在技术学习的道路上,往往最常见、用的最多地方,却有着容易忽略的技术细节。某个时间点蓦然回首,才发现最应该了解和掌握的技术基础,却由于缺少总结和记录、或者是因为常态思维固化缺少场景去思考,却显得那么陌生。
这篇文章将从作者自身的角度,去重新认识SharedPreference和ContentProvider这两个控件,并且以后也会在博客中有意识的记录类似的技术细节,防止这些基础的技术细节问题再次被遗忘和忽略。
SharedPreference和ContentProvider 细节知识加固
开始前,先来自述下作者对安卓系统这两个控件以往一般的认识。
SharedPreference
- Android平台轻量级数据存储方式,底层系统封装了xml文件存储,基于key-value键值对数据。
- SharedPreference提供了多种读取模式,可支持多进程读取,但是并不保证多进程并发读写安全。
- 底层xml文件的初始化加载和Edit的apply方法是在子线程中进行。
ContentProvider
- Android平台四大组件之一,数据共享控件,多用于多个进程间的数据传递。
- 启动要优先于Application的OnCreat方法(查看Application初始化源码得之),尽量不要在App初次启动,(启动进程中)使用多个ContentProvider。
- ContentProvider进程间IPC调用,用到了Binder获取文件描述符(FileDescriptor),通过文件描述符实现了高效的共享内存,使得进程间可传递大量数据(Binder接口有1M大小限制)。
一、SharedPreference 知识加固
SharedPreference文件保存在App私有目录中,可以在root手机或者调试模式run-as进入查看。
1、SharedPreference在多进程并发读取时,数据是不安全,那单进程多个线程并发情况呢?
平时遇到SharedPreference的数据读取问题,几乎都是多进程并发场景。多进程问题项目中,基本都是封装一层ContentProvider,将数据的读写共享到其他进程,来保证数据的正确性(当然也可以自定义SharedPref的实现类,去保证多进程并发逻辑)。
那单进程情况下SharedPref多线程的并发,却是容易忽略的地方(虽然很久之前,看过SharedPref的部分源码,但是还是忘了系统是有针对并发加锁的)。
其实简单分析下,平时没有遇到单线程的并发问题,也没有针对单线程的SharedPref读写做相关锁操作,理论上应该是线程安全的,那么我们深入源码看看系统到底是如何保证并发安全的~
从ContextImpl开始去获取一个SharedPref实例:
@Override public SharedPreferences getSharedPreferences(File file, int mode) { checkMode(mode); SharedPreferencesImpl sp; //真正的系统默认实现类 synchronized (ContextImpl.class) { final ArrayMapcache = getSharedPreferencesCacheLocked(); sp = cache.get(file); if (sp == null) { sp = new SharedPreferencesImpl(file, mode); cache.put(file, sp); //首次加载缓存 return sp; } } if ((mode & Context.MODE_MULTI_PROCESS) != 0 || // 多进程模式去写,每次方法被调用时,会ReLoad一次xml文件。 // 这种机制在多进程并发严重时,数据是不安全的。 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; }复制代码
系统中默认的SharedPref实现类是SharedPreferenceImpl,找到这个类相关读写方法,会看到系统已经针对并发加类锁机制。
public Editor putLong(String key, long value) { synchronized (this) { mModified.put(key, value); return this; } } public long getLong(String key, long defValue) { synchronized (this) { awaitLoadedLocked();//注意:这里可能会wait Long v = (Long)mMap.get(key); return v != null ? v : defValue; } }复制代码
由此可见,虽然系统默认实现的SharedPreferencesImpl支持的多进程的去写模式,但并不保证数据安全。方法读取时,系统已经帮我们针对并发加类锁机制。
2、Xml文件的初始化加载和Edit的apply方法是在子线程中进行,那么是否意味着,我们可以不用考虑卡UI,去放心的加载大Shared文件和apply的频繁调用?
A、先来分析Xml的加载问题
还是从源码开始,上文中SharedPreferencesImpl的获取是从下面这段代码开始的:
private static ArrayMap> sSharedPrefsCache;//这里居然是静态的(为了读取速度) synchronized (ContextImpl.class) { final ArrayMap cache = getSharedPreferencesCacheLocked();//入口 sp = cache.get(file); if (sp == null) { sp = new SharedPreferencesImpl(file, mode);//开始加载 cache.put(file, sp); return sp; } } private ArrayMap getSharedPreferencesCacheLocked() { if (sSharedPrefsCache == null) { sSharedPrefsCache = new ArrayMap<>(); } final String packageName = getPackageName(); ArrayMap packagePrefs = sSharedPrefsCache.get(packageName); if (packagePrefs == null) { packagePrefs = new ArrayMap<>(); sSharedPrefsCache.put(packageName, packagePrefs); } return packagePrefs; }复制代码
这里看到,系统为了快速读取已经加载到内存的SharedPref文件,会将读进来的文件保存在一个静态的Map中。这样空间换时间,虽然读取速度有了一定优化,但是读到内存的对象,不会被释放。
SharedPreferencesImpl构造器中会开始加载目标file,如下:
private void startLoadFromDisk() { synchronized (this) { mLoaded = false;//是否加载完成的标记 } new Thread("SharedPreferencesImpl-load") { public void run() { loadFromDisk();// 异步加载 } }.start(); }复制代码
这里主要看mloaded标记,用来标记文件是否已经加载完成,主要用在*awaitLoadedLocked()*方法中。
private void awaitLoadedLocked() { ~~~ ··· while (!mLoaded) { try { wait(); } catch (InterruptedException unused) { } } }复制代码
而awaitLoadedLocked()方法会在每次读和保存时调用,以为着在加载过程中,如果主线程有读写操作,依然会卡UI。
So,Xml的加载本身虽然不卡UI,但是读取操作会等待加载过程,依然有可能卡UI的。并且SharedPref尽量分类去保存,避免一次加载很大的无用文件到内存(可能引起GC,依然会卡UI),耗时又耗内存。
B、Apply方法是否会引起卡顿?
Show me the code
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);//关键:新加一个任务到队列 Runnable postWriteRunnable = new Runnable() { public void run() { awaitCommit.run(); QueuedWork.remove(awaitCommit);//当保存任务执行后会从队列中移除 } }; SharedPreferencesImpl.this.enqueueDiskWrite(mcr, postWriteRunnable);//在线程中执行 // 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); } private void enqueueDiskWrite(final MemoryCommitResult mcr, final Runnable postWriteRunnable) { final Runnable writeToDiskRunnable = new Runnable() { public void run() { synchronized (mWritingToDiskLock) {//支持并发 writeToFile(mcr);//真正的写入文件 } synchronized (SharedPreferencesImpl.this) { mDiskWritesInFlight--; } if (postWriteRunnable != null) { postWriteRunnable.run(); } } }; 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) { writeToDiskRunnable.run(); return; } } QueuedWork.singleThreadExecutor().execute(writeToDiskRunnable);//单线程池中执行 }复制代码
然后只到发现Activity的一段代码()
private void handleStopActivity(IBinder token, boolean show, int configChanges, int seq) { ··· // Make sure any pending writes are now committed. if (!r.isPreHoneycomb()) { QueuedWork.waitToFinish(); } ··· } public static void waitToFinish() { Runnable toFinish; while ((toFinish = sPendingWorkFinishers.poll()) != null) { toFinish.run(); }}复制代码
如果Activity在stop时,apply到QueuedWork中任务未执行完,就会在引起主线的等待造成卡顿。
所以,尽量不要频繁的调用apply,可以将多个提交合成一个apply,并且注意规避activity stop时候的SharedPref保存逻辑,避免卡顿。
二、ContentProvider 知识加固
1、ContentProvider为多进程数据的读取提供类机制,那么单进程下使用ContentProvider,能保证线程并发安全吗?
平时项目中常用到ContentProvider的地方见的最多的可能就是DB的和SharedPreference的数据封装类,由于Sqlite和SharedPref都是自身可以支持线程并发问题的,并没有注意到ContentProvider自身方法的并发特性。
提高ContentProvider,工程中更多的注意力放在类多进程的交互方式上,而忽略了自身方法调用的细节问题。
ContentResolver resolver = content.getContentResolver();复制代码
通常,在使用ContentProvider时,会通过以上代码去获取ContentResolver引用,然后调用相关接口方法。
private final ApplicationContentResolver mContentResolver; @Override public ContentResolver getContentResolver() { return mContentResolver; }复制代码
同样,接口方法依然在ContextImpl中,可以看到ContentResolver的实现类是ApplicationContentResolver。 这里以query方法为例,深入分析下:
public final @Nullable Cursor query(final @RequiresPermission.Read @NonNull Uri uri, @Nullable String[] projection, @Nullable String selection, @Nullable String[] selectionArgs, @Nullable String sortOrder, @Nullable CancellationSignal cancellationSignal) { IContentProvider unstableProvider = acquireUnstableProvider(uri);//抽象方法,ApplicationContentResolver中实现 if (unstableProvider == null) { return null; } IContentProvider stableProvider = null; Cursor qCursor = null; try { long startTime = SystemClock.uptimeMillis(); ICancellationSignal remoteCancellationSignal = null; if (cancellationSignal != null) { cancellationSignal.throwIfCanceled(); remoteCancellationSignal = unstableProvider.createCancellationSignal(); cancellationSignal.setRemote(remoteCancellationSignal); } try { qCursor = unstableProvider.query(mPackageName, uri, projection,//缓存Binder处理 selection, selectionArgs, sortOrder, remoteCancellationSignal); } catch (DeadObjectException e) { // The remote process has died... but we only hold an unstable // reference though, so we might recover!!! Let's try!!!! // This is exciting!!1!!1!!!!1 unstableProviderDied(unstableProvider);//Provider进程死掉逻辑处理 stableProvider = acquireProvider(uri);//抽象方法,ApplicationContentResolver中实现 if (stableProvider == null) { return null; } qCursor = stableProvider.query(mPackageName, uri, projection,//再次query selection, selectionArgs, sortOrder, remoteCancellationSignal); } if (qCursor == null) { return null; } return wrapper; } catch (RemoteException e) { // Arbitrary and not worth documenting, as Activity // Manager will kill this process shortly anyway. return null; } finally { //··· } }复制代码
主要跟踪下IContentProvider接口如何在acquireProvider和acquireUnstableProvider中的实现,最终会委托给ActivityThread的acquireProvider方法,如下:
public final IContentProvider acquireProvider( Context c, String auth, int userId, boolean stable) { final IContentProvider provider = acquireExistingProvider(c, auth, userId, stable);//缓存中读取 if (provider != null) { return provider; } IActivityManager.ContentProviderHolder holder = null; try { holder = ActivityManagerNative.getDefault().getContentProvider( getApplicationThread(), auth, userId, stable);//缓存没有读到,则跨进程AMS中读取 } catch (RemoteException ex) { } if (holder == null) { Slog.e(TAG, "Failed to find provider info for " + auth); return null; } holder = installProvider(c, holder, holder.info, true /*noisy*/, holder.noReleaseNeeded, stable);//安装 return holder.provider; }复制代码
方法的最后一个参数stable代表着ContentProvider能否可靠拿到一个进程存活的Provider。首先调用了acquireExistingProvider,去mProviderMap中获取一次接口。
public final IContentProvider acquireExistingProvider( Context c, String auth, int userId, boolean stable) { synchronized (mProviderMap) { final ProviderKey key = new ProviderKey(auth, userId); final ProviderClientRecord pr = mProviderMap.get(key);//从缓存中先读取 if (pr == null) { return null; } IContentProvider provider = pr.mProvider; IBinder jBinder = provider.asBinder(); if (!jBinder.isBinderAlive()) { // The hosting process of the provider has died; we can't // use this one. Log.i(TAG, "Acquiring provider " + auth + " for user " + userId + ": existing object's process dead"); handleUnstableProviderDiedLocked(jBinder, true); return null; } // Only increment the ref count if we have one. If we don't then the // provider is not reference counted and never needs to be released. ProviderRefCount prc = mProviderRefCountMap.get(jBinder); if (prc != null) { incProviderRefLocked(prc, stable); } return provider; } }复制代码
mProviderMap中,这个map的类型是ArrayMap,那么mProviderMap中的数据是哪里的呢?
查下mProvider的put方法,只有installProviderAuthoritiesLocked中调用,并且它是在又是在installProvider的调用的,那么重的分析下installProvider方法。
installProvider方法主要会在handleBindApplication和acquireProvider方法中调用,意味着:本进程和AMS获取到的Provider都会调用installProvider最后会放到mProviderMap缓存中,
private IActivityManager.ContentProviderHolder installProvider(Context context, IActivityManager.ContentProviderHolder holder, ProviderInfo info, boolean noisy, boolean noReleaseNeeded, boolean stable) { ContentProvider localProvider = null; IContentProvider provider; if (holder == null || holder.provider == null) { //本地进程provider安装 try { final java.lang.ClassLoader cl = c.getClassLoader(); localProvider = (ContentProvider)cl. loadClass(info.name).newInstance(); provider = localProvider.getIContentProvider(); if (provider == null) { Slog.e(TAG, "Failed to instantiate class " + info.name + " from sourceDir " + info.applicationInfo.sourceDir); return null; } localProvider.attachInfo(c, info); } catch (java.lang.Exception e) { return null; } } else { provider = holder.provider; //跨进程 } IActivityManager.ContentProviderHolder retHolder; synchronized (mProviderMap) { //获取可跨进程的Binder对象 IBinder jBinder = provider.asBinder(); if (localProvider != null) { ComponentName cname = new ComponentName(info.packageName, info.name); ProviderClientRecord pr = mLocalProvidersByName.get(cname); if (pr != null) { provider = pr.mProvider; } else { holder = new IActivityManager.ContentProviderHolder(info); holder.provider = provider; holder.noReleaseNeeded = true; pr = installProviderAuthoritiesLocked(provider, localProvider, holder); mLocalProviders.put(jBinder, pr); mLocalProvidersByName.put(cname, pr); } retHolder = pr.mHolder; } else { ProviderRefCount prc = mProviderRefCountMap.get(jBinder); if (prc != null) { if (!noReleaseNeeded) { incProviderRefLocked(prc, stable); try { ActivityManagerNative.getDefault().removeContentProvider( holder.connection, stable); } catch (RemoteException e) { //do nothing content provider object is dead any way } } } else { ProviderClientRecord client = installProviderAuthoritiesLocked( provider, localProvider, holder); if (noReleaseNeeded) { prc = new ProviderRefCount(holder, client, 1000, 1000); } else { prc = stable ? new ProviderRefCount(holder, client, 1, 0) : new ProviderRefCount(holder, client, 0, 1); } mProviderRefCountMap.put(jBinder, prc); } retHolder = prc.holder; } } return retHolder; }复制代码
分析到这里可以看出,如何Provider是在同一个进程,会通过Binder进口先查下本地对象,然后返回本地Provider实例。
回到起初的query方法,就是直接调用目标对象的query,然后系统对query方法的调用并没有做并发处理,所以可得出结论,ContentProvider的方法调用是非并发安全的。
2、ContentProvider启动时机?
上文分析中在App进程启动时,通过thread.attach()方法会调用到handleBindApplication,方法内初始化当前进程的application和provider。
private void handleBindApplication(AppBindData data) { //··· final StrictMode.ThreadPolicy savedPolicy = StrictMode.allowThreadDiskWrites(); try { // If the app is being launched for full backup or restore, bring it up in // a restricted environment with the base application class. Application app = data.info.makeApplication(data.restrictedBackupMode, null);//注:这里第二个参数是null,意味着Application只会调用attachBaseContext方法 mInitialApplication = app; // don't bring up providers in restricted mode; they may depend on the // app's custom Application class if (!data.restrictedBackupMode) { if (!ArrayUtils.isEmpty(data.providers)) { installContentProviders(app, data.providers); //本进程内Provider的启动安装 // For process that contains content providers, we want to // ensure that the JIT is enabled "at some point". mH.sendEmptyMessageDelayed(H.ENABLE_JIT, 10*1000); } } try { mInstrumentation.callApplicationOnCreate(app);// Provider启动后才会继续回调Application的onCreate } catch (Exception e) { } } finally { StrictMode.setThreadPolicy(savedPolicy); } }复制代码
由此可见,本进程的Provider的安装是在Application初始化调用attact方法后,调用onCreate前启动。所以在App启动时注意规避本进程内的Provider,避免启动耗时。
三、总结
技术成长的道路上,不要忽略基础知识的积累,特别是一些细节问题,是不同阶段程序员修炼内功需要坚持的习惯,也是深入技术研究的基础之路。
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