一、概述
SharedPreferences简称SP,我相信大家对它很熟悉，开发时经常会用到。它主要的用途是以key-value(键值对)的形式存储一些轻量级的数据(比如应用的配置信息等),数据的保存格式为xml。
1.1 SP的特性/使用规范
a、只能存储少量数据
b、不要频繁调用edit()方法/不要频繁实例化Editor
c、commit()运行在主线程，用于同步提交数据，可以拿到返回结果；apply()运行在子线程，用于异步提交数据，不会返回数据提交结果。
了解了SP的特性后，我们可以在它的特性前加个“why”，然后带着这些疑问去探索它的源码。
二、SharedPreferences源码分析
2.1 初始化SharedPreferences
我们使用SP时，初始化方式如下：
//方式一
SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(context);
//方式二
SharedPreferences sharedPreferences = getSharedPreferences("test", MODE_PRIVATE);
//方式三
......
上面的初始化方式最终都是调用new SharedPreferencesImpl(file,model)方法来进行实例化的，因此我们可直接查看SharedPreferencesImpl的源码。SharedPreferences是接口，SharedPreferencesImpl是它的实现类
final class SharedPreferencesImpl implements SharedPreferences {
@GuardedBy("mLock")
private boolean mLoaded = false;//标记是否已读取SP文件(是否已加载到内存)
//构造方法
SharedPreferencesImpl(File file, int mode) {
mFile = file;
mBackupFile = makeBackupFile(file);
mMode = mode;
mLoaded = false;
mMap = null;
mThrowable = null;
//从磁盘读取sp文件
startLoadFromDisk();
}
private void startLoadFromDisk() {
synchronized (mLock) {
mLoaded = false;
}
//创建线程从磁盘加载sp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");
}
//用于存放从sp中读取的数据
Map<String, Object> map = null;
StructStat stat = null;
Throwable thrown = null;
try {
stat = Os.stat(mFile.getPath());
if (mFile.canRead()) {
BufferedInputStream str = null;
try {
str = new BufferedInputStream(
new FileInputStream(mFile), 16 * 1024);
//xml文件解析
map = (Map<String, Object>) XmlUtils.readMapXml(str);
} catch (Exception e) {
Log.w(TAG, "Cannot read " + mFile.getAbsolutePath(), e);
} finally {
IoUtils.closeQuietly(str);
}
}
} catch (ErrnoException e) {
// An errno exception means the stat failed. Treat as empty/non-existing by
// ignoring.
} catch (Throwable t) {
thrown = t;
}
synchronized (mLock) {
mLoaded = true;
mThrowable = thrown;
// It's important that we always signal waiters, even if we'll make
// them fail with an exception. The try-finally is pretty wide, but
// better safe than sorry.
try {
if (thrown == null) {
if (map != null) {
mMap = map;
mStatTimestamp = stat.st_mtim;//最近一次更改时间
mStatSize = stat.st_size;//文件大小
} else {
mMap = new HashMap<>();
}
}
// In case of a thrown exception, we retain the old map. That allows
// any open editors to commit and store updates.
} catch (Throwable t) {
mThrowable = t;
} finally {
//唤醒处于等待状态的线程
mLock.notifyAll();
}
}
}
}
从上述源码中(尤其是我加注释的地方)我们可以得到如下2点结论：
- 数据以xml格式保存
- 当实例化SP时系统会将SP的内容全部读取到内存中，这也是sp为什么只能存储少量数据的原因
另外我们注意到系统是异步(开了新线程)加载sp到内存的，那我们在使用getxxx()、setxxx()以及edit()时并没有判断sp是否已加载到内存，那为什么从没报过错呢？难道是运气？答案是否定的，因为这些方法是阻塞等待的(详见2.2小节)，直到sp被加载后内存后调用mLock.notifyAll()来唤醒它们时，它们才会操作内存
2.2 获取数据 getXXX()
SP的获取方式笔者就不赘述了，此处以getString()方法为例进行分析
@Override
@Nullable
public String getString(String key, @Nullable String defValue) {
synchronized (mLock) {
awaitLoadedLocked();//检测SP是否加载到内存
String v = (String)mMap.get(key);
return v != null ? v : defValue;
}
}
@GuardedBy("mLock")
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 {
mLock.wait();//SP未加载到内存时则进入等待状态
} catch (InterruptedException unused) {
}
}
if (mThrowable != null) {
throw new IllegalStateException(mThrowable);
}
}
三、Editor 源码分析
3.1 实例化Editor
我们在实例化Editor时一般都是调用SharedPreferences的edit()方法，下面我们先来看一下该源码：
@Override
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 (mLock) {
awaitLoadedLocked();//判断sp是否加载到内存
}
return new EditorImpl();//
}
从上面可以看出实例化Editor时返回的是EditorImpl对象，EditorImpl是SharedPreferences的内部类，是Editor的实现类。
从这里我们也可以看出不建议多次调用edit()方法的原因，因未每调用一次就会new一个对象，太频繁的话会引起内存抖动，严重的话还会OOM
3.2 插入数据 putxxx()
此处以putString()为例
public final class EditorImpl implements Editor {
private final Object mEditorLock = new Object();
@GuardedBy("mEditorLock")
private final Map<String, Object> mModified = new HashMap<>();
@GuardedBy("mEditorLock")
private boolean mClear = false;
@Override
public Editor putString(String key, @Nullable String value) {
synchronized (mEditorLock) {
mModified.put(key, value);//将数据存放到mModified
return this;
}
}
}
从上面我们可以看到putxxx()方法只是将数据存放到mModified中，所以才会有下面的提交方法
3.2 提交数据 commit()/apply()
3.2.1 commit()
同步提交，有返回结果
public final class EditorImpl implements Editor {
private final Object mEditorLock = new Object();
@GuardedBy("mEditorLock")
private final Map<String, Object> mModified = new HashMap<>();
@GuardedBy("mEditorLock")
private boolean mClear = false;
@Override
public boolean commit() {
long startTime = 0;
if (DEBUG) {
startTime = System.currentTimeMillis();
}
MemoryCommitResult mcr = commitToMemory();//将数据提交到内存
//将内存数据同步到文件
SharedPreferencesImpl.this.enqueueDiskWrite(
mcr, null /* sync write on this thread okay */);
try {
mcr.writtenToDiskLatch.await();//进入等待状态, 直到写入文件的操作完成
} catch (InterruptedException e) {
return false;
} finally {
if (DEBUG) {
Log.d(TAG, mFile.getName() + ":" + mcr.memoryStateGeneration
+ " committed after " + (System.currentTimeMillis() - startTime)
+ " ms");
}
}
//通知监听者, 并在主线程回调onSharedPreferenceChanged()方法
notifyListeners(mcr);
return mcr.writeToDiskResult;// 返回提交结果
}
//将数据提交到内存
private MemoryCommitResult commitToMemory() {
long memoryStateGeneration;
List<String> keysModified = null;
Set<OnSharedPreferenceChangeListener> listeners = null;
Map<String, Object> mapToWriteToDisk; //此map中的所有数据都会写入磁盘
synchronized (SharedPreferencesImpl.this.mLock) {
// We optimistically don't make a deep copy until
// a memory commit comes in when we're already
// writing to disk.
//mDiskWritesInFlight表示当前有多少个正在进行的写入磁盘的操作，如果mDiskWritesInFlight>0，表示在前面有某个写操作正把 mMap 的内容写到硬盘。此时我们不能直接修改 mMap，否则硬盘的数据的一致性会有问题（比方说，部分 key 是旧的，部分是新的）。拷贝一份 mMap 后，我们就可以安全地进行修改了。
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);//构造一个与mMap 具有相同映射的 HashMap
}
//将mMap赋值给mapToWriteToDisk,即浅拷贝(其中一个数据发生变化后另一个数据也会发生相应的改变)，因此每次提交数据时sp会将所有//的数据全部更新一次，因此sp中的数据越多，sp提交数据所需时间会越久
mapToWriteToDisk = mMap;
mDiskWritesInFlight++;
boolean hasListeners = mListeners.size() > 0;
if (hasListeners) {
keysModified = new ArrayList<String>();
listeners = new HashSet<OnSharedPreferenceChangeListener>(mListeners.keySet());
}
synchronized (mEditorLock) {
boolean changesMade = false;
if (mClear) {//如果mClear为true,则清空mapToWriteToDisk
if (!mapToWriteToDisk.isEmpty()) {
changesMade = true;
mapToWriteToDisk.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.
//删除特殊数据
if (v == this || v == null) {
if (!mapToWriteToDisk.containsKey(k)) {
continue;
}
mapToWriteToDisk.remove(k);
} else {
//判断数据是否发生变化
if (mapToWriteToDisk.containsKey(k)) {
Object existingValue = mapToWriteToDisk.get(k);
if (existingValue != null && existingValue.equals(v)) {
continue;//数据没发生更改则跳出此次循环
}
}
//更新数据
mapToWriteToDisk.put(k, v);
}
//标记数据是否变化
changesMade = true;
if (hasListeners) {
keysModified.add(k);
}
}
mModified.clear();//清空EditorImpl中的mModified数据
if (changesMade) {
//数据发生更改后将此数加加，后文写入文件时会用到
mCurrentMemoryStateGeneration++;
}
memoryStateGeneration = mCurrentMemoryStateGeneration;
}
}
return new MemoryCommitResult(memoryStateGeneration, keysModified, listeners,
mapToWriteToDisk);
}
//启动磁盘写入任务
private void enqueueDiskWrite(final MemoryCommitResult mcr,
final Runnable postWriteRunnable) {
//因为由commit()方法进入，postWriteRunnable为null
final boolean isFromSyncCommit = (postWriteRunnable == null);
final Runnable writeToDiskRunnable = new Runnable() {
@Override
public void run() {
synchronized (mWritingToDiskLock) {
//写入磁盘
writeToFile(mcr, isFromSyncCommit);
}
synchronized (mLock) {
mDiskWritesInFlight--;
}
if (postWriteRunnable != null) {
postWriteRunnable.run();
}
}
};
// commit()方法会进入此判断
if (isFromSyncCommit) {
boolean wasEmpty = false;
synchronized (mLock) {
//commitToMemory过程会加1,则wasEmpty=true
wasEmpty = mDiskWritesInFlight == 1;
}
if (wasEmpty) {
//调用新线程进行文件写入
writeToDiskRunnable.run();
return;
}
}
//此处不会执行，因为上面调用了return
QueuedWork.queue(writeToDiskRunnable, !isFromSyncCommit);
}
//文件写入
@GuardedBy("mWritingToDiskLock")
private void writeToFile(MemoryCommitResult mcr, boolean isFromSyncCommit) {
long startTime = 0;
long existsTime = 0;
long backupExistsTime = 0;
long outputStreamCreateTime = 0;
long writeTime = 0;
long fsyncTime = 0;
long setPermTime = 0;
long fstatTime = 0;
long deleteTime = 0;
if (DEBUG) {
startTime = System.currentTimeMillis();
}
boolean fileExists = mFile.exists();
if (DEBUG) {
existsTime = System.currentTimeMillis();
// Might not be set, hence init them to a default value
backupExistsTime = existsTime;
}
// Rename the current file so it may be used as a backup during the next read
if (fileExists) {
boolean needsWrite = false;
// Only need to write if the disk state is older than this commit
//判断数据是否需要写入
if (mDiskStateGeneration < mcr.memoryStateGeneration) {
if (isFromSyncCommit) {
needsWrite = true;
} else {
synchronized (mLock) {
// No need to persist intermediate states. Just wait for the latest state to
// be persisted.
//apply()会执行此处代码，3.2.2小节会介绍apply()方法会延迟100毫秒后才执行写入操作，延迟的目的是为了避免不必要的写//入操作，例：如果我们频繁调用apply()且间隔时间小于100毫秒，那么只有最后一次的apply()操作才满足此处的条件，才会将//needsWrite设为true
if (mCurrentMemoryStateGeneration == mcr.memoryStateGeneration) {
needsWrite = true;
}
}
}
}
if (!needsWrite) {
mcr.setDiskWriteResult(false, true);
return;
}
boolean backupFileExists = mBackupFile.exists();
if (DEBUG) {
backupExistsTime = System.currentTimeMillis();
}
if (!backupFileExists) {
if (!mFile.renameTo(mBackupFile)) {
Log.e(TAG, "Couldn't rename file " + mFile
+ " to backup file " + mBackupFile);
mcr.setDiskWriteResult(false, 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 (DEBUG) {
outputStreamCreateTime = System.currentTimeMillis();
}
if (str == null) {
mcr.setDiskWriteResult(false, false);
return;
}
//写入数据
XmlUtils.writeMapXml(mcr.mapToWriteToDisk, str);
writeTime = System.currentTimeMillis();
FileUtils.sync(str);
fsyncTime = System.currentTimeMillis();
str.close();
ContextImpl.setFilePermissionsFromMode(mFile.getPath(), mMode, 0);
if (DEBUG) {
setPermTime = System.currentTimeMillis();
}
try {
final StructStat stat = Os.stat(mFile.getPath());
synchronized (mLock) {
mStatTimestamp = stat.st_mtim;
mStatSize = stat.st_size;
}
} catch (ErrnoException e) {
// Do nothing
}
if (DEBUG) {
fstatTime = System.currentTimeMillis();
}
// Writing was successful, delete the backup file if there is one.
mBackupFile.delete();
if (DEBUG) {
deleteTime = System.currentTimeMillis();
}
mDiskStateGeneration = mcr.memoryStateGeneration;
//返回写入成功并唤醒等待线程
mcr.setDiskWriteResult(true, true);
if (DEBUG) {
Log.d(TAG, "write: " + (existsTime - startTime) + "/"
+ (backupExistsTime - startTime) + "/"
+ (outputStreamCreateTime - startTime) + "/"
+ (writeTime - startTime) + "/"
+ (fsyncTime - startTime) + "/"
+ (setPermTime - startTime) + "/"
+ (fstatTime - startTime) + "/"
+ (deleteTime - startTime));
}
long fsyncDuration = fsyncTime - writeTime;
mSyncTimes.add((int) fsyncDuration);
mNumSync++;
if (DEBUG || mNumSync % 1024 == 0 || fsyncDuration > MAX_FSYNC_DURATION_MILLIS) {
mSyncTimes.log(TAG, "Time required to fsync " + mFile + ": ");
}
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);
}
}
//返回写入失败并唤醒等待线程
mcr.setDiskWriteResult(false, false);
}
}
3.2.2 apply()
异步提交数据，没有返回结果
public final class EditorImpl implements Editor {
private final Object mEditorLock = new Object();
@GuardedBy("mEditorLock")
private final Map<String, Object> mModified = new HashMap<>();
@GuardedBy("mEditorLock")
private boolean mClear = false;
@Override
public void apply() {
final long startTime = System.currentTimeMillis();
//提交到内存 详见3.2.1
final MemoryCommitResult mcr = commitToMemory();
final Runnable awaitCommit = new Runnable() {
@Override
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");
}
}
};
//将awaitCommit加入到排队队列中
QueuedWork.addFinisher(awaitCommit);
Runnable postWriteRunnable = new Runnable() {
@Override
public void run() {
awaitCommit.run();
//awaitCommit执行完成后被移除
QueuedWork.removeFinisher(awaitCommit);
}
};
//异步写入磁盘
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);
}
//启动磁盘写入任务
private void enqueueDiskWrite(final MemoryCommitResult mcr,
final Runnable postWriteRunnable) {
//因为由apply()方法进入，postWriteRunnable不为null
final boolean isFromSyncCommit = (postWriteRunnable == null);
final Runnable writeToDiskRunnable = new Runnable() {
@Override
public void run() {
synchronized (mWritingToDiskLock) {
//写入磁盘
writeToFile(mcr, isFromSyncCommit);
}
synchronized (mLock) {
mDiskWritesInFlight--;
}
if (postWriteRunnable != null) {
postWriteRunnable.run();
}
}
};
// apply()方法不会进入此判断
if (isFromSyncCommit) {
boolean wasEmpty = false;
synchronized (mLock) {
wasEmpty = mDiskWritesInFlight == 1;
}
if (wasEmpty) {
writeToDiskRunnable.run();
return;
}
}
//将写入磁盘的任务交给工作队列负责
QueuedWork.queue(writeToDiskRunnable, !isFromSyncCommit);
}
}
QueuedWork 是一个全局的工作队列，addFinisher 添加进去的 runnable 不会立即执行，仅仅是放到一个链表里。当某些操作需要等待 QueuedWork 所有工作执行完毕时，就调用 QueuedWork.waitToFinish()，在这个方法里面会取出之前添加的所有 addFinisher 的任务，一个一个执行。
在 awaitCommit 里面，我们调用了 mcr.writtenToDiskLatch.await() 来等待数据写入硬盘，所以这里的把 awaitCommit 放到 QueuedWork 里，就提供了一种机制，让外界等待文件的写入操作。读者可以到 ActivityThread 中搜一下 QueuedWork.waitToFinish，会发现在 activity/service stop 的时候，都会执行这个操作，从而保证在应用退出前 SP 已经写入硬盘。
QueuedWork相关源码：
public class QueuedWork {
private static final long DELAY = 100;
@GuardedBy("sLock")
private static boolean sCanDelay = true;
public static void queue(Runnable work, boolean shouldDelay) {
Handler handler = getHandler();//getHandler返回的是一个HandlerThread对象
synchronized (sLock) {
sWork.add(work);
//因为是由apply()方法进入，因此shouldDelay为true
if (shouldDelay && sCanDelay) {
//默认延迟100毫秒后处理写入操作
handler.sendEmptyMessageDelayed(QueuedWorkHandler.MSG_RUN, DELAY);
} else {
handler.sendEmptyMessage(QueuedWorkHandler.MSG_RUN);
}
}
}
private static Handler getHandler() {
synchronized (sLock) {
if (sHandler == null) {
HandlerThread handlerThread = new HandlerThread("queued-work-looper",
Process.THREAD_PRIORITY_FOREGROUND);
handlerThread.start();
sHandler = new QueuedWorkHandler(handlerThread.getLooper());
}
return sHandler;
}
}
}
友情提示：代码中的注释均为重点
四、总结
至此，SP的源码已阅读完毕，我想前文中所提示的3个疑问应该都已找到答案，我们在这里再进行一些简单的总结：
1、 SP只能存储少量数据
2、不要频繁调用edit()方法/不要频繁实例化Editor
3、commit()运行在主线程，用于同步提交数据，可以拿到返回结果；
apply()运行在子线程，用于异步提交数据，不会返回数据提交结果。
另外数据每次提交时都会将之前所有的数据重新写入，因此一定不要存储大量数据
4、对于一些需要频繁写入的场景推荐使用Tencent的MMKV组件，它借助mmap提高了IO效率并且可以实现增量更新操作
点此查看MMVK
