SharePreference属于轻量级的键值存储方式，以XML文件方式保存数据。花2两个小时看下源码还是很有收获的。
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老规矩，先上图：
SharedPreference.png
获取SharedPreferences
我们一般有两种方式获取SharedPreference:
Activity的public SharedPreferences getPreferences(int mode)方法
或者
ContextImpl的public SharedPreferences getSharedPreferences(String name, int mode)方法
Activity的public SharedPreferences getPreferences(int mode)方法实际上调用了ContextImpl的public SharedPreferences getSharedPreferences(String name, int mode)方法：
public SharedPreferences getPreferences(int mode) {
return getSharedPreferences(getLocalClassName(), mode);
}
public abstract SharedPreferences getSharedPreferences(String name, int mode)是Context抽象类的抽象方法，它的实现类是哪个呢？这个涉及到Activity的启动，感兴趣的可以看下ActivityThread的performLaunchActivity方法。这个Context的实现类是ContextImpl类。
看下它的getSharedPreferences方法。
@Override
public SharedPreferences getSharedPreferences(String name, int mode) {
SharedPreferencesImpl sp;
synchronized (ContextImpl.class) {
if (sSharedPrefs == null) {
sSharedPrefs = new ArrayMap<String, ArrayMap<String, SharedPreferencesImpl>>(); //创建一个map，键是包名，值还是一个map(j键是传入的name，值是SharedPreferencesImpl对象)
}
final String packageName = getPackageName();
ArrayMap<String, SharedPreferencesImpl> packagePrefs = sSharedPrefs.get(packageName); //获取当前包名对应的map
if (packagePrefs == null) { //若为空则创建一个
packagePrefs = new ArrayMap<String, SharedPreferencesImpl>();
sSharedPrefs.put(packageName, packagePrefs);
}
// 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";
}
}
sp = packagePrefs.get(name); //根据传入的name获取SharedPreferencesImpl对象
if (sp == null) { //若为空，则新建一个SharedPreferencesImpl对象
File prefsFile = getSharedPrefsFile(name); //使用name创建一个文件
sp = new SharedPreferencesImpl(prefsFile, mode); //新建一个SharedPreferencesImpl对象
packagePrefs.put(name, sp); //放入当前包名对应的map中
return sp; //方法SharedPreferencesImpl对象
}
}
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;
}
这里建议看下类图，弄明白sSharedPrefs这个map的含义。这个方法从sSharedPrefs中根据包名获取一个和这个包名对应的map，然后从这个map中获取和传入的name关联的SharedPreferencesImpl对象，若不存在则创建这个对象。我们看下这个类。
final class SharedPreferencesImpl implements SharedPreferences
------------------------------成员变量-----------------------------------
private final File mFile; //保存数据的文件
private final File mBackupFile;
private final int mMode;
private Map<String, Object> mMap; //保存和该name关联的键值对
------------------------------构造方法--------------------------------------
SharedPreferencesImpl(File file, int mode) {
mFile = file;
mBackupFile = makeBackupFile(file);
mMode = mode;
mLoaded = false;
mMap = null;
startLoadFromDisk(); //从文件获取map数据
}
private void startLoadFromDisk() {
synchronized (this) {
mLoaded = false;
}
new Thread("SharedPreferencesImpl-load") {
public void run() {
synchronized (SharedPreferencesImpl.this) {
loadFromDiskLocked();//子线程中加载文件中的数据到成员变量map中
}
}
}.start();
}
private void loadFromDiskLocked() {
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); //获取文件数据流
map = XmlUtils.readMapXml(str); //解析xml数据，保存到map中
} catch (XmlPullParserException e) {
Log.w(TAG, "getSharedPreferences", e);
} catch (FileNotFoundException e) {
Log.w(TAG, "getSharedPreferences", e);
} catch (IOException e) {
Log.w(TAG, "getSharedPreferences", e);
} finally {
IoUtils.closeQuietly(str);
}
}
} catch (ErrnoException e) {
}
mLoaded = true;
if (map != null) {
mMap = map; //赋值给成员变量
mStatTimestamp = stat.st_mtime;
mStatSize = stat.st_size;
} else {
mMap = new HashMap<String, Object>();
}
notifyAll();
}
创建SharedPreferencesImpl对象的同时，会从对应的xml文件中解析数据，保存到成员变量mMap中。
到这里，我就就创建好了SharedPreferences，所以我们调用getSharedPreferences后返回的SharedPreferences就是SharedPreferencesImpl对象。
读写数据
首先我们要获取一个Editor对象
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();
}
这个Editor的实现是EditorImpl，我们看下这个类：
public final class EditorImpl implements Editor
-----------------------------成员变量-------------------------------
private final Map<String, Object> mModified = Maps.newHashMap();
----------------------------核心方法---------------------------
public Editor putBoolean(String key, boolean value)
等
比如要写入一个boolean值我们调用Editor的如下方法：
public Editor putBoolean(String key, boolean value) {
synchronized (this) {
mModified.put(key, value);
return this;
}
}
很简单，将本次增加的键值对保存到mModified中，暂时缓存在内存中。
同步数据到文件
想要把数据同步到文件要调用commit()方法：
public boolean commit() {
MemoryCommitResult mcr = commitToMemory(); //mModified和mMap中的数据一并保存到MemoryCommitResult对象中
SharedPreferencesImpl.this.enqueueDiskWrite( //写入到文件
mcr, null);
try {
mcr.writtenToDiskLatch.await();
} catch (InterruptedException e) {
return false;
}
notifyListeners(mcr);
return mcr.writeToDiskResult;
}
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);
}
});
}
}
}
这里注意commitToMemory方法，该方法将本次修改的数据mModified和以前的数据mMap一并保存到MemoryCommitResult对象中，然后调用enqueueDiskWrite方法将内存中的数据写入到文件。
看下commitToMemory方法：
private MemoryCommitResult commitToMemory() {
MemoryCommitResult mcr = new MemoryCommitResult(); //新建MemoryCommitResult对象
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; //将mMap保存到mcr的mapToWriteToDisk这个map中
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); //将本次修改增加的数据也加入到mMap中
}
mcr.changesMade = true;
if (hasListeners) {
mcr.keysModified.add(k);
}
}
mModified.clear();
}
}
return mcr;
}
接着看下enqueueDiskWrite是如何将数据写入文件的：
private void enqueueDiskWrite(final MemoryCommitResult mcr,
final Runnable postWriteRunnable) {
final Runnable writeToDiskRunnable = new Runnable() {
public void run() {
synchronized (mWritingToDiskLock) {
writeToFile(mcr); //将mcr写到文件，mcr保存着键值对
}
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(); //commit走这里，在当前线程执行
return;
}
}
QueuedWork.singleThreadExecutor().execute(writeToDiskRunnable);
}
commit方法会直接调用 writeToDiskRunnable.run()，也就是说在当前线程进行写文件操作，我们看下writeToFile方法：
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); //获取文件流
if (str == null) {
mcr.setDiskWriteResult(false);
return;
}
XmlUtils.writeMapXml(mcr.mapToWriteToDisk, str); //使用XmlUtils.writeMapXml方法将内存中的键值对写入文件
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);
}
这里主要关注两步，首先获取文件流，然后使用XmlUtils.writeMapXml将map写入文件。到这里，内存中的数据就写入到了文件。
总结
SharedPreference保存数据的形式是xml文件，并且创建时不同的name对应不同的xml文件。只有执行commit()操作才会将数据同步到文件，并且commit是同步的，会阻塞当前线程。想异步写入可以考虑apply方法。
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