Android: /cache中的檔案是怎麼消失的

自己放某個檔案到/cache分區，重啟後發現檔案消失了，那麼是怎麼消失的呢?

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這兩個目錄外的檔案都刪掉

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目錄中，除last_開頭的檔案都刪掉

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開機走完這兩步後倖存的檔案只有 /cache/lost+found目錄中的檔案，及/cache/recovery/last_ 開頭的檔案了