获取SharePreference对象的途径
Context的实例方法
@Override
public SharedPreferences getSharedPreferences(String name, int mode) {
return mBase.getSharedPreferences(name, mode);
}
Activity的实例方法
public SharedPreferences getPreferences(int mode) {
return getSharedPreferences(getLocalClassName(), mode);
}
PreferenceManager的静态方法
public static SharedPreferences getDefaultSharedPreferences(Context context) {
return context.getSharedPreferences(getDefaultSharedPreferencesName(context),
getDefaultSharedPreferencesMode());
}上面的获取SharePreference实例的三个方法,本质上的实现是一样的，唯一的不同之处在于SharePreference本地存储文件xml的文件名称,它们最终调用的都是ContextImpl类中的getSharedPreferences(String name, int mode)方法实现的
ContextImpl中获取SharePreference的源码
@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);
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;
}
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;
}
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");
}
}
}
上面的源码做了5件事情:
- SharePreference是个接口，并不是一个具体的类,SharePreferenceImpl实现了该接口
使用了synchronized关键字修饰从缓存中读取SharePreferenceImpl对象或者是创建该对象,表明在多线程中获取SharePreference对象是线程安全的。
只有第一次获取SharePreference实例对象时，需要new SharePreferenceImpl()出来,然后保存到以packageName为key的ArrayMap
SharedPreferencesImpl(File file, int mode) {
mFile = file;
mBackupFile = makeBackupFile(file);
mMode = mode;
mLoaded = false;
mMap = null;
startLoadFromDisk();
}
private void startLoadFromDisk() {
synchronized (this) {
mLoaded = false;
}
new Thread("SharedPreferencesImpl-load") {
public void run() {
loadFromDisk();
}
}.start();
}
private void loadFromDisk() {
synchronized (SharedPreferencesImpl.this) {
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 map = null;
StructStat stat = null;
try {
stat = Os.stat(mFile.getPath());
if (mFile.canRead()) {
BufferedInputStream str = null;
try {
str = new BufferedInputStream(
new FileInputStream(mFile), 16*1024);
map = XmlUtils.readMapXml(str);
} catch (XmlPullParserException | IOException e) {
Log.w(TAG, "getSharedPreferences", e);
} finally {
IoUtils.closeQuietly(str);
}
}
} catch (ErrnoException e) {
/* ignore */
}
synchronized (SharedPreferencesImpl.this) {
mLoaded = true;
if (map != null) {
mMap = map;
mStatTimestamp = stat.st_mtime;
mStatSize = stat.st_size;
} else {
mMap = new HashMap<>();
}
notifyAll();
}
}
上面的源码做了4件事情:
- 实例化SharePreferenceImpl对象时为一些基础数据赋值,并开启了一个子线程去读取硬盘中的数据，保存到mMap集合中.
- mBackFile表示备份文件它是以.bak为结尾的文件,从硬盘的时候如果发现备份存在(表示上次的写操作出现了异常,正常情况下在每次开启写操作时,都会将原来的xml文件备份一次，然后把数据写入新的文件，写入成功后就删除备份文件,否则删除新建的写文件,保留备份文件)就删除mFile文件,并重命名备份文件为mFile文件的名称,这样做的目的就是数据的恢复。
- 读取成功后会修改mLoaded=true(这个值很重要,后续会讲到),并调用notifyAll()通知其他处于wait()等待中的线程，表示已经从硬盘获取数据成功可以继续执行使用数据了。
- 更新内存中的mStatTimestamp，mStatSize为硬盘上的值,保持一致.
@Nullable
public String getString(String key, @Nullable String defValue) {
synchronized (this) {
awaitLoadedLocked();
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 {
wait();
} catch (InterruptedException unused) {
}
}
}上面通过SharePreference的getString方法获取value的时候根据标志位mLoaded决定的,如果mLoadad=false，就执行wait操作，等待数据从硬盘加载完成，才能读取内存中mMap的value值，如果loadfromDisk加载完成会调用notifyAll()方法解除阻塞，从这里可以看出通过getXXX方法获取key值对应的value都是从内存中读取的并且 getXXX方法是会阻塞线程执行的。所以不能在sharePreference文件中保存大数据，如果数据大，那么第一次读取硬盘操作会很耗时,进而通过getXXX获取value会造成阻塞，如果是在主线程中调用getXXX有可能造成ANR异常的。
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 (this) {
awaitLoadedLocked();
}
return new EditorImpl();
}
public final class EditorImpl implements Editor {
private final Map<String, Object> mModified = Maps.newHashMap();
private boolean mClear = false;
public Editor putString(String key, @Nullable String value) {
synchronized (this) {
mModified.put(key, value);
return this;
}
}
当我们通过putXXX存储数据的时候，实际上调用的是EditorImpl类中的方法，它实现了Editor接口,并实现了里面的方法。
通过上面的源码分析我们可以得出如下的结论:
- 调用putXXX并不是真的把数据保存在了硬盘,只是暂时存放在Map
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;
}
// Returns true if any changes were made
private MemoryCommitResult commitToMemory() {
MemoryCommitResult mcr = new MemoryCommitResult();
synchronized (SharedPreferencesImpl.this) {
// 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);
}
mcr.mapToWriteToDisk = mMap;
mDiskWritesInFlight++;
boolean hasListeners = mListeners.size() > 0;
if (hasListeners) {
mcr.keysModified = new ArrayList<String>();
mcr.listeners =
new HashSet<OnSharedPreferenceChangeListener>(mListeners.keySet());
}
synchronized (this) {
if (mClear) {
if (!mMap.isEmpty()) {
mcr.changesMade = true;
mMap.clear();
}
mClear = false;
}
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 (!mMap.containsKey(k)) {
continue;
}
mMap.remove(k);
} else {
if (mMap.containsKey(k)) {
Object existingValue = mMap.get(k);
if (existingValue != null && existingValue.equals(v)) {
continue;
}
}
mMap.put(k, v);
}
mcr.changesMade = true;
if (hasListeners) {
mcr.keysModified.add(k);
}
}
mModified.clear();
}
}
return mcr;
}
// Return value from EditorImpl#commitToMemory()
private static class MemoryCommitResult {
public boolean changesMade; // any keys different?
public List<String> keysModified; // may be null
public Set<OnSharedPreferenceChangeListener> listeners; // may be null
public Map<?, ?> mapToWriteToDisk;
public final CountDownLatch writtenToDiskLatch = new CountDownLatch(1);
public volatile boolean writeToDiskResult = false;
public void setDiskWriteResult(boolean result) {
writeToDiskResult = result;
writtenToDiskLatch.countDown();
}
}
上面的commitToMemory()方法，是创造一个适合内存存储的对象，这个方法很重要是核心。
commitToMemory主要做了4件事情:
- 创建MemoryCommitResult实例对象mcr
- 过滤mModified集合中的数据并放入mMap集合内存中
- 将mMap集合的数据赋值给mcr的mapToWriteToDisk，等待写入磁盘。
- mDiskWritesInFlight等待写入磁盘的数据执行逻辑运算符++
- 判断mListeners中是否有registerOnSharedPreferenceChangeListener的注册对象,如果有就将WeakHashMap
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();
}
}
};
final boolean isFromSyncCommit = (postWriteRunnable == null);
// Typical #commit() path with fewer allocations, doing a write on
// the current thread.
if (isFromSyncCommit) {
boolean wasEmpty = false;
synchronized (SharedPreferencesImpl.this) {
wasEmpty = mDiskWritesInFlight == 1;
}
if (wasEmpty) {
writeToDiskRunnable.run();
return;
}
}
QueuedWork.singleThreadExecutor().execute(writeToDiskRunnable);
}
// Note: must hold mWritingToDiskLock
private void writeToFile(MemoryCommitResult mcr) {
// Rename the current file so it may be used as a backup during the next read
if (mFile.exists()) {
if (!mcr.changesMade) {
// If the file already exists, but no changes were
// made to the underlying map, it's wasteful to
// re-write the file. Return as if we wrote it
// out.
mcr.setDiskWriteResult(true);
return;
}
if (!mBackupFile.exists()) {
if (!mFile.renameTo(mBackupFile)) {
Log.e(TAG, "Couldn't rename file " + mFile
+ " to backup file " + mBackupFile);
mcr.setDiskWriteResult(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 (str == null) {
mcr.setDiskWriteResult(false);
return;
}
XmlUtils.writeMapXml(mcr.mapToWriteToDisk, str);
FileUtils.sync(str);
str.close();
ContextImpl.setFilePermissionsFromMode(mFile.getPath(), mMode, 0);
try {
final StructStat stat = Os.stat(mFile.getPath());
synchronized (this) {
mStatTimestamp = stat.st_mtime;
mStatSize = stat.st_size;
}
} catch (ErrnoException e) {
// Do nothing
}
// Writing was successful, delete the backup file if there is one.
mBackupFile.delete();
mcr.setDiskWriteResult(true);
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);
}
}
调用上面的方法存储到硬盘。enqueueDiskWrite方法中的postWriteRunnable参数，用来区分是commit还是apply提交方式.
commit方式提交分析:
- 此时isFromSyncCommit=true,如果当前只有一个editor等待提交到硬盘,调用writeToDiskRunnable.run()方法并返回,否则开启一个新线程池异步提交到硬盘。
- commit方法中有mcr.writtenToDiskLatch.await();表示需要等到writeToFile方法提交到磁盘后的结果才能继续执行notifyListeners.所以有可能会造成线程阻塞。这里用到了countDownlatchjava多线程(3)之join与countDownLatch,这里不在赘述。
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);
// 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);
}
private void notifyListeners(final MemoryCommitResult mcr) {
if (mcr.listeners == null || mcr.keysModified == null ||
mcr.keysModified.size() == 0) {
return;
}
if (Looper.myLooper() == Looper.getMainLooper()) {
for (int i = mcr.keysModified.size() - 1; i >= 0; i--) {
final String key = mcr.keysModified.get(i);
for (OnSharedPreferenceChangeListener listener : mcr.listeners) {
if (listener != null) {
listener.onSharedPreferenceChanged(SharedPreferencesImpl.this, key);
}
}
}
} else {
// Run this function on the main thread.
ActivityThread.sMainThreadHandler.post(new Runnable() {
public void run() {
notifyListeners(mcr);
}
});
}
}
}
apply提交方式分析
- 开启了一个新的线程池提交数据到磁盘，不需要等待返回的结果，立即调用了notifyListeners(mcr)操作
- 在调用writeToFile存储到硬盘时,需要注意每次数据存储前需要备份数据,如果操作成功了就删除备份mBackFile文件,保留mFile文件,失败了就保留mBackFile文件,删除mFile文件,便于下次数据读取时的恢复。
- notifyListeners用于通知注册了监听器的key值的变化，如果是在主线程中调用的就直接触发监听事件,如果不是就调用ActivityThread.sMainThreadHandler.post方式转换到主线程中来,这个思路便于我们以后的采纳。
欢迎大家指出理解有误的地方，本人会不胜感激。
