寫這篇部落格，其實我自己還有很多地方原理沒有看懂（像Binder機制達到目前程序跟AMS進行通信），但并不是很影響看懂App啟動主要做了一些什麼事。
看源碼我是帶着1個問題
問題：Application具體在什麼時候建立以及執行的onCreate
衆所周知App應用是從ActivityThread的main方法開始（以友善閱讀我删掉了一些次要代碼）
public static void main(String[] args) {
//從這裡我們可以知道 App以消息來驅動App的各種事件（Handler機制嘛，有機會我會寫一遍關于Handler機制源碼分析的部落格）
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
//接下來，看看ActivityThread的attach的實作
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
private void attach(boolean system) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
// ...省略
RuntimeInit.setApplicationObject(mAppThread.asBinder());
//這裡拿到的IActivityManager,這裡先打住忘下看。我搞清楚mgr它真正是什麼，在哪裡執行個體，搞清這個情況接下來看看ActivityManager的getService()
final IActivityManager mgr = ActivityManager.getService();
try {
//這裡等一等，先搞清楚mgr
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
// Watch for getting close to heap limit.
BinderInternal.addGcWatcher(new Runnable() {
@Override public void run() {
....
});
} else {
//...省略
}
// ...省略
}
/**
* @hide
*/
public static IActivityManager getService() {
//赫赫...點選IActivityManagerSingleton就到了下面
return IActivityManagerSingleton.get();
}
//這裡...
private static final Singleton<IActivityManager> IActivityManagerSingleton =
new Singleton<IActivityManager>() {
@Override
protected IActivityManager create() {
//這裡先記住這個Context.ACTIVITY_SERVICE,
final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
final IActivityManager am = IActivityManager.Stub.asInterface(b);
return am;
//看完這裡，是不是有點什麼感覺，這塊Binder機制來實作擷取到遠端的一個代理類 am
//到此還是沒搞懂這個am真正的執行個體在哪裡，它的實作類是什麼鬼
//要弄清這個問題，我要飙車了，直接去看一下ActivityManagerService的一個方法，請往下看
}
};
//這快的代碼我是在ActivityManagerService拷貝過來的
public void setSystemProcess() {
try {
//這裡，看Context.ACTIVITY_SERVICE 注冊的AMS,看到這裡我們應該知道上面的哪個am
//真正實作就是ActivityManagerService
//别問我怎麼知道的，我也是看了《深入了解android核心設計》AMS專題才知道的，
//要不然鬼知道AMS在這裡注冊的哈
ServiceManager.addService(Context.ACTIVITY_SERVICE, this, true);
ServiceManager.addService(ProcessStats.SERVICE_NAME, mProcessStats);
ServiceManager.addService("meminfo", new MemBinder(this));
ServiceManager.addService("gfxinfo", new GraphicsBinder(this));
ServiceManager.addService("dbinfo", new DbBinder(this));
if (MONITOR_CPU_USAGE) {
ServiceManager.addService("cpuinfo", new CpuBinder(this));
}
ServiceManager.addService("permission", new PermissionController(this));
ServiceManager.addService("processinfo", new ProcessInfoService(this));
//...省略
}
}
OK，回到剛開始的ActivityThread的attach那裡
final IActivityManager mgr = ActivityManager.getService();
try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
到此已經知道了mgr他的真正實作是ActivityManagerService(我直接說AMS了),我們直接去去看看attachApplication(...)的實作（哦對了，先留意一下mAppThread它是目前程序跟遠端AMS通信的接口）
下面是AMS相關實作
@Override
public final void attachApplication(IApplicationThread thread) {
synchronized (this) {
int callingPid = Binder.getCallingPid();
final long origId = Binder.clearCallingIdentity();
//繼續跟，往下看
attachApplicationLocked(thread, callingPid);
Binder.restoreCallingIdentity(origId);
}
}
private final boolean attachApplicationLocked(IApplicationThread thread,
int pid) {
//...此處省略一堆代碼
checkTime(startTime, "attachApplicationLocked: immediately before bindApplication");
mStackSupervisor.mActivityMetricsLogger.notifyBindApplication(app);
//這裡AMS通過收集資訊，然後通過程序通信把資訊回傳App程序。OK,下面我們再往會看看ActivityThread的一個内部類實作的IApplicationThread的bindApplication的方法
if (app.instr != null) {
thread.bindApplication(processName, appInfo, providers,
app.instr.mClass,
profilerInfo, app.instr.mArguments,
app.instr.mWatcher,
app.instr.mUiAutomationConnection, testMode,
mBinderTransactionTrackingEnabled, enableTrackAllocation,
isRestrictedBackupMode || !normalMode, app.persistent,
new Configuration(getGlobalConfiguration()), app.compat,
getCommonServicesLocked(app.isolated),
mCoreSettingsObserver.getCoreSettingsLocked(),
buildSerial);
} else {
thread.bindApplication(processName, appInfo, providers, null, profilerInfo,
null, null, null, testMode,
mBinderTransactionTrackingEnabled, enableTrackAllocation,
isRestrictedBackupMode || !normalMode, app.persistent,
new Configuration(getGlobalConfiguration()), app.compat,
getCommonServicesLocked(app.isolated),
mCoreSettingsObserver.getCoreSettingsLocked(),
buildSerial);
}
// See if the top visible activity is waiting to run in this process...
if (normalMode) {
try {//這裡會有驚喜，這次暫時不做分析這塊流程
if (mStackSupervisor.attachApplicationLocked(app)) {
didSomething = true;
}
} catch (Exception e) {
Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
badApp = true;
}
}
//...省略一路代碼
return true;
}
回到ActivityThread的ApplicationThread（這裡是App程序哦，不是AMS所在程序）
public final void bindApplication(String processName, ApplicationInfo appInfo,
List<ProviderInfo> providers, ComponentName instrumentationName,
ProfilerInfo profilerInfo, Bundle instrumentationArgs,
IInstrumentationWatcher instrumentationWatcher,
IUiAutomationConnection instrumentationUiConnection, int debugMode,
boolean enableBinderTracking, boolean trackAllocation,
boolean isRestrictedBackupMode, boolean persistent, Configuration config,
CompatibilityInfo compatInfo, Map services, Bundle coreSettings,
String buildSerial) {
if (services != null) {
// Setup the service cache in the ServiceManager
ServiceManager.initServiceCache(services);
}
setCoreSettings(coreSettings);
AppBindData data = new AppBindData();
data.processName = processName;
data.appInfo = appInfo;
data.providers = providers;
data.instrumentationName = instrumentationName;
data.instrumentationArgs = instrumentationArgs;
data.instrumentationWatcher = instrumentationWatcher;
data.instrumentationUiAutomationConnection = instrumentationUiConnection;
data.debugMode = debugMode;
data.enableBinderTracking = enableBinderTracking;
data.trackAllocation = trackAllocation;
data.restrictedBackupMode = isRestrictedBackupMode;
data.persistent = persistent;
data.config = config;
data.compatInfo = compatInfo;
data.initProfilerInfo = profilerInfo;
data.buildSerial = buildSerial;
//這裡，sendMessage,然後跟進去其實就是ActivityThread一個内部Handler執行個體，繼續
sendMessage(H.BIND_APPLICATION, data);
}
根據sendMessage的H.BIND_APPLICATION,我跟到了這裡
public void handleMessage(Message msg) {
//...省略其它case
case BIND_APPLICATION:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
AppBindData data = (AppBindData)msg.obj;
handleBindApplication(data);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
//...省略其它case
}
接下來看看handleBindApplication（data）實作
Application app;
try {
//看到這裡，Application在此處執行個體化（次于怎麼建立的感興趣的同學跟進去看看哈）
app = data.info.makeApplication(data.restrictedBackupMode, null);
mInitialApplication = app;
// don't bring up providers in restricted mode; they may depend on the
// app's custom Application class
if (!data.restrictedBackupMode) {
if (!ArrayUtils.isEmpty(data.providers)) {
installContentProviders(app, data.providers);
// For process that contains content providers, we want to
// ensure that the JIT is enabled "at some point".
mH.sendEmptyMessageDelayed(H.ENABLE_JIT, 10*1000);
}
}
//...省略
try {
//這裡跟進去就會發現 Application的onCreate再此處調用
mInstrumentation.callApplicationOnCreate(app);
} catch (Exception e) {
if (!mInstrumentation.onException(app, e)) {
throw new RuntimeException(
"Unable to create application " + app.getClass().getName()
+ ": " + e.toString(), e);
}
}
} finally {
if (data.appInfo.targetSdkVersion <= Build.VERSION_CODES.O
|| StrictMode.getThreadPolicy().equals(writesAllowedPolicy)) {
StrictMode.setThreadPolicy(savedPolicy);
}
}
到此Application的啟動流程夢遊了一遍，看源碼需謹慎帶着問題去源碼裡面走可以不容易迷失在源碼裡面
其實看看這些東西，有些問題可能會不言自明，比如App啟動優化為什麼在Application的onCreate進行優化...
最近一個多月公司大部分精力放在了小程式開發這塊，時間充足，接下來要是有空我會寫幾遍關于App換皮膚，插件化的幹貨部落格附帶Demo
![與專家面對面：Android開發入門問與答[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)