ActivityStackSupervisor#realStartActivityLocked（以下源码都是基于API25）
realStartActivityLocked方法中有一段非常重要的代码
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(task.mOverrideConfig), r.compat, r.launchedFromPackage,
task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,
newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo);
//...
}
- 其中app是realStartActivityLocked方法传入的参数，其类型是ProcessRecord（包含当前正在运行的特定进程的完整信息）。app.thread的类型则是IApplicationThread。
- IApplicationThread接口继承了IInterface接口，所以它是一个Binder类型的接口。该接口内部包含了大量启动、停止Activity的方法。该接口的最终实现者是ActivityThread的内部类ApplicationThread
public final class ActivityThread {
private class ApplicationThread extends ApplicationThreadNative {...}
}
public abstract class ApplicationThreadNative extends Binder
implements IApplicationThread {...}
ActivityThread.ApplicationThread#scheduleLaunchActivity
// we use token to identify this activity without having to send the
// activity itself back to the activity manager. (matters more with ipc)
@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 pendingResults, List pendingNewIntents,
boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {
updateProcessState(procState, false);
ActivityClientRecord r = new ActivityClientRecord();
//一些r的配置
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r); //H是ActivityThread的一个内部类，其继承自Handler
}
该方法的实现很简单，就是发送一个启动activity的消息交给H处理。
ActivityThread.H#handleMessage
private class H extends Handler {
//...
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
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"); //继续调用
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
//...
}
}
H对这条消息的处理是：调用ActivityThread的handleLaunchActivity方法启动activity
ActivityThread#handleLaunchActivity
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
//...
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
//...
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
} else {
//...
}
}
其中，performLaunchActivity方法最终完成了activity对象的创建和启动，并且通过handleResumeActivity方法来调用该activity对象的onResume这一生命周期方法。
ActivityThread#performLaunchActivity
该方法主要完成了这几件事：
1. 从ActivityClientRecord中获取待启动的activity的组件信息
2. 通过Instrumentation的newActivity方法使用类加载器创建Activity对象
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent); //创建Activity对象并最终返回该对象
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
//...
}
3. 通过LoadedApk的makeApplication方法来尝试创建Application对象
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
//r是一个ActivityClientRecord（ActivityThread的静态内部类）对象
//r.packageInfo是一个LoadedApk对象
LoadedApk#makeApplication
public Application makeApplication(boolean forceDefaultAppClass,
Instrumentation instrumentation) {
if (mApplication != null) { //如果已经创建过Application，就直接返回该Application
return mApplication;
}
Application app = null;
try {
//...
app = mActivityThread.mInstrumentation.newApplication(
cl, appClass, appContext);
//与创建Activity对象类似，创建Application对象也是通过Instrumentation并使用类加载器完成的
} catch (Exception e) {
//...
}
mActivityThread.mAllApplications.add(app);
mApplication = app;
if (instrumentation != null) {
try {
instrumentation.callApplicationOnCreate(app); //调用Application的onCreate方法
} catch (Exception e) {
//...
}
}
return app;
}
可以看出，如果之前已经创建过Application对象，就直接返回该对象，这保证了一个应用只有一个Application对象。Application对象创建完毕后，会通过Instrumentation的callApplicationOnCreate方法来调用Application的onCreate方法
4. 创建ContextImpl对象并通过Activity的attach方法来完成一项重要数据的初始化
Context appContext = createBaseContextForActivity(r, activity); //最终返回的是ContextImpl对象
//...
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);
- ContextImpl是Context抽象类的具体实现，ContextImpl是通过Activity的attach方法来和Activity建立关联的。
- attach方法除了关联ContextImpl和Activity，还会完成Window的创建并建立Activity和Windwo的关联，这样当Window接收到外部输入事件后就可以将事件传递给Activity。
5. 调用Activity的onCreate方法
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
Activity的onCreate方法的调用，也意味着Activity已经完成了整个启动过程。
参考
- 《Android开发艺术探索》