保存键值对数据¶

原文：https://developer.android.com/training/data-storage/shared-preferences

如果您有想要保存的相对较小键值对集合，则应使用 SharedPreferences API。SharedPreferences 对象指向包含键值对的文件，并提供读写这些键值对的简单方法。每个 SharedPreferences 文件均由框架进行管理，可以是私有文件，也可以是共享文件。

{% hint style="info" %} 注意：SharedPreferences API 用于读写键值对，不要将它们与 Preference API 混淆，后者可帮助您构建用于显示应用设置的界面（虽然它们也使用 SharedPreferences 保存用户设置）。如需了解 Preference API，请参阅设置开发者指南。

获取SharedPreferences¶

您可以通过调用以下方法之一创建新的共享偏好设置文件或访问已有共享偏好设置文件：

getSharedPreferences() - 如果您需要多个由名称（使用第一个参数指定）标识的共享偏好设置文件，则使用此方法。您可以从您的应用中的任何 Context 调用此方法。
getPreferences() - 如果您只需使用 Activity 的一个共享首选项，请从 Activity 中使用此方法。由于这会检索属于该 Activity 的默认共享偏好设置文件，因此您无需提供名称。
如果您使用 SharedPreferences API 保存应用设置，则应改用PreferenceManager 的getDefaultSharedPreferences() 方法获取整个应用的默认共享偏好设置文件。如需了解详情，请参阅设置开发者指南。

//在私有模式下打开该文件，确保只有您的应用可以访问该文件：
val sharedPreferences1 = getSharedPreferences("file1", Context.MODE_PRIVATE)
sharedPreferences1.edit().putInt("value1",1).apply()
val sharedPreferences2 = getPreferences(Context.MODE_PRIVATE)
sharedPreferences2.edit().putInt("value2",2).apply()
val sharedPreferences3 = PreferenceManager.getDefaultSharedPreferences(this)
sharedPreferences3.edit().putInt("value3",3).apply()

{% hint style="info" %} 注意：自 API 级别 17 起，MODE_WORLD_READABLE 和 MODE_WORLD_WRITEABLE 模式已被弃用。从 Android 7.0（API 级别 24）开始，如果您使用这些模式，Android 会抛出 SecurityException。如果您的应用需要与其他应用共享私有文件，可以通过 FLAG_GRANT_READ_URI_PERMISSION 使用 FileProvider。如需了解详情，另请参阅共享文件。

写入共享偏好设置¶

如需写入共享偏好设置文件，请通过对您的 SharedPreferences 调用 edit() 以创建一个 SharedPreferences.Editor。

{% hint style="info" %} 注意：通过对 EncryptedSharedPreferences 对象（而不是 SharedPreferences 对象）调用 edit() 方法，您可以更安全地修改共享偏好设置。如需了解详情，请参阅有关如何更安全地处理数据的指南。

传递您想要使用 putInt() 和 putString() 等方法写入的键和值。然后，调用 apply() 或 commit() 以保存更改。例如：

SharedPreferences sharedPref = getActivity().getPreferences(Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPref.edit();
editor.putInt(getString(R.string.saved_high_score_key), newHighScore);
editor.commit();

apply() 会立即更改内存中的 SharedPreferences 对象，但会将更新异步写入磁盘。或者，您也可以使用 commit() 将数据同步写入磁盘。但是，由于 commit() 是同步的，您应避免从主线程调用它，因为它可能会暂停您的界面呈现。

从共享偏好设置中读取¶

如需从共享偏好设置文件中检索值，请调用 getInt() 和 getString() 等方法，为您想要的值提供键；如果键不存在，则可以选择返回默认值。例如：

SharedPreferences sharedPref = getActivity().getPreferences(Context.MODE_PRIVATE);
int defaultValue = getResources().getInteger(R.integer.saved_high_score_default_key);
int highScore = sharedPref.getInt(getString(R.string.saved_high_score_key), defaultValue);

获取SharedPreferences源码分析¶

getSharedPreferences(String name, int mode)¶

//Activity的getPreferences方法，以当前类名为文件名
public SharedPreferences getPreferences(@Context.PreferencesMode int mode) {
    return getSharedPreferences(getLocalClassName(), mode);
}
//PreferenceManager的getDefaultSharedPreferences方法
public static SharedPreferences getDefaultSharedPreferences(Context context) {
    return context.getSharedPreferences(getDefaultSharedPreferencesName(context),
            getDefaultSharedPreferencesMode());
}
//文件名 包名_preferences
public static String getDefaultSharedPreferencesName(Context context) {
    return context.getPackageName() + "_preferences";
}
//以上两个方法本质上都是调用的Context的getSharedPreferences方法

以上两个方法本质上都是调用的Context的getSharedPreferences方法。

//ContextWrapper.java
@Override
public SharedPreferences getSharedPreferences(String name, int mode) {
    return mBase.getSharedPreferences(name, mode);
}
//ContextImpl.java
private ArrayMap<String, File> mSharedPrefsPaths;
@Override
public SharedPreferences getSharedPreferences(String name, int mode) {
    // At least one application in the world actually passes in a null
    // name.  This happened to work because when we generated the file name
    // we would stringify it to "null.xml".  Nice.
    if (mPackageInfo.getApplicationInfo().targetSdkVersion <
            Build.VERSION_CODES.KITKAT) {
        if (name == null) {
            name = "null";
        }
    }
    File file;
    synchronized (ContextImpl.class) {
        if (mSharedPrefsPaths == null) {
            mSharedPrefsPaths = new ArrayMap<>();
        }
        //查询是否存在相应文件
        file = mSharedPrefsPaths.get(name);
        if (file == null) {
            //如果不存在则创建相应文件 并存入Map中
            file = getSharedPreferencesPath(name);
            mSharedPrefsPaths.put(name, file);
        }
    }
    return getSharedPreferences(file, mode);
}

getSharedPreferencesPath¶

@Override
public File getSharedPreferencesPath(String name) {
    return makeFilename(getPreferencesDir(), name + ".xml");
}
@UnsupportedAppUsage
private File getPreferencesDir() {
    synchronized (mSync) {
        if (mPreferencesDir == null) {
            mPreferencesDir = new File(getDataDir(), "shared_prefs");
        }
        return ensurePrivateDirExists(mPreferencesDir);
    }
}
@Override
public File getDataDir() {
    if (mPackageInfo != null) {
        File res = null;
        if (isCredentialProtectedStorage()) {
            res = mPackageInfo.getCredentialProtectedDataDirFile();
        } else if (isDeviceProtectedStorage()) {
            res = mPackageInfo.getDeviceProtectedDataDirFile();
        } else {
            res = mPackageInfo.getDataDirFile();
        }
        if (res != null) {
            if (!res.exists() && android.os.Process.myUid() == android.os.Process.SYSTEM_UID) {
                Log.wtf(TAG, "Data directory doesn't exist for package " + getPackageName(),
                        new Throwable());
            }
            return res;
        } else {
            throw new RuntimeException(
                    "No data directory found for package " + getPackageName());
        }
    } else {
        throw new RuntimeException(
                "No package details found for package " + getPackageName());
    }
}
//确保文件是否存在
private static File ensurePrivateDirExists(File file) {
    return ensurePrivateDirExists(file, 0771, -1, null);
}
private static File ensurePrivateCacheDirExists(File file, String xattr) {
    final int gid = UserHandle.getCacheAppGid(Process.myUid());
    return ensurePrivateDirExists(file, 02771, gid, xattr);
}
private static File ensurePrivateDirExists(File file, int mode, int gid, String xattr) {
    if (!file.exists()) {
        final String path = file.getAbsolutePath();
        try {
            Os.mkdir(path, mode);
            Os.chmod(path, mode);
            if (gid != -1) {
                Os.chown(path, -1, gid);
            }
        } catch (ErrnoException e) {
            if (e.errno == OsConstants.EEXIST) {
                // We must have raced with someone; that's okay
            } else {
                Log.w(TAG, "Failed to ensure " + file + ": " + e.getMessage());
            }
        }
        if (xattr != null) {
            try {
                final StructStat stat = Os.stat(file.getAbsolutePath());
                final byte[] value = new byte[8];
                Memory.pokeLong(value, 0, stat.st_ino, ByteOrder.nativeOrder());
                Os.setxattr(file.getParentFile().getAbsolutePath(), xattr, value, 0);
            } catch (ErrnoException e) {
                Log.w(TAG, "Failed to update " + xattr + ": " + e.getMessage());
            }
        }
    }
    return file;
}

getSharedPreferences(File file, int mode)¶

@Override
public SharedPreferences getSharedPreferences(File file, int mode) {
    SharedPreferencesImpl sp;
    synchronized (ContextImpl.class) {
        final ArrayMap<File, SharedPreferencesImpl> cache = getSharedPreferencesCacheLocked();
        sp = cache.get(file);
        if (sp == null) {
            checkMode(mode);
            if (getApplicationInfo().targetSdkVersion >= android.os.Build.VERSION_CODES.O) {
                if (isCredentialProtectedStorage()
                        && !getSystemService(UserManager.class)
                                .isUserUnlockingOrUnlocked(UserHandle.myUserId())) {
                    throw new IllegalStateException("SharedPreferences in credential encrypted "
                            + "storage are not available until after user is unlocked");
                }
            }
            //创建SharedPreferencesImpl
            sp = new SharedPreferencesImpl(file, mode);
            cache.put(file, sp);
            return sp;
        }
    }
    //指定多进程模式，则当文件被其他进程修改时，则会重新加载
    if ((mode & Context.MODE_MULTI_PROCESS) != 0 ||
        getApplicationInfo().targetSdkVersion < android.os.Build.VERSION_CODES.HONEYCOMB) {
        // If somebody else (some other process) changed the prefs
        // file behind our back, we reload it.  This has been the
        // historical (if undocumented) behavior.
        sp.startReloadIfChangedUnexpectedly();
    }
    return sp;
}

private void checkMode(int mode) {
     //7.0之后使用MODE_WORLD_READABLE和MODE_WORLD_WRITEABLE会抛异常
    if (getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.N) {
        if ((mode & MODE_WORLD_READABLE) != 0) {
            throw new SecurityException("MODE_WORLD_READABLE no longer supported");
        }
        if ((mode & MODE_WORLD_WRITEABLE) != 0) {
            throw new SecurityException("MODE_WORLD_WRITEABLE no longer supported");
        }
    }
}

getSharedPreferencesCacheLocked¶

@GuardedBy("ContextImpl.class")
private ArrayMap<File, SharedPreferencesImpl> getSharedPreferencesCacheLocked() {
    if (sSharedPrefsCache == null) {
        sSharedPrefsCache = new ArrayMap<>();
    }
    final String packageName = getPackageName();
    ArrayMap<File, SharedPreferencesImpl> packagePrefs = sSharedPrefsCache.get(packageName);
    if (packagePrefs == null) {
        packagePrefs = new ArrayMap<>();
        sSharedPrefsCache.put(packageName, packagePrefs);
    }
    return packagePrefs;
}

SharedPreferencesImpl分析¶

SharedPreferencesImpl构造函数¶

@UnsupportedAppUsage
SharedPreferencesImpl(File file, int mode) {
    //赋值给全局变量
    mFile = file;
    //创建以.bak为后缀的备份文件 发生异常时，可通过备份文件来恢复数据
    mBackupFile = makeBackupFile(file);
    mMode = mode;
    mLoaded = false;
    mMap = null;
    mThrowable = null;
    startLoadFromDisk();
}

startLoadFromDisk¶

@UnsupportedAppUsage
private void startLoadFromDisk() {
    //mLoaded用于标记SP文件已经加载到内存中
    synchronized (mLock) {
        mLoaded = false;
    }
    //开启新线程从硬盘加载
    new Thread("SharedPreferencesImpl-load") {
        public void run() {
            loadFromDisk();
        }
    }.start();
}

loadFromDisk¶

private void loadFromDisk() {
    synchronized (mLock) {
        if (mLoaded) {
            return;
        }
        if (mBackupFile.exists()) {
            mFile.delete();
            mBackupFile.renameTo(mFile);
        }
    }

    // Debugging
    if (mFile.exists() && !mFile.canRead()) {
        Log.w(TAG, "Attempt to read preferences file " + mFile + " without permission");
    }

    Map<String, Object> map = null;
    StructStat stat = null;
    Throwable thrown = null;
    try {
        stat = Os.stat(mFile.getPath());
        if (mFile.canRead()) {
            BufferedInputStream str = null;
            try {
                //读取流
                str = new BufferedInputStream(
                        new FileInputStream(mFile), 16 * 1024);
                //解析xml
                map = (Map<String, Object>) XmlUtils.readMapXml(str);
            } catch (Exception e) {
                Log.w(TAG, "Cannot read " + mFile.getAbsolutePath(), e);
            } finally {
                IoUtils.closeQuietly(str);
            }
        }
    } catch (ErrnoException e) {
        // An errno exception means the stat failed. Treat as empty/non-existing by
        // ignoring.
    } catch (Throwable t) {
        thrown = t;
    }

    synchronized (mLock) {
        mLoaded = true;
        mThrowable = thrown;

        // It's important that we always signal waiters, even if we'll make
        // them fail with an exception. The try-finally is pretty wide, but
        // better safe than sorry.
        try {
            if (thrown == null) {
                if (map != null) {
                    mMap = map;
                    mStatTimestamp = stat.st_mtim;
                    mStatSize = stat.st_size;
                } else {
                    mMap = new HashMap<>();
                }
            }
            // In case of a thrown exception, we retain the old map. That allows
            // any open editors to commit and store updates.
        } catch (Throwable t) {
            mThrowable = t;
        } finally {
            mLock.notifyAll();
        }
    }
}

获取数据分析¶

//SharedPreferencesImpl.java
@Override
@Nullable
public String getString(String key, @Nullable String defValue) {
    synchronized (mLock) {
       //检查是否加载完成
        awaitLoadedLocked();
        String v = (String)mMap.get(key);
        return v != null ? v : defValue;
    }
}
@GuardedBy("mLock")
private void awaitLoadedLocked() {
    if (!mLoaded) {
        // Raise an explicit StrictMode onReadFromDisk for this
        // thread, since the real read will be in a different
        // thread and otherwise ignored by StrictMode.
        BlockGuard.getThreadPolicy().onReadFromDisk();
    }
    while (!mLoaded) {
        try {
            //当没有加载完成，则进入等待状态
            mLock.wait();
        } catch (InterruptedException unused) {
        }
    }
    if (mThrowable != null) {
        throw new IllegalStateException(mThrowable);
    }
}

EditorImpl分析¶

//SharedPreferencesImpl的edit方法
@Override
public Editor edit() {
    // TODO: remove the need to call awaitLoadedLocked() when
    // requesting an editor.  will require some work on the
    // Editor, but then we should be able to do:
    //
    //      context.getSharedPreferences(..).edit().putString(..).apply()
    //
    // ... all without blocking.
    synchronized (mLock) {
        awaitLoadedLocked();
    }
    return new EditorImpl();
}
//EditorImpl是SharedPreferencesImpl的内部类
public final class EditorImpl implements Editor {
        private final Object mEditorLock = new Object();
        //创建EditorImpl会创建一个Map，并把这些put的数据存储到Map中，当commit的时候，遍历
        //map，并存到mMap中
        @GuardedBy("mEditorLock")
        private final Map<String, Object> mModified = new HashMap<>();

        @GuardedBy("mEditorLock")
        private boolean mClear = false;

        @Override
        public Editor putString(String key, @Nullable String value) {
            synchronized (mEditorLock) {
                mModified.put(key, value);
                return this;
            }
        }
        @Override
        public Editor remove(String key) {
            synchronized (mEditorLock) {
                mModified.put(key, this);
                return this;
            }
        }


        @Override
        public Editor clear() {
            synchronized (mEditorLock) {
                mClear = true;
                return this;
            }
        }
}

commit()¶

@Override
public boolean commit() {
    long startTime = 0;
    if (DEBUG) {
        startTime = System.currentTimeMillis();
    }
    //将数据更新到内存
    MemoryCommitResult mcr = commitToMemory();
    //将内存数据同步到文件
    SharedPreferencesImpl.this.enqueueDiskWrite(
        mcr, null /* sync write on this thread okay */);
    try {
        //进入等待状态，直到写入文件的操作完成
        mcr.writtenToDiskLatch.await();
    } catch (InterruptedException e) {
        return false;
    } finally {
        if (DEBUG) {
            Log.d(TAG, mFile.getName() + ":" + mcr.memoryStateGeneration
                    + " committed after " + (System.currentTimeMillis() - startTime)
                    + " ms");
        }
    }
    //通知监听
    notifyListeners(mcr);
    //返回文件操作的结果数据
    return mcr.writeToDiskResult;
}

commitToMemory¶

// Returns true if any changes were made
private MemoryCommitResult commitToMemory() {
    long memoryStateGeneration;
    boolean keysCleared = false;
    List<String> keysModified = null;
    Set<OnSharedPreferenceChangeListener> listeners = null;
    Map<String, Object> mapToWriteToDisk;

    synchronized (SharedPreferencesImpl.this.mLock) {
        // We optimistically don't make a deep copy until
        // a memory commit comes in when we're already
        // writing to disk.
        if (mDiskWritesInFlight > 0) {
            // We can't modify our mMap as a currently
            // in-flight write owns it.  Clone it before
            // modifying it.
            // noinspection unchecked
            mMap = new HashMap<String, Object>(mMap);
        }
        //赋值
        mapToWriteToDisk = mMap;
        mDiskWritesInFlight++;

        boolean hasListeners = mListeners.size() > 0;
        if (hasListeners) {
            keysModified = new ArrayList<String>();
            listeners = new HashSet<OnSharedPreferenceChangeListener>(mListeners.keySet());
        }

        synchronized (mEditorLock) {
            boolean changesMade = false;

            if (mClear) {
                if (!mapToWriteToDisk.isEmpty()) {
                    changesMade = true;
                    mapToWriteToDisk.clear();
                }
                keysCleared = true;
                mClear = false;
            }
            //遍历
            for (Map.Entry<String, Object> e : mModified.entrySet()) {
                String k = e.getKey();
                Object v = e.getValue();
                // "this" is the magic value for a removal mutation. In addition,
                // setting a value to "null" for a given key is specified to be
                // equivalent to calling remove on that key.
                if (v == this || v == null) {
                    //不包含跳出本次循环
                    if (!mapToWriteToDisk.containsKey(k)) {
                        continue;
                    }
                    //包含，调用remove方法
                    mapToWriteToDisk.remove(k);
                } else {
                    //存在key 并且存入的value和将要存入的值相同则跳出本次循环
                    if (mapToWriteToDisk.containsKey(k)) {
                        Object existingValue = mapToWriteToDisk.get(k);
                        if (existingValue != null && existingValue.equals(v)) {
                            continue;
                        }
                    }
                    mapToWriteToDisk.put(k, v);
                }
                //如果执行到这里 说明发生改变
                changesMade = true;
                if (hasListeners) {
                    keysModified.add(k);
                }
            }
            //清理
            mModified.clear();
            //发生改变++
            if (changesMade) {
                mCurrentMemoryStateGeneration++;
            }

            memoryStateGeneration = mCurrentMemoryStateGeneration;
        }
    }
    return new MemoryCommitResult(memoryStateGeneration, keysCleared, keysModified,
            listeners, mapToWriteToDisk);
}

enqueueDiskWrite¶

private void enqueueDiskWrite(final MemoryCommitResult mcr,
                              final Runnable postWriteRunnable) {
    //判断是否是同步
    final boolean isFromSyncCommit = (postWriteRunnable == null);
    final Runnable writeToDiskRunnable = new Runnable() {
            @Override
            public void run() {
                synchronized (mWritingToDiskLock) {
                    //执行文件写入操作
                    writeToFile(mcr, isFromSyncCommit);
                }
                synchronized (mLock) {
                    mDiskWritesInFlight--;
                }
                //commit传入为null不执行该方法
                if (postWriteRunnable != null) {
                    postWriteRunnable.run();
                }
            }
        };
    // Typical #commit() path with fewer allocations, doing a write on
    // the current thread.
    if (isFromSyncCommit) {
        boolean wasEmpty = false;
        synchronized (mLock) {
            wasEmpty = mDiskWritesInFlight == 1;
        }
        if (wasEmpty) {
            writeToDiskRunnable.run();
            return;
        }
    }
    //同步不执行该方法
    QueuedWork.queue(writeToDiskRunnable, !isFromSyncCommit);
}

writeToFile¶

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) {
              //判断是否需要写入
            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;
                        }
                    }
                }
            }
            //不需要写入直接return
            if (!needsWrite) {
                mcr.setDiskWriteResult(false, true);
                return;
            }

            boolean backupFileExists = mBackupFile.exists();

            if (DEBUG) {
                backupExistsTime = System.currentTimeMillis();
            }
            //备份文件不存在 将当前文件名修改为备份文件
            if (!backupFileExists) {
                if (!mFile.renameTo(mBackupFile)) {
                    Log.e(TAG, "Couldn't rename file " + mFile
                          + " to backup file " + mBackupFile);
                    mcr.setDiskWriteResult(false, false);
                    return;
                }
            } else {
                //存在删除文件
                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.
            //写入成功删除备份文件
            mBackupFile.delete();

            if (DEBUG) {
                deleteTime = System.currentTimeMillis();
            }

            mDiskStateGeneration = mcr.memoryStateGeneration;
            //返回写入成功，唤醒等待线程
            mcr.setDiskWriteResult(true, true);

            if (DEBUG) {
                Log.d(TAG, "write: " + (existsTime - startTime) + "/"
                        + (backupExistsTime - startTime) + "/"
                        + (outputStreamCreateTime - startTime) + "/"
                        + (writeTime - startTime) + "/"
                        + (fsyncTime - startTime) + "/"
                        + (setPermTime - startTime) + "/"
                        + (fstatTime - startTime) + "/"
                        + (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
        //文件写入失败，则删除未成功写入的文件
        if (mFile.exists()) {
            if (!mFile.delete()) {
                Log.e(TAG, "Couldn't clean up partially-written file " + mFile);
            }
        }
        mcr.setDiskWriteResult(false, false);
    }

apply()¶

@Override
public void apply() {
    final long startTime = System.currentTimeMillis();

    final MemoryCommitResult mcr = commitToMemory();
    final Runnable awaitCommit = new Runnable() {
            @Override
            public void run() {
                try {
                    //进入等待状态
                    mcr.writtenToDiskLatch.await();
                } catch (InterruptedException ignored) {
                }

                if (DEBUG && mcr.wasWritten) {
                    Log.d(TAG, mFile.getName() + ":" + mcr.memoryStateGeneration
                            + " applied after " + (System.currentTimeMillis() - startTime)
                            + " ms");
                }
            }
        };

    QueuedWork.addFinisher(awaitCommit);

    Runnable postWriteRunnable = new Runnable() {
            @Override
            public void run() {
                awaitCommit.run();
                QueuedWork.removeFinisher(awaitCommit);
            }
        };

    SharedPreferencesImpl.this.enqueueDiskWrite(mcr, postWriteRunnable);

    // Okay to notify the listeners before it's hit disk
    // because the listeners should always get the same
    // SharedPreferences instance back, which has the
    // changes reflected in memory.
    notifyListeners(mcr);
}

apply 与commit的对比

apply没有返回值, commit有返回值能知道修改是否提交成功
apply是将修改提交到内存，再异步提交到磁盘文件; commit是同步的提交到磁盘文件;
多并发的提交commit时，需等待正在处理的commit数据更新到磁盘文件后才会继续往下执行，从而降低效率; 而apply只是原子更新到内存，后调用apply函数会直接覆盖前面内存数据，从一定程度上提高很多效率。

获取SP与Editor:

getSharedPreferences()是从ContextImpl.sSharedPrefsCache唯一的SPI对象;
edit()每次都是创建新的EditorImpl对象.

缺点¶

虽然 SharedPreferences 使用非常简便，但也是我们诟病比较多的存储方法。它的性能问题比较多，我可以轻松地说出它的“七宗罪”。

跨进程不安全。由于没有使用跨进程的锁，就算使用MODE_MULTI_PROCESS，SharedPreferences 在跨进程频繁读写有可能导致数据全部丢失。根据线上统计，SP 大约会有万分之一的损坏率。
加载缓慢。SharedPreferences 文件的加载使用了异步线程，而且加载线程并没有设置线程优先级，如果这个时候主线程读取数据就需要等待文件加载线程的结束。这就导致出现主线程等待低优先级线程锁的问题，比如一个 100KB 的 SP 文件读取等待时间大约需要 50~100ms，我建议提前用异步线程预加载启动过程用到的 SP 文件。
全量写入。无论是调用 commit() 还是 apply()，即使我们只改动其中的一个条目，都会把整个内容全部写到文件。而且即使我们多次写入同一个文件，SP 也没有将多次修改合并为一次，这也是性能差的重要原因之一。
卡顿。由于提供了异步落盘的 apply 机制，在崩溃或者其他一些异常情况可能会导致数据丢失。所以当应用收到系统广播，或者被调用 onPause 等一些时机，系统会强制把所有的 SharedPreferences 对象数据落地到磁盘。如果没有落地完成，这时候主线程会被一直阻塞。这样非常容易造成卡顿，甚至是 ANR，从线上数据来看 SP 卡顿占比一般会超过 5%。

参考¶

****保存键值对数据****
全面剖析SharedPreferences