[学习笔记]Android开发艺术探索：四大组件的工作过程之ContentProvider
ContentProvider是一个内存共享型组件，他通过Binder向其他组件乃至其他应用提供数据，当ContentProvider所在的进程启动的时候，ContentProvider会同时启动并且发布到AMS中，需要注意的是，这个时候ContentProvider的onCreate要先于Application的onCreate执行，这是四大组件一个少有的现象一个应用启动的时候，入口方法在Ac.
ContentProvider是一个内存共享型组件，他通过Binder向其他组件乃至其他应用提供数据，当ContentProvider所在的进程启动的时候，ContentProvider会同时启动并且发布到AMS中，需要注意的是，这个时候ContentProvider的onCreate要先于Application的onCreate执行，这是四大组件一个少有的现象
一个应用启动的时候，入口方法在ActivityThread中的main里，main是一个静态方法，在main中会创建ActivityThread的实例并创建主线程的消息队列，然后在ActivityThread的attach中远程调用AMS的attachApplication方法并将ActivityThread提供给AMS，ActivityThread是一个Binder对象，他的Binder接口是IApplicationThread，他主要用于ActivityThread和AMS之间的通信，会调用ApplicationThread的bindApplication方法，注意这个过程同样是跨进程完成的，bindApplication的逻辑是经过ActivityThread中的mH Hander切换到ActivityThread执行，具体是方法是handlerBindApplication,在handlerApplication方法中，ActivityThread会加载一个ContentProvider，然后再调用Application的onCreate。
ContentProvider启动后，外界就可以通过他所提供的增删改查四个接口来操作ContentProvider的数据源，insert,delete,update,query，这四个方法都是通过Binder来调用的，外界无法直接访问ContentProvider，他只能通过AMS提供的Uri来获取对应的ContentProvider的Binder接口和IContentProvider，然后通过IContentProvider来访问ContentProvider的数据源。
一般来说，ContentProvider都应该是单实例的，ContentProvider到底是不是单实例的，这是由他的android:multiprocess来决定的，当他为false的时候为多实例，默认为true，在实际的开发当中，我们并没有发现多实例的案例，官方文档的解释是避免进程间通信的开销，但是在实际开发中仍然缺少使用价值，因此我们可以简单认为ContentProvider就是单实例，接下来我们来分析下ContentProvider的启动过程
党文ContentProvider需要通过ContentResolver，ContentResolver是一个抽象类，通过Context的getContentResolver方法获取的实际上是ApplicationContentResolver对象，当ContentProvider所在的进程未启动的时候，第一次访问他时就会触发ContentProvider的创建，这也就伴随着ContentProvider所在的进程启动，通过ContentProvider的第四个方法的任何一个都能启动，这里我们通过query来讲解
ContentProvider的query方法，首先获取的IContentProvider对象，不管是通过acquireUnstableProvider方法还是直接通过acquireProvider方法，他们的本质都是一样的，最终都是通过acquireProvider的方法来获取ContentProvider，下面是ApplicationContentProvider的acquireProvider具体的实现方式：
protected IContentProvider acquireProvider(Context context,String auth){
return mMainThread.acquireProvider(context,
ContentResolver.getAuthorityWithoutUserId(auth),
resolveUserIdFromAuthority(auth),true);
}
ApplicationContentResolver的acquireProvider方法并没有处理任何逻辑，他直接调用了ActivityThread的acquireProvider方法，acquireProvider的这个方法源码如下
public final IContentProvider acquireProvider(
Context c, String auth, int userId, boolean stable) {
final IContentProvider provider = acquireExistingProvider(c, auth, userId, stable);
if (provider != null) {
return provider;
}
// There is a possible race here. Another thread may try to acquire
// the same provider at the same time. When this happens, we want to ensure
// that the first one wins.
// Note that we cannot hold the lock while acquiring and installing the
// provider since it might take a long time to run and it could also potentially
// be re-entrant in the case where the provider is in the same process.
IActivityManager.ContentProviderHolder holder = null;
try {
holder = ActivityManagerNative.getDefault().getContentProvider(
getApplicationThread(), auth, userId, stable);
} catch (RemoteException ex) {
}
if (holder == null) {
Slog.e(TAG, "Failed to find provider info for " + auth);
return null;
}
// Install provider will increment the reference count for us, and break
// any ties in the race.
holder = installProvider(c, holder, holder.info,
true /*noisy*/, holder.noReleaseNeeded, stable);
return holder.provider;
}
上面的代码首先会从ActivityThread中超找是否已经存在目标的ContentProvider，如果存在就直接返回，ActivityThread中通过mProviderMap来存储已经启动的ContentProvider
// The lock of mProviderMap protects the following variables.
final ArrayMap<ProviderKey, ProviderClientRecord> mProviderMap
= new ArrayMap<ProviderKey, ProviderClientRecord>();
如果目前ContentProvider并没有启动，那么久发送一个进程间请求给AMS让其启动目标ContentProvider，最后通过installProvider方法来修改引用计数，那么AMS是如何启动ContentProvider的呢？我们知道，ContentProvider被启动时伴随着进程也会被启动，那么AMS重，首先会启动ContentProvider所在的进程，然后再ContentProvider，启动进程是由AMS的startProcessLocked方法来完成的，其内部主要是通过Process的start方法来完成一系列的新进程启动，新进程启动后其入口方法为ActivityThread的main方法：
public static void main(String[] args) {
SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
Security.addProvider(new AndroidKeyStoreProvider());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
可以看到，ActivityThread的main方法是一个静态方法，他在内部先创建了一个实例并且调用了attach方法来进行一系列的初始化，接着开始消息循环了，最终会将ApplicationThread对象通过AMS的attchApplication方法跨进程传递给AMS，最终AMS会将ContentProvider的创建过程
try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
// Ignore
}
AMS的attachApplication方法又调用attachApplicationLocked方法，attachApplicationLocked中又调用了bindApplication，注意这个过程也是进程间调用
ActivityThread的bindApplication会发送一个BIND_APPLICATION类型的消息给mH,他是一个Handler，他收到消息后会调用ActivityThread，过程如下：
AppBindData data = new AppBindData();
data.processName = processName;
data.appInfo = appInfo;
data.providers = providers;
data.instrumentationName = instrumentationName;
data.instrumentationArgs = instrumentationArgs;
data.instrumentationWatcher = instrumentationWatcher;
data.instrumentationUiAutomationConnection = instrumentationUiConnection;
data.debugMode = debugMode;
data.enableOpenGlTrace = enableOpenGlTrace;
data.restrictedBackupMode = isRestrictedBackupMode;
data.persistent = persistent;
data.config = config;
data.compatInfo = compatInfo;
data.initProfilerInfo = profilerInfo;
sendMessage(H.BIND_APPLICATION, data);
ActivityThread的bindBaseApplication则完成了Application的创建以及ContentProvider的创建，可以分为如下四个步骤：
1.创建ContextImpl和instrumentation
ContextImpl instrContext = ContextImpl.createAppContext(this, pi);
try {
java.lang.ClassLoader cl = instrContext.getClassLoader();
mInstrumentation = (Instrumentation)
cl.loadClass(data.instrumentationName.getClassName()).newInstance();
} catch (Exception e) {
throw new RuntimeException(
"Unable to instantiate instrumentation "
+ data.instrumentationName + ": " + e.toString(), e);
}
mInstrumentation.init(this, instrContext, appContext,
new ComponentName(ii.packageName, ii.name), data.instrumentationWatcher,
data.instrumentationUiAutomationConnection);
2.创建Application对象
Application app = data.info.makeApplication(data.restrictedBackupMode, null);
mInitialApplication = app;
3.启动当前进程的ContentProvider并调用onCreate方法
if (!data.restrictedBackupMode) {
List<ProviderInfo> providers = data.providers;
if (providers != null) {
installContentProviders(app, providers);
// 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);
}
}
installContentProviders完成了ContentProvider的启动工作，他的实现，首先是遍历ProviderInfo的列表并一一调用installProvider方法来启动他们，接着将以及启动的ContentProvider发送到AMS中，AMS会把他们存储在ProviderMap中，这样一来外部调用者可以直接从AMS中获取ContentProvider了
private void installContentProviders(
Context context, List<ProviderInfo> providers) {
final ArrayList<IActivityManager.ContentProviderHolder> results =
new ArrayList<IActivityManager.ContentProviderHolder>();
for (ProviderInfo cpi : providers) {
if (DEBUG_PROVIDER) {
StringBuilder buf = new StringBuilder(128);
buf.append("Pub ");
buf.append(cpi.authority);
buf.append(": ");
buf.append(cpi.name);
Log.i(TAG, buf.toString());
}
IActivityManager.ContentProviderHolder cph = installProvider(context, null, cpi,
false /*noisy*/, true /*noReleaseNeeded*/, true /*stable*/);
if (cph != null) {
cph.noReleaseNeeded = true;
results.add(cph);
}
}
try {
ActivityManagerNative.getDefault().publishContentProviders(
getApplicationThread(), results);
} catch (RemoteException ex) {
}
}
下面看一下ContentProvider对象的创建过程，在installProvider方法中有下面一段代码，并通过类加载器完成了ContentProvider对象的创建：
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;
}
if (DEBUG_PROVIDER) Slog.v(
TAG, "Instantiating local provider " + info.name);
// XXX Need to create the correct context for this provider.
localProvider.attachInfo(c, info);
在上述代码中，除了完成ContentProvider对象的创建，还会通过ContentProvider的attachInfo方法来调用他的onCreate方法，如下所示：
private void attachInfo(Context context, ProviderInfo info, boolean testing) {
mNoPerms = testing;
/*
* Only allow it to be set once, so after the content service gives
* this to us clients can't change it.
*/
if (mContext == null) {
mContext = context;
if (context != null) {
mTransport.mAppOpsManager = (AppOpsManager) context.getSystemService(
Context.APP_OPS_SERVICE);
}
mMyUid = Process.myUid();
if (info != null) {
setReadPermission(info.readPermission);
setWritePermission(info.writePermission);
setPathPermissions(info.pathPermissions);
mExported = info.exported;
mSingleUser = (info.flags & ProviderInfo.FLAG_SINGLE_USER) != 0;
setAuthorities(info.authority);
}
ContentProvider.this.onCreate();
}
}
到此为止，ContentProvider已经被创建并且其onCreate方法也被调用了，这意味着他已经启动完成了
4.调用Application的onCreate方法
try {
mInstrumentation.callApplicationOnCreate(app);
} catch (Exception e) {
if (!mInstrumentation.onException(app, e)) {
throw new RuntimeException(
"Unable to create application " + app.getClass().getName()
+ ": " + e.toString(), e);
}
}
经过上面的四个步骤，ContentProvider已经启动了，并且在其所在进程的Application也已经启动了，这意味着ContentProvider所在的进程已经完成了整个的启动过程，然后其他应用通过AMS来访问这个ContentProvider，我们来看下query的实现
@Override
public Cursor query(String callingPkg, Uri uri, String[] projection,
String selection, String[] selectionArgs, String sortOrder,
ICancellationSignal cancellationSignal) {
validateIncomingUri(uri);
uri = getUriWithoutUserId(uri);
if (enforceReadPermission(callingPkg, uri, null) != AppOpsManager.MODE_ALLOWED) {
return rejectQuery(uri, projection, selection, selectionArgs, sortOrder,
CancellationSignal.fromTransport(cancellationSignal));
}
final String original = setCallingPackage(callingPkg);
try {
return ContentProvider.this.query(
uri, projection, selection, selectionArgs, sortOrder,
CancellationSignal.fromTransport(cancellationSignal));
} finally {
setCallingPackage(original);
}
}
很显然，ContentProvider.Transport的query方法调用了ContentProvider的query方法，query方法的执行结果再通过Binder返回给调用者，这样一来整个调用过程就完成了，除了query方法，其他方法也是类似的，这里就不去深究了
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