Android文件存储SharedPreferences源码解析

        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;
        //读取文件中存储的key value，并存到全局变量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方法就是从这个map中读取。
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) {
            //将要修改的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存储了文件中所有的键值对。
           mcr.mapToWriteToDisk = mMap;
           对要新增或修改的键值对进行遍历。并添加到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;
   }

  注意第二个参数，commit传的是null。apply传的是一个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();
                       }
                   }
               };
           //根据 postWriteRunnable 是不是null来区分是commit方式还是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，上面的注释很也说明了commit是在当前线程执行的文件存储。
           if (isFromSyncCommit) {
               boolean wasEmpty = false;
               synchronized (SharedPreferencesImpl.this) {
                   wasEmpty = mDiskWritesInFlight == 1;
               }
               if (wasEmpty) {
                   //直接调用Runnable的run方法。在当前线程执行文件的存储。所以是同步方式
                   writeToDiskRunnable.run();
                   return;
               }
           }
            // 如果是applay方式，上面代码不会执行，也就不会return。
            //则会把存储文件的方法放到一个线程池中去执行
           QueuedWork.singleThreadExecutor().execute(writeToDiskRunnable);
       }

 private void writeToFile(MemoryCommitResult mcr) {
            //在写文件前，先将源文件进行一个备份
             if (!mBackupFile.exists()) {
                 if (!mFile.renameTo(mBackupFile)) {
                     mcr.setDiskWriteResult(false);
                     return;
                 }
             } else { //如果备份文件存在，则将源文件删掉
                 mFile.delete();
             }
          FileOutputStream str = createFileOutputStream(mFile);
            //将文件中所有的keyvalue，保护要修改的，全量存入新的文件中。
            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.
            //删除备份的文件
            mBackupFile.delete();
            mcr.setDiskWriteResult(true);
 }