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:

1. /**
2. * Removes files in the systemcache and downloads data dir without corresponding entries in
3. * the downloads database.
4. * This can occur if a delete is done on the database but the file is not removed from the
5. * filesystem (due to sudden death of the process, for example).
6. * This is not a very common occurrence. So, do this only once in a while.
7. */
8. private void removeSpuriousFiles() {
9. if (true || Constants.LOGV) {
10. Log.i(Constants.TAG, “in removeSpuriousFiles”);
11. }
12. // get a list of all files in system cache dir and downloads data dir
13. List<File> files = new ArrayList<File>();
14. File[] listOfFiles = mSystemCacheDir.listFiles();
15. if (listOfFiles != null) {
16. files.addAll(Arrays.asList(listOfFiles));
17. }
18. listOfFiles = mDownloadDataDir.listFiles();
19. if (listOfFiles != null) {
20. files.addAll(Arrays.asList(listOfFiles));
21. }
22. if (files.size() == 0) {
23. return;
24. }
25. Cursor cursor = mContext.getContentResolver().query(
26. Downloads.Impl.ALL_DOWNLOADS_CONTENT_URI,
27. new String[] { Downloads.Impl._DATA }, null, null, null);
28. try {
29. if (cursor != null) {
30. while (cursor.moveToNext()) {
31. String filename = cursor.getString(0);
32. if (!TextUtils.isEmpty(filename)) {
33. if (true || Constants.LOGV) {
34. Log.i(Constants.TAG, “in removeSpuriousFiles, preserving file ” +
35. filename);
36. }
37. files.remove(new File(filename));
38. }
39. }
40. }
41. } finally {
42. if (cursor != null) {
43. cursor.close();
44. }
45. }
46. // delete the files not found in the database
47. for (File file : files) {
48. if (file.getName().equals(Constants.KNOWN_SPURIOUS_FILENAME) ||
49. file.getName().equalsIgnoreCase(Constants.RECOVERY_DIRECTORY)) {
50. continue;
51. }
52. if (true || Constants.LOGV) {
53. Log.i(Constants.TAG, “deleting spurious file ” + file.getAbsolutePath());
54. }
55. file.delete();
56. }
57. }

1. /** A magic filename that is allowed to exist within the system cache */
2. public static final String KNOWN_SPURIOUS_FILENAME = "lost+found";
3. /** A magic filename that is allowed to exist within the system cache */
4. 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:

1. /**
2. * Called after booting to process and remove recovery-related files.
3. * @return the log file from recovery, or null if none was found.
4. *
5. * @hide
6. */
7. public static String handleAftermath() {
8. // Record the tail of the LOG_FILE
9. String log = null;
10. try {
11. log = FileUtils.readTextFile(LOG_FILE, -LOG_FILE_MAX_LENGTH, "...\n");
12. } catch (FileNotFoundException e) {
13. Log.i(TAG, "No recovery log file");
14. } catch (IOException e) {
15. Log.e(TAG, "Error reading recovery log", e);
16. }
17. // Delete everything in RECOVERY_DIR except those beginning
18. // with LAST_PREFIX
19. String[] names = RECOVERY_DIR.list();
20. for (int i = 0; names != null && i < names.length; i++) {
21. if (names[i].startsWith(LAST_PREFIX)) continue;
22. File f = new File(RECOVERY_DIR, names[i]);
23. if (!f.delete()) {
24. Log.e(TAG, "Can't delete: " + f);
25. } else {
26. Log.i(TAG, "Deleted: " + f);
27. }
28. }
29. return log;
30. }

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