在这个方法中，首先通过判断 postWriteRunnable 是否为 null 来判断是 apply() 还是 commit()。然后定义了一个 Runnable 任务，在 Runnable 中先调用了 writeToFile() 进行了写入和计数器更新的操作。
然后我们再来看看这个 writeToFile() 方法做了些什么。
@GuardedBy("mWritingToDiskLock") private void writeToFile(MemoryCommitResult mcr, boolean isFromSyncCommit) { long startTime = 0; long existsTime = 0; long backupExistsTime = 0; long outputStreamCreateTime = 0; long writeTime = 0; long fsyncTime = 0; long setPermTime = 0; long fstatTime = 0; long deleteTime = 0; if (DEBUG) { startTime = System.currentTimeMillis(); } boolean fileExists = mFile.exists(); if (DEBUG) { existsTime = System.currentTimeMillis(); // Might not be set, hence init them to a default value backupExistsTime = existsTime; } // Rename the current file so it may be used as a backup during the next read if (fileExists) { boolean needsWrite = false; // Only need to write if the disk state is older than this commit if (mDiskStateGeneration < mcr.memoryStateGeneration) { if (isFromSyncCommit) { needsWrite = true; } else { synchronized (mLock) { // No need to persist intermediate states. Just wait for the latest state to // be persisted. if (mCurrentMemoryStateGeneration == mcr.memoryStateGeneration) { needsWrite = true; } } } } if (!needsWrite) { mcr.setDiskWriteResult(false, true); return; } boolean backupFileExists = mBackupFile.exists(); if (DEBUG) { backupExistsTime = System.currentTimeMillis(); } // 此处需要注意一下 if (!backupFileExists) { if (!mFile.renameTo(mBackupFile)) { Log.e(TAG, "Couldn't rename file " + mFile + " to backup file " + mBackupFile); mcr.setDiskWriteResult(false, false); return; } } else { mFile.delete(); } } // Attempt to write the file, delete the backup and return true as atomically as // possible. If any exception occurs, delete the new file; next time we will restore // from the backup. try { FileOutputStream str = createFileOutputStream(mFile); if (DEBUG) { outputStreamCreateTime = System.currentTimeMillis(); } if (str == null) { mcr.setDiskWriteResult(false, false); return; } XmlUtils.writeMapXml(mcr.mapToWriteToDisk, str); writeTime = System.currentTimeMillis(); FileUtils.sync(str); fsyncTime = System.currentTimeMillis(); str.close(); ContextImpl.setFilePermissionsFromMode(mFile.getPath(), mMode, 0); if (DEBUG) { setPermTime = System.currentTimeMillis(); } try { final StructStat stat = Os.stat(mFile.getPath()); synchronized (mLock) { mStatTimestamp = stat.st_mtim; mStatSize = stat.st_size; } } catch (ErrnoException e) { // Do nothing } if (DEBUG) { fstatTime = System.currentTimeMillis(); } // Writing was successful, delete the backup file if there is one. mBackupFile.delete(); if (DEBUG) { deleteTime = System.currentTimeMillis(); } mDiskStateGeneration = mcr.memoryStateGeneration; mcr.setDiskWriteResult(true, true); if (DEBUG) { Log.d(TAG, "write: " + (existsTime - startTime) + "http://www.likecs.com/" + (backupExistsTime - startTime) + "http://www.likecs.com/" + (outputStreamCreateTime - startTime) + "http://www.likecs.com/" + (writeTime - startTime) + "http://www.likecs.com/" + (fsyncTime - startTime) + "http://www.likecs.com/" + (setPermTime - startTime) + "http://www.likecs.com/" + (fstatTime - startTime) + "http://www.likecs.com/" + (deleteTime - startTime)); } long fsyncDuration = fsyncTime - writeTime; mSyncTimes.add((int) fsyncDuration); mNumSync++; if (DEBUG || mNumSync % 1024 == 0 || fsyncDuration > MAX_FSYNC_DURATION_MILLIS) { mSyncTimes.log(TAG, "Time required to fsync " + mFile + ": "); } return; } catch (XmlPullParserException e) { Log.w(TAG, "writeToFile: Got exception:", e); } catch (IOException e) { Log.w(TAG, "writeToFile: Got exception:", e); } // Clean up an unsuccessfully written file if (mFile.exists()) { if (!mFile.delete()) { Log.e(TAG, "Couldn't clean up partially-written file " + mFile); } } mcr.setDiskWriteResult(false, false); }代码比较长，做了一些时间的记录和 XML 的相关处理，但最值得我们关注的还是其中打了标注的对于 mBackupFile 的处理。我们可以明显地看到，在我们写入文件的时候，我们会把此前的 XML 文件改名为一个备份文件，然后再将要写入的数据写入到一个新的文件中。如果这个过程执行成功的话，就会把备份文件删除。由此可见：即使我们每次只是添加一个键值对，也会重新写入整个文件的数据，这也说明了 SharedPreferences 只适合保存少量数据，文件太大会有性能问题。