本文深入解析Android中ContentProvider的启动流程与工作原理，包括其如何在进程启动时初始化，与Application对象创建的先后顺序，以及跨进程调用机制。重点介绍了ContentProvider的onCreate方法执行时机，以及其与Application onCreate方法的关系。
转自： https://blog.youkuaiyun.com/tianmi1988/article/details/51077378
ContentProvider启动
ContentProvider是一种内容共享型组件，实际上它是通过Binder向其它应用提供数据。当ContentProvider所在的进程启动时，ContentProvider会同时启动并被发布到AMS中，需要特别注意的是ContentProvider的onCreate方法要早于Application的onCreate方法执行。
废话不多说先看源码,As we all known,每个进程的入口都是ActivityThread.main
-
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();
-
}
-
-
AsyncTask.init();
-
-
if (
false) {
-
Looper.myLooper().setMessageLogging(
new
-
LogPrinter(Log.DEBUG,
"ActivityThread"));
-
}
-
-
Looper.loop();
-
-
throw
new RuntimeException(
"Main thread loop unexpectedly exited");
-
}
主要看attach方法
-
private void attach(boolean system) {
-
...
-
final IActivityManager mgr = ActivityManagerNative.getDefault();
-
try {
-
mgr.attachApplication(mAppThread);
-
}
catch (RemoteException ex) {
-
// Ignore
-
}
-
...
-
}
-
@Override
-
public final void attachApplication(IApplicationThread thread) {
-
synchronized (
this) {
-
int callingPid = Binder.getCallingPid();
-
final
long origId = Binder.clearCallingIdentity();
-
attachApplicationLocked(thread, callingPid);
-
Binder.restoreCallingIdentity(origId);
-
}
-
}
AMS.attachApplicationLocked这个方法也是很长，这里截取最关键的
-
private final boolean attachApplicationLocked(IApplicationThread thread,
-
int pid) {
-
...
-
ProfilerInfo profilerInfo = profileFile ==
null ?
null
-
:
new ProfilerInfo(profileFile, profileFd, samplingInterval, profileAutoStop);
-
thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
-
profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
-
app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace,
-
isRestrictedBackupMode || !normalMode, app.persistent,
-
new Configuration(mConfiguration), app.compat, getCommonServicesLocked(),
-
mCoreSettingsObserver.getCoreSettingsLocked());
-
updateLruProcessLocked(app,
false,
null);
-
...
-
}
这里又IPC通信，返回客户端处理去了
总结一下，跟ContentProvider相关的事件
1.创建ActivityThread实例，并创建消息队列
2.ActivityThread的attach方法远程调用AMS的attachApplication方法，并将ApplicationThread对象传递给AMS。
3.AMS的attachApplication方法中会调用ApplicationThread的bingApplication（IPC调用）进而通过H对象切换到ActivityThread中执行handleBindApplication方法
这个方法有200多行，这里截取了最后比较关键的部分
-
private void handleBindApplication(AppBindData data) {
-
...
-
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);
-
mInitialApplication = app;
-
-
// don't bring up providers in restricted mode; they may depend on the
-
// app's custom Application class
-
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);
-
}
-
}
-
-
// Do this after providers, since instrumentation tests generally start their
-
// test thread at this point, and we don't want that racing.
-
try {
-
mInstrumentation.onCreate(data.instrumentationArgs);
-
}
-
catch (Exception e) {
-
throw
new RuntimeException(
-
"Exception thrown in onCreate() of "
-
+ data.instrumentationName +
": " + e.toString(), e);
-
}
-
-
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);
-
}
-
}
-
}
finally {
-
StrictMode.setThreadPolicy(savedPolicy);
-
}
-
}
其中依次会调用到
Application app = data.info.makeApplication(data.restrictedBackupMode, null);和installContentProviders(app, providers);方法，前者就是创建Application对象，而后者则是创建ContentProvider,创建ContentProvider之后则调用Application的onCreate方法。
-
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) {
-
}
-
}
创建好后通过IPC通信，发布到AMS中,这里还没完接着往下挖，ActivityThread.installProvider
-
private IActivityManager.
ContentProviderHolder installProvider(Context context,
-
IActivityManager.ContentProviderHolder holder, ProviderInfo info,
-
boolean noisy, boolean noReleaseNeeded, boolean stable) {
-
...
-
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);
-
}
-
...
-
}
-
private void attachInfo(Context context, ProviderInfo info, boolean testing) {
-
/*
-
* We may be using AsyncTask from binder threads. Make it init here
-
* so its static handler is on the main thread.
-
*/
-
AsyncTask.init();
-
-
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();
-
}
-
}
这部分的逻辑，我在书上看到的一幅图我觉得无可挑剔，直接拍下来放在博客上
ContentProvider的启动过程：
1.当一个应用启动时，入口方法是ActivityThread的main方法，其中创建ActivityThread的实例并创建主线程的消息队列；
2.ActivityThread的attach方法中会远程调用ActivityManagerService的attachApplication，并将ApplicationThread提供给AMS，ApplicationThread主要用于ActivityThread和AMS之间的通信；
3.ActivityManagerService的attachApplication会调用ApplicationThread的bindApplication方法，这个方法会通过H切换到ActivityThread中去执行，即调用handleBindApplication方法；
4.handleBindApplication方法会创建Application对象并加载ContentProvider，注意是先加载ContentProvider，然后调用Application的onCreate方法。
(3)ContentProvider的android:multiprocess属性决定它是否是单实例，默认值是false，也就是默认是单实例。当设置为true时，每个调用者的进程中都存在一个ContentProvider对象。实际使用中并未发现这样的使用场景，所以一般我们可以认为其为单实例的。
ContentProvider工作原理
当调用ContentProvider的insert、delete、update、query方法中的任何一个时，如果ContentProvider所在的进程没有启动的话，那么就会触发ContentProvider的创建，并伴随着ContentProvider所在进程的启动。
这里直接从query方法顺藤摸瓜,我们的一般使用就是在Context中使用getContentResolver.query方法来获取数据
这里的Context.getContentResolver获取的是ApplicationContentResolver对象，它是ContextImpl的内部类，继承了ContentResolver并实现了其抽象方法。
代码在ContextImpl.java中
-
private
static
final
class ApplicationContentResolver extends ContentResolver {
-
private
final ActivityThread mMainThread;
-
private
final UserHandle mUser;
-
-
public ApplicationContentResolver(
-
Context context, ActivityThread mainThread, UserHandle user) {
-
super(context);
-
mMainThread = Preconditions.checkNotNull(mainThread);
-
mUser = Preconditions.checkNotNull(user);
-
}
-
-
@Override
-
protected IContentProvider acquireProvider(Context context, String auth) {
-
return mMainThread.acquireProvider(context,
-
ContentProvider.getAuthorityWithoutUserId(auth),
-
resolveUserIdFromAuthority(auth),
true);
-
}
-
-
@Override
-
protected IContentProvider acquireExistingProvider(Context context, String auth) {
-
return mMainThread.acquireExistingProvider(context,
-
ContentProvider.getAuthorityWithoutUserId(auth),
-
resolveUserIdFromAuthority(auth),
true);
-
}
-
-
@Override
-
public boolean releaseProvider(IContentProvider provider) {
-
return mMainThread.releaseProvider(provider,
true);
-
}
-
-
@Override
-
protected IContentProvider acquireUnstableProvider(Context c, String auth) {
-
return mMainThread.acquireProvider(c,
-
ContentProvider.getAuthorityWithoutUserId(auth),
-
resolveUserIdFromAuthority(auth),
false);
-
}
-
-
@Override
-
public boolean releaseUnstableProvider(IContentProvider icp) {
-
return mMainThread.releaseProvider(icp,
false);
-
}
-
-
@Override
-
public void unstableProviderDied(IContentProvider icp) {
-
mMainThread.handleUnstableProviderDied(icp.asBinder(),
true);
-
}
-
-
@Override
-
public void appNotRespondingViaProvider(IContentProvider icp) {
-
mMainThread.appNotRespondingViaProvider(icp.asBinder());
-
}
-
-
/** @hide */
-
protected int resolveUserIdFromAuthority(String auth) {
-
return ContentProvider.getUserIdFromAuthority(auth, mUser.getIdentifier());
-
}
-
}
-
public final Cursor query(final Uri uri, String[] projection,
-
String selection, String[] selectionArgs, String sortOrder,
-
CancellationSignal cancellationSignal) {
-
IContentProvider unstableProvider = acquireUnstableProvider(uri);
-
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,
-
selection, selectionArgs, sortOrder, remoteCancellationSignal);
-
}
-
...
-
...
通过上面ApplicationContentResolver类的定义可以看出直接调用了ActivityThread的acquireProvider
-
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;
-
}
以上代码首先会在ActivityThread中查找是否已经存在目标ContentProvider，如果有则直接返回。成员变量mProviderMap就是用来存储ContentProvider的
-
final ArrayMap<ProviderKey, ProviderClientRecord> mProviderMap
-
=
new ArrayMap<ProviderKey, ProviderClientRecord>();
ActivityManagerNative.getDefault().getContentProvider( getApplicationThread(), auth, userId, stable);
接下来重点关注，AMS怎样启动ContentProvider所在进程的，沿着IPC调用往下挖 AMS.getContentProvider
-
@Override
-
public final ContentProviderHolder getContentProvider(
-
IApplicationThread caller, String name, int userId, boolean stable) {
-
enforceNotIsolatedCaller(
"getContentProvider");
-
if (caller ==
null) {
-
String msg =
"null IApplicationThread when getting content provider "
-
+ name;
-
Slog.w(TAG, msg);
-
throw
new SecurityException(msg);
-
}
-
// The incoming user check is now handled in checkContentProviderPermissionLocked() to deal
-
// with cross-user grant.
-
return getContentProviderImpl(caller, name,
null, stable, userId);
-
}
-
private final ContentProviderHolder getContentProviderImpl(IApplicationThread caller,
-
String name, IBinder token, boolean stable, int userId) {
-
ContentProviderRecord cpr;
-
...
-
if (!providerRunning) {
-
...
-
ProcessRecord proc = getProcessRecordLocked(
-
cpi.processName, cpr.appInfo.uid,
false);
-
if (proc !=
null && proc.thread !=
null) {
-
if (DEBUG_PROVIDER) {
-
Slog.d(TAG,
"Installing in existing process " + proc);
-
}
-
checkTime(startTime,
"getContentProviderImpl: scheduling install");
-
proc.pubProviders.put(cpi.name, cpr);
-
try {
-
proc.thread.scheduleInstallProvider(cpi);
-
}
catch (RemoteException e) {
-
}}
else {
-
checkTime(startTime,
"getContentProviderImpl: before start process");
-
proc = startProcessLocked(cpi.processName,
-
cpr.appInfo,
false,
0,
"content provider",
-
new ComponentName(cpi.applicationInfo.packageName,
-
cpi.name),
false,
false,
false);
-
checkTime(startTime,
"getContentProviderImpl: after start process");
-
if (proc ==
null) {
-
Slog.w(TAG,
"Unable to launch app "
-
+ cpi.applicationInfo.packageName +
"/"
-
+ cpi.applicationInfo.uid +
" for provider "
-
+ name +
": process is bad");
-
return
null;
-
}
-
}
-
cpr.launchingApp = proc;
-
mLaunchingProviders.add(cpr);
-
...
-
...
-
}
启动ContentProvider所在进程的就是AMS.startProcessLocked,其内部主要是通过Process的start方法来完成新进程的启动的，对这一部分好奇的读者可以移步到老罗的博客：Android应用程序进程启动过程的源代码分析。新进程启动后入口就是ActivityThread.main方法，这跟前面ContentProvider启动部分一致了。
就此总结一下前面ActivityThread.handleBindApplication方法
1.创建ContextImpl和Instrumentation
2.创建Application对象
3.启动ContentProvider所在进程并调用OnCreate方法
4.调用Application的OnCreate方法
到这里ContentProvider所在进程已经启动成功，其他应用也就可以通过它提供的接口方法来访问它了。需要注意的是其他应用拿到的并不是ContentProvider对象，而是IContentProvider对象，了解过AIDL的童鞋都知道这里的IContentProvider就是ContentProvider的Binder对象，IContentProvider的具体实现是ContentProviderNative和ContentProvider.Transport,而其中ContentProvider.Transport继承了ContentProviderNative。我们可以从AMN.getDefault()可以找到类似的设计。
其他应用通过AMS获取到ContentProvider的Binder引用IContentProvider，接着通过IPC调用执行ContentProvider所在进程方法
ContentProvider.Transport
-
class Transport extends ContentProviderNative {
-
AppOpsManager mAppOpsManager =
null;
-
int mReadOp = AppOpsManager.OP_NONE;
-
int mWriteOp = AppOpsManager.OP_NONE;
-
-
ContentProvider getContentProvider() {
-
return ContentProvider.
this;
-
}
-
-
@Override
-
public String getProviderName() {
-
return getContentProvider().getClass().getName();
-
}
-
-
@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) != 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);
-
}
-
}
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