor.java
void startSpecificActivity(ActivityRecord r, boolean andResume, boolean checkConfig) {
// Is this activity's application already running?
final WindowProcessController wpc =
mService.getProcessController(r.processName, r.info.applicationInfo.uid);
boolean knownToBeDead = false;
//进程存在
if (wpc != null && wpc.hasThread()) {
try {
//真正启动Activity方法
realStartActivityLocked(r, wpc, 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.
knownToBeDead = true;
}
r.notifyUnknownVisibilityLaunchedForKeyguardTransition();
final boolean isTop = andResume && r.isTopRunningActivity();
//进程不存在 mService =ATMS
mService.startProcessAsync(r, knownToBeDead, isTop, isTop ? "top-activity" : "activity");
}
startSpecificActivity() 这个方法是关键方法，如果进程已经存在直接执行 realStartActivityLocked 去启动 Activity，进程不存在则通过AMS去创建 Socket，然后通知 Zygote 去 fork 进程。由于这里第一次创建，所以会走到 startProcessAsync() 。
Launcher启动流程调用栈：
launcher启动流程调用栈.png
创建Socket建立连接
ActivityTaskManagerService.startProcessAsync()
void startProcessAsync(ActivityRecord activity, boolean knownToBeDead, boolean isTop,
String hostingType) {
try {
if (Trace.isTagEnabled(TRACE_TAG_WINDOW_MANAGER)) {
Trace.traceBegin(TRACE_TAG_WINDOW_MANAGER, "dispatchingStartProcess:"
+ activity.processName);
}
// Post message to start process to avoid possible deadlock of calling into AMS with the
// ATMS lock held.
final Message m = PooledLambda.obtainMessage(ActivityManagerInternal::startProcess,
mAmInternal, activity.processName, activity.info.applicationInfo, knownToBeDead,
isTop, hostingType, activity.intent.getComponent());
mH.sendMessage(m);
} finally {
Trace.traceEnd(TRACE_TAG_WINDOW_MANAGER);
}
}
通过handler发消息执行ActivityManagerInternal.startProcess() ，内部又转调用ActivityManagerService.startProcessLocked() :
@GuardedBy("this")
final ProcessRecord startProcessLocked(String processName,
ApplicationInfo info, boolean knownToBeDead, int intentFlags,
HostingRecord hostingRecord, int zygotePolicyFlags, boolean allowWhileBooting,
boolean isolated, boolean keepIfLarge) {
return mProcessList.startProcessLocked(processName, info, knownToBeDead, intentFlags,
hostingRecord, zygotePolicyFlags, allowWhileBooting, isolated, 0 /* isolatedUid */,
keepIfLarge, null /* ABI override */, null /* entryPoint */,
null /* entryPointArgs */, null /* crashHandler */);
}
mProcessList类型是ProcessList，这是一个进程管理类，描述了进程的adj值，当系统资源吃紧时，就会根据这里描述的adj去判断杀死哪个应用来释放资源，可以通过adb shell dumpsys meminfo来查看当前所有进程的分类情况，接着来看下ProcessList.startProcessLocked():
// ProcessList.java
final ProcessRecord startProcessLocked(String processName, ApplicationInfo info,
boolean knownToBeDead, int intentFlags, HostingRecord hostingRecord,
boolean allowWhileBooting, boolean isolated, int isolatedUid, boolean keepIfLarge,
String abiOverride, String entryPoint, String[] entryPointArgs, Runnable crashHandler) {
ProcessRecord app;
... ...
checkSlow(startTime, "startProcess: stepping in to startProcess");
final boolean success = startProcessLocked(app, hostingRecord, abiOverride);
checkSlow(startTime, "startProcess: done starting proc!");
return success ? app : null;
}
boolean startProcessLocked(ProcessRecord app, HostingRecord hostingRecord,
boolean disableHiddenApiChecks, boolean mountExtStorageFull,
String abiOverride) {
... ...
try {
... ...
// Start the process. It will either succeed and return a result containing
// the PID of the new process, or else throw a RuntimeException.
final String entryPoint = "android.app.ActivityThread";
return startProcessLocked(hostingRecord, entryPoint, app, uid, gids,
runtimeFlags, mountExternal, seInfo, requiredAbi, instructionSet, invokeWith,
startTime);
} catch (RuntimeException e) {
Slog.e(ActivityManagerService.TAG, "Failure starting process " + app.processName, e);
// Something went very wrong while trying to start this process; one
// common case is when the package is frozen due to an active
// upgrade. To recover, clean up any active bookkeeping related to
// starting this process. (We already invoked this method once when
// the package was initially frozen through KILL_APPLICATION_MSG, so
// it doesn't hurt to use it again.)
mService.forceStopPackageLocked(app.info.packageName, UserHandle.getAppId(app.uid),
false, false, true, false, false, app.userId, "start failure");
return false;
}
}
boolean startProcessLocked(HostingRecord hostingRecord,String entryPoint,
ProcessRecord app, int uid, int[] gids, int runtimeFlags, int mountExternal,
String seInfo, String requiredAbi, String instructionSet, String invokeWith,long startTime) {
... ...
final Process.ProcessStartResult startResult = startProcess(app.hostingRecord,entryPoint, app, app.startUid, gids, runtimeFlags, mountExternal,app.seInfo, requiredAbi, instructionSet, invokeWith, app.startTime);
... ...
}
private Process.ProcessStartResult startProcess(HostingRecord hostingRecord, String entryPoint,
ProcessRecord app, int uid, int[] gids, int runtimeFlags, int mountExternal,
String seInfo, String requiredAbi, String instructionSet, String invokeWith,
long startTime) {
try {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "Start proc: " +
app.processName);
checkSlow(startTime, "startProcess: asking zygote to start proc");
final Process.ProcessStartResult startResult;
if (hostingRecord.usesWebviewZygote()) {
... ...
} else if (hostingRecord.usesAppZygote()) {
final AppZygote appZygote = createAppZygoteForProcessIfNeeded(app);
startResult = appZygote.getProcess().start(entryPoint,
app.processName, uid, uid, gids, runtimeFlags, mountExternal,
app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
app.info.dataDir, null, app.info.packageName,
/*useUsapPool=*/ false,
new String[] {PROC_START_SEQ_IDENT + app.startSeq});
} else {
... ...
}
checkSlow(startTime, "startProcess: returned from zygote!");
return startResult;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
}
}
这里有几个我们需要关注的点，一个是第二个方法定义的entryPoint="android.app.ActivityThread"，这个就是后面创建进程时会通过反射调用到的类，再来看最后一个方法，这里会执行else if语句，也就是执行ZygoteProcess的start（）方法。最终到 ZygoteProcess 的attemptUsapSendArgsAndGetResult 通过 Socket 通信，fork 一个新的 Launcher 进程，调用过程如图：
创建Socket建立连接调用流程.png
ZygoteProcess.attemptZygoteSendArgsAndGetResult()
// ZygoteProcess.java
private Process.ProcessStartResult attemptZygoteSendArgsAndGetResult(
ZygoteState zygoteState, String msgStr) throws ZygoteStartFailedEx {
try {
final BufferedWriter zygoteWriter = zygoteState.mZygoteOutputWriter;
final DataInputStream zygoteInputStream = zygoteState.mZygoteInputStream;
zygoteWriter.write(msgStr);把应用进程的一些参数写给前面连接的zygote进程
zygoteWriter.flush();//进入Zygote进程，处于阻塞状态
//从socket中得到zygote创建的应用pid，赋值给 ProcessStartResult的对象
Process.ProcessStartResult result = new Process.ProcessStartResult();
result.pid = zygoteInputStream.readInt();
result.usingWrapper = zygoteInputStream.readBoolean();
return result;
} catch (IOException ex) {
zygoteState.close();
}
}
Zygote进程处理
ZygoteInit.main
public static void main(String argv[]) {
Runnable caller;
if (startSystemServer) {
//Zygote Fork出的第一个进程 SystmeServer
Runnable r = forkSystemServer(abiList, zygoteSocketName, zygoteServer);
if (r != null) {
r.run();
return;
}
}
//循环等待fork出其他的应用进程，比如Launcher
caller = zygoteServer.runSelectLoop(abiList);
...
if (caller != null) {
caller.run(); //执行runSelectLoop返回的Runnable对象，进入子进程
}
}
ZygoteServer.runSelectLoop
Runnable runSelectLoop(String abiList) {
while (true) {
int pollReturnValue;
try {
//epoll机制 循环
pollReturnValue = Os.poll(pollFDs, pollTimeoutMs);
} catch (ErrnoException ex) {
throw new RuntimeException("poll failed", ex);
}
...
//来了消息后，调用processOneCommand()来进行进程的处理
final Runnable command = connection.processOneCommand(this);
}
}
ZygoteConnection.processOneCommand
Runnable processOneCommand(ZygoteServer zygoteServer) {
//fork进程，得到新进程pid
//pid=0 表示Zygote fork子进程成功
//pid > 0 表示子进程 的真正的PID
pid = Zygote.forkAndSpecialize(parsedArgs.mUid, parsedArgs.mGid, parsedArgs.mGids,
parsedArgs.mRuntimeFlags, rlimits, parsedArgs.mMountExternal, parsedArgs.mSeInfo,
parsedArgs.mNiceName, fdsToClose, fdsToIgnore, parsedArgs.mStartChildZygote,
parsedArgs.mInstructionSet, parsedArgs.mAppDataDir, parsedArgs.mIsTopApp,
parsedArgs.mPkgDataInfoList, parsedArgs.mWhitelistedDataInfoList,
parsedArgs.mBindMountAppDataDirs, parsedArgs.mBindMountAppStorageDirs);
try {
//fork成功，第一次返回的pid = 0
if (pid == 0) {
return handleChildProc(parsedArgs, childPipeFd, parsedArgs.mStartChildZygote);
} else {
handleParentProc(pid, serverPipeFd);
return null;
}
}
** ZygoteConnection.handleChildProc**
这里主要执行到 ZygoteInit.zygoteInit，zygoteInit 进行一些环境的初始化、启动Binder进程等操作，最终反射执行 ActivityThread.main
private Runnable handleChildProc(ZygoteArguments parsedArgs,
FileDescriptor pipeFd, boolean isZygote) {
closeSocket();
Zygote.setAppProcessName(parsedArgs, TAG);
if (!isZygote) {
//App进程将会调用到这里，最终反射执行ActivityThread.main
return ZygoteInit.zygoteInit(parsedArgs.mTargetSdkVersion,
parsedArgs.mDisabledCompatChanges,
parsedArgs.mRemainingArgs, null /* classLoader */);
} else {
return ZygoteInit.childZygoteInit(parsedArgs.mTargetSdkVersion,
parsedArgs.mRemainingArgs, null /* classLoader */);
}
}
Zygote进程调用栈:
Zygote进程调用栈.png
ActivityThread中处理
Zygote fork出了 Launcher 的进程，并把接下来的 Launcher 启动任务交给了 ActivityThread 来进行，接下来我们就从 ActivityThread.main方法来分析 Launcher 的创建过程。
// ActivityThread.java
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
... ...
//创建主线程Looper
Looper.prepareMainLooper();
... ...
ActivityThread thread = new ActivityThread();
//执行attach
... ...
thread.attach(false, startSeq);
//开启循环
... ...
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
这里就是android层面划分为一个应用进程的开始，初始化一个looper，也就是android中说的主线程，并开始looper循环，这里调用到了ActivitThread.attach() :
// ActivityThread.java
private void attach(boolean system, long startSeq) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
... ...
// 这里会拿到ActivityManagerService的代理
final IActivityManager mgr = ActivityManager.getService();
try {
//应用的句柄发给AMS，从而使AMS可以管理新进程
mgr.attachApplication(mAppThread, startSeq);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
... ...
} else {
// 在前面讲Zygote进程的时候，里面创建ActivityThread就会执行到这里
}
//当系统配置发生变更时会执行这个回调
ViewRootImpl.ConfigChangedCallback configChangedCallback
= (Configuration globalConfig) -> {
... ...
};
ViewRootImpl.addConfigCallback(configChangedCallback);
}
这里会执行if语句里面的内容，mAppThread的类型是ApplicationThread，这个类的主要作用是在ActivityManagerService中回调回ActivityThread中来，mgr是ActivityManagerService的代理，在执行它的ActivityManagerService.attachApplication() 方法：
// ActivityManagerService.java
public final void attachApplication(IApplicationThread thread, long startSeq) {
synchronized (this) {
int callingPid = Binder.getCallingPid();
final int callingUid = Binder.getCallingUid();
final long origId = Binder.clearCallingIdentity();
attachApplicationLocked(thread, callingPid, callingUid, startSeq);
Binder.restoreCallingIdentity(origId);
}
}
ActivityManagerService.attachApplicationLocked()
调用AMS的 attachApplication()，最终层层调用到 ActivityStackSupervisor 的 realStartActivityLocked，真正准备去启动Activity。
private final boolean attachApplicationLocked(IApplicationThread thread,int pid, int callingUid, long startSeq) {
... ...
//这里会去调用ActivityThrea的bindApplication(),也就是会去创建Application
thread.bindApplication(... ...)
... ...
//如果当前的Application记录仍然依附到之前的进程中，则清理掉
if (app.thread != null) {
handleAppDiedLocked(app, true, true);
}
... ...
// See if the top visible activity is waiting to run in this process...
if (normalMode) {
try {
//调用ActivityTaskManagerService的attachApplication()，最终层层调用到ActivityStackSupervisor.java的 realStartActivityLocked()
didSomething = mAtmInternal.attachApplication(app.getWindowProcessController());
} catch (Exception e) {
Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
badApp = true;
}
}
... ...
}
这里通过AMS最终会调用到ActivityStackSupervisor.realStartActivityLocked()：
// ActivityStackSupervisor.java
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
boolean andResume, boolean checkConfig) throws RemoteException {
//确保所有的Activity执行了onPause才会往下继续执行
if (!mRootActivityContainer.allPausedActivitiesComplete()) {
// While there are activities pausing we skipping starting any new activities until
// pauses are complete. NOTE: that we also do this for activities that are starting in
// the paused state because they will first be resumed then paused on the client side.
if (DEBUG_SWITCH || DEBUG_PAUSE || DEBUG_STATES) Slog.v(TAG_PAUSE,
"realStartActivityLocked: Skipping start of r=" + r
+ " some activities pausing...");
return false;
}
... ...
try {
... ...
try{
... ...
// Create activity launch transaction.
final ClientTransaction clientTransaction = ClientTransaction.obtain(
proc.getThread(), r.appToken);
final DisplayContent dc = r.getDisplay().mDisplayContent;
//在回调序列的末尾添加一条消息
clientTransaction.addCallback(LaunchActivityItem.obtain(... ...));
// Set desired final state.
final ActivityLifecycleItem lifecycleItem;
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
//添加最后执行的生命周期状态
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// 这里其实就是执行ClientTransaction的schedule()方法
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
... ...
} catch (RemoteException e) {
if (r.launchFailed) {
// 第二次启动失败，finish activity并放弃启动
Slog.e(TAG, "Second failure launching "
+ r.intent.getComponent().flattenToShortString() + ", giving up", e);
proc.appDied();
stack.requestFinishActivityLocked(r.appToken, Activity.RESULT_CANCELED, null,
"2nd-crash", false);
return false;
}
//第一次启动失败，尝试重启进程
r.launchFailed = true;
proc.removeActivity(r);
throw e;
}
} finally {
endDeferResume();
}
... ...
return true;
}
ActivityThread调用栈:
ActivityThread调用栈.png
小结
至此已经走到 realStartActivityLocked，接下来就是 Activity 的启动流程，Activity 启动下篇文章会单独分析。总结一下 Launcher 启动流程，Launcher的启动经过了三个阶段：
1、SystemServer 创建AMS和ATMS，通过 SystemReady 进入 LauncherActivity 的调用
2、Zygote 进程 fork 出 Launcher 进程，通过 Socket 进行通信
3、进入 ActivityThread 的处理，完成 Launcher 的 Acitivty 启动
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