SharedPreferences是我们在android开发中使用的比较频繁的轻量级数据存储方案。如果使用不当可能会造成一些困扰。通过源码分析，可以帮助我们更好的了解它内部的运行机制。
使用方式
SharedPreferences sp = getSharedPreferences("test", Context.MODE_PRIVATE);
//添加数据改变的监听
sp.registerOnSharedPreferenceChangeListener(mListener);
Set<String> set = new HashSet<>();
set.add("一班");
set.add("二班");
//写入
sp.edit().putString("name", "张三").putStringSet("grade", set).apply();
//读取
String name = sp.getString("name","");
//删除
sp.edit().remove("name").apply();
//清空
sp.edit().clear().apply();数据会以test.xml文件形式，存储在设备上data/data/程序包名/shared_prefs文件夹下。内容如下：
<?xml version='1.0' encoding='utf-8' standalone='yes' ?>
<map>
<string name="name">张三</string>
<set name="grade">
<string>一班</string>
<string>二班</string>
</set>
</map>源码解析
获取SharedPreferences对象
在ContextImpl调用getSharedPreferences()获取，源码
@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<>();
}
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) {
checkMode(mode);
if (getApplicationInfo().targetSdkVersion >= android.os.Build.VERSION_CODES.O) {
if (isCredentialProtectedStorage()
&& !getSystemService(StorageManager.class).isUserKeyUnlocked(
UserHandle.myUserId())
&& !isBuggy()) {
throw new IllegalStateException("SharedPreferences in credential encrypted "
+ "storage are not available until after user is unlocked");
}
}
SharedPreferencesImpl sp;
synchronized (ContextImpl.class) {
final ArrayMap<File, SharedPreferencesImpl> cache = getSharedPreferencesCacheLocked();
sp = cache.get(file);
if (sp == null) {
sp = new SharedPreferencesImpl(file, mode);
cache.put(file, sp);
return sp;
}
}
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;
}- SharedPreferences是一个接口，其实现类为SharedPreferencesImpl 。
- SharedPreferencesImpl 在第一次创建的时候会被缓存。
SharedPreferencesImpl 初始化 源码
SharedPreferencesImpl(File file, int mode) {
mFile = file;
//创建备份文件
mBackupFile = makeBackupFile(file);
mMode = mode;
mLoaded = false;
mMap = null;
startLoadFromDisk();
}
private void startLoadFromDisk() {
synchronized (mLock) {
mLoaded = false;
}
//开启线程加载文件
new Thread("SharedPreferencesImpl-load") {
public void run() {
loadFromDisk();
}
}.start();
}
private void loadFromDisk() {
synchronized (mLock) {
//如果已经加载过就直接返回
if (mLoaded) {
return;
}
if (mBackupFile.exists()) {
//如果备份文件存在，证明之前的文件操作出现过异常，需要删除，然后从备份文件恢复
mFile.delete();
mBackupFile.renameTo(mFile);
}
}
// Debugging
if (mFile.exists() && !mFile.canRead()) {
Log.w(TAG, "Attempt to read preferences file " + mFile + " without permission");
}
Map map = null;
StructStat stat = null;
try {
//获取文件信息结构体
stat = Os.stat(mFile.getPath());
if (mFile.canRead()) {
BufferedInputStream str = null;
try {
str = new BufferedInputStream(
new FileInputStream(mFile), 16*1024);
//将xml文件解析为map
map = XmlUtils.readMapXml(str);
} catch (Exception e) {
Log.w(TAG, "Cannot read " + mFile.getAbsolutePath(), e);
} finally {
IoUtils.closeQuietly(str);
}
}
} catch (ErrnoException e) {
/* ignore */
}
synchronized (mLock) {
mLoaded = true;
if (map != null) {
//数据缓存到内存map
mMap = map;
//存储上次访问时间
mStatTimestamp = stat.st_mtime;
//存储文件长度
mStatSize = stat.st_size;
} else {
mMap = new HashMap<>();
}
//唤醒对象的等待池中的所有线程
mLock.notifyAll();
}
}- 查询是否有备份文件，如果有就证明上一次的文件写入出现了问题，因此会读取备份的文件。如果没有就读取原始文件。
- 开启一个线程读取文件内容，并缓存到内存map。
- 所有操作完成后通过mLock.notifyAll()唤醒对象的等待池中的所有线程。
通过getXXX读取数据
由于get方法大都一样，这里分析getString。
@Nullable
public String getString(String key, @Nullable String defValue) {
synchronized (mLock) {
//加锁等待异步加载线程把数据加载完毕
awaitLoadedLocked();
//直接从内存map读取数据
String v = (String)mMap.get(key);
return v != null ? v : defValue;
}
}
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 {
//等待mLock.notifyAll()
mLock.wait();
} catch (InterruptedException unused) {
}
}
}- 数据直接从内存map读取。
- 数据读取之前通过mLock.wait()保证数据已经从文件到内存加载完毕。如果xml文件过大，导致解析时间过长，此时进行读取有可能导致界面卡顿或者ANR。
- 数据读取是线程安全的。
通过putXXX写入数据
数据的写入是通过SharedPreferences的内部类Editor完成的，Editor也是一个接口，其实现类为SharedPreferencesImpl 的内部类EditorImpl 。源码
public final class EditorImpl implements Editor {
private final Object mLock = new Object();
//临时变量，存储修改未提交的数据
@GuardedBy("mLock")
private final Map<String, Object> mModified = Maps.newHashMap();
@GuardedBy("mLock")
private boolean mClear = false;
public Editor putString(String key, @Nullable String value) {
synchronized (mLock) {
//将数据保存到临时map
mModified.put(key, value);
return this;
}
}
...
public Editor remove(String key) {
synchronized (mLock) {
//删除数据传入this
mModified.put(key, this);
return this;
}
}
public Editor clear() {
synchronized (mLock) {
//标记为清空
mClear = true;
return this;
}
}
public void apply() {
final long startTime = System.currentTimeMillis();
//将修改先提交到内存
final MemoryCommitResult mcr = commitToMemory();
final Runnable awaitCommit = new Runnable() {
public void run() {
try {
//阻塞等待
mcr.writtenToDiskLatch.await();
} catch (InterruptedException ignored) {
}
if (DEBUG && mcr.wasWritten) {
Log.d(TAG, mFile.getName() + ":" + mcr.memoryStateGeneration
+ " applied after " + (System.currentTimeMillis() - startTime)
+ " ms");
}
}
};
//添加一个等待任务到QueuedWork
QueuedWork.addFinisher(awaitCommit);
Runnable postWriteRunnable = new Runnable() {
public void run() {
awaitCommit.run();
//移除等待任务
QueuedWork.removeFinisher(awaitCommit);
}
};
//将提交到内存后的结果写入磁盘
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.
//回调监听
notifyListeners(mcr);
}
public boolean commit() {
long startTime = 0;
if (DEBUG) {
startTime = System.currentTimeMillis();
}
//将修改先提交到内存
MemoryCommitResult mcr = commitToMemory();
//将提交到内存后的结果写入磁盘
SharedPreferencesImpl.this.enqueueDiskWrite(
mcr, null /* sync write on this thread okay */);
try {
//阻塞等待
mcr.writtenToDiskLatch.await();
} catch (InterruptedException e) {
return false;
} finally {
if (DEBUG) {
Log.d(TAG, mFile.getName() + ":" + mcr.memoryStateGeneration
+ " committed after " + (System.currentTimeMillis() - startTime)
+ " ms");
}
}
//回调监听
notifyListeners(mcr);
//返回写入结果
return mcr.writeToDiskResult;
}- 当我们调用putXXX方法的时候，数据会先被缓存到EditorImpl 中的临时map。然后，无论我们调用commit还是apply修改都会先提交到内存 commitToMemory。
- commitToMemory完成后无论是commit和apply都会调用enqueueDiskWrite将数据写入磁盘。区别是commit会阻塞等待写入磁盘完成，传入的第二个参数为null；apply会传入一个Runnable 。
- 如果我们设置了监听，最后会回调监听。如果是commit，回调监听一定发生在写入磁盘之后。如果是apply，修改提交到内存后就会回调，并不保证数据已经写入磁盘。
接下来看看最重要的两个函数commitToMemory和enqueueDiskWrite。
commitToMemory
// Return value from EditorImpl#commitToMemory()
private static class MemoryCommitResult {
final long memoryStateGeneration;
//修改的key集合
@Nullable final List<String> keysModified;
//设置的监听
@Nullable final Set<OnSharedPreferenceChangeListener> listeners;
//需要写入磁盘的数据
final Map<String, Object> mapToWriteToDisk;
//同步锁
final CountDownLatch writtenToDiskLatch = new CountDownLatch(1);
@GuardedBy("mWritingToDiskLock")
volatile boolean writeToDiskResult = false;
boolean wasWritten = false;
private MemoryCommitResult(long memoryStateGeneration, @Nullable List<String> keysModified,
@Nullable Set<OnSharedPreferenceChangeListener> listeners,
Map<String, Object> mapToWriteToDisk) {
//正在发生的内存修改次数
this.memoryStateGeneration = memoryStateGeneration;
//修改的key的集合
this.keysModified = keysModified;
//监听哪些key被修改
this.listeners = listeners;
//需要写入磁盘的数据
this.mapToWriteToDisk = mapToWriteToDisk;
}
void setDiskWriteResult(boolean wasWritten, boolean result) {
this.wasWritten = wasWritten;
writeToDiskResult = result;
//释放锁
writtenToDiskLatch.countDown();
}
}// Returns true if any changes were made
private MemoryCommitResult commitToMemory() {
long memoryStateGeneration;
List<String> keysModified = null;
Set<OnSharedPreferenceChangeListener> listeners = null;
Map<String, Object> mapToWriteToDisk;
//读写互斥
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);
}
//将源数据赋值给临时变量以便后面遍历修改
mapToWriteToDisk = mMap;
//正在排队的磁盘写入任务数量
mDiskWritesInFlight++;
boolean hasListeners = mListeners.size() > 0;
if (hasListeners) {
keysModified = new ArrayList<String>();
listeners = new HashSet<OnSharedPreferenceChangeListener>(mListeners.keySet());
}
synchronized (mEditorLock) {
boolean changesMade = false;
//如果有被标记为清空
if (mClear) {
if (!mapToWriteToDisk.isEmpty()) {
changesMade = true;
//将源数据清空
mapToWriteToDisk.clear();
}
mClear = false;
}
//遍历被修改的数据
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.
//这里别忘了，在remove的时候传入了this，标记删除
if (v == this || v == null) {
//源数据没有就跳过有就删除
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;
if (hasListeners) {
//只有设置了监听者才缓存修改的key
keysModified.add(k);
}
}
//清空临时数据
mModified.clear();
if (changesMade) {
//将正在发生的内存修改加1，写入磁盘的时候会根据这个值来确定是否有必要写入
mCurrentMemoryStateGeneration++;
}
memoryStateGeneration = mCurrentMemoryStateGeneration;
}
}
//返回内存修改结果
return new MemoryCommitResult(memoryStateGeneration, keysModified, listeners,
mapToWriteToDisk);
}- 将源数据与mModified遍历组合生成真正需要写入磁盘的数据，数据会全量写入xml文件。
- 通过mCurrentMemoryStateGeneration来记录正在发生的内存修改次数，用来保证最后写入磁盘的数据是最后一次修改的，减少IO操作的数量。
- 用mDiskWritesInFlight来标记正在排队的磁盘写入任务数量。
- 清空标记清空的是源数据，并没有清空缓存数据。例如： sp.edit().putString(“name”, “张三”).clear().apply()；存在临时map里面的“name”对应的数据不会被清空，最终会被写入磁盘。
- 只有我们设置了监听者，才有必要缓存修改的key。同时需要注意，我们设置的listener是通过弱引用缓存的，如果我们直接通过匿名内部类进行设置很有可能无法达到预期。
@GuardedBy("mLock")
private final WeakHashMap<OnSharedPreferenceChangeListener, Object> mListeners =
new WeakHashMap<OnSharedPreferenceChangeListener, Object>();enqueueDiskWrite
/**
* Enqueue an already-committed-to-memory result to be written
* to disk.
*
* They will be written to disk one-at-a-time in the order
* that they're enqueued.
*
* @param postWriteRunnable if non-null, we're being called
* from apply() and this is the runnable to run after
* the write proceeds. if null (from a regular commit()),
* then we're allowed to do this disk write on the main
* thread (which in addition to reducing allocations and
* creating a background thread, this has the advantage that
* we catch them in userdebug StrictMode reports to convert
* them where possible to apply() ...)
*/
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) {
//写入到磁盘文件
writeToFile(mcr, isFromSyncCommit);
}
synchronized (mLock) {
//需要排队写入的任务数量减一
mDiskWritesInFlight--;
}
if (postWriteRunnable != null) {
//只有apply才不为null
postWriteRunnable.run();
}
}
};
// Typical #commit() path with fewer allocations, doing a write on
// the current thread.
if (isFromSyncCommit) {
//如果是通过调用commit写入
boolean wasEmpty = false;
synchronized (mLock) {
//是否存在多个写入任务
wasEmpty = mDiskWritesInFlight == 1;
}
if (wasEmpty) {
//当前任务只有一个，那就是自己，直接同步调用run方法写入文件即可
writeToDiskRunnable.run();
return;
}
}
//mDiskWritesInFlight大于1，表示自己之前还有需要执行的写入任务，那就把自己加入队列。
//这里8.0以上和8.0以下实现方式有些不一样。在7.0上是调用的是
//QueuedWork.singleThreadExecutor().execute(writeToDiskRunnable);
QueuedWork.queue(writeToDiskRunnable, !isFromSyncCommit);
}
private static FileOutputStream createFileOutputStream(File file) {
FileOutputStream str = null;
try {
str = new FileOutputStream(file);
} catch (FileNotFoundException e) {
File parent = file.getParentFile();
if (!parent.mkdir()) {
Log.e(TAG, "Couldn't create directory for SharedPreferences file " + file);
return null;
}
FileUtils.setPermissions(
parent.getPath(),
FileUtils.S_IRWXU|FileUtils.S_IRWXG|FileUtils.S_IXOTH,
-1, -1);
try {
str = new FileOutputStream(file);
} catch (FileNotFoundException e2) {
Log.e(TAG, "Couldn't create SharedPreferences file " + file, e2);
}
}
return str;
}
@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();
}
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) + "/"
+ (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
if (mFile.exists()) {
if (!mFile.delete()) {
Log.e(TAG, "Couldn't clean up partially-written file " + mFile);
}
}
mcr.setDiskWriteResult(false, false);
}
}虽然代码行数比较多，但是逻辑很简单。这里贴以下QueuedWork的部分源码
/**
* Lazily create a handler on a separate thread.
*
* @return the handler
*/
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;
}
}
/**
* Queue a work-runnable for processing asynchronously.
*
* @param work The new runnable to process
* @param shouldDelay If the message should be delayed
*/
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);
}
}
}- 当我们调用的是commit提交数据，如果当前没有正在执行的磁盘写入任务，就直接同步调用写文件操作；如果存在，就将任务加入到QueuedWork通过HandlerThread在新线程里执行。commit会通过mcr.writtenToDiskLatch.await()加锁等待，当磁盘写入完成后通过mcr.setDiskWriteResult(true, true)通知commit释放锁，继续往后执行。
- 当调用apply提交数据，会先提交一个等待任务到QueuedWork.addFinisher(awaitCommit)，任务中包含了等待锁mcr.writtenToDiskLatch.await()，只有写入磁盘完成后才会释放该锁，并移除等待任务。然后和commit一样，将任务加入队列，异步执行。
- 当Activity、service生命周期变化的时候，ActivityThread 的handleSleeping 、handleServiceArgs、handleStopService、handleStopActivity等方法会被调用，最终会调用到QueuedWork.waitToFinish()，如果我们通过QueuedWork.addFinisher(awaitCommit)加入的等待任务超时未释放等待锁，有可能会导致ANR。美团的这篇文章有详细介绍，文章链接。
虽然SharedPreferences有它的缺陷和不足。如：
- 调用getXXX获取数据可能会导致线程阻塞。
- SharedPreferences为了提高性能会将数据保存在内存里面，加载内存负担。
- 文件写入是全量写入，如果xml文件过大会影响性能。
- apply虽然是异步，也有可能导致ARN。
但通过分析源码，我们能够找到它出现这些问题的原因，并给出一定的解决方案：
- 通过异步或者延迟加载的方式首次加载数据到内存。
- 尽量将需要频繁修改的数据单独保存为一个文件，减小全量写入带来的性能损耗。
- 尽量多次调用putXXX后再一次apply或commit，减少IO操作。
如有不足，欢迎指正。
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