最近在項目中使用了SharePreferences，因此看了一下SharePreferences相關的源碼，在此進行一下記錄
SharePreferences在Android提供的幾種數據存儲方式中屬於輕量級的鍵值存儲方式，以XML文件方式保存數據，通常用來存儲一些用戶行爲開關狀態等，一般的存儲一些常見的數據類型。
SharePreferences保存的xml文件存放於 /data/data/應用程序包/shared_prefs 的目錄下
SharePreferences是接口，位於android.content包中
SharePreferencesImpl是對於SharePreferences的實現，位於android.app包下
以下是一段SharePreferences的寫入以及讀取的例子：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
SharedPreferences sp = getSharedPreferences("data", MODE_PRIVATE);
SharedPreferences.Editor editor = sp.edit();
editor.putString("name", "username");
editor.commit();
String s = sp.getString("name", "");
}第6行調用了ContextWrapper中的getSharedPreferences方法：
@Override
public SharedPreferences getSharedPreferences(String name, int mode) {
return mBase.getSharedPreferences(name, mode);
}這裏調用了mBase的getSharedPreferences，mBase是ContextWrapper持有的Context變量，很明顯實際類型即是ContextImpl（Context的實現類）：
@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;
}上面代碼首先檢查了是否緩存過對應的SharedPreferences，如果沒有緩存則創建SharedPreferencesImpl對象（第9行）：
SharedPreferencesImpl(File file, int mode) {
mFile = file;
mBackupFile = makeBackupFile(file);
mMode = mode;
mLoaded = false;
mMap = null;
startLoadFromDisk();
}簡單說一下這裏的幾個變量：
- mFile：當前的xml文件，即/data/data/應用程序包/shared_prefs/xxx.xml
- mBackupFile：當前xml備份文件，名爲xxx.xml.bak
- mMode：int類型，表示打開的模式
- mLoaded：boolean類型，表示是否已經加載xml文件
- mMap：xml的數據再內存中的儲存形式
在SharedPreferencesImpl構造函數中，調用了startLoadFromDisk方法加載xml中的數據：
private void startLoadFromDisk() {
synchronized (this) {
mLoaded = false;
}
new Thread("SharedPreferencesImpl-load") {
public void run() {
synchronized (SharedPreferencesImpl.this) {
loadFromDiskLocked();
}
}
}.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);
map = XmlUtils.readMapXml(str);
} 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;
mStatTimestamp = stat.st_mtime;
mStatSize = stat.st_size;
} else {
mMap = new HashMap<String, Object>();
}
notifyAll();
}在startLoadFromDisk方法中，SharedPreferencesImpl開啓了新的子線程，並持有對象鎖來調用loadFromDiskLocked方法，通過XmlUtils工具類把xml讀取爲內存中的Map形式，最後通過notifyAll通知所有wait的地方
看完SharedPreferencesImpl的初始化，我們來看一下它的get方法，以例子中的getString爲例：
public String getString(String key, 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) {
}
}
}該方法在持有對象鎖的情況下，通過awaitLoadedLocked方法檢查是否已經從xml加載到內存map中，然後直接讀取map中的數據即可
awaitLoadedLocked方法處理了一些與StrictMode相關的問題，在此不做討論，在它發現SharedPreferencesImpl還沒有加載完畢時，會調用wait方法把當前線程掛起，直到加載完畢通過notifyAll喚醒
看完了SharedPreferencesImpl的get方法，我們再來看一下如何向其中插入新的數據：
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();
}當我們調用edit方法時，會返回一個新的Edit接口的實現類EditorImpl，EditorImpl使用的是默認構造函數，它包含了兩個成員變量：
- mModified：Map類型，用於存儲修改的鍵值對
- mClear：boolean類型，用於標識是否清空SharedPreferences
獲取EditorImpl實例後，我們會調用其put或者clear方法修改SharedPreferences：
public Editor putString(String key, String value) {
synchronized (this) {
mModified.put(key, value);
return this;
}
}
public Editor clear() {
synchronized (this) {
mClear = true;
return this;
}
}put方法會把我們的修改存入mModified變量，而clear方法會把mClear變量置爲true
當我們修改SharedPreferences完成後，會調用Editor的apply或者commit方法（apply爲異步提交，commit爲同步提交）：
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);
}
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;
}兩個方法都首先調用了commitToMemory方法，把修改同步到內存中，然後都把寫入文件中的任務加入到了隊列當中等待執行
寫入文件的任務使用的同步工具是CounDownLatch，commit在當前線程調用了await方法等待寫入完成並返回Boolean表示寫入狀態，而apply在把寫入任務加入隊列後則返回了
在看commitToMemory方法之前，我們先來看看他的返回結果：MemoryCommitResult類
// 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();
}
}MemoryCommitResult中：writeToDiskResult用於表示寫入文件的結果，mapToWriteToDisk表示寫入文件的map，即Editor更新後的map
commitToMemory方法如下：
// 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;
}在第26行我們可以看到，Editor首先通過mClear變量決定是否清空mMap，然後通過mModified變量決定如何修改mMap