学步园

自己放某个文件到/cache分区，重启后发现文件消失了，那么是怎么消失的呢?

Step 1. 

packages\providers\DownloadProvider\src\com\android\providers\downloads\StorageManager.java:

    /**
     * Removes files in the systemcache and downloads data dir without corresponding entries in
     * the downloads database.
     * This can occur if a delete is done on the database but the file is not removed from the
     * filesystem (due to sudden death of the process, for example).
     * This is not a very common occurrence. So, do this only once in a while.
     */
    private void removeSpuriousFiles() {
        if (true || Constants.LOGV) {
            Log.i(Constants.TAG, "in removeSpuriousFiles");
        }
        // get a list of all files in system cache dir and downloads data dir
        List<File> files = new ArrayList<File>();
        File[] listOfFiles = mSystemCacheDir.listFiles();
        if (listOfFiles != null) {
            files.addAll(Arrays.asList(listOfFiles));
        }
        listOfFiles = mDownloadDataDir.listFiles();
        if (listOfFiles != null) {
            files.addAll(Arrays.asList(listOfFiles));
        }
        if (files.size() == 0) {
            return;
        }
        Cursor cursor = mContext.getContentResolver().query(
                Downloads.Impl.ALL_DOWNLOADS_CONTENT_URI,
                new String[] { Downloads.Impl._DATA }, null, null, null);
        try {
            if (cursor != null) {
                while (cursor.moveToNext()) {
                    String filename = cursor.getString(0);
                    if (!TextUtils.isEmpty(filename)) {
                        if (true || Constants.LOGV) {
                            Log.i(Constants.TAG, "in removeSpuriousFiles, preserving file " +
                                    filename);
                        }
                        files.remove(new File(filename));
                    }
                }
            }
        } finally {
            if (cursor != null) {
                cursor.close();
            }
        }
        // delete the files not found in the database
        for (File file : files) {
            if (file.getName().equals(Constants.KNOWN_SPURIOUS_FILENAME) ||
                    file.getName().equalsIgnoreCase(Constants.RECOVERY_DIRECTORY)) {
                continue;
            }
            if (true || Constants.LOGV) {
                Log.i(Constants.TAG, "deleting spurious file " + file.getAbsolutePath());
            }
            file.delete();
        }
    }

    /** A magic filename that is allowed to exist within the system cache */
    public static final String KNOWN_SPURIOUS_FILENAME = "lost+found";

    /** A magic filename that is allowed to exist within the system cache */
    public static final String RECOVERY_DIRECTORY = "recovery";

除 lost+found, recovery这两个目录外的文件都删掉

Setp 2.

frameworks\base\core\java\android\os\RecoverySystem.java:

    /**
     * Called after booting to process and remove recovery-related files.
     * @return the log file from recovery, or null if none was found.
     *
     * @hide
     */
    public static String handleAftermath() {
        // Record the tail of the LOG_FILE
        String log = null;
        try {
            log = FileUtils.readTextFile(LOG_FILE, -LOG_FILE_MAX_LENGTH, "...\n");
        } catch (FileNotFoundException e) {
            Log.i(TAG, "No recovery log file");
        } catch (IOException e) {
            Log.e(TAG, "Error reading recovery log", e);
        }

        // Delete everything in RECOVERY_DIR except those beginning
        // with LAST_PREFIX
        String[] names = RECOVERY_DIR.list();
        for (int i = 0; names != null && i < names.length; i++) {
            if (names[i].startsWith(LAST_PREFIX)) continue;
            File f = new File(RECOVERY_DIR, names[i]);
            if (!f.delete()) {
                Log.e(TAG, "Can't delete: " + f);
            } else {
                Log.i(TAG, "Deleted: " + f);
            }
        }

        return log;
    }

private static String LAST_PREFIX = "last_";

/cache/recovery目录中，除last_开头的文件都删掉

------------------------------------------------

开机走完这两步后幸存的文件只有 /cache/lost+found目录中的文件，及/cache/recovery/last_ 开头的文件了