sharedpreference实现源代码走读
1、sharedpreference的使用：
SharedPreferences sh = getSharedPreferences("shared.xml",0);
int testValue = sh.getInt("testValue",0);
SharedPreferences.Editor ed = sh.edit();
ed.putInt("testValue",9);
ed.commit();
使用非常简单，是android中常用的保存本地数据的方式，其中数据多以键值对形式保存于本地的xml文件中，因sharedpreference实现原理中涉及文件操作，已经会在内存中保留一份数据键值对列表，因此，使用sharedpreference不建议保存数据量较大的数据，比如超长字符串，超大json，也不建议调用commit过于频繁。
2、文件获取方式：一个应用可同时打开多个sharedpreference文件，保存在sSharedPrefsCache中，如果没有，则新建。
private static ArrayMap<String, ArrayMap<File, SharedPreferencesImpl>> sSharedPrefsCache;
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;
}
3、新建SharedPreferencesImpl，
新建.bak备份文件，将问价中的键值对加载到内存中保存在mMap中，mLoaded设置为true。
SharedPreferencesImpl(File file, int mode) {
mFile = file;
mBackupFile = makeBackupFile(file);
mMode = mode;
mLoaded = false;
mMap = null;
startLoadFromDisk();
}
static File makeBackupFile(File prefsFile) {
return new File(prefsFile.getPath() + ".bak");
}
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 {
//将本地xml文件中的数据读取到mMap列表中
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) {
//将从文件读取出来的数据列表赋值到全局变量mMap中，并且设置xml文件大小和最后修改时间，mLoaded设置为true，这里以后会用到。
mLoaded = true;
if (map != null) {
mMap = map;
mStatTimestamp = stat.st_mtime;
mStatSize = stat.st_size;
} else {
mMap = new HashMap<>();
}
notifyAll();
}
}
4、数据读取, 以getInt为例：
public int getInt(String key, int defValue) {
synchronized (this) {
awaitLoadedLocked();
//直接从内存中的mMap变量获取
Integer v = (Integer)mMap.get(key);
return v != null ? v : defValue;
}
}
private void awaitLoadedLocked() {
//检测mLoaded是否为false，如果是false则表示当前还在读取本地xml文件的过程中，会一直等待，知道设置为true，为true表示本地文件已经读取完成。
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) {
}
}
}
5、数据存储，以putInt为例，忽略其他设置类型代码：
public Editor edit() {
// 一样的需要先等待本地xml文件读取完成才能执行
synchronized (this) {
awaitLoadedLocked();
}
return new EditorImpl();
}
public final class EditorImpl implements Editor {
//新建一列表用于暂时存储写入的键值数据对
private final Map<String, Object> mModified = Maps.newHashMap();
//是否清除标记
private boolean mClear = false;
public Editor putInt(String key, int value) {
synchronized (this) {
mModified.put(key, value);
return this;
}
}
public Editor remove(String key) {
synchronized (this) {
mModified.put(key, this);
return this;
}
}
public Editor clear() {
synchronized (this) {
mClear = true;
return this;
}
}
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);
}
// 复制mMap中的数据，将其赋值给mapToWriteToDisk，如果clear为true，则清空mMap，之后将mModified中的数据与mMap中数据对比，如果value为空或者this魔数，则清空mMap中的对应数据，否则将数据添加到mMap中去。
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;
}
public boolean commit() {
MemoryCommitResult mcr = commitToMemory();
//启动线程操作mMap中数据，将其写入到本地文件中
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;
}
}
6、文件写入：
commit则直接启动写入线程，apply则将线程交给单例线程池，依次进行
private void enqueueDiskWrite(final MemoryCommitResult mcr,
final Runnable postWriteRunnable) {
final Runnable writeToDiskRunnable = new Runnable() {
public void run() {
synchronized (mWritingToDiskLock) {
//写入文件方法
writeToFile(mcr);
}
synchronized (SharedPreferencesImpl.this) {
mDiskWritesInFlight--;
}
if (postWriteRunnable != null) {
postWriteRunnable.run();
}
}
};
//commit方法适合该值为true
final boolean isFromSyncCommit = (postWriteRunnable == null);
if (isFromSyncCommit) {
boolean wasEmpty = false;
synchronized (SharedPreferencesImpl.this) {
//mDiskWritesInFlight初始值为0，在commitToMemory中++，若首次进入则该处返回true。这样可以防止多个线程同时对文件进行操作
wasEmpty = mDiskWritesInFlight == 1;
}
//如当前并没有线程在写入文件，就启动线程写入文件
if (wasEmpty) {
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 {
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;
}
//将数据写入xml文件中
XmlUtils.writeMapXml(mcr.mapToWriteToDisk, str);
FileUtils.sync(str);
str.close();
//重新设置文件权限
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);
}