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Android SharedPreferences解析
引:在使用SharedPreferences过程中,觉得Android这种轻量级的存储思想用起来挺方便的.本着知其然知其所以然的想法.尝试阅读SharedPreferences源码
入口函数
public SharedPreferences getPreferences(int mode) {
return getSharedPreferences(getLocalClassName(), mode);
}public SharedPreferences getSharedPreferences(String name, int mode) {
SharedPreferencesImpl sp;
synchronized (sSharedPrefs) {
sp = sSharedPrefs.get(name);
if (sp == null) {
File prefsFile = getSharedPrefsFile(name);
sp = new SharedPreferencesImpl(prefsFile, mode);
sSharedPrefs.put(name, 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;
}很简单的两个入口函数.
public SharedPreferences getPreferences(int mode)定义在Activity.class类
public SharedPreferences getSharedPreferences(String name, int mode) 定义在ContextImpl.class类
两者之间的关系,getPreferences通过调用getSharedPreferences来实现.
public SharedPreferences getPreferences(int mode) {
return getSharedPreferences(getLocalClassName(), mode);
}public String getLocalClassName() {
final String pkg = getPackageName();
final String cls = mComponent.getClassName();
int packageLen = pkg.length();
if (!cls.startsWith(pkg) || cls.length() <= packageLen
|| cls.charAt(packageLen) != '.') {
return cls;
}
return cls.substring(packageLen+1);
}getPreferences(int mode)函数通过调用getLocalClassName()传递的是类名进去.然而他们两区别就是这里了.
getSharedPreferences函数
getSharedPreferences函数返回的是一个SharedPreferencesImpl实例.使用过SharedPreferences的应该都知道,我们操作都是通过SharedPreferences实例进行操作的.
SharedPreferencesImpl继承SharedPreferences
ContextImpl.class有个sSharedPrefs静态变量.sSharedPrefs保存的是键值对.每当调用getSharedPreferences函数首先会用name查询sSharedPrefs里面有没保存对于的SharedPreferencesImpl实例.如果有那么直接返回name对于的SharedPreferencesImpl实例.如果没找到那么创建一个SharedPreferencesImpl实例并且已键值对的形式插入到sSharedPrefs.以便以后在通过name获取SharedPreferencesImpl实例.
- 也就说调用getSharedPreferences函数传递进来的name是一个索引.通过索引返回SharedPreferencesImpl实例.
private static final HashMap<String, SharedPreferencesImpl> sSharedPrefs =
new HashMap<String, SharedPreferencesImpl>();public SharedPreferences getSharedPreferences(String name, int mode) {
SharedPreferencesImpl sp;
synchronized (sSharedPrefs) {
//通过name查找SharedPreferencesImpl实例
sp = sSharedPrefs.get(name);
if (sp == null) {
//为空,第一次读取.没有根据name产生SharedPreferencesImpl实例
//getSharedPrefsFile()构造Xml文件File
File prefsFile = getSharedPrefsFile(name);
//创建SharedPreferencesImpl实例,默认在”/data/data/包名/shared_prefs/”目录里面创建XML文件
sp = new SharedPreferencesImpl(prefsFile, mode);
//保存SharedPreferencesImpl实例到sSharedPrefs,name作为索引
sSharedPrefs.put(name, sp);
return sp;
}
}
//这部分忽略,Context.MODE_MULTI_PROCESS getSharedPreferences是用于跨线程
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;接下来这三个很好理解,getSharedPrefsFile就是返回Xml文件的File.所以说SharedPreferences的实现是基于Xml文件.从这里可以看出.
- 例如:
- 会生成xml文件
- /data/data/com.dsliang.SharedPreferencesDemo/shared_prefs/ds.xml
public File getSharedPrefsFile(String name) {
return makeFilename(getPreferencesDir(), name + ".xml");
}private File getPreferencesDir() {
synchronized (mSync) {
if (mPreferencesDir == null) {
mPreferencesDir = new File(getDataDirFile(), "shared_prefs");
}
return mPreferencesDir;
}
} private File getDataDirFile() {
if (mPackageInfo != null) {
return mPackageInfo.getDataDirFile();
}
throw new RuntimeException("Not supported in system context");
}SharedPreferencesImpl类
理解了SharedPreferencesImpl类基本对SharedPreferences就明白一大半了.
构造函数
SharedPreferencesImpl(File file, int mode) {
//Xml文件的File
mFile = file;
//Xml文件的副本(备份)
mBackupFile = makeBackupFile(file);
//SharedPreferences模式
mMode = mode;
//是否加载文件内容到内存(加载到sSharedPrefs)
mLoaded = false;
//SharedPreferences键值对哈希表,最终获取数据就是在这表里面获取
mMap = null;
//异步读取文件并且保存数据到sSharedPrefs
startLoadFromDisk();
} private void startLoadFromDisk() {
synchronized (this) {
//标识文件未读取到内存
//在getString/getBoolean一系列的读取函数中会调用awaitLoadedLocked()函数会判断mLoaded.false - awaitLoadedLocked()函数阻塞等待mLoaded为true.
//还没读取完Xml文件哪里来读取呢?关键就在这变量.没赋值为true之前所有读取操作都是阻塞等待.
mLoaded = false;
}
//子线程执行读取Xml文件操作
new Thread("SharedPreferencesImpl-load") {
public void run() {
synchronized (SharedPreferencesImpl.this) {
loadFromDiskLocked();
}
}
}.start();
}
private void loadFromDiskLocked() {
if (mLoaded) {
return;
}
//如果存在Xml备份文件,删除Xml文件.用备份文件替换.
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;
FileStatus stat = new FileStatus();
if (FileUtils.getFileStatus(mFile.getPath(), stat) && mFile.canRead()) {
try {
BufferedInputStream str = new BufferedInputStream(
//通过XmlPull解析Xml文件
new FileInputStream(mFile), 16*1024);
//返回的数据是Map.有兴趣可以进去看看.里面很多递归调用,看到头都晕.知道是解析Xml文件并且返回Map就可以了.
map = XmlUtils.readMapXml(str);
str.close();
} catch (XmlPullParserException e) {
Log.w(TAG, "getSharedPreferences", e);
} catch (FileNotFoundException e) {
Log.w(TAG, "getSharedPreferences", e);
} catch (IOException e) {
Log.w(TAG, "getSharedPreferences", e);
}
}
//到这里文件已经读取出来并且保持到内存当中.
mLoaded = true;
if (map != null) {
//mMap,SharedPreferencesImpl内部用来存放Map的实例
mMap = map;
//文件读取时间
mStatTimestamp = stat.mtime;
//文件大小
mStatSize = stat.size;
} else {
//文件不存在创建一个空的Map
mMap = new HashMap<String, Object>();
}
//通知等待线程
notifyAll();
}
SharedPreferencesImpl类解析,在构造函数会创建子线程读取Xml文件解析成Map保存到SharedPreferencesImpl类内部的mMap.在读取解析Xml文件这过程当中对该SharedPreferencesImpl实例进行操作都会阻塞.(awaitLoadedLocked()函数导致)
可以知道SharedPreferences是会将Xml文件的全部数据都读取到内存.
读取数据
在SharedPreferencesImpl.class内部,mMap哈希表用来存放SharedPreferences键值对.
private Map<String, Object> mMap; getInt/getLong/getString等.
这一系列函数本质上没多少区别,就是通过key在mMap查询,查不到责返回默认值.
public String getString(String key, String defValue) {
synchronized (this) {
awaitLoadedLocked();
String v = (String)mMap.get(key);
return v != null ? v : defValue;
}
public int getInt(String key, int defValue) {
synchronized (this) {
awaitLoadedLocked();
Integer v = (Integer)mMap.get(key);
return v != null ? v : defValue;
}
}
public long getLong(String key, long defValue) {
synchronized (this) {
awaitLoadedLocked();
Long v = (Long)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) {
}
}
}
读取数据函数主要是要关注awaitLoadedLocked()函数.如果文件还没加载完成会导致操作阻塞.
修改数据-Editor类
调用SharedPreferencesImpl类的edit()方法返回EditorImpl类.EditorImpl类封装对数据操作的函数.putString/putBoolean等一系列方法.这里需要注意几个问题.
- putString/putBoolean等一系列方法操作EditorImpl类内部的一个Map并不是操作SharedPreferencesImpl类的Map.所以要生效是需要调用EditorImpl类的commit()方法.
- EditorImpl类的clear(0方法是清空EditorImpl类内部的Map实例.
以下操作不会导致SharedPreferences清楚数据.
edit().clear().commit();- 正确姿势如下:
edit().remove("Save")..remove("PassWord")commit(); 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();
}最后就是commit()方法源代码.
//记录Commit()方法结果的类
private static class MemoryCommitResult {
//数据是否需要更新
public boolean changesMade; // any keys different?
//Map实例
public List<String> keysModified; // may be null
//回调函数,有可能为null
public Set<OnSharedPreferenceChangeListener> listeners; // may be null
public Map<?, ?> mapToWriteToDisk;
public final CountDownLatch writtenToDiskLatch = new CountDownLatch(1);
//结果
public volatile boolean writeToDiskResult = false;
public void setDiskWriteResult(boolean result) {
writeToDiskResult = result;
writtenToDiskLatch.countDown();
}
}
// 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.
//mDiskWritesInFlight等处理的提交个数
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);
}
//SharedPreferences的Map保存到mcr.mapToWriteToDisk
mcr.mapToWriteToDisk = mMap;
mDiskWritesInFlight++;
//回调函数
boolean hasListeners = mListeners.size() > 0;
if (hasListeners) {
mcr.keysModified = new ArrayList<String>();
mcr.listeners =
new HashSet<OnSharedPreferenceChangeListener>(mListeners.keySet());
}
synchronized (this) {
//是否有清空,调用clear()以后设置mClear=true.就是当调用clear()以后在put数据进去也不会生效.
if (mClear) {
if (!mMap.isEmpty()) {
mcr.changesMade = true;
mMap.clear();
}
mClear = false;
}
//处理edit的Map和SharedPreferences的Map
for (Map.Entry<String, Object> e : mModified.entrySet()) {
String k = e.getKey();
Object v = e.getValue();
//调用romve以后,Value会设置成this.用来标识此key需要删除.很犀利的技巧.如果不设置没法知道哪一个需要删除.
if (v == this) { // magic value for a removal mutation
if (!mMap.containsKey(k)) {
continue;
}
//从SharedPreferences的Map移除
mMap.remove(k);
} else {
boolean isSame = false;
if (mMap.containsKey(k)) {
Object existingValue = mMap.get(k);
if (existingValue != null && existingValue.equals(v)) {
continue;
}
}
//键值对插入SharedPreferences的Map
mMap.put(k, v);
}
//设置修改位,标志已经是脏的.需要更新
mcr.changesMade = true;
if (hasListeners) {
mcr.keysModified.add(k);
}
}
//edit的Map已经处理完.可以清空.此时SharedPreferences的Map数据已经是最新的
mModified.clear();
}
}
return mcr;
}
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();
}
}
};
/.通过commit()方法调用,此处isFromSyncCommit为true
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) {
//回写
writeToDiskRunnable.run();
return;
}
}
QueuedWork.singleThreadExecutor().execute(writeToDiskRunnable);
}
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 {
//如果备份Xml文件存在,把mFile删除就可以了.
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;
}
//回写到文件
XmlUtils.writeMapXml(mcr.mapToWriteToDisk, str);
//刷新
FileUtils.sync(str);
str.close();
//设置权限ContextImpl.setFilePermissionsFromMode(mFile.getPath(), mMode, 0);
FileStatus stat = new FileStatus();
if (FileUtils.getFileStatus(mFile.getPath(), stat)) {
synchronized (this) {
mStatTimestamp = stat.mtime;
mStatSize = stat.size;
}
}
// Writing was successful, delete the backup file if there is one.
//成功后,把备份文件删除
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);
}
}
public boolean commit() {
//提交修改
MemoryCommitResult mcr = commitToMemory();
//这里就是回写函数
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;
}
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);
}
});
}
}
}SharedPreferences代码总结
SharedPreferences本质是通过Xml文件实现.使用的时候会把Xml文件解析成Map保存到内存中.要知道它会将Xml文件全部数据都解析到内存当中.免不了会占用内存.其次对SharedPreferences修改都会导致整个Xml文件再次回写.
