Android应用程序启动过程源代码分析

前文简要介绍了Android应用程序的Activity的启动过程。在Android系统中，应用程序是由Activity组成的，因此，应用程序的启动过程实际上就是应用程序中的默认Activity的启动过程，本文将详细分析应用程序框架层的源代码，了解Android应用程序的启动过程。

在上一篇文章Android应用程序的Activity启动过程简要介绍和学习计划中，我们举例子说明了启动Android应用程序中的Activity的两种情景，其中，在手机屏幕中点击应用程序图标的情景就会引发Android应用程序中的默认Activity的启动，从而把应用程序启动起来。这种启动方式的特点是会启动一个新的进程来加载相应的Activity。这里，我们继续以这个例子为例来说明Android应用程序的启动过程，即MainActivity的启动过程。

MainActivity的启动过程如下图所示：

点击查看大图

下面详细分析每一步是如何实现的。

Step 1. Launcher.startActivitySafely

在Android系统中，应用程序是由Launcher启动起来的，其实，Launcher本身也是一个应用程序，其它的应用程序安装后，就会Launcher的界面上出现一个相应的图标，点击这个图标时，Launcher就会对应的应用程序启动起来。

Launcher的源代码工程在packages/apps/Launcher2目录下，负责启动其它应用程序的源代码实现在src/com/android/launcher2/Launcher.java文件中：

/**
* Default launcher application.
*/
public final class Launcher extends Activity
		implements View.OnClickListener, OnLongClickListener, LauncherModel.Callbacks, AllAppsView.Watcher {

	......

	/**
	* Launches the intent referred by the clicked shortcut.
	*
	* @param v The view representing the clicked shortcut.
	*/
	public void onClick(View v) {
		Object tag = v.getTag();
		if (tag instanceof ShortcutInfo) {
			// Open shortcut
			final Intent intent = ((ShortcutInfo) tag).intent;
			int[] pos = new int[2];
			v.getLocationOnScreen(pos);
			intent.setSourceBounds(new Rect(pos[0], pos[1],
				pos[0] + v.getWidth(), pos[1] + v.getHeight()));
			startActivitySafely(intent, tag);
		} else if (tag instanceof FolderInfo) {
			......
		} else if (v == mHandleView) {
			......
		}
	}

	void startActivitySafely(Intent intent, Object tag) {
		intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
		try {
			startActivity(intent);
		} catch (ActivityNotFoundException e) {
			......
		} catch (SecurityException e) {
			......
		}
	}

	......

}

回忆一下前面一篇文章Android应用程序的Activity启动过程简要介绍和学习计划说到的应用程序Activity，它的默认Activity是MainActivity，这里是AndroidManifest.xml文件中配置的：

<activity android:name=".MainActivity"  
      android:label="@string/app_name">  
       <intent-filter>  
        <action android:name="android.intent.action.MAIN" />  
        <category android:name="android.intent.category.LAUNCHER" />  
    </intent-filter>  
</activity>  

因此，这里的intent包含的信息为：action = "android.intent.action.Main"，category="android.intent.category.LAUNCHER", cmp="shy.luo.activity/.MainActivity"，表示它要启动的Activity为shy.luo.activity.MainActivity。Intent.FLAG_ACTIVITY_NEW_TASK表示要在一个新的Task中启动这个Activity，注意，Task是Android系统中的概念，它不同于进程Process的概念。简单地说，一个Task是一系列Activity的集合，这个集合是以堆栈的形式来组织的，遵循后进先出的原则。事实上，Task是一个非常复杂的概念，有兴趣的读者可以到官网http://developer.android.com/guide/topics/manifest/activity-element.html查看相关的资料。这里，我们只要知道，这个MainActivity要在一个新的Task中启动就可以了。

Step 2. Activity.startActivity

在Step 1中，我们看到，Launcher继承于Activity类，而Activity类实现了startActivity函数，因此，这里就调用了Activity.startActivity函数，它实现在frameworks/base/core/java/android/app/Activity.java文件中：

public class Activity extends ContextThemeWrapper
		implements LayoutInflater.Factory,
		Window.Callback, KeyEvent.Callback,
		OnCreateContextMenuListener, ComponentCallbacks {

	......

	@Override
	public void startActivity(Intent intent) {
		startActivityForResult(intent, -1);
	}

	......

}

这个函数实现很简单，它调用startActivityForResult来进一步处理，第二个参数传入-1表示不需要这个Actvity结束后的返回结果。

Step 3.Activity.startActivityForResult

这个函数也是实现在frameworks/base/core/java/android/app/Activity.java文件中：

public class Activity extends ContextThemeWrapper
		implements LayoutInflater.Factory,
		Window.Callback, KeyEvent.Callback,
		OnCreateContextMenuListener, ComponentCallbacks {

	......

	public void startActivityForResult(Intent intent, int requestCode) {
		if (mParent == null) {
			Instrumentation.ActivityResult ar =
				mInstrumentation.execStartActivity(
				this, mMainThread.getApplicationThread(), mToken, this,
				intent, requestCode);
			......
		} else {
			......
		}


	......

}

这里的mInstrumentation是Activity类的成员变量，它的类型是Intrumentation，定义在frameworks/base/core/java/android/app/Instrumentation.java文件中，它用来监控应用程序和系统的交互。

这里的mMainThread也是Activity类的成员变量，它的类型是ActivityThread，它代表的是应用程序的主线程，我们在Android系统在新进程中启动自定义服务过程（startService）的原理分析一文中已经介绍过了。这里通过mMainThread.getApplicationThread获得它里面的ApplicationThread成员变量，它是一个Binder对象，后面我们会看到，ActivityManagerService会使用它来和ActivityThread来进行进程间通信。这里我们需注意的是，这里的mMainThread代表的是Launcher应用程序运行的进程。

这里的mToken也是Activity类的成员变量，它是一个Binder对象的远程接口。

Step 4. Instrumentation.execStartActivity
这个函数定义在frameworks/base/core/java/android/app/Instrumentation.java文件中：

public class Instrumentation {

	......

	public ActivityResult execStartActivity(
	Context who, IBinder contextThread, IBinder token, Activity target,
	Intent intent, int requestCode) {
		IApplicationThread whoThread = (IApplicationThread) contextThread;
		if (mActivityMonitors != null) {
			......
		}
		try {
			int result = ActivityManagerNative.getDefault()
				.startActivity(whoThread, intent,
				intent.resolveTypeIfNeeded(who.getContentResolver()),
				null, 0, token, target != null ? target.mEmbeddedID : null,
				requestCode, false, false);
			......
		} catch (RemoteException e) {
		}
		return null;
	}

	......

}

这里的ActivityManagerNative.getDefault返回ActivityManagerService的远程接口，即ActivityManagerProxy接口，具体可以参考Android系统在新进程中启动自定义服务过程（startService）的原理分析一文。

这里的intent.resolveTypeIfNeeded返回这个intent的MIME类型，在这个例子中，没有AndroidManifest.xml设置MainActivity的MIME类型，因此，这里返回null。

这里的target不为null，但是target.mEmbddedID为null，我们不用关注。

Step 5. ActivityManagerProxy.startActivity

这个函数定义在frameworks/base/core/java/android/app/ActivityManagerNative.java文件中：

class ActivityManagerProxy implements IActivityManager
{

	......

	public int startActivity(IApplicationThread caller, Intent intent,
			String resolvedType, Uri[] grantedUriPermissions, int grantedMode,
			IBinder resultTo, String resultWho,
			int requestCode, boolean onlyIfNeeded,
			boolean debug) throws RemoteException {
		Parcel data = Parcel.obtain();
		Parcel reply = Parcel.obtain();
		data.writeInterfaceToken(IActivityManager.descriptor);
		data.writeStrongBinder(caller != null ? caller.asBinder() : null);
		intent.writeToParcel(data, 0);
		data.writeString(resolvedType);
		data.writeTypedArray(grantedUriPermissions, 0);
		data.writeInt(grantedMode);
		data.writeStrongBinder(resultTo);
		data.writeString(resultWho);
		data.writeInt(requestCode);
		data.writeInt(onlyIfNeeded ? 1 : 0);
		data.writeInt(debug ? 1 : 0);
		mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);
		reply.readException();
		int result = reply.readInt();
		reply.recycle();
		data.recycle();
		return result;
	}

	......

}

这里的参数比较多，我们先整理一下。从上面的调用可以知道，这里的参数resolvedType、grantedUriPermissions和resultWho均为null；参数caller为ApplicationThread类型的Binder实体；参数resultTo为一个Binder实体的远程接口，我们先不关注它；参数grantedMode为0，我们也先不关注它；参数requestCode为-1；参数onlyIfNeeded和debug均空false。

Step 6. ActivityManagerService.startActivity

上一步Step 5通过Binder驱动程序就进入到ActivityManagerService的startActivity函数来了，它定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNative
		implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {

	......

	public final int startActivity(IApplicationThread caller,
			Intent intent, String resolvedType, Uri[] grantedUriPermissions,
			int grantedMode, IBinder resultTo,
			String resultWho, int requestCode, boolean onlyIfNeeded,
			boolean debug) {
		return mMainStack.startActivityMayWait(caller, intent, resolvedType,
			grantedUriPermissions, grantedMode, resultTo, resultWho,
			requestCode, onlyIfNeeded, debug, null, null);
	}


	......

}

这里只是简单地将操作转发给成员变量mMainStack的startActivityMayWait函数，这里的mMainStack的类型为ActivityStack。

Step 7.ActivityStack.startActivityMayWait

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	final int startActivityMayWait(IApplicationThread caller,
			Intent intent, String resolvedType, Uri[] grantedUriPermissions,
			int grantedMode, IBinder resultTo,
			String resultWho, int requestCode, boolean onlyIfNeeded,
			boolean debug, WaitResult outResult, Configuration config) {

		......

		boolean componentSpecified = intent.getComponent() != null;

		// Don't modify the client's object!
		intent = new Intent(intent);

		// Collect information about the target of the Intent.
		ActivityInfo aInfo;
		try {
			ResolveInfo rInfo =
				AppGlobals.getPackageManager().resolveIntent(
				intent, resolvedType,
				PackageManager.MATCH_DEFAULT_ONLY
				| ActivityManagerService.STOCK_PM_FLAGS);
			aInfo = rInfo != null ? rInfo.activityInfo : null;
		} catch (RemoteException e) {
			......
		}

		if (aInfo != null) {
			// Store the found target back into the intent, because now that
			// we have it we never want to do this again.  For example, if the
			// user navigates back to this point in the history, we should
			// always restart the exact same activity.
			intent.setComponent(new ComponentName(
				aInfo.applicationInfo.packageName, aInfo.name));
			......
		}

		synchronized (mService) {
			int callingPid;
			int callingUid;
			if (caller == null) {
				......
			} else {
				callingPid = callingUid = -1;
			}

			mConfigWillChange = config != null
				&& mService.mConfiguration.diff(config) != 0;

			......

			if (mMainStack && aInfo != null &&
				(aInfo.applicationInfo.flags&ApplicationInfo.FLAG_CANT_SAVE_STATE) != 0) {
				  
		              ......

			}

			int res = startActivityLocked(caller, intent, resolvedType,
				grantedUriPermissions, grantedMode, aInfo,
				resultTo, resultWho, requestCode, callingPid, callingUid,
				onlyIfNeeded, componentSpecified);

			if (mConfigWillChange && mMainStack) {
				......
			}

			......

			if (outResult != null) {
				......
			}

			return res;
		}

	}

	......

}

注意，从Step 6传下来的参数outResult和config均为null，此外，表达式(aInfo.applicationInfo.flags&ApplicationInfo.FLAG_CANT_SAVE_STATE) != 0为false，因此，这里忽略了无关代码。

下面语句对参数intent的内容进行解析，得到MainActivity的相关信息，保存在aInfo变量中：

    ActivityInfo aInfo;
    try {
	ResolveInfo rInfo =
	AppGlobals.getPackageManager().resolveIntent(
		intent, resolvedType,
		PackageManager.MATCH_DEFAULT_ONLY
		| ActivityManagerService.STOCK_PM_FLAGS);
	aInfo = rInfo != null ? rInfo.activityInfo : null;
    } catch (RemoteException e) {
		......
    }

解析之后，得到的aInfo.applicationInfo.packageName的值为"shy.luo.activity"，aInfo.name的值为"shy.luo.activity.MainActivity"，这是在这个实例的配置文件AndroidManifest.xml里面配置的。

此外，函数开始的地方调用intent.getComponent()函数的返回值不为null，因此，这里的componentSpecified变量为true。

接下去就调用startActivityLocked进一步处理了。

Step 8. ActivityStack.startActivityLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	final int startActivityLocked(IApplicationThread caller,
		    Intent intent, String resolvedType,
		    Uri[] grantedUriPermissions,
		    int grantedMode, ActivityInfo aInfo, IBinder resultTo,
	            String resultWho, int requestCode,
		    int callingPid, int callingUid, boolean onlyIfNeeded,
		    boolean componentSpecified) {
	        int err = START_SUCCESS;

		ProcessRecord callerApp = null;
		if (caller != null) {
			callerApp = mService.getRecordForAppLocked(caller);
			if (callerApp != null) {
				callingPid = callerApp.pid;
				callingUid = callerApp.info.uid;
			} else {
				......
			}
		}

		......

		ActivityRecord sourceRecord = null;
		ActivityRecord resultRecord = null;
		if (resultTo != null) {
			int index = indexOfTokenLocked(resultTo);
			
			......
				
			if (index >= 0) {
				sourceRecord = (ActivityRecord)mHistory.get(index);
				if (requestCode >= 0 && !sourceRecord.finishing) {
					......
				}
			}
		}

		int launchFlags = intent.getFlags();

		if ((launchFlags&Intent.FLAG_ACTIVITY_FORWARD_RESULT) != 0
			&& sourceRecord != null) {
			......
		}

		if (err == START_SUCCESS && intent.getComponent() == null) {
			......
		}

		if (err == START_SUCCESS && aInfo == null) {
			......
		}

		if (err != START_SUCCESS) {
			......
		}

		......

		ActivityRecord r = new ActivityRecord(mService, this, callerApp, callingUid,
			intent, resolvedType, aInfo, mService.mConfiguration,
			resultRecord, resultWho, requestCode, componentSpecified);

		......

		return startActivityUncheckedLocked(r, sourceRecord,
			grantedUriPermissions, grantedMode, onlyIfNeeded, true);
	}


	......

}

从传进来的参数caller得到调用者的进程信息，并保存在callerApp变量中，这里就是Launcher应用程序的进程信息了。

前面说过，参数resultTo是Launcher这个Activity里面的一个Binder对象，通过它可以获得Launcher这个Activity的相关信息，保存在sourceRecord变量中。
再接下来，创建即将要启动的Activity的相关信息，并保存在r变量中：

ActivityRecord r = new ActivityRecord(mService, this, callerApp, callingUid,
	intent, resolvedType, aInfo, mService.mConfiguration,
	resultRecord, resultWho, requestCode, componentSpecified);

接着调用startActivityUncheckedLocked函数进行下一步操作。

Step 9. ActivityStack.startActivityUncheckedLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	final int startActivityUncheckedLocked(ActivityRecord r,
		ActivityRecord sourceRecord, Uri[] grantedUriPermissions,
		int grantedMode, boolean onlyIfNeeded, boolean doResume) {
		final Intent intent = r.intent;
		final int callingUid = r.launchedFromUid;

		int launchFlags = intent.getFlags();

		// We'll invoke onUserLeaving before onPause only if the launching
		// activity did not explicitly state that this is an automated launch.
		mUserLeaving = (launchFlags&Intent.FLAG_ACTIVITY_NO_USER_ACTION) == 0;
		
		......

		ActivityRecord notTop = (launchFlags&Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP)
			!= 0 ? r : null;

		// If the onlyIfNeeded flag is set, then we can do this if the activity
		// being launched is the same as the one making the call...  or, as
		// a special case, if we do not know the caller then we count the
		// current top activity as the caller.
		if (onlyIfNeeded) {
			......
		}

		if (sourceRecord == null) {
			......
		} else if (sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
			......
		} else if (r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE
			|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK) {
			......
		}

		if (r.resultTo != null && (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
			......
		}

		boolean addingToTask = false;
		if (((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0 &&
			(launchFlags&Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)
			|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK
			|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
				// If bring to front is requested, and no result is requested, and
				// we can find a task that was started with this same
				// component, then instead of launching bring that one to the front.
				if (r.resultTo == null) {
					// See if there is a task to bring to the front.  If this is
					// a SINGLE_INSTANCE activity, there can be one and only one
					// instance of it in the history, and it is always in its own
					// unique task, so we do a special search.
					ActivityRecord taskTop = r.launchMode != ActivityInfo.LAUNCH_SINGLE_INSTANCE
						? findTaskLocked(intent, r.info)
						: findActivityLocked(intent, r.info);
					if (taskTop != null) {
						......
					}
				}
		}

		......

		if (r.packageName != null) {
			// If the activity being launched is the same as the one currently
			// at the top, then we need to check if it should only be launched
			// once.
			ActivityRecord top = topRunningNonDelayedActivityLocked(notTop);
			if (top != null && r.resultTo == null) {
				if (top.realActivity.equals(r.realActivity)) {
					......
				}
			}

		} else {
			......
		}

		boolean newTask = false;

		// Should this be considered a new task?
		if (r.resultTo == null && !addingToTask
			&& (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
				// todo: should do better management of integers.
				mService.mCurTask++;
				if (mService.mCurTask <= 0) {
					mService.mCurTask = 1;
				}
				r.task = new TaskRecord(mService.mCurTask, r.info, intent,
					(r.info.flags&ActivityInfo.FLAG_CLEAR_TASK_ON_LAUNCH) != 0);
				......
				newTask = true;
				if (mMainStack) {
					mService.addRecentTaskLocked(r.task);
				}

		} else if (sourceRecord != null) {
			......
		} else {
			......
		}

		......

		startActivityLocked(r, newTask, doResume);
		return START_SUCCESS;
	}

	......

}

函数首先获得intent的标志值，保存在launchFlags变量中。

这个intent的标志值的位Intent.FLAG_ACTIVITY_NO_USER_ACTION没有置位，因此 ，成员变量mUserLeaving的值为true。

这个intent的标志值的位Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP也没有置位，因此，变量notTop的值为null。

由于在这个例子的AndroidManifest.xml文件中，MainActivity没有配置launchMode属值，因此，这里的r.launchMode为默认值0，表示以标准（Standard，或者称为ActivityInfo.LAUNCH_MULTIPLE）的方式来启动这个Activity。Activity的启动方式有四种，其余三种分别是ActivityInfo.LAUNCH_SINGLE_INSTANCE、ActivityInfo.LAUNCH_SINGLE_TASK和ActivityInfo.LAUNCH_SINGLE_TOP，具体可以参考官方网站http://developer.android.com/reference/android/content/pm/ActivityInfo.html。

传进来的参数r.resultTo为null，表示Launcher不需要等这个即将要启动的MainActivity的执行结果。

由于这个intent的标志值的位Intent.FLAG_ACTIVITY_NEW_TASK被置位，而且Intent.FLAG_ACTIVITY_MULTIPLE_TASK没有置位，因此，下面的if语句会被执行：

    if (((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0 &&
	(launchFlags&Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)
	|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK
	|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
		// If bring to front is requested, and no result is requested, and
		// we can find a task that was started with this same
		// component, then instead of launching bring that one to the front.
		if (r.resultTo == null) {
			// See if there is a task to bring to the front.  If this is
			// a SINGLE_INSTANCE activity, there can be one and only one
			// instance of it in the history, and it is always in its own
			// unique task, so we do a special search.
			ActivityRecord taskTop = r.launchMode != ActivityInfo.LAUNCH_SINGLE_INSTANCE
				? findTaskLocked(intent, r.info)
				: findActivityLocked(intent, r.info);
			if (taskTop != null) {
				......
			}
		}
    }

这段代码的逻辑是查看一下，当前有没有Task可以用来执行这个Activity。由于r.launchMode的值不为ActivityInfo.LAUNCH_SINGLE_INSTANCE，因此，它通过findTaskLocked函数来查找存不存这样的Task，这里返回的结果是null，即taskTop为null，因此，需要创建一个新的Task来启动这个Activity。

接着往下看：

    if (r.packageName != null) {
	// If the activity being launched is the same as the one currently
	// at the top, then we need to check if it should only be launched
	// once.
	ActivityRecord top = topRunningNonDelayedActivityLocked(notTop);
	if (top != null && r.resultTo == null) {
		if (top.realActivity.equals(r.realActivity)) {
			......
		}
	}

    } 

这段代码的逻辑是看一下，当前在堆栈顶端的Activity是否就是即将要启动的Activity，有些情况下，如果即将要启动的Activity就在堆栈的顶端，那么，就不会重新启动这个Activity的别一个实例了，具体可以参考官方网站http://developer.android.com/reference/android/content/pm/ActivityInfo.html。现在处理堆栈顶端的Activity是Launcher，与我们即将要启动的MainActivity不是同一个Activity，因此，这里不用进一步处理上述介绍的情况。

执行到这里，我们知道，要在一个新的Task里面来启动这个Activity了，于是新创建一个Task：

   if (r.resultTo == null && !addingToTask
	&& (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
	// todo: should do better management of integers.
	mService.mCurTask++;
	if (mService.mCurTask <= 0) {
		mService.mCurTask = 1;
	}
	r.task = new TaskRecord(mService.mCurTask, r.info, intent,
		(r.info.flags&ActivityInfo.FLAG_CLEAR_TASK_ON_LAUNCH) != 0);
	......
	newTask = true;
	if (mMainStack) {
		mService.addRecentTaskLocked(r.task);
	}

    }

新建的Task保存在r.task域中，同时，添加到mService中去，这里的mService就是ActivityManagerService了。

最后就进入startActivityLocked(r, newTask, doResume)进一步处理了。这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	private final void startActivityLocked(ActivityRecord r, boolean newTask,
			boolean doResume) {
		final int NH = mHistory.size();

		int addPos = -1;

		if (!newTask) {
			......
		}

		// Place a new activity at top of stack, so it is next to interact
		// with the user.
		if (addPos < 0) {
			addPos = NH;
		}

		// If we are not placing the new activity frontmost, we do not want
		// to deliver the onUserLeaving callback to the actual frontmost
		// activity
		if (addPos < NH) {
			......
		}

		// Slot the activity into the history stack and proceed
		mHistory.add(addPos, r);
		r.inHistory = true;
		r.frontOfTask = newTask;
		r.task.numActivities++;
		if (NH > 0) {
			// We want to show the starting preview window if we are
			// switching to a new task, or the next activity's process is
			// not currently running.
			......
		} else {
			// If this is the first activity, don't do any fancy animations,
			// because there is nothing for it to animate on top of.
			......
		}
		
		......

		if (doResume) {
			resumeTopActivityLocked(null);
		}
	}

	......

}

这里的NH表示当前系统中历史任务的个数，这里肯定是大于0，因为Launcher已经跑起来了。当NH>0时，并且现在要切换新任务时，要做一些任务切的界面操作，这段代码我们就不看了，这里不会影响到下面启Activity的过程，有兴趣的读取可以自己研究一下。

这里传进来的参数doResume为true，于是调用resumeTopActivityLocked进一步操作。

Step 10. Activity.resumeTopActivityLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	/**
	* Ensure that the top activity in the stack is resumed.
	*
	* @param prev The previously resumed activity, for when in the process
	* of pausing; can be null to call from elsewhere.
	*
	* @return Returns true if something is being resumed, or false if
	* nothing happened.
	*/
	final boolean resumeTopActivityLocked(ActivityRecord prev) {
		// Find the first activity that is not finishing.
		ActivityRecord next = topRunningActivityLocked(null);

		// Remember how we'll process this pause/resume situation, and ensure
		// that the state is reset however we wind up proceeding.
		final boolean userLeaving = mUserLeaving;
		mUserLeaving = false;

		if (next == null) {
			......
		}

		next.delayedResume = false;

		// If the top activity is the resumed one, nothing to do.
		if (mResumedActivity == next && next.state == ActivityState.RESUMED) {
			......
		}

		// If we are sleeping, and there is no resumed activity, and the top
		// activity is paused, well that is the state we want.
		if ((mService.mSleeping || mService.mShuttingDown)
			&& mLastPausedActivity == next && next.state == ActivityState.PAUSED) {
			......
		}

		......

		// If we are currently pausing an activity, then don't do anything
		// until that is done.
		if (mPausingActivity != null) {
			......
		}

		......

		// We need to start pausing the current activity so the top one
		// can be resumed...
		if (mResumedActivity != null) {
			......
			startPausingLocked(userLeaving, false);
			return true;
		}

		......
	}

	......

}

函数先通过调用topRunningActivityLocked函数获得堆栈顶端的Activity，这里就是MainActivity了，这是在上面的Step 9设置好的，保存在next变量中。

接下来把mUserLeaving的保存在本地变量userLeaving中，然后重新设置为false，在上面的Step 9中，mUserLeaving的值为true，因此，这里的userLeaving为true。

这里的mResumedActivity为Launcher，因为Launcher是当前正被执行的Activity。

当我们处理休眠状态时，mLastPausedActivity保存堆栈顶端的Activity，因为当前不是休眠状态，所以mLastPausedActivity为null。

有了这些信息之后，下面的语句就容易理解了：

    // If the top activity is the resumed one, nothing to do.
    if (mResumedActivity == next && next.state == ActivityState.RESUMED) {
	......
    }

    // If we are sleeping, and there is no resumed activity, and the top
    // activity is paused, well that is the state we want.
    if ((mService.mSleeping || mService.mShuttingDown)
	&& mLastPausedActivity == next && next.state == ActivityState.PAUSED) {
	......
    }

它首先看要启动的Activity是否就是当前处理Resumed状态的Activity，如果是的话，那就什么都不用做，直接返回就可以了；否则再看一下系统当前是否休眠状态，如果是的话，再看看要启动的Activity是否就是当前处于堆栈顶端的Activity，如果是的话，也是什么都不用做。

上面两个条件都不满足，因此，在继续往下执行之前，首先要把当处于Resumed状态的Activity推入Paused状态，然后才可以启动新的Activity。但是在将当前这个Resumed状态的Activity推入Paused状态之前，首先要看一下当前是否有Activity正在进入Pausing状态，如果有的话，当前这个Resumed状态的Activity就要稍后才能进入Paused状态了，这样就保证了所有需要进入Paused状态的Activity串行处理。

这里没有处于Pausing状态的Activity，即mPausingActivity为null，而且mResumedActivity也不为null，于是就调用startPausingLocked函数把Launcher推入Paused状态去了。

Step 11. ActivityStack.startPausingLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	private final void startPausingLocked(boolean userLeaving, boolean uiSleeping) {
		if (mPausingActivity != null) {
			......
		}
		ActivityRecord prev = mResumedActivity;
		if (prev == null) {
			......
		}
		......
		mResumedActivity = null;
		mPausingActivity = prev;
		mLastPausedActivity = prev;
		prev.state = ActivityState.PAUSING;
		......

		if (prev.app != null && prev.app.thread != null) {
			......
			try {
				......
				prev.app.thread.schedulePauseActivity(prev, prev.finishing, userLeaving,
					prev.configChangeFlags);
				......
			} catch (Exception e) {
				......
			}
		} else {
			......
		}

		......
	
	}

	......

}

函数首先把mResumedActivity保存在本地变量prev中。在上一步Step 10中，说到mResumedActivity就是Launcher，因此，这里把Launcher进程中的ApplicationThread对象取出来，通过它来通知Launcher这个Activity它要进入Paused状态了。当然，这里的prev.app.thread是一个ApplicationThread对象的远程接口，通过调用这个远程接口的schedulePauseActivity来通知Launcher进入Paused状态。

参数prev.finishing表示prev所代表的Activity是否正在等待结束的Activity列表中，由于Laucher这个Activity还没结束，所以这里为false；参数prev.configChangeFlags表示哪些config发生了变化，这里我们不关心它的值。

Step 12.ApplicationThreadProxy.schedulePauseActivity

这个函数定义在frameworks/base/core/java/android/app/ApplicationThreadNative.java文件中：

class ApplicationThreadProxy implements IApplicationThread {
	
	......

	public final void schedulePauseActivity(IBinder token, boolean finished,
	boolean userLeaving, int configChanges) throws RemoteException {
		Parcel data = Parcel.obtain();
		data.writeInterfaceToken(IApplicationThread.descriptor);
		data.writeStrongBinder(token);
		data.writeInt(finished ? 1 : 0);
		data.writeInt(userLeaving ? 1 :0);
		data.writeInt(configChanges);
		mRemote.transact(SCHEDULE_PAUSE_ACTIVITY_TRANSACTION, data, null,
			IBinder.FLAG_ONEWAY);
		data.recycle();
	}

	......

}

这个函数通过Binder进程间通信机制进入到ApplicationThread.schedulePauseActivity函数中。

Step 13. ApplicationThread.schedulePauseActivity

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中，它是ActivityThread的内部类：

public final class ActivityThread {
	
	......

	private final class ApplicationThread extends ApplicationThreadNative {
		
		......

		public final void schedulePauseActivity(IBinder token, boolean finished,
				boolean userLeaving, int configChanges) {
			queueOrSendMessage(
				finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY,
				token,
				(userLeaving ? 1 : 0),
				configChanges);
		}

		......

	}

	......

}

这里调用的函数queueOrSendMessage是ActivityThread类的成员函数。

上面说到，这里的finished值为false，因此，queueOrSendMessage的第一个参数值为H.PAUSE_ACTIVITY，表示要暂停token所代表的Activity，即Launcher。

Step 14. ActivityThread.queueOrSendMessage

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {
	
	......

	private final void queueOrSendMessage(int what, Object obj, int arg1) {
		queueOrSendMessage(what, obj, arg1, 0);
	}

	private final void queueOrSendMessage(int what, Object obj, int arg1, int arg2) {
		synchronized (this) {
			......
			Message msg = Message.obtain();
			msg.what = what;
			msg.obj = obj;
			msg.arg1 = arg1;
			msg.arg2 = arg2;
			mH.sendMessage(msg);
		}
	}

	......

}

这里首先将相关信息组装成一个msg，然后通过mH成员变量发送出去，mH的类型是H，继承于Handler类，是ActivityThread的内部类，因此，这个消息最后由H.handleMessage来处理。

Step 15. H.handleMessage

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {
	
	......

	private final class H extends Handler {

		......

		public void handleMessage(Message msg) {
			......
			switch (msg.what) {
			
			......
			
			case PAUSE_ACTIVITY:
				handlePauseActivity((IBinder)msg.obj, false, msg.arg1 != 0, msg.arg2);
				maybeSnapshot();
				break;

			......

			}
		......

	}

	......

}

这里调用ActivityThread.handlePauseActivity进一步操作，msg.obj是一个ActivityRecord对象的引用，它代表的是Launcher这个Activity。
Step 16.ActivityThread.handlePauseActivity

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {
	
	......

	private final void handlePauseActivity(IBinder token, boolean finished,
			boolean userLeaving, int configChanges) {

		ActivityClientRecord r = mActivities.get(token);
		if (r != null) {
			//Slog.v(TAG, "userLeaving=" + userLeaving + " handling pause of " + r);
			if (userLeaving) {
				performUserLeavingActivity(r);
			}

			r.activity.mConfigChangeFlags |= configChanges;
			Bundle state = performPauseActivity(token, finished, true);

			// Make sure any pending writes are now committed.
			QueuedWork.waitToFinish();

			// Tell the activity manager we have paused.
			try {
				ActivityManagerNative.getDefault().activityPaused(token, state);
			} catch (RemoteException ex) {
			}
		}
	}

	......

}

函数首先将Binder引用token转换成ActivityRecord的远程接口ActivityClientRecord，然后做了三个事情：1. 如果userLeaving为true，则通过调用performUserLeavingActivity函数来调用Activity.onUserLeaveHint通知Activity，用户要离开它了；2. 调用performPauseActivity函数来调用Activity.onPause函数，我们知道，在Activity的生命周期中，当它要让位于其它的Activity时，系统就会调用它的onPause函数；3. 它通知ActivityManagerService，这个Activity已经进入Paused状态了，ActivityManagerService现在可以完成未竟的事情，即启动MainActivity了。

Step 17. ActivityManagerProxy.activityPaused

这个函数定义在frameworks/base/core/java/android/app/ActivityManagerNative.java文件中：

class ActivityManagerProxy implements IActivityManager
{
	......

	public void activityPaused(IBinder token, Bundle state) throws RemoteException
	{
		Parcel data = Parcel.obtain();
		Parcel reply = Parcel.obtain();
		data.writeInterfaceToken(IActivityManager.descriptor);
		data.writeStrongBinder(token);
		data.writeBundle(state);
		mRemote.transact(ACTIVITY_PAUSED_TRANSACTION, data, reply, 0);
		reply.readException();
		data.recycle();
		reply.recycle();
	}

	......

}

这里通过Binder进程间通信机制就进入到ActivityManagerService.activityPaused函数中去了。

Step 18.ActivityManagerService.activityPaused

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNative
			implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
	......

	public final void activityPaused(IBinder token, Bundle icicle) {
		
		......

		final long origId = Binder.clearCallingIdentity();
		mMainStack.activityPaused(token, icicle, false);
		
		......
	}

	......

}

这里，又再次进入到ActivityStack类中，执行activityPaused函数。

Step 19.ActivityStack.activityPaused

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	final void activityPaused(IBinder token, Bundle icicle, boolean timeout) {
		
		......

		ActivityRecord r = null;

		synchronized (mService) {
			int index = indexOfTokenLocked(token);
			if (index >= 0) {
				r = (ActivityRecord)mHistory.get(index);
				if (!timeout) {
					r.icicle = icicle;
					r.haveState = true;
				}
				mHandler.removeMessages(PAUSE_TIMEOUT_MSG, r);
				if (mPausingActivity == r) {
					r.state = ActivityState.PAUSED;
					completePauseLocked();
				} else {
					......
				}
			}
		}
	}

	......

}

这里通过参数token在mHistory列表中得到ActivityRecord，从上面我们知道，这个ActivityRecord代表的是Launcher这个Activity，而我们在Step 11中，把Launcher这个Activity的信息保存在mPausingActivity中，因此，这里mPausingActivity等于r，于是，执行completePauseLocked操作。

Step 20.ActivityStack.completePauseLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	private final void completePauseLocked() {
		ActivityRecord prev = mPausingActivity;
		
		......

		if (prev != null) {

			......

			mPausingActivity = null;
		}

		if (!mService.mSleeping && !mService.mShuttingDown) {
			resumeTopActivityLocked(prev);
		} else {
			......
		}

		......
	}

	......

}

函数首先把mPausingActivity变量清空，因为现在不需要它了，然后调用resumeTopActivityLokced进一步操作，它传入的参数即为代表Launcher这个Activity的ActivityRecord。

Step 21.ActivityStack.resumeTopActivityLokced
这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	final boolean resumeTopActivityLocked(ActivityRecord prev) {
		......

		// Find the first activity that is not finishing.
		ActivityRecord next = topRunningActivityLocked(null);

		// Remember how we'll process this pause/resume situation, and ensure
		// that the state is reset however we wind up proceeding.
		final boolean userLeaving = mUserLeaving;
		mUserLeaving = false;

		......

		next.delayedResume = false;

		// If the top activity is the resumed one, nothing to do.
		if (mResumedActivity == next && next.state == ActivityState.RESUMED) {
			......
			return false;
		}

		// If we are sleeping, and there is no resumed activity, and the top
		// activity is paused, well that is the state we want.
		if ((mService.mSleeping || mService.mShuttingDown)
			&& mLastPausedActivity == next && next.state == ActivityState.PAUSED) {
			......
			return false;
		}

		.......


		// We need to start pausing the current activity so the top one
		// can be resumed...
		if (mResumedActivity != null) {
			......
			return true;
		}

		......


		if (next.app != null && next.app.thread != null) {
			......

		} else {
			......
			startSpecificActivityLocked(next, true, true);
		}

		return true;
	}


	......

}

通过上面的Step 9，我们知道，当前在堆栈顶端的Activity为我们即将要启动的MainActivity，这里通过调用topRunningActivityLocked将它取回来，保存在next变量中。之前最后一个Resumed状态的Activity，即Launcher，到了这里已经处于Paused状态了，因此，mResumedActivity为null。最后一个处于Paused状态的Activity为Launcher，因此，这里的mLastPausedActivity就为Launcher。前面我们为MainActivity创建了ActivityRecord后，它的app域一直保持为null。有了这些信息后，上面这段代码就容易理解了，它最终调用startSpecificActivityLocked进行下一步操作。

Step 22. ActivityStack.startSpecificActivityLocked
这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	private final void startSpecificActivityLocked(ActivityRecord r,
			boolean andResume, boolean checkConfig) {
		// Is this activity's application already running?
		ProcessRecord app = mService.getProcessRecordLocked(r.processName,
			r.info.applicationInfo.uid);

		......

		if (app != null && app.thread != null) {
			try {
				realStartActivityLocked(r, app, andResume, checkConfig);
				return;
			} catch (RemoteException e) {
				......
			}
		}

		mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
			"activity", r.intent.getComponent(), false);
	}


	......

}

注意，这里由于是第一次启动应用程序的Activity，所以下面语句：

ProcessRecord app = mService.getProcessRecordLocked(r.processName,
	r.info.applicationInfo.uid);

取回来的app为null。在Activity应用程序中的AndroidManifest.xml配置文件中，我们没有指定Application标签的process属性，系统就会默认使用package的名称，这里就是"shy.luo.activity"了。每一个应用程序都有自己的uid，因此，这里uid + process的组合就可以为每一个应用程序创建一个ProcessRecord。当然，我们可以配置两个应用程序具有相同的uid和package，或者在AndroidManifest.xml配置文件的application标签或者activity标签中显式指定相同的process属性值，这样，不同的应用程序也可以在同一个进程中启动。

函数最终执行ActivityManagerService.startProcessLocked函数进行下一步操作。

Step 23.ActivityManagerService.startProcessLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNative
		implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {

	......

	final ProcessRecord startProcessLocked(String processName,
			ApplicationInfo info, boolean knownToBeDead, int intentFlags,
			String hostingType, ComponentName hostingName, boolean allowWhileBooting) {

		ProcessRecord app = getProcessRecordLocked(processName, info.uid);
		
		......

		String hostingNameStr = hostingName != null
			? hostingName.flattenToShortString() : null;

		......

		if (app == null) {
			app = new ProcessRecordLocked(null, info, processName);
			mProcessNames.put(processName, info.uid, app);
		} else {
			// If this is a new package in the process, add the package to the list
			app.addPackage(info.packageName);
		}

		......

		startProcessLocked(app, hostingType, hostingNameStr);
		return (app.pid != 0) ? app : null;
	}

	......

}

这里再次检查是否已经有以process + uid命名的进程存在，在我们这个情景中，返回值app为null，因此，后面会创建一个ProcessRecord，并存保存在成员变量mProcessNames中，最后，调用另一个startProcessLocked函数进一步操作：

public final class ActivityManagerService extends ActivityManagerNative
		implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {

	......

	private final void startProcessLocked(ProcessRecord app,
				String hostingType, String hostingNameStr) {

		......

		try {
			int uid = app.info.uid;
			int[] gids = null;
			try {
				gids = mContext.getPackageManager().getPackageGids(
					app.info.packageName);
			} catch (PackageManager.NameNotFoundException e) {
				......
			}
			
			......

			int debugFlags = 0;
			
			......
			
			int pid = Process.start("android.app.ActivityThread",
				mSimpleProcessManagement ? app.processName : null, uid, uid,
				gids, debugFlags, null);
			
			......

		} catch (RuntimeException e) {
			
			......

		}
	}

	......

}

这里主要是调用Process.start接口来创建一个新的进程，新的进程会导入android.app.ActivityThread类，并且执行它的main函数，这就是为什么我们前面说每一个应用程序都有一个ActivityThread实例来对应的原因。

Step 24. ActivityThread.main

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {

	......

	private final void attach(boolean system) {
		......

		mSystemThread = system;
		if (!system) {

			......

			IActivityManager mgr = ActivityManagerNative.getDefault();
			try {
				mgr.attachApplication(mAppThread);
			} catch (RemoteException ex) {
			}
		} else {

			......

		}
	}

	......

	public static final void main(String[] args) {
		
		.......

		ActivityThread thread = new ActivityThread();
		thread.attach(false);

		......

		Looper.loop();

		.......

		thread.detach();
		
		......
	}
}

这个函数在进程中创建一个ActivityThread实例，然后调用它的attach函数，接着就进入消息循环了，直到最后进程退出。

函数attach最终调用了ActivityManagerService的远程接口ActivityManagerProxy的attachApplication函数，传入的参数是mAppThread，这是一个ApplicationThread类型的Binder对象，它的作用是用来进行进程间通信的。

Step 25.ActivityManagerProxy.attachApplication

这个函数定义在frameworks/base/core/java/android/app/ActivityManagerNative.java文件中：

class ActivityManagerProxy implements IActivityManager
{
	......

	public void attachApplication(IApplicationThread app) throws RemoteException
	{
		Parcel data = Parcel.obtain();
		Parcel reply = Parcel.obtain();
		data.writeInterfaceToken(IActivityManager.descriptor);
		data.writeStrongBinder(app.asBinder());
		mRemote.transact(ATTACH_APPLICATION_TRANSACTION, data, reply, 0);
		reply.readException();
		data.recycle();
		reply.recycle();
	}

	......

}

这里通过Binder驱动程序，最后进入ActivityManagerService的attachApplication函数中。

Step 26.ActivityManagerService.attachApplication

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNative
		implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {

	......

	public final void attachApplication(IApplicationThread thread) {
		synchronized (this) {
			int callingPid = Binder.getCallingPid();
			final long origId = Binder.clearCallingIdentity();
			attachApplicationLocked(thread, callingPid);
			Binder.restoreCallingIdentity(origId);
		}
	}

	......

}

这里将操作转发给attachApplicationLocked函数。

Step 27.ActivityManagerService.attachApplicationLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNative
		implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {

	......

	private final boolean attachApplicationLocked(IApplicationThread thread,
			int pid) {
		// Find the application record that is being attached...  either via
		// the pid if we are running in multiple processes, or just pull the
		// next app record if we are emulating process with anonymous threads.
		ProcessRecord app;
		if (pid != MY_PID && pid >= 0) {
			synchronized (mPidsSelfLocked) {
				app = mPidsSelfLocked.get(pid);
			}
		} else if (mStartingProcesses.size() > 0) {
			......
		} else {
			......
		}

		if (app == null) {
			......
			return false;
		}

		......

		String processName = app.processName;
		try {
			thread.asBinder().linkToDeath(new AppDeathRecipient(
				app, pid, thread), 0);
		} catch (RemoteException e) {
			......
			return false;
		}

		......

		app.thread = thread;
		app.curAdj = app.setAdj = -100;
		app.curSchedGroup = Process.THREAD_GROUP_DEFAULT;
		app.setSchedGroup = Process.THREAD_GROUP_BG_NONINTERACTIVE;
		app.forcingToForeground = null;
		app.foregroundServices = false;
		app.debugging = false;

		......

		boolean normalMode = mProcessesReady || isAllowedWhileBooting(app.info);

		......

		boolean badApp = false;
		boolean didSomething = false;

		// See if the top visible activity is waiting to run in this process...
		ActivityRecord hr = mMainStack.topRunningActivityLocked(null);
		if (hr != null && normalMode) {
			if (hr.app == null && app.info.uid == hr.info.applicationInfo.uid
				&& processName.equals(hr.processName)) {
					try {
						if (mMainStack.realStartActivityLocked(hr, app, true, true)) {
							didSomething = true;
						}
					} catch (Exception e) {
						......
					}
			} else {
				......
			}
		}

		......

		return true;
	}

	......

}

在前面的Step 23中，已经创建了一个ProcessRecord，这里首先通过pid将它取回来，放在app变量中，然后对app的其它成员进行初始化，最后调用mMainStack.realStartActivityLocked执行真正的Activity启动操作。这里要启动的Activity通过调用mMainStack.topRunningActivityLocked(null)从堆栈顶端取回来，这时候在堆栈顶端的Activity就是MainActivity了。

Step 28. ActivityStack.realStartActivityLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {

	......

	final boolean realStartActivityLocked(ActivityRecord r,
			ProcessRecord app, boolean andResume, boolean checkConfig)
			throws RemoteException {
		
		......

		r.app = app;

		......

		int idx = app.activities.indexOf(r);
		if (idx < 0) {
			app.activities.add(r);
		}
		
		......

		try {
			......

			List<ResultInfo> results = null;
			List<Intent> newIntents = null;
			if (andResume) {
				results = r.results;
				newIntents = r.newIntents;
			}
	
			......
			
			app.thread.scheduleLaunchActivity(new Intent(r.intent), r,
				System.identityHashCode(r),
				r.info, r.icicle, results, newIntents, !andResume,
				mService.isNextTransitionForward());

			......

		} catch (RemoteException e) {
			......
		}

		......

		return true;
	}

	......

}

这里最终通过app.thread进入到ApplicationThreadProxy的scheduleLaunchActivity函数中，注意，这里的第二个参数r，是一个ActivityRecord类型的Binder对象，用来作来这个Activity的token值。

Step 29.ApplicationThreadProxy.scheduleLaunchActivity
这个函数定义在frameworks/base/core/java/android/app/ApplicationThreadNative.java文件中：

class ApplicationThreadProxy implements IApplicationThread {

	......

	public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
			ActivityInfo info, Bundle state, List<ResultInfo> pendingResults,
			List<Intent> pendingNewIntents, boolean notResumed, boolean isForward)
			throws RemoteException {
		Parcel data = Parcel.obtain();
		data.writeInterfaceToken(IApplicationThread.descriptor);
		intent.writeToParcel(data, 0);
		data.writeStrongBinder(token);
		data.writeInt(ident);
		info.writeToParcel(data, 0);
		data.writeBundle(state);
		data.writeTypedList(pendingResults);
		data.writeTypedList(pendingNewIntents);
		data.writeInt(notResumed ? 1 : 0);
		data.writeInt(isForward ? 1 : 0);
		mRemote.transact(SCHEDULE_LAUNCH_ACTIVITY_TRANSACTION, data, null,
			IBinder.FLAG_ONEWAY);
		data.recycle();
	}

	......

}

这个函数最终通过Binder驱动程序进入到ApplicationThread的scheduleLaunchActivity函数中。

Step 30.ApplicationThread.scheduleLaunchActivity
这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {

	......

	private final class ApplicationThread extends ApplicationThreadNative {

		......

		// we use token to identify this activity without having to send the
		// activity itself back to the activity manager. (matters more with ipc)
		public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
				ActivityInfo info, Bundle state, List<ResultInfo> pendingResults,
				List<Intent> pendingNewIntents, boolean notResumed, boolean isForward) {
			ActivityClientRecord r = new ActivityClientRecord();

			r.token = token;
			r.ident = ident;
			r.intent = intent;
			r.activityInfo = info;
			r.state = state;

			r.pendingResults = pendingResults;
			r.pendingIntents = pendingNewIntents;

			r.startsNotResumed = notResumed;
			r.isForward = isForward;

			queueOrSendMessage(H.LAUNCH_ACTIVITY, r);
		}

		......

	}

	......
}

函数首先创建一个ActivityClientRecord实例，并且初始化它的成员变量，然后调用ActivityThread类的queueOrSendMessage函数进一步处理。

Step 31.ActivityThread.queueOrSendMessage
这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {

	......

	private final class ApplicationThread extends ApplicationThreadNative {

		......

		// if the thread hasn't started yet, we don't have the handler, so just
		// save the messages until we're ready.
		private final void queueOrSendMessage(int what, Object obj) {
			queueOrSendMessage(what, obj, 0, 0);
		}

		......

		private final void queueOrSendMessage(int what, Object obj, int arg1, int arg2) {
			synchronized (this) {
				......
				Message msg = Message.obtain();
				msg.what = what;
				msg.obj = obj;
				msg.arg1 = arg1;
				msg.arg2 = arg2;
				mH.sendMessage(msg);
			}
		}

		......

	}

	......
}

函数把消息内容放在msg中，然后通过mH把消息分发出去，这里的成员变量mH我们在前面已经见过，消息分发出去后，最后会调用H类的handleMessage函数。

Step 32. H.handleMessage

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {

	......

	private final class H extends Handler {

		......

		public void handleMessage(Message msg) {
			......
			switch (msg.what) {
			case LAUNCH_ACTIVITY: {
				ActivityClientRecord r = (ActivityClientRecord)msg.obj;

				r.packageInfo = getPackageInfoNoCheck(
					r.activityInfo.applicationInfo);
				handleLaunchActivity(r, null);
			} break;
			......
			}

		......

	}

	......
}

这里最后调用ActivityThread类的handleLaunchActivity函数进一步处理。

Step 33.ActivityThread.handleLaunchActivity

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {

	......

	private final void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
		......

		Activity a = performLaunchActivity(r, customIntent);

		if (a != null) {
			r.createdConfig = new Configuration(mConfiguration);
			Bundle oldState = r.state;
			handleResumeActivity(r.token, false, r.isForward);

			......
		} else {
			......
		}
	}

	......
}

这里首先调用performLaunchActivity函数来加载这个Activity类，即shy.luo.activity.MainActivity，然后调用它的onCreate函数，最后回到handleLaunchActivity函数时，再调用handleResumeActivity函数来使这个Activity进入Resumed状态，即会调用这个Activity的onResume函数，这是遵循Activity的生命周期的。

Step 34.ActivityThread.performLaunchActivity
这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {

	......

	private final Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
		
		ActivityInfo aInfo = r.activityInfo;
		if (r.packageInfo == null) {
			r.packageInfo = getPackageInfo(aInfo.applicationInfo,
				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);
		}

		Activity activity = null;
		try {
			java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
			activity = mInstrumentation.newActivity(
				cl, component.getClassName(), r.intent);
			r.intent.setExtrasClassLoader(cl);
			if (r.state != null) {
				r.state.setClassLoader(cl);
			}
		} catch (Exception e) {
			......
		}

		try {
			Application app = r.packageInfo.makeApplication(false, mInstrumentation);

			......

			if (activity != null) {
				ContextImpl appContext = new ContextImpl();
				appContext.init(r.packageInfo, r.token, this);
				appContext.setOuterContext(activity);
				CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
				Configuration config = new Configuration(mConfiguration);
				......
				activity.attach(appContext, this, getInstrumentation(), r.token,
					r.ident, app, r.intent, r.activityInfo, title, r.parent,
					r.embeddedID, r.lastNonConfigurationInstance,
					r.lastNonConfigurationChildInstances, config);

				if (customIntent != null) {
					activity.mIntent = customIntent;
				}
				r.lastNonConfigurationInstance = null;
				r.lastNonConfigurationChildInstances = null;
				activity.mStartedActivity = false;
				int theme = r.activityInfo.getThemeResource();
				if (theme != 0) {
					activity.setTheme(theme);
				}

				activity.mCalled = false;
				mInstrumentation.callActivityOnCreate(activity, r.state);
				......
				r.activity = activity;
				r.stopped = true;
				if (!r.activity.mFinished) {
					activity.performStart();
					r.stopped = false;
				}
				if (!r.activity.mFinished) {
					if (r.state != null) {
						mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
					}
				}
				if (!r.activity.mFinished) {
					activity.mCalled = false;
					mInstrumentation.callActivityOnPostCreate(activity, r.state);
					if (!activity.mCalled) {
						throw new SuperNotCalledException(
							"Activity " + r.intent.getComponent().toShortString() +
							" did not call through to super.onPostCreate()");
					}
				}
			}
			r.paused = true;

			mActivities.put(r.token, r);

		} catch (SuperNotCalledException e) {
			......

		} catch (Exception e) {
			......
		}

		return activity;
	}

	......
}

函数前面是收集要启动的Activity的相关信息，主要package和component信息：

   ActivityInfo aInfo = r.activityInfo;
   if (r.packageInfo == null) {
        r.packageInfo = getPackageInfo(aInfo.applicationInfo,
                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);
   }

然后通过ClassLoader将shy.luo.activity.MainActivity类加载进来：

   Activity activity = null;
   try {
	java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
	activity = mInstrumentation.newActivity(
		cl, component.getClassName(), r.intent);
	r.intent.setExtrasClassLoader(cl);
	if (r.state != null) {
		r.state.setClassLoader(cl);
	}
   } catch (Exception e) {
	......
   }

接下来是创建Application对象，这是根据AndroidManifest.xml配置文件中的Application标签的信息来创建的：

   Application app = r.packageInfo.makeApplication(false, mInstrumentation);

后面的代码主要创建Activity的上下文信息，并通过attach方法将这些上下文信息设置到MainActivity中去：

   activity.attach(appContext, this, getInstrumentation(), r.token,
	r.ident, app, r.intent, r.activityInfo, title, r.parent,
	r.embeddedID, r.lastNonConfigurationInstance,
	r.lastNonConfigurationChildInstances, config);

最后还要调用MainActivity的onCreate函数：

   mInstrumentation.callActivityOnCreate(activity, r.state);

这里不是直接调用MainActivity的onCreate函数，而是通过mInstrumentation的callActivityOnCreate函数来间接调用，前面我们说过，mInstrumentation在这里的作用是监控Activity与系统的交互操作，相当于是系统运行日志。

Step 35. MainActivity.onCreate

这个函数定义在packages/experimental/Activity/src/shy/luo/activity/MainActivity.java文件中，这是我们自定义的app工程文件：

public class MainActivity extends Activity  implements OnClickListener {
	
	......

	@Override
	public void onCreate(Bundle savedInstanceState) {
		......

		Log.i(LOG_TAG, "Main Activity Created.");
	}

	......

}

这样，MainActivity就启动起来了，整个应用程序也启动起来了。

整个应用程序的启动过程要执行很多步骤，但是整体来看，主要分为以下五个阶段：

一. Step1 - Step 11：Launcher通过Binder进程间通信机制通知ActivityManagerService，它要启动一个Activity；

二. Step 12 - Step 16：ActivityManagerService通过Binder进程间通信机制通知Launcher进入Paused状态；

三. Step 17 - Step 24：Launcher通过Binder进程间通信机制通知ActivityManagerService，它已经准备就绪进入Paused状态，于是ActivityManagerService就创建一个新的进程，用来启动一个ActivityThread实例，即将要启动的Activity就是在这个ActivityThread实例中运行；

四. Step 25 - Step 27：ActivityThread通过Binder进程间通信机制将一个ApplicationThread类型的Binder对象传递给ActivityManagerService，以便以后ActivityManagerService能够通过这个Binder对象和它进行通信；

五. Step 28 - Step 35：ActivityManagerService通过Binder进程间通信机制通知ActivityThread，现在一切准备就绪，它可以真正执行Activity的启动操作了。

这里不少地方涉及到了Binder进程间通信机制，相关资料请参考Android进程间通信（IPC）机制Binder简要介绍和学习计划一文。

这样，应用程序的启动过程就介绍完了，它实质上是启动应用程序的默认Activity，在下一篇文章中，我们将介绍在应用程序内部启动另一个Activity的过程，即新的Activity与启动它的Activity将会在同一个进程（Process）和任务（Task）运行，敬请关注。

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