突然想到个问题，SharedPreferences线程安全么？有没有使用缓存相关的技术？
首先想到的是Activity里面的：
public abstract SharedPreferences getSharedPreferences(String name, int mode);
在android.content.Context中，我们首先找到简单的解释：
The single SharedPreferences instance that can be used to retrieve and modify the preference values.
关键字：单例，继续
1 // android.app.ContextImpl.java
2 private static final HashMap<String, SharedPreferencesImpl> sSharedPrefs = new HashMap<String, SharedPreferencesImpl>();
3 @Override
4 public SharedPreferences getSharedPreferences(String name, int mode) {
5 SharedPreferencesImpl sp;
6 File prefsFile;
7 boolean needInitialLoad = false;
8 synchronized (sSharedPrefs) {
9 sp = sSharedPrefs.get(name);
10 if (sp != null && !sp.hasFileChangedUnexpectedly()) {
11 return sp;
12 }
13 prefsFile = getSharedPrefsFile(name);
14 if (sp == null) {
15 sp = new SharedPreferencesImpl(prefsFile, mode, null);
16 sSharedPrefs.put(name, sp);
17 needInitialLoad = true;
18 }
19 }
20
21 synchronized (sp) {
22 if (needInitialLoad && sp.isLoaded()) {
23 // lost the race to load; another thread handled it
24 return sp;
25 }
26 File backup = makeBackupFile(prefsFile);
27 if (backup.exists()) {
28 prefsFile.delete();
29 backup.renameTo(prefsFile);
30 }
31
32 // Debugging
33 if (prefsFile.exists() && !prefsFile.canRead()) {
34 Log.w(TAG, "Attempt to read preferences file " + prefsFile + " without permission");
35 }
36
37 Map map = null;
38 FileStatus stat = new FileStatus();
39 if (FileUtils.getFileStatus(prefsFile.getPath(), stat) && prefsFile.canRead()) {
40 try {
41 FileInputStream str = new FileInputStream(prefsFile);
42 map = XmlUtils.readMapXml(str);
43 str.close();
44 } catch (org.xmlpull.v1.XmlPullParserException e) {
45 Log.w(TAG, "getSharedPreferences", e);
46 } catch (FileNotFoundException e) {
47 Log.w(TAG, "getSharedPreferences", e);
48 } catch (IOException e) {
49 Log.w(TAG, "getSharedPreferences", e);
50 }
51 }
52 sp.replace(map, stat);
53 }
54 return sp;
55 }
认识1：getSharedPreferences通过Map保证SharedPreferences单例。
继续
//SharedPreferencesImpl.java
52 final class More ...SharedPreferencesImpl implements SharedPreferences {
53 private static final String TAG = "SharedPreferencesImpl";
54 private static final boolean DEBUG = false;
55
56 // Lock ordering rules:
57 // - acquire SharedPreferencesImpl.this before EditorImpl.this
58 // - acquire mWritingToDiskLock before EditorImpl.this
59
60 private final File mFile;
61 private final File mBackupFile;
62 private final int mMode;
63
64 private Map<String, Object> mMap; // guarded by ‘this‘
mMap是不是editor的缓存？
//SharedPreferencesImpl.java
273 public Editor edit() {
274 // TODO: remove the need to call awaitLoadedLocked() when
275 // requesting an editor. will require some work on the
276 // Editor, but then we should be able to do:
277 //
278 // context.getSharedPreferences(..).edit().putString(..).apply()
279 //
280 // ... all without blocking.
281 synchronized (this) {
282 awaitLoadedLocked();
283 }
284
285 return new EditorImpl();
286 }
303 public final class EditorImpl implements Editor {
304 private final Map<String, Object> mModified = Maps.newHashMap();
305 private boolean mClear = false;
306
307 public Editor putString(String key, String value) {
308 synchronized (this) {
309 mModified.put(key, value);
310 return this;
311 }
312 }
认识2：每次edit会new EditorImpl ，并且EditorImpl 自带mModified缓存。
388 // Returns true if any changes were made
389 private MemoryCommitResult commitToMemory() {
390 MemoryCommitResult mcr = new MemoryCommitResult();
391 synchronized (SharedPreferencesImpl.this) {
392 // We optimistically don‘t make a deep copy until
393 // a memory commit comes in when we‘re already
394 // writing to disk.
395 if (mDiskWritesInFlight > 0) {
396 // We can‘t modify our mMap as a currently
397 // in-flight write owns it. Clone it before
398 // modifying it.
399 // noinspection unchecked
400 mMap = new HashMap<String, Object>(mMap);
401 }
402 mcr.mapToWriteToDisk = mMap;
403 mDiskWritesInFlight++;
404
405 boolean hasListeners = mListeners.size() > 0;
406 if (hasListeners) {
407 mcr.keysModified = new ArrayList<String>();
408 mcr.listeners =
409 new HashSet<OnSharedPreferenceChangeListener>(mListeners.keySet());
410 }
411
412 synchronized (this) {
413 if (mClear) {
414 if (!mMap.isEmpty()) {
415 mcr.changesMade = true;
416 mMap.clear();
417 }
418 mClear = false;
419 }
420
421 for (Map.Entry<String, Object> e : mModified.entrySet()) {
422 String k = e.getKey();
423 Object v = e.getValue();
424 if (v == this) { // magic value for a removal mutation
425 if (!mMap.containsKey(k)) {
426 continue;
427 }
428 mMap.remove(k);
429 } else {
430 boolean isSame = false;
431 if (mMap.containsKey(k)) {
432 Object existingValue = mMap.get(k);
433 if (existingValue != null && existingValue.equals(v)) {
434 continue;
435 }
436 }
437 mMap.put(k, v);
438 }
439
440 mcr.changesMade = true;
441 if (hasListeners) {
442 mcr.keysModified.add(k);
443 }
444 }
445
446 mModified.clear();
447 }
448 }
449 return mcr;
450 }
452 public boolean commit() {
453 MemoryCommitResult mcr = commitToMemory();
454 SharedPreferencesImpl.this.enqueueDiskWrite(
455 mcr, null /* sync write on this thread okay */);
456 try {
457 mcr.writtenToDiskLatch.await();
458 } catch (InterruptedException e) {
459 return false;
460 }
461 notifyListeners(mcr);
462 return mcr.writeToDiskResult;
463 }
commit时，editor先把mModified同步到mMap，然后再写入File。认识3：editor.commit是线程安全的。
564 private void writeToFile(MemoryCommitResult mcr) {
565 // Rename the current file so it may be used as a backup during the next read
566 if (mFile.exists()) {
567 if (!mcr.changesMade) {
568 // If the file already exists, but no changes were
569 // made to the underlying map, it‘s wasteful to
570 // re-write the file. Return as if we wrote it
571 // out.
572 mcr.setDiskWriteResult(true);
573 return;
574 }
575 if (!mBackupFile.exists()) {
576 if (!mFile.renameTo(mBackupFile)) {
577 Log.e(TAG, "Couldn‘t rename file " + mFile
578 + " to backup file " + mBackupFile);
579 mcr.setDiskWriteResult(false);
580 return;
581 }
582 } else {
583 mFile.delete();
584 }
585 }
586
587 // Attempt to write the file, delete the backup and return true as atomically as
588 // possible. If any exception occurs, delete the new file; next time we will restore
589 // from the backup.
590 try {
591 FileOutputStream str = createFileOutputStream(mFile);
592 if (str == null) {
593 mcr.setDiskWriteResult(false);
594 return;
595 }
596 XmlUtils.writeMapXml(mcr.mapToWriteToDisk, str);
597 FileUtils.sync(str);
598 str.close();
599 ContextImpl.setFilePermissionsFromMode(mFile.getPath(), mMode, 0);
600 try {
601 final StructStat stat = Libcore.os.stat(mFile.getPath());
602 synchronized (this) {
603 mStatTimestamp = stat.st_mtime;
604 mStatSize = stat.st_size;
605 }
606 } catch (ErrnoException e) {
607 // Do nothing
608 }
609 // Writing was successful, delete the backup file if there is one.
610 mBackupFile.delete();
611 mcr.setDiskWriteResult(true);
612 return;
613 } catch (XmlPullParserException e) {
614 Log.w(TAG, "writeToFile: Got exception:", e);
615 } catch (IOException e) {
616 Log.w(TAG, "writeToFile: Got exception:", e);
617 }
618 // Clean up an unsuccessfully written file
619 if (mFile.exists()) {
620 if (!mFile.delete()) {
621 Log.e(TAG, "Couldn‘t clean up partially-written file " + mFile);
622 }
623 }
624 mcr.setDiskWriteResult(false);
625 }
认识4：存在mBackupFile，直接操作mFile，如果出现问题，恢复备份。
总结：SharedPreferences使用时，单例，线程安全，存在缓存（二次加载快速）。
时间： 2024-07-29 00:25:11