Android檔案儲存SharedPreferences原始碼解析

        SharedPreferences sp = getSharedPreferences("fileName", Context.MODE_PRIVATE);
        SharedPreferences.Editor editor = sp.edit();
        editor.putString("key","value");
        editor.commit();

    SharedPreferences sp = getSharedPreferences("fileName", Context.MODE_PRIVATE);
        SharedPreferences.Editor editor = sp.edit();
        editor.putString("key","value");
        editor.apply();

ArrayMap<String, SharedPreferencesImpl> packagePrefs = sSharedPrefs.get(packageName);
public SharedPreferences getSharedPreferences(String name, int mode) {
	   if (sp == null) {
		 File prefsFile = getSharedPrefsFile(name);
		 sp = new SharedPreferencesImpl(prefsFile, mode);
		 packagePrefs.put(name, sp);
		 return sp;
     }
}

  SharedPreferencesImpl(File file, int mode) {
        mFile = file;
        mBackupFile = makeBackupFile(file);
        mMode = mode;
        mLoaded = false;
        mMap = null;
        //讀取檔案中儲存的key value，並存到全域性變數mMap中
        startLoadFromDisk();
    }

  private void loadFromDiskLocked() {
    .......
         str = new BufferedInputStream(
                            new FileInputStream(mFile), 16*1024);
         map = XmlUtils.readMapXml(str);
        if (map != null) {
               mMap = map;
               mStatTimestamp = stat.st_mtime;
               mStatSize = stat.st_size;
       } else {
           mMap = new HashMap<String, Object>();
       }
    }

    get方法就是從這個map中讀取。
public String getString(String key, String defValue) {
        synchronized (this) {
            awaitLoadedLocked();
            String v = (String)mMap.get(key);
            return v != null ? v : defValue;
        }
    }

 SharedPreferences.Editor editor = sp.edit();
 public Editor edit() {
     return new EditorImpl();
 }

  private final Map<String, Object> mModified = Maps.newHashMap();
 public Editor putString(String key, String value) {
        synchronized (this) {
            //將要修改的key value，存放到map中
            mModified.put(key, value);
            return this;
        }
}

  public boolean commit() {
	MemoryCommitResult mcr = commitToMemory();
	SharedPreferencesImpl.this.enqueueDiskWrite(
		mcr, null /* sync write on this thread okay */);
	try {
		mcr.writtenToDiskLatch.await();
	} catch (InterruptedException e) {
		return false;
	}
	notifyListeners(mcr);
	return mcr.writeToDiskResult;
}

public void apply() {
	final MemoryCommitResult mcr = commitToMemory();
	final Runnable awaitCommit = new Runnable() {
			public void run() {
				try {
					mcr.writtenToDiskLatch.await();
				} catch (InterruptedException ignored) {
				}
			}
		};
	QueuedWork.add(awaitCommit);
	Runnable postWriteRunnable = new Runnable() {
			public void run() {
				awaitCommit.run();
				QueuedWork.remove(awaitCommit);
			}
		};
	SharedPreferencesImpl.this.enqueueDiskWrite(mcr, postWriteRunnable);
	notifyListeners(mcr);
}

   private MemoryCommitResult commitToMemory() {
          MemoryCommitResult mcr = new MemoryCommitResult();
           //mMap儲存了檔案中所有的鍵值對。
           mcr.mapToWriteToDisk = mMap;
           對要新增或修改的鍵值對進行遍歷。並新增到mMap中
            for (Map.Entry<String, Object> e : mModified.entrySet()) {
                       String k = e.getKey();
                       Object v = e.getValue();
                if (mMap.containsKey(k)) {
                   Object existingValue = mMap.get(k);
                   if (existingValue != null && existingValue.equals(v)) {
                       continue;
                   }
               }
               mMap.put(k, v);
            }
            mcr.changesMade = true;
            mModified.clear();
         return mcr;
   }

  注意第二個引數，commit傳的是null。apply傳的是一個postWriteRunnable
   private void enqueueDiskWrite(final MemoryCommitResult mcr,
                                     final Runnable postWriteRunnable) {
           final Runnable writeToDiskRunnable = new Runnable() {
                   public void run() {
                       synchronized (mWritingToDiskLock) {
                           writeToFile(mcr);
                       }
                       synchronized (SharedPreferencesImpl.this) {
                           mDiskWritesInFlight--;
                       }
                       if (postWriteRunnable != null) {
                           postWriteRunnable.run();
                       }
                   }
               };
           //根據 postWriteRunnable 是不是null來區分是commit方式還是apply方式
           final boolean isFromSyncCommit = (postWriteRunnable == null);
           // Typical #commit() path with fewer allocations, doing a write on
           // the current thread.
           //如果是commit方式，上面的註釋很也說明了commit是在當前執行緒執行的檔案儲存。
           if (isFromSyncCommit) {
               boolean wasEmpty = false;
               synchronized (SharedPreferencesImpl.this) {
                   wasEmpty = mDiskWritesInFlight == 1;
               }
               if (wasEmpty) {
                   //直接呼叫Runnable的run方法。在當前執行緒執行檔案的儲存。所以是同步方式
                   writeToDiskRunnable.run();
                   return;
               }
           }
            // 如果是applay方式，上面程式碼不會執行，也就不會return。
            //則會把儲存檔案的方法放到一個執行緒池中去執行
           QueuedWork.singleThreadExecutor().execute(writeToDiskRunnable);
       }

 private void writeToFile(MemoryCommitResult mcr) {
            //在寫檔案前，先將原始檔進行一個備份
             if (!mBackupFile.exists()) {
                 if (!mFile.renameTo(mBackupFile)) {
                     mcr.setDiskWriteResult(false);
                     return;
                 }
             } else { //如果備份檔案存在，則將原始檔刪掉
                 mFile.delete();
             }
          FileOutputStream str = createFileOutputStream(mFile);
            //將檔案中所有的keyvalue，保護要修改的，全量存入新的檔案中。
            XmlUtils.writeMapXml(mcr.mapToWriteToDisk, str);
            FileUtils.sync(str);
            str.close();
            ContextImpl.setFilePermissionsFromMode(mFile.getPath(), mMode, 0);
            // Writing was successful, delete the backup file if there is one.
            //刪除備份的檔案
            mBackupFile.delete();
            mcr.setDiskWriteResult(true);
 }