com.android.settings.MasterClear

private void showFinalConfirmation() {
        Preference preference = new Preference(getActivity());
        preference.setFragment(MasterClearConfirm.class.getName());
        preference.setTitle(R.string.master_clear_confirm_title);
        preference.getExtras().putBoolean(ERASE_EXTERNAL_EXTRA, mExternalStorage.isChecked());
        ((SettingsActivity) getActivity()).onPreferenceStartFragment(null, preference);
    }

MasterClearConfirm.java

Intent intent = new Intent(Intent.ACTION_MASTER_CLEAR);

    private void doMasterClear() {
        if (mEraseSdCard) {
            Intent intent = new Intent(ExternalStorageFormatter.FORMAT_AND_FACTORY_RESET);
            intent.putExtra(Intent.EXTRA_REASON, "WipeAllFlash");
            intent.setComponent(ExternalStorageFormatter.COMPONENT_NAME);
            getActivity().startService(intent);
        } else {
            Intent intent = new Intent(Intent.ACTION_MASTER_CLEAR);
            intent.addFlags(Intent.FLAG_RECEIVER_FOREGROUND);
            intent.putExtra(Intent.EXTRA_REASON, "MasterClearConfirm");
            getActivity().sendBroadcast(intent);
            // Intent handling is asynchronous -- assume it will happen soon.
        }
    }

core/res/AndroidManifest.xml: <receiver android:name="com.android.server.MasterClearReceiver"

系统注册一个receiver

<receiver android:name="com.android.server.MasterClearReceiver"
            android:permission="android.permission.MASTER_CLEAR">
            <intent-filter
                    android:priority="100" >
                <!-- For Checkin, Settings, etc.: action=MASTER_CLEAR -->
                <action android:name="android.intent.action.MASTER_CLEAR" />

                <!-- MCS always uses REMOTE_INTENT: category=MASTER_CLEAR -->
                <action android:name="com.google.android.c2dm.intent.RECEIVE" />
                <category android:name="android.intent.category.MASTER_CLEAR" />
            </intent-filter>
        </receiver>

services/core/java/com/android/server/MasterClearReceiver.java

Thread thr = new Thread("Reboot") {
            @Override
            public void run() {
                try {
                    RecoverySystem.rebootWipeUserData(context, shutdown, reason);
                    Log.wtf(TAG, "Still running after master clear?!");
                } catch (IOException e) {
                    Slog.e(TAG, "Can't perform master clear/factory reset", e);
                } catch (SecurityException e) {
                    Slog.e(TAG, "Can't perform master clear/factory reset", e);
                }
            }
        };
        thr.start();

格式化sdcard，执行umount命令

core/res/AndroidManifest.xml: <service android:name="com.android.internal.os.storage.ExternalStorageFormatter“

<service android:name="com.android.internal.os.storage.ExternalStorageFormatter"
            android:permission="android.permission.MASTER_CLEAR"
            android:exported="true" />

services/core/java/com/android/server/MountService.java

public void unmountVolume(String path, boolean force, boolean removeEncryption) {
        validatePermission(android.Manifest.permission.MOUNT_UNMOUNT_FILESYSTEMS);
        waitForReady();

        String volState = getVolumeState(path);
        if (DEBUG_UNMOUNT) {
            Slog.i(TAG, "Unmounting " + path
                    + " force = " + force
                    + " removeEncryption = " + removeEncryption);
        }
        if (Environment.MEDIA_UNMOUNTED.equals(volState) ||
                Environment.MEDIA_REMOVED.equals(volState) ||
                Environment.MEDIA_SHARED.equals(volState) ||
                Environment.MEDIA_UNMOUNTABLE.equals(volState)) {
            // Media already unmounted or cannot be unmounted.
            // TODO return valid return code when adding observer call back.
            return;
        }
        UnmountCallBack ucb = new UnmountCallBack(path, force, removeEncryption);
        mHandler.sendMessage(mHandler.obtainMessage(H_UNMOUNT_PM_UPDATE, ucb));
    }
    
    private int doUnmountVolume(String path, boolean force, boolean removeEncryption) {
        if (!getVolumeState(path).equals(Environment.MEDIA_MOUNTED)) {
            return VoldResponseCode.OpFailedVolNotMounted;
        }

        /*
         * Force a GC to make sure AssetManagers in other threads of the
         * system_server are cleaned up. We have to do this since AssetManager
         * instances are kept as a WeakReference and it's possible we have files
         * open on the external storage.
         */
        Runtime.getRuntime().gc();

        // Redundant probably. But no harm in updating state again.
        if (path.equals(mExternalStoragePath)) //only flash
        {
            mPms.updateExternalMediaStatus(false, false);
        }
        try {
            final Command cmd = new Command("volume", "unmount", path);
            if (removeEncryption) {
                cmd.appendArg("force_and_revert");
            } else if (force) {
                cmd.appendArg("force");
            }
            mConnector.execute(cmd);
            // We unmounted the volume. None of the asec containers are available now.
            synchronized (mAsecMountSet) {
                mAsecMountSet.clear();
            }
            return StorageResultCode.OperationSucceeded;
        } catch (NativeDaemonConnectorException e) {
            // Don't worry about mismatch in PackageManager since the
            // call back will handle the status changes any way.
            int code = e.getCode();
            if (code == VoldResponseCode.OpFailedVolNotMounted) {
                return StorageResultCode.OperationFailedStorageNotMounted;
            } else if (code == VoldResponseCode.OpFailedStorageBusy) {
                return StorageResultCode.OperationFailedStorageBusy;
            } else {
                return StorageResultCode.OperationFailedInternalError;
            }
        }
    }