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前文簡要介紹了Android應用程序的Activity的啟動過程。在Android系統(tǒng)中，應用程序是由Activity組成的，因此，應用程序的啟動過程實際上就是應用程序中的默認Activity的啟動過程，本文將詳細分析應用程序框架層的源代碼，了解Android應用程序的啟動過程。

在上一篇文章 Android應用程序的Activity啟動過程簡要介紹和學習計劃中，我們舉例子說明了啟動Android應用程序中的Activity的兩種情景，其中，在手機屏幕中點擊應用程序圖標的情景就會引發(fā)Android應用程序中的默認Activity的啟動，從而把應用程序啟動起來。這種啟動方式的特點是會啟動一個新的進程來加載相應的Activity。這里，我們繼續(xù)以這個例子為例來說明Android應用程序的啟動過程，即MainActivity的啟動過程。

MainActivity的啟動過程如下圖所示：

點擊查看大圖

下面詳細分析每一步是如何實現的。

Step 1. Launcher.startActivitySafely

在Android系統(tǒng)中，應用程序是由Launcher啟動起來的，其實，Launcher本身也是一個應用程序，其它的應用程序安裝后，就會Launcher的界面上出現一個相應的圖標，點擊這個圖標時，Launcher就會對應的應用程序啟動起來。

Launcher的源代碼工程在packages/apps/Launcher2目錄下，負責啟動其它應用程序的源代碼實現在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) {
			......
		}
	}

	......

}

回憶一下前面一篇文章 Android應用程序的Activity啟動過程簡要介紹和學習計劃說到的應用程序Activity，它的默認Activity是MainActivity，這里是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，這里的intent包含的信息為：action = "android.intent.action.Main"，category="android.intent.category.LAUNCHER", cmp="shy.luo.activity/.MainActivity"，表示它要啟動的Activity為shy.luo.activity.MainActivity。Intent.FLAG_ACTIVITY_NEW_TASK表示要在一個新的Task中啟動這個Activity，注意，Task是Android系統(tǒng)中的概念，它不同于進程Process的概念。簡單地說，一個Task是一系列Activity的集合，這個集合是以堆棧的形式來組織的，遵循后進先出的原則。事實上，Task是一個非常復雜的概念，有興趣的讀者可以到官網 http://developer.android.com/guide/topics/manifest/activity-element.html 查看相關的資料。這里，我們只要知道，這個MainActivity要在一個新的Task中啟動就可以了。

Step 2. Activity.startActivity

在Step 1中，我們看到，Launcher繼承于Activity類，而Activity類實現了startActivity函數，因此，這里就調用了Activity.startActivity函數，它實現在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);
	}

	......

}

這個函數實現很簡單，它調用startActivityForResult來進一步處理，第二個參數傳入-1表示不需要這個Actvity結束后的返回結果。

Step 3.Activity.startActivityForResult

這個函數也是實現在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 {
			......
		}


	......

}

這里的mInstrumentation是Activity類的成員變量，它的類型是Intrumentation，定義在frameworks/base/core/java/android/app/Instrumentation.java文件中，它用來監(jiān)控應用程序和系統(tǒng)的交互。

這里的mMainThread也是Activity類的成員變量，它的類型是ActivityThread，它代表的是應用程序的主線程，我們在 Android系統(tǒng)在新進程中啟動自定義服務過程（startService）的原理分析一文中已經介紹過了。這里通過mMainThread.getApplicationThread獲得它里面的ApplicationThread成員變量，它是一個Binder對象，后面我們會看到，ActivityManagerService會使用它來和ActivityThread來進行進程間通信。這里我們需注意的是，這里的mMainThread代表的是Launcher應用程序運行的進程。

這里的mToken也是Activity類的成員變量，它是一個Binder對象的遠程接口。

Step 4. Instrumentation.execStartActivity
這個函數定義在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;
	}

	......

}

這里的ActivityManagerNative.getDefault返回ActivityManagerService的遠程接口，即ActivityManagerProxy接口，具體可以參考 Android系統(tǒng)在新進程中啟動自定義服務過程（startService）的原理分析一文。

這里的intent.resolveTypeIfNeeded返回這個intent的MIME類型，在這個例子中，沒有AndroidManifest.xml設置MainActivity的MIME類型，因此，這里返回null。

這里的target不為null，但是target.mEmbddedID為null，我們不用關注。

Step 5. ActivityManagerProxy.startActivity

這個函數定義在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;
	}

	......

}

這里的參數比較多，我們先整理一下。從上面的調用可以知道，這里的參數resolvedType、grantedUriPermissions和resultWho均為null；參數caller為ApplicationThread類型的Binder實體；參數resultTo為一個Binder實體的遠程接口，我們先不關注它；參數grantedMode為0，我們也先不關注它；參數requestCode為-1；參數onlyIfNeeded和debug均空false。

Step 6. ActivityManagerService.startActivity

上一步Step 5通過Binder驅動程序就進入到ActivityManagerService的startActivity函數來了，它定義在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);
	}


	......

}

這里只是簡單地將操作轉發(fā)給成員變量mMainStack的startActivityMayWait函數，這里的mMainStack的類型為ActivityStack。

Step 7.ActivityStack.startActivityMayWait

這個函數定義在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，從Step 6傳下來的參數outResult和config均為null，此外，表達式(aInfo.applicationInfo.flags&ApplicationInfo.FLAG_CANT_SAVE_STATE) != 0為false，因此，這里忽略了無關代碼。

下面語句對參數intent的內容進行解析，得到MainActivity的相關信息，保存在aInfo變量中：

        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的值為"shy.luo.activity"，aInfo.name的值為"shy.luo.activity.MainActivity"，這是在這個實例的配置文件AndroidManifest.xml里面配置的。

此外，函數開始的地方調用intent.getComponent()函數的返回值不為null，因此，這里的componentSpecified變量為true。

接下去就調用startActivityLocked進一步處理了。

Step 8. ActivityStack.startActivityLocked

這個函數定義在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);
	}


	......

}

從傳進來的參數caller得到調用者的進程信息，并保存在callerApp變量中，這里就是Launcher應用程序的進程信息了。

前面說過，參數resultTo是Launcher這個Activity里面的一個Binder對象，通過它可以獲得Launcher這個Activity的相關信息，保存在sourceRecord變量中。
再接下來，創(chuàng)建即將要啟動的Activity的相關信息，并保存在r變量中：

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

接著調用startActivityUncheckedLocked函數進行下一步操作。

Step 9. ActivityStack.startActivityUncheckedLocked

這個函數定義在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;
	}

	......

}

函數首先獲得intent的標志值，保存在launchFlags變量中。

這個intent的標志值的位Intent.FLAG_ACTIVITY_NO_USER_ACTION沒有置位，因此，成員變量mUserLeaving的值為true。

這個intent的標志值的位Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP也沒有置位，因此，變量notTop的值為null。

由于在這個例子的AndroidManifest.xml文件中，MainActivity沒有配置launchMode屬值，因此，這里的r.launchMode為默認值0，表示以標準（Standard，或者稱為ActivityInfo.LAUNCH_MULTIPLE）的方式來啟動這個Activity。Activity的啟動方式有四種，其余三種分別是ActivityInfo.LAUNCH_SINGLE_INSTANCE、ActivityInfo.LAUNCH_SINGLE_TASK和ActivityInfo.LAUNCH_SINGLE_TOP，具體可以參考官方網站 http://developer.android.com/reference/android/content/pm/ActivityInfo.html 。

傳進來的參數r.resultTo為null，表示Launcher不需要等這個即將要啟動的MainActivity的執(zhí)行結果。

由于這個intent的標志值的位Intent.FLAG_ACTIVITY_NEW_TASK被置位，而且Intent.FLAG_ACTIVITY_MULTIPLE_TASK沒有置位，因此，下面的if語句會被執(zhí)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) {
				......
			}
		}
    }

這段代碼的邏輯是查看一下，當前有沒有Task可以用來執(zhí)行這個Activity。由于r.launchMode的值不為ActivityInfo.LAUNCH_SINGLE_INSTANCE，因此，它通過findTaskLocked函數來查找存不存這樣的Task，這里返回的結果是null，即taskTop為null，因此，需要創(chuàng)建一個新的Task來啟動這個Activity。

接著往下看：

        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)) {
			......
		}
	}

    }

這段代碼的邏輯是看一下，當前在堆棧頂端的Activity是否就是即將要啟動的Activity，有些情況下，如果即將要啟動的Activity就在堆棧的頂端，那么，就不會重新啟動這個Activity的別一個實例了，具體可以參考官方網站 http://developer.android.com/reference/android/content/pm/ActivityInfo.html 。現在處理堆棧頂端的Activity是Launcher，與我們即將要啟動的MainActivity不是同一個Activity，因此，這里不用進一步處理上述介紹的情況。

執(zhí)行到這里，我們知道，要在一個新的Task里面來啟動這個Activity了，于是新創(chuàng)建一個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，同時，添加到mService中去，這里的mService就是ActivityManagerService了。

最后就進入startActivityLocked(r, newTask, doResume)進一步處理了。這個函數定義在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);
		}
	}

	......

}

這里的NH表示當前系統(tǒng)中歷史任務的個數，這里肯定是大于0，因為Launcher已經跑起來了。當NH>0時，并且現在要切換新任務時，要做一些任務切的界面操作，這段代碼我們就不看了，這里不會影響到下面啟Activity的過程，有興趣的讀取可以自己研究一下。

這里傳進來的參數doResume為true，于是調用resumeTopActivityLocked進一步操作。

Step 10. Activity.resumeTopActivityLocked

這個函數定義在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;
		}

		......
	}

	......

}

函數先通過調用topRunningActivityLocked函數獲得堆棧頂端的Activity，這里就是MainActivity了，這是在上面的Step 9設置好的，保存在next變量中。

接下來把mUserLeaving的保存在本地變量userLeaving中，然后重新設置為false，在上面的Step 9中，mUserLeaving的值為true，因此，這里的userLeaving為true。

這里的mResumedActivity為Launcher，因為Launcher是當前正被執(zhí)行的Activity。

當我們處理休眠狀態(tài)時，mLastPausedActivity保存堆棧頂端的Activity，因為當前不是休眠狀態(tài)，所以mLastPausedActivity為null。

有了這些信息之后，下面的語句就容易理解了：

        // 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) {
	......
    }

它首先看要啟動的Activity是否就是當前處理Resumed狀態(tài)的Activity，如果是的話，那就什么都不用做，直接返回就可以了；否則再看一下系統(tǒng)當前是否休眠狀態(tài)，如果是的話，再看看要啟動的Activity是否就是當前處于堆棧頂端的Activity，如果是的話，也是什么都不用做。

上面兩個條件都不滿足，因此，在繼續(xù)往下執(zhí)行之前，首先要把當處于Resumed狀態(tài)的Activity推入Paused狀態(tài)，然后才可以啟動新的Activity。但是在將當前這個Resumed狀態(tài)的Activity推入Paused狀態(tài)之前，首先要看一下當前是否有Activity正在進入Pausing狀態(tài)，如果有的話，當前這個Resumed狀態(tài)的Activity就要稍后才能進入Paused狀態(tài)了，這樣就保證了所有需要進入Paused狀態(tài)的Activity串行處理。

這里沒有處于Pausing狀態(tài)的Activity，即mPausingActivity為null，而且mResumedActivity也不為null，于是就調用startPausingLocked函數把Launcher推入Paused狀態(tài)去了。

Step 11. ActivityStack.startPausingLocked

這個函數定義在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 {
			......
		}

		......
	
	}

	......

}

函數首先把mResumedActivity保存在本地變量prev中。在上一步Step 10中，說到mResumedActivity就是Launcher，因此，這里把Launcher進程中的ApplicationThread對象取出來，通過它來通知Launcher這個Activity它要進入Paused狀態(tài)了。當然，這里的prev.app.thread是一個ApplicationThread對象的遠程接口，通過調用這個遠程接口的schedulePauseActivity來通知Launcher進入Paused狀態(tài)。

參數prev.finishing表示prev所代表的Activity是否正在等待結束的Activity列表中，由于Laucher這個Activity還沒結束，所以這里為false；參數prev.configChangeFlags表示哪些config發(fā)生了變化，這里我們不關心它的值。

Step 12.ApplicationThreadProxy.schedulePauseActivity

這個函數定義在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();
	}

	......

}

這個函數通過Binder進程間通信機制進入到ApplicationThread.schedulePauseActivity函數中。

Step 13. ApplicationThread.schedulePauseActivity

這個函數定義在frameworks/base/core/java/android/app/ActivityThread.java文件中，它是ActivityThread的內部類：

    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);
		}

		......

	}

	......

}

這里調用的函數queueOrSendMessage是ActivityThread類的成員函數。

上面說到，這里的finished值為false，因此，queueOrSendMessage的第一個參數值為H.PAUSE_ACTIVITY，表示要暫停token所代表的Activity，即Launcher。

Step 14. ActivityThread.queueOrSendMessage

這個函數定義在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);
		}
	}

	......

}

這里首先將相關信息組裝成一個msg，然后通過mH成員變量發(fā)送出去，mH的類型是H，繼承于Handler類，是ActivityThread的內部類，因此，這個消息最后由H.handleMessage來處理。

Step 15. H.handleMessage

這個函數定義在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;

			......

			}
		......

	}

	......

}

這里調用ActivityThread.handlePauseActivity進一步操作，msg.obj是一個ActivityRecord對象的引用，它代表的是Launcher這個Activity。
Step 16.ActivityThread.handlePauseActivity

這個函數定義在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) {
			}
		}
	}

	......

}

函數首先將Binder引用token轉換成ActivityRecord的遠程接口ActivityClientRecord，然后做了三個事情：1. 如果userLeaving為true，則通過調用performUserLeavingActivity函數來調用Activity.onUserLeaveHint通知Activity，用戶要離開它了；2. 調用performPauseActivity函數來調用Activity.onPause函數，我們知道，在Activity的生命周期中，當它要讓位于其它的Activity時，系統(tǒng)就會調用它的onPause函數；3. 它通知ActivityManagerService，這個Activity已經進入Paused狀態(tài)了，ActivityManagerService現在可以完成未竟的事情，即啟動MainActivity了。

Step 17. ActivityManagerProxy.activityPaused

這個函數定義在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();
	}

	......

}

這里通過Binder進程間通信機制就進入到ActivityManagerService.activityPaused函數中去了。

Step 18.ActivityManagerService.activityPaused

這個函數定義在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);
		
		......
	}

	......

}

這里，又再次進入到ActivityStack類中，執(zhí)行activityPaused函數。

Step 19.ActivityStack.activityPaused

這個函數定義在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 {
					......
				}
			}
		}
	}

	......

}

這里通過參數token在mHistory列表中得到ActivityRecord，從上面我們知道，這個ActivityRecord代表的是Launcher這個Activity，而我們在Step 11中，把Launcher這個Activity的信息保存在mPausingActivity中，因此，這里mPausingActivity等于r，于是，執(zhí)行completePauseLocked操作。

Step 20.ActivityStack.completePauseLocked

這個函數定義在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 {
			......
		}

		......
	}

	......

}

函數首先把mPausingActivity變量清空，因為現在不需要它了，然后調用resumeTopActivityLokced進一步操作，它傳入的參數即為代表Launcher這個Activity的ActivityRecord。

Step 21.ActivityStack.resumeTopActivityLokced
這個函數定義在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;
	}


	......

}

通過上面的Step 9，我們知道，當前在堆棧頂端的Activity為我們即將要啟動的MainActivity，這里通過調用topRunningActivityLocked將它取回來，保存在next變量中。之前最后一個Resumed狀態(tài)的Activity，即Launcher，到了這里已經處于Paused狀態(tài)了，因此，mResumedActivity為null。最后一個處于Paused狀態(tài)的Activity為Launcher，因此，這里的mLastPausedActivity就為Launcher。前面我們?yōu)镸ainActivity創(chuàng)建了ActivityRecord后，它的app域一直保持為null。有了這些信息后，上面這段代碼就容易理解了，它最終調用startSpecificActivityLocked進行下一步操作。

Step 22. ActivityStack.startSpecificActivityLocked
這個函數定義在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，這里由于是第一次啟動應用程序的Activity，所以下面語句：

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

取回來的app為null。在Activity應用程序中的AndroidManifest.xml配置文件中，我們沒有指定Application標簽的process屬性，系統(tǒng)就會默認使用package的名稱，這里就是"shy.luo.activity"了。每一個應用程序都有自己的uid，因此，這里uid + process的組合就可以為每一個應用程序創(chuàng)建一個ProcessRecord。當然，我們可以配置兩個應用程序具有相同的uid和package，或者在AndroidManifest.xml配置文件的application標簽或者activity標簽中顯式指定相同的process屬性值，這樣，不同的應用程序也可以在同一個進程中啟動。

函數最終執(zhí)行ActivityManagerService.startProcessLocked函數進行下一步操作。

Step 23.ActivityManagerService.startProcessLocked

這個函數定義在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;
	}

	......

}

這里再次檢查是否已經有以process + uid命名的進程存在，在我們這個情景中，返回值app為null，因此，后面會創(chuàng)建一個ProcessRecord，并存保存在成員變量mProcessNames中，最后，調用另一個startProcessLocked函數進一步操作：

    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) {
			
			......

		}
	}

	......

}

這里主要是調用Process.start接口來創(chuàng)建一個新的進程，新的進程會導入android.app.ActivityThread類，并且執(zhí)行它的main函數，這就是為什么我們前面說每一個應用程序都有一個ActivityThread實例來對應的原因。

Step 24. ActivityThread.main

這個函數定義在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();
		
		......
	}
}

這個函數在進程中創(chuàng)建一個ActivityThread實例，然后調用它的attach函數，接著就進入消息循環(huán)了，直到最后進程退出。

函數attach最終調用了ActivityManagerService的遠程接口ActivityManagerProxy的attachApplication函數，傳入的參數是mAppThread，這是一個ApplicationThread類型的Binder對象，它的作用是用來進行進程間通信的。

Step 25.ActivityManagerProxy.attachApplication

這個函數定義在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();
	}

	......

}

這里通過Binder驅動程序，最后進入ActivityManagerService的attachApplication函數中。

Step 26.ActivityManagerService.attachApplication

這個函數定義在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);
		}
	}

	......

}

這里將操作轉發(fā)給attachApplicationLocked函數。

Step 27.ActivityManagerService.attachApplicationLocked

這個函數定義在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，已經創(chuàng)建了一個ProcessRecord，這里首先通過pid將它取回來，放在app變量中，然后對app的其它成員進行初始化，最后調用mMainStack.realStartActivityLocked執(zhí)行真正的Activity啟動操作。這里要啟動的Activity通過調用mMainStack.topRunningActivityLocked(null)從堆棧頂端取回來，這時候在堆棧頂端的Activity就是MainActivity了。

Step 28. ActivityStack.realStartActivityLocked

這個函數定義在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;
	}

	......

}

這里最終通過app.thread進入到ApplicationThreadProxy的scheduleLaunchActivity函數中，注意，這里的第二個參數r，是一個ActivityRecord類型的Binder對象，用來作來這個Activity的token值。

Step 29.ApplicationThreadProxy.scheduleLaunchActivity
這個函數定義在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();
	}

	......

}

這個函數最終通過Binder驅動程序進入到ApplicationThread的scheduleLaunchActivity函數中。

Step 30.ApplicationThread.scheduleLaunchActivity
這個函數定義在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);
		}

		......

	}

	......
}

函數首先創(chuàng)建一個ActivityClientRecord實例，并且初始化它的成員變量，然后調用ActivityThread類的queueOrSendMessage函數進一步處理。

Step 31.ActivityThread.queueOrSendMessage
這個函數定義在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);
			}
		}

		......

	}

	......
}

函數把消息內容放在msg中，然后通過mH把消息分發(fā)出去，這里的成員變量mH我們在前面已經見過，消息分發(fā)出去后，最后會調用H類的handleMessage函數。

Step 32. H.handleMessage

這個函數定義在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;
			......
			}

		......

	}

	......
}

這里最后調用ActivityThread類的handleLaunchActivity函數進一步處理。

Step 33.ActivityThread.handleLaunchActivity

這個函數定義在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 {
			......
		}
	}

	......
}

這里首先調用performLaunchActivity函數來加載這個Activity類，即shy.luo.activity.MainActivity，然后調用它的onCreate函數，最后回到handleLaunchActivity函數時，再調用handleResumeActivity函數來使這個Activity進入Resumed狀態(tài)，即會調用這個Activity的onResume函數，這是遵循Activity的生命周期的。

Step 34.ActivityThread.performLaunchActivity
這個函數定義在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;
	}

	......
}

函數前面是收集要啟動的Activity的相關信息，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);
   }

然后通過ClassLoader將shy.luo.activity.MainActivity類加載進來：

       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) {
	......
   }

接下來是創(chuàng)建Application對象，這是根據AndroidManifest.xml配置文件中的Application標簽的信息來創(chuàng)建的：

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

后面的代碼主要創(chuàng)建Activity的上下文信息，并通過attach方法將這些上下文信息設置到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);

最后還要調用MainActivity的onCreate函數：

       mInstrumentation.callActivityOnCreate(activity, r.state);

這里不是直接調用MainActivity的onCreate函數，而是通過mInstrumentation的callActivityOnCreate函數來間接調用，前面我們說過，mInstrumentation在這里的作用是監(jiān)控Activity與系統(tǒng)的交互操作，相當于是系統(tǒng)運行日志。

Step 35. MainActivity.onCreate

這個函數定義在packages/experimental/Activity/src/shy/luo/activity/MainActivity.java文件中，這是我們自定義的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.");
	}

	......

}

這樣，MainActivity就啟動起來了，整個應用程序也啟動起來了。

整個應用程序的啟動過程要執(zhí)行很多步驟，但是整體來看，主要分為以下五個階段：

一. Step1 - Step 11：Launcher通過Binder進程間通信機制通知ActivityManagerService，它要啟動一個Activity；

二. Step 12 - Step 16：ActivityManagerService通過Binder進程間通信機制通知Launcher進入Paused狀態(tài)；

三. Step 17 - Step 24：Launcher通過Binder進程間通信機制通知ActivityManagerService，它已經準備就緒進入Paused狀態(tài)，于是ActivityManagerService就創(chuàng)建一個新的進程，用來啟動一個ActivityThread實例，即將要啟動的Activity就是在這個ActivityThread實例中運行；

四. Step 25 - Step 27：ActivityThread通過Binder進程間通信機制將一個ApplicationThread類型的Binder對象傳遞給ActivityManagerService，以便以后ActivityManagerService能夠通過這個Binder對象和它進行通信；

五. Step 28 - Step 35：ActivityManagerService通過Binder進程間通信機制通知ActivityThread，現在一切準備就緒，它可以真正執(zhí)行Activity的啟動操作了。

這里不少地方涉及到了Binder進程間通信機制，相關資料請參考 Android進程間通信（IPC）機制Binder簡要介紹和學習計劃一文。

這樣，應用程序的啟動過程就介紹完了，它實質上是啟動應用程序的默認Activity，在下一篇文章中，我們將介紹在應用程序內部啟動另一個Activity的過程，即新的Activity與啟動它的Activity將會在同一個進程（Process）和任務（Task）運行，敬請關注。

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Android應用程序啟動過程源代碼分析

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