Android软件的主入口也是main，这个main方法定义在ActivityThread中：
public static void main(String[] args) {
...
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
...
Looper.loop();
...
}
这里主要用到了Handler机制，由于本文主要讲述Android加载机制，所有不过多分析Handler，main方法中主要有一个Looper，一直在循环等待消息（Message），main方法中大多数都是主线程Handler的一些操作，除此之外最重要的就是：
thread.attach(false);
查看attach方法
private void attach(boolean system) {
...
final IActivityManager mgr = ActivityManager.getService(); //1
try {
mgr.attachApplication(mAppThread); //2
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
...
}
1处获得了IActivityManager对象mgr，该对象是系统通过Binder机制创建的，是Activity的管理类，该类由系统服务所调用管理，并且通过在binder接口当中进行调用。
2处的ApplicationThread对象mAppThread是ActivityThread的成员变量，该类也是ActivityThread的内部类
private class ApplicationThread extends IApplicationThread.Stub {
public final void scheduleResumeActivity(IBinder token, int processState,
boolean isForward, Bundle resumeArgs) {
...
sendMessage(H.RESUME_ACTIVITY, token, isForward ? 1 : 0, 0, seq);
}
// 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<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {
...
sendMessage(H.LAUNCH_ACTIVITY, r);
}
...
}
也就是说mgr与操作系统（由系统服务调用）有关，而ApplicationThread只是ActivityThread的内部类，因此调用mgr的attachApplication让android应用建立起与操作系统的关系，操作系统在收到对应的事件后，通过ApplicationThread的方法向app发送系统信息。例如当用户打开app的时候，系统会通过mgr间接调用scheduleLaunchActivity，该方法向主线程Handler发送信息：
sendMessage(H.LAUNCH_ACTIVITY, r);
查看Handler的handlerMessage方法
public void handleMessage(Message msg) {
...
switch (msg.what) {
case LAUNCH_ACTIVITY: {
...
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;
handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
} break;
...
}
}
ActivityClientRecord的对象保存了系统传过来的参数，查看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);
...
}
...
}
该方法主要执行Activity的加载（Launch）或者Resume工作，继续查看performLaunchActivity：
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
try {
...
if (activity != null) {
...
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
...
}
}
return activity;
}
查看mInstrumentation对象的callActivityOnCreate方法
public void callActivityOnCreate(Activity activity, Bundle icicle,
PersistableBundle persistentState) {
prePerformCreate(activity);
activity.performCreate(icicle, persistentState);
postPerformCreate(activity);
}
从上面三个方法名来看，该方法主要做了Activity的Create，已经Create的前后（pre和post）工作，查看performCreate方法
final void performCreate(Bundle icicle, PersistableBundle persistentState) {
...
if (persistentState != null) {
onCreate(icicle, persistentState);
} else {
onCreate(icicle);
}
...
}
到这就恍然大悟了，Handler在接收到对应信息（Message）后执行操作，最后会调用Activity的onCreate方法，至此完成了Activity的Launch过程。