SharedPreferences
1、加载/初始化
维护spName–>file,file–>sharedPreferencesImpl两个ArrayMap内存缓存
ContextImpl.java
@Override
public SharedPreferences getSharedPreferences(String name, int mode) {
File file;
synchronized (ContextImpl.class) {
if (mSharedPrefsPaths == null) {
mSharedPrefsPaths = new ArrayMap<>();
}
file = mSharedPrefsPaths.get(name);
if (file == null) {
file = getSharedPreferencesPath(name);
mSharedPrefsPaths.put(name, file);
}
}
return getSharedPreferences(file, 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);
sp = new SharedPreferencesImpl(file, mode);
cache.put(file, sp);
return sp;
}
}
return sp;
}
SharedPreferencesImpl构造方法切子线程loadFromDisk,得到Map<String, Object> mMap
SharedPreferencesImpl.java
SharedPreferencesImpl(File file, int mode) {
mFile = file;
mBackupFile = makeBackupFile(file);
mMode = mode;
mLoaded = false;
mMap = null;
mThrowable = null;
startLoadFromDisk();
}
private void startLoadFromDisk() {
new Thread("SharedPreferencesImpl-load") {
public void run() {
loadFromDisk();
}
}.start();
}
private void loadFromDisk() {
synchronized (mLock) {
if (mLoaded) {
return;
}
if (mBackupFile.exists()) {
mFile.delete();
mBackupFile.renameTo(mFile);
}
}
str = new BufferedInputStream(new FileInputStream(mFile), 16 * 1024);
map = (Map<String, Object>) XmlUtils.readMapXml(str);
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 {
mMap = map;
} catch (Throwable t) {
mThrowable = t;
} finally {
mLock.notifyAll();
}
}
edit: wait util loaded
@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();
}
awaitLoadedLocked
@GuardedBy("mLock")
private void awaitLoadedLocked() {
while (!mLoaded) {
try {
mLock.wait();
} catch (InterruptedException unused) {
}
}
}
putXxx: mModified.put(key, value)
@Override
public Editor putString(String key, @Nullable String value) {
synchronized (mEditorLock) {
mModified.put(key, value);
return this;
}
}
getXXX() 导致ANR
public String getString(String key, @Nullable String defValue) {
synchronized (mLock) {
awaitLoadedLocked();
String v = (String)mMap.get(key);
return v != null ? v : defValue;
}
}
2、编辑提交
2.1、 commit()流程
@Override
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;
}
2.1.1 commitToMemory
// Returns true if any changes were made
private MemoryCommitResult commitToMemory() {
long memoryStateGeneration;
List<String> keysModified = null;
Set<OnSharedPreferenceChangeListener> listeners = null;
Map<String, Object> mapToWriteToDisk;
synchronized (SharedPreferencesImpl.this.mLock) {
mapToWriteToDisk = mMap;
synchronized (mEditorLock) {
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) {
if (!mapToWriteToDisk.containsKey(k)) {
continue;
}
mapToWriteToDisk.remove(k);
} else {
if (mapToWriteToDisk.containsKey(k)) {
Object existingValue = mapToWriteToDisk.get(k);
if (existingValue != null && existingValue.equals(v)) {
continue;
}
}
mapToWriteToDisk.put(k, v);
}
changesMade = true;
}
return new MemoryCommitResult(memoryStateGeneration, keysModified, listeners,
mapToWriteToDisk);
}
}
2.2.2 enqueueDiskWrite
private void enqueueDiskWrite(final MemoryCommitResult mcr,
final Runnable postWriteRunnable) {
final boolean isFromSyncCommit = (postWriteRunnable == null);
final Runnable writeToDiskRunnable = new Runnable() {
@Override
public void run() {
synchronized (mWritingToDiskLock) {
writeToFile(mcr, isFromSyncCommit);
}
synchronized (mLock) {
mDiskWritesInFlight--;
}
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;
}
}
QueuedWork.queue(writeToDiskRunnable, !isFromSyncCommit);//add to sWork
}
QueuedWork.queue
- 当apply()方式提交的时候，默认消息会延迟发送100毫秒，避免频繁的磁盘写入操作。
- 当commit()方式，调用QueuedWork的queue()时，会立即向handler()发送Message。
/** Delay for delayed runnables, as big as possible but low enough to be barely perceivable */
private static final long DELAY = 100;
public static void queue(Runnable work, boolean shouldDelay) {
Handler handler = getHandler();
synchronized (sLock) {
sWork.add(work);
if (shouldDelay && sCanDelay) {
handler.sendEmptyMessageDelayed(QueuedWorkHandler.MSG_RUN, DELAY);
} else {
handler.sendEmptyMessage(QueuedWorkHandler.MSG_RUN);
}
}
}
getHandler
/**
* Lazily create a handler on a separate thread.
*/
private static Handler getHandler() {
synchronized (sLock) {
if (sHandler == null) {
HandlerThread handlerThread = new HandlerThread("queued-work-looper",
Process.THREAD_PRIORITY_FOREGROUND);
handlerThread.start();
sHandler = new QueuedWorkHandler(handlerThread.getLooper());
}
return sHandler;
}
}
private static class QueuedWorkHandler extends Handler {
static final int MSG_RUN = 1;
QueuedWorkHandler(Looper looper) {
super(looper);
}
public void handleMessage(Message msg) {
if (msg.what == MSG_RUN) {
processPendingWork();
}
}
}
processPendingWork
private static void processPendingWork() {
long startTime = 0;
synchronized (sProcessingWork) {
LinkedList<Runnable> work;
synchronized (sLock) {
work = (LinkedList<Runnable>) sWork.clone();
sWork.clear();
// Remove all msg-s as all work will be processed now
getHandler().removeMessages(QueuedWorkHandler.MSG_RUN);
}
if (work.size() > 0) {
for (Runnable w : work) {
w.run();
}
}
}
}
writeToFile
private void writeToFile(MemoryCommitResult mcr, boolean isFromSyncCommit) {
boolean fileExists = mFile.exists();
// Rename the current file so it may be used as a backup during the next read
if (fileExists) {
boolean backupFileExists = mBackupFile.exists();
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);
XmlUtils.writeMapXml(mcr.mapToWriteToDisk, str);
FileUtils.sync(str);
str.close();
// Writing was successful, delete the backup file if there is one.
mBackupFile.delete();
mcr.setDiskWriteResult(true, true);
void setDiskWriteResult(boolean wasWritten, boolean result) {
this.wasWritten = wasWritten;
writeToDiskResult = result;
writtenToDiskLatch.countDown();
}
2.2、 apply()流程
public void apply() {
final MemoryCommitResult mcr = commitToMemory();
final Runnable awaitCommit = new Runnable() {
@Override
public void run() {
try {
mcr.writtenToDiskLatch.await();
} catch (InterruptedException ignored) {
}
}
};
QueuedWork.addFinisher(awaitCommit);
Runnable postWriteRunnable = new Runnable() {
@Override
public void run() {
awaitCommit.run();
QueuedWork.removeFinisher(awaitCommit);
}
};
SharedPreferencesImpl.this.enqueueDiskWrite(mcr, postWriteRunnable);
notifyListeners(mcr);
}
apply 接口整体的详细设计思路如下图（基于 Android8.0 及以下版本分析）：
尽管 Google 官方在 Android 8.0 及以后版本对 sp 写入逻辑进行优化，期望是在上述步骤 6 中 UI 线程不是傻傻的等，而是帮助子线程一起写入，但是由于是保守协助，并没有很好的解决这个问题。
2.3 主线程堵塞ANR
为了保证异步任务及时完成，当生命周期处于 handleStopService() 、handlePauseActivity() 、 handleStopActivity() 的时候会调用QueuedWork.waitToFinish() 会等待写入任务执行完毕。
waitToFinish，processPendingWork
You don’t need to worry about Android component lifecycles and their interaction with apply() writing to disk. The framework makes sure in-flight disk writes from apply() complete before switching states.
//QueuedWork.java
/**
* Trigger queued work to be processed immediately. The queued work is processed on a separate
* thread asynchronous. While doing that run and process all finishers on this thread. The
* finishers can be implemented in a way to check weather the queued work is finished.
*
* Is called from the Activity base class's onPause(), after BroadcastReceiver's onReceive,
* after Service command handling, etc. (so async work is never lost)
*/
public static void waitToFinish() {
processPendingWork();//write to disk and run postWriteRunnable, will block main thread
while (true) {
Runnable finisher;
synchronized (sLock) {
finisher = sFinishers.poll();
}
finisher.run();
}
}
waitToFinish()会将，储存在QueuedWork的操作一并处理掉。什么时候呢？在Activiy的 onPause()、BroadcastReceiver的onReceive()以及Service的onStartCommand()方法之前都会调用waitToFinish()。大家知道这些方法都是执行在主线程中，一旦waitToFinish()执行超时，就会抛出ANR。
至于waitToFinish调用具体时机，查看ActivityThread.java类文件。这里只是说本质原理
线程安全
多操作线程安全
为了保证SharedPreferences是线程安全的，Google的设计者一共使用了3把锁：
对于简单的 读操作 而言，我们知道其原理是读取内存中mMap的值并返回，那么为了保证线程安全，只需要加一把锁保证mMap的线程安全即可：
mMap相关的mLock锁
public String getString(String key, @Nullable String defValue) {
synchronized (mLock) {
String v = (String)mMap.get(key);
return v != null ? v : defValue;
}
}
写操作线程安全
对于写操作而言，每次putXXX()并不能立即更新在mMap中，这是理所当然的，如果开发者没有调用apply()方法，那么这些数据的更新理所当然应该被抛弃掉，但是如果直接更新在mMap中，那么数据就难以恢复。
因此，Editor本身也应该持有一个mEditorMap对象，用于存储数据的更新；只有当调用apply()时，才尝试将mEditorMap与mMap进行合并，以达到数据更新的目的。
因此，这里我们还需要另外一把锁保证mEditorMap的线程安全，笔者认为，不和mMap公用同一把锁的原因是，在apply()被调用之前，getXXX和putXXX理应是没有冲突的。
代码实现参考如下：
EditorImpl相关的mEditorLock锁
public final class EditorImpl implements Editor {
@Override
public Editor putString(String key, String value) {
synchronized (mEditorLock) {
mEditorMap.put(key, value);
return this;
}
}
}
而当真正需要执行apply()进行写操作时，mEditorMap与mMap进行合并，这时必须通过2把锁保证mEditorMap与mMap的线程安全，保证mMap最终能够更新成功，最终向对应的xml文件中进行更新。
文件的更新理所当然也需要加一把锁：
写文件时的锁mWritingToDiskLock
// SharedPreferencesImpl.EditorImpl.enqueueDiskWrite()
synchronized (mWritingToDiskLock) {
writeToFile(mcr, isFromSyncCommit);
}
最终，我们一共通过使用了3把锁，对整个写操作的线程安全进行了保证。
篇幅限制，本文不对源码进行详细引申，有兴趣的读者可参考 SharedPreferencesImpl.EditorImpl 类的apply()源码。
3、跨进程操作的解决方案
//ContextImpl
private void checkMode(int mode) {
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");
}
}
}
Andorid 7.0及以上会抛出异常，Sharepreferences不再支持多进程模式。多进程共享文件会出现问题的本质在于，因为不同进程，所以线程同步会失效。要解决这个问题，可尝试跨进程解决方案，如ContentProvider、AIDL、AIDL、Service。
4、替代方案
MMKV
Jetpack DataStore
5、 小结
通过本文我们了解了SharedPreferences的基本原理。再回头看看文章开头的那几个问题，是不是有答案了。
- commit()方法和apply()方法的区别：commit()方法是同步的有返回结果，同步保证使用Countdownlatch，即使同步但不保证往磁盘的写入是发生在当前线程的。apply()方法是异步的具体发生在QueuedWork中，里面维护了一个单线程去执行磁盘写入操作。
- commit()和apply()方法其实都是Block主线程。commit()只要在主线程调用就会堵塞主线程;apply（）方法磁盘写入操作虽然是异步的，但是当组件(Activity Service BroadCastReceiver)这些系统组件特定状态转换的时候，会把QueuedWork中未完成的那些磁盘写入操作放在主线程执行，且如果比较耗时会产生ANR。
- 跨进程操作，需要借助Android平台常规的IPC手段（如，AIDL ContentProvider等来封装一层sp数据处理流程）来完成。
- 替代解决方案:看4。