sp 初始化.jpg
使用
创建
SharedPreferences sp = getSharedPreferences("sp_config", MODE_PRIVATE);
SharedPreferences.Editor edit = sp.edit();
edit.putString("sp_string","SharedPreferences");
edit.putInt("sp_int",100);
edit.apply();
调用
SharedPreferences sp = getSharedPreferences("sp_config", MODE_PRIVATE);
Map<String, ?> map = sp.getAll();
for (Map.Entry<String, ?> entry:map.entrySet()){
Log.d("TAG","Key="+entry.getKey()+" Values="+entry.getValue());
}
log
D/TAG: Key=sp_string Values=SharedPreferences
D/TAG: Key=sp_int Values=100
源码分析
/**
* Proxying implementation of Context that simply delegates all of its calls to
* another Context. Can be subclassed to modify behavior without changing
* the original Context.
*/
//context 的代理类，context 是个抽象类，具体实现为contextImpl
public class ContextWrapper extends Context {
Context mBase;
public ContextWrapper(Context base) {
mBase = base;
}
@Override
public SharedPreferences getSharedPreferences(String name, int mode) {
return mBase.getSharedPreferences(name, mode);
}
}
/**
* Common implementation of Context API, which provides the base
* context object for Activity and other application components.
*/
//Context 具体的实现类，为Activity 和application 提供基础组件
class ContextImpl extends Context {
@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) {
//创建sp
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.
//如果其他人（其他一些进程）更改了我们后面的prefs文件，我们会重新加载它。 这是历史（无证）行为。
sp.startReloadIfChangedUnexpectedly();
}
return sp;
}
}
以应用包名为Key 缓存SharedPrefsCache，所以不要存放大的key和value在SharedPreferences中，否则会一直存储在内存中得不到释放，可能造成内存溢出，内存使用过高会频发引发GC（ garbage collection），导致界面丢帧甚至ANR（Application Not Responding）。
class ContextImpl extends Context {
/**
* Map from package name, to preference name, to cached preferences.
*/
@GuardedBy("ContextImpl.class")
private static ArrayMap<String, ArrayMap<File, SharedPreferencesImpl>> sSharedPrefsCache;
private ArrayMap<File, SharedPreferencesImpl> getSharedPreferencesCacheLocked() {
if (sSharedPrefsCache == null) {
sSharedPrefsCache = new ArrayMap<>();
}
final String packageName = getPackageName();
//以应用包名为Key 缓存SharedPrefsCache
ArrayMap<File, SharedPreferencesImpl> packagePrefs = sSharedPrefsCache.get(packageName);
if (packagePrefs == null) {
packagePrefs = new ArrayMap<>();
sSharedPrefsCache.put(packageName, packagePrefs);
}
return packagePrefs;
}
}
final class SharedPreferencesImpl implements SharedPreferences {
SharedPreferencesImpl(File file, int mode) {
mFile = file;
mBackupFile = makeBackupFile(file);
mMode = mode;
mLoaded = false;
mMap = null;
startLoadFromDisk();
}
private void startLoadFromDisk() {
synchronized (mLock) {
mLoaded = false;
}
//开启线程，从磁盘加载文件
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);
}
}
// 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);
//从输入流里解析xml
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) {
mMap = map;
mStatTimestamp = stat.st_mtime;
mStatSize = stat.st_size;
} else {
mMap = new HashMap<>();
}
//唤醒所有线程在这个对象上的锁机制
mLock.notifyAll();
}
}
sp.edit().putString("sp_string","SharedPreferences").apply();
final class SharedPreferencesImpl implements SharedPreferences {
public void apply() {
final long startTime = System.currentTimeMillis();
//把数据提交到内存中，所有不用担心在使用apply异步操作出现读取脏数据
final MemoryCommitResult mcr = commitToMemory();
//创建一个异步线程，这里也是commit（） 提交方式的区别，commit 是同步操作，而apply是异步操作
final Runnable awaitCommit = new Runnable() {
public void run() {
try {
mcr.writtenToDiskLatch.await();
} catch (InterruptedException ignored) {
}
if (DEBUG && mcr.wasWritten) {
Log.d(TAG, mFile.getName() + ":" + mcr.memoryStateGeneration
+ " applied after " + (System.currentTimeMillis() - startTime)
+ " ms");
}
}
};
QueuedWork.addFinisher(awaitCommit);
Runnable postWriteRunnable = new Runnable() {
public void run() {
awaitCommit.run();
QueuedWork.removeFinisher(awaitCommit);
}
};
//为写入操作开启了工作线程，不错造成UI线程阻塞
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);
}
}
commit()
public boolean commit() {
long startTime = 0;
if (DEBUG) {
startTime = System.currentTimeMillis();
}
MemoryCommitResult mcr = commitToMemory();
//postWriteRunnable 为null，为写入操作并没开启线程，所以是同步操作
SharedPreferencesImpl.this.enqueueDiskWrite(
mcr, null /* sync write on this thread okay */);
try {
mcr.writtenToDiskLatch.await();
} catch (InterruptedException e) {
return false;
} finally {
if (DEBUG) {
Log.d(TAG, mFile.getName() + ":" + mcr.memoryStateGeneration
+ " committed after " + (System.currentTimeMillis() - startTime)
+ " ms");
}
}
notifyListeners(mcr);
return mcr.writeToDiskResult;
}
总结：
- apply 是异步的， commit 是同步的；
平时使用的时候，尽量使用 apply 避免卡主主线程。因为写入前都已经更新修改到缓存了，不用担心读到脏数据。 - 初次读取数据会有概率的阻塞，是因为线程锁（SharePreferencesImpl.this）的原因，在初始化加载文件的时候，和读取数据的时候都会用到这个锁，而加载xml loadFromDisk 是 IO 耗时操作,虽然 loadFromDisk 操作被分配到另一个线程执行，但因为读取数据的时候，争用了这个锁，会发生概率卡顿。
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