【Android】应用程序启动过程源码分析

  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。
  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。

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,Launcher的界面上就会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) {
            ......
        }
    }

    ......

}
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    MainActivity在AndroidManifest.xml文件中是这样配置的:
<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>  
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    因此,这里的intent包含的信息为:action = "android.intent.action.Main",category="android.intent.category.LAUNCHER",表示它要启动的Activity为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。

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);
    }

    ......

}
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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 {
            ......
        }


    ......

}
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  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。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;
    }

    ......

}
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  这里的ActivityManagerNative.getDefault返回ActivityManagerService的远程接口,即ActivityManagerProxy接口

  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。

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;
    }

    ......

}
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  从上面的调用可以知道,这里的参数

  1. resolvedType、grantedUriPermissions和resultWho均为null;
  2. 参数caller为ApplicationThread类型的Binder实体;
  3. 参数resultTo为一个Binder实体的远程接口,先不关注它;
  4. 参数grantedMode为0,也先不关注它;
  5. 参数requestCode为-1;
  6. 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);
    }


    ......

}
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  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;
        }

    }

    ......

}
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  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

  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) {
        ......
    }
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  解析之后,得到的aInfo.applicationInfo.packageName和aInfo.name即为配置文件AndroidManifest.xml里面配置的参数。

  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);
    }


    ......

}
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  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);
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  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;
    }

    ......

}
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  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

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) {
                ......
            }
        }
    }
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  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)) {
            ......
        }
    }

    } 
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  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。

  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);
    }

    }
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  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);
        }
    }

    ......

}
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  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,要做一些任务切换的界面操作,这里不会影响到下面启Activity的过程,有兴趣的童鞋可以自己研究一下。

  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;
        }

        ......
    }

    ......

}
View Code

  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) {
    ......
    }
View Code

  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 {
            ......
        }

        ......
    
    }

    ......

}
View Code

  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();
    }

    ......

}
View Code

  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);
        }

        ......

    }

    ......

}
View Code

  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);
        }
    }

    ......

}
View Code

  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;

            ......

            }
        ......

    }

    ......

}
View Code

  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) {
            }
        }
    }

    ......

}
View Code

  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();
    }

    ......

}
View Code

  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);
        
        ......
    }

    ......

}
View Code

  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 {
                    ......
                }
            }
        }
    }

    ......

}
View Code

  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 {
            ......
        }

        ......
    }

    ......

}
View Code

  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;
    }


    ......

}
View Code

  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);
    }


    ......

}
View Code

  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);
View Code

  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。

  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;
    }

    ......

}
View Code

  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) {
            
            ......

        }
    }

    ......

}
View Code

  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();
        
        ......
    }
}
View Code
  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();
    }

    ......

}
View Code

  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);
        }
    }

    ......

}
View Code

  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;
    }

    ......

}
View Code

  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;
    }

    ......

}
View Code

  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,用来作为这个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();
    }

    ......

}
View Code

  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);
        }

        ......

    }

    ......
}
View Code

  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);
            }
        }

        ......

    }

    ......
}
View Code

  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;
            ......
            }

        ......

    }

    ......
}
View Code

  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 {
            ......
        }
    }

    ......
}
View Code

  这里首先调用performLaunchActivity函数来加载这个Activity类,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;
    }

    ......
}
View Code

  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);
   }
View Code

   然后通过ClassLoader将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) {
    ......
   }
View Code

  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);  
View Code

  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);
View Code

  最后还要调用MainActivity的onCreate函数:

mInstrumentation.callActivityOnCreate(activity, r.state);  
View Code

  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

  这个函数就是我们自定义的app工程文件。

  这样,MainActivity就启动起来了,整个应用程序也启动起来了。


总结

  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:

  1. Step1 - Step 11:Launcher通过Binder进程间通信机制通知ActivityManagerService,它要启动一个Activity;
  2. Step 12 - Step 16:ActivityManagerService通过Binder进程间通信机制通知Launcher进入Paused状态;
  3. 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;
  4. 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;
  5. 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
 
 

参考文章

  http://blog.csdn.net/luoshengyang/article/details/6689748
 
 
posted @ 2014-10-03 22:06  Leo.cheng  阅读(529)  评论(0编辑  收藏