Android应用程序启动过程源代码分析

文章转载至CSDN社区罗升阳的安卓之旅，原文地址：http://blog.csdn.net/luoshengyang/article/details/6689748

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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