通常情况下，我们在显式调用Activity的情况下，只需要通过如下代码就能启动一个Activity：
Intent intent = new Intent(this, TestActivity.class);
startActivity(intent);
通过上面的代码就能启动一个Activity，之后新的Activity就能被系统展示给用户。
那么系统是如何启动一个Activity？
新的Activity对象是在什么情况下被创建的？
onCreate是在什么时机被系统回调的？
带着这些问题，我们来一起分析下Activity的启动过程。
我们通过startActivity开始分析：
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
@Nullable Bundle options) {
if (mParent == null) {
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);
}
}
}
通过上面的代码可以发现调用了mInstrumentation.execStartActivity(this, mMainThread.getApplicationThread(), mToken, this,intent, requestCode, options)这个方法：
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);
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 = 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 new RuntimeException("Failure from system", e);
}
return null;
}
通过上面的代码可以发现，启动Activity的真正实现是ActivityManangerNative.getDefault()的startActivity方法来完成的。
public abstract class ActivityManagerNative extends Binder implements IActivityManager{
}
通过观察ActivityManangerNative可以发现继承了Binder并实现了IActivityManager，所以ActivityManangerNative是一个Binder对象：
private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
protected IActivityManager create() {
IBinder b = ServiceManager.getService("activity");
if (false) {
Log.v("ActivityManager", "default service binder = " + b);
}
IActivityManager am = asInterface(b);
if (false) {
Log.v("ActivityManager", "default service = " + am);
}
return am;
}
};
通过ActivityManangerNative.getDefault()方法可以看到这是一个单例的封装：
IBinder b = ServiceManager.getService("activity");
通过ServiceManger来获取到真正的Binder对象并返回，而这个对象就是ActivityManagerService，那么怎么确定就是它呢？可以参考PackageManagerService的创建过程。
到此为止，Activity的启动过程就转到了ActivityManagerService中(不同版本的源码略有区别)：
ActivityManagerService.startActivity
-> ActivityManagerService.startActivityAsUser
-> ActivityStarter.startActivityMayWait
-> ActivityStarter.startActivityLocked
-> ActivityStarter.startActivityUnchecked
-> ActivityStackSupervisor.resumeFocusedStackTopActivityLocked
-> ActivityStack.resumeTopActivityUncheckedLocked
-> ActivityStack.resumeTopActivityInnerLocked
-> ActivityStackSupervisor.startSpecificActivityLocked
-> ActivityStackSupervisor.realStartActivityLocked
通过不断的跳转最终跳转到了ActivityStackSupervisor.realStartActivityLocked方法中：
app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),
new Configuration(task.mOverrideConfig), r.compat, r.launchedFromPackage,
task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,
newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo);
app.thread是IApplicationThread的类型：
public interface IApplicationThread extends IInterface {}
而IApplicationThread是IInterface类型，所以它也是一个Binder类型的接口，从IApplicationThread的声明可以看出，其中包含了大量的启动、停止Activity的接口，还包含启动和停止Service的接口。通过声明可以猜测这个Binder接口完成了大量的Activity和Service启动和停止相关的功能。
那么IApplicationThread的实现又是什么呢？
我们通过追溯thread可以知道是ApplicationThread：
@Override
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
int procState, Bundle state, PersistableBundle persistentState,
List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {
updateProcessState(procState, false);
ActivityClientRecord r = new ActivityClientRecord();
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;
r.pendingIntents = pendingNewIntents;
r.startsNotResumed = notResumed;
r.isForward = isForward;
r.profilerInfo = profilerInfo;
r.overrideConfig = overrideConfig;
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r);
}
这个方法很简单，就是构造了ActivityClientRecord并赋值，之后发送一个消息给Handler，这个Handler就是主线程的H：
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, "LAUNCH_ACTIVITY");
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
Activity a = performLaunchActivity(r, customIntent);
}
最终performLaunchActivity完成了Activity对象的创建和启动过程。
这个方法主要完成了这几个操作：
-
通过
ActivityClientRecord获取到需要启动的Activity的组件信息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); } -
通过类加载器创建Activity
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); } } public Activity newActivity(ClassLoader cl, String className, Intent intent) throws InstantiationException, IllegalAccessException, ClassNotFoundException { return (Activity)cl.loadClass(className).newInstance(); } -
通过makeApplication尝试创建Application
Application app = r.packageInfo.makeApplication(false, mInstrumentation);从makeApplication中也可以看到如果Application已经存在了，那么就不会在创建Application，保证一个应用只有一个Application对象。Application也是通过类加载器来创建的。
instrumentation.callApplicationOnCreate(app);当Application第一次创建之后就会调用Application的onCreate方法，这就是为什么当启动一个App的时候，Application会被首先调用的原因。
-
创建ContextImpl对象并通过Activity的attach方法来完成一系列操作
Context appContext = createBaseContextForActivity(r, activity); CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager()); Configuration config = new Configuration(mCompatConfiguration); if (r.overrideConfig != null) { config.updateFrom(r.overrideConfig); } if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity " + r.activityInfo.name + " with config " + config); Window window = null; if (r.mPendingRemoveWindow != null && r.mPreserveWindow) { window = r.mPendingRemoveWindow; r.mPendingRemoveWindow = null; r.mPendingRemoveWindowManager = null; } activity.attach(appContext, this, getInstrumentation(), r.token, r.ident, app, r.intent, r.activityInfo, title, r.parent, r.embeddedID, r.lastNonConfigurationInstances, config, r.referrer, r.voiceInteractor, window);attach方法会关联ContextImpl对象，还会关联Window对象，并建立自己和Window对象的关联。
-
调用Activity的onCreate方法
mInstrumentation.callActivityOnCreate(activity, r.state); -> activity.performCreate(icicle); final void performCreate(Bundle icicle) { restoreHasCurrentPermissionRequest(icicle); onCreate(icicle); mActivityTransitionState.readState(icicle); performCreateCommon(); }最终调用了我们熟悉的onCreate的生命周期中，此时Activity就完成了整个的启动过程。
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