前言
Content Provider做为四大组件之一，通常情况下并没有其他的组件使用频繁，但这不能作为我们不去深入学习它的理由。关于Content Provider一篇文章是写不完的，这一篇文章先来介绍它的启动过程。
1.query方法到AMS的调用过程
在Android IPC机制（四）用ContentProvider进行进程间通信这篇文章我举了一个Content Provider使用的例子，在Activity中我是使用如下代码调用Content Provider的：
public class ContentProviderActivity extends AppCompatActivity {
private final static String TAG = "ContentProviderActivity";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_content_provider);
Uri uri = Uri.parse("content://com.example.liuwangshu.mooncontentprovide.GameProvider");
ContentValues mContentValues = new ContentValues();
mContentValues.put("_id", 2);
mContentValues.put("name", "大航海时代ol");
mContentValues.put("describe", "最好玩的航海网游");
getContentResolver().insert(uri, mContentValues);//1
Cursor gameCursor = getContentResolver().query(uri, new String[]{"name", "describe"}, null, null, null);
...
}
}要想调用Content Provider，首先需要使用注释1处的getContentResolver方法，如下所示。
frameworks/base/core/Java/android/content/ContextWrapper.java
@Override
public ContentResolver getContentResolver() {
return mBase.getContentResolver();
}这里mBase指的是ContextImpl，ContextImpl的getContentResolver方法如下所示。
frameworks/base/core/java/android/app/ContextImpl.java
@Override
public ContentResolver getContentResolver() {
return mContentResolver;
}上面的代码return了ApplicationContentResolver类型的mContentResolver对象，ApplicationContentResolver是ContextImpl中的静态内部类，继承自ContentResolver，它在ContextImpl的构造方法中被创建。
当我们调用ContentResolver的insert、query、update等方法时就会启动Content Provider，这里拿query方法来进行举例。
query方法的实现在ApplicationContentResolver的父类ContentResolver中，代码如下所示。
frameworks/base/core/java/android/content/ContentResolver.java
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) {
Preconditions.checkNotNull(uri, "uri");
IContentProvider unstableProvider = acquireUnstableProvider(uri);//1
...
try {
...
try {
qCursor = unstableProvider.query(mPackageName, uri, projection,
selection, selectionArgs, sortOrder, remoteCancellationSignal);//2
} catch (DeadObjectException e) {
...
}
...
}在注释1处通过acquireUnstableProvider方法返回IContentProvider类型的unstableProvider对象，在注释2处调用unstableProvider的query方法。我们查看acquireUnstableProvider方法做了什么，如下所示。
frameworks/base/core/java/android/content/ContentResolver.java
public final IContentProvider acquireUnstableProvider(Uri uri) {
if (!SCHEME_CONTENT.equals(uri.getScheme())) {//1
return null;
}
String auth = uri.getAuthority();
if (auth != null) {
return acquireUnstableProvider(mContext, uri.getAuthority());//2
}
return null;
}注释1处用来检查Uri的scheme是否等于"content"，如果不是则返回null。注释2处调用了acquireUnstableProvider方法，这是个抽象方法，它的实现在ContentResolver的子类ApplicationContentResolver中：
frameworks/base/core/java/android/app/ContextImpl.java
@Override
protected IContentProvider acquireUnstableProvider(Context c, String auth) {
return mMainThread.acquireProvider(c,
ContentProvider.getAuthorityWithoutUserId(auth),
resolveUserIdFromAuthority(auth), false);
}return了ActivityThread类型的mMainThread对象的acquireProvider方法：
frameworks/base/core/java/android/app/ActivityThread.java
public final IContentProvider acquireProvider(
Context c, String auth, int userId, boolean stable) {
final IContentProvider provider = acquireExistingProvider(c, auth, userId, stable);//1
if (provider != null) {
return provider;
}
IActivityManager.ContentProviderHolder holder = null;
try {
holder = ActivityManagerNative.getDefault().getContentProvider(
getApplicationThread(), auth, userId, stable);//2
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
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);//3
return holder.provider;
}注释1处检查ActivityThread中的ArrayMap类型的mProviderMap中是否有目标ContentProvider存在，有则返回，没有就会在注释2处调用AMP的getContentProvider方法，最终会调用AMS的getContentProvider方法。注释3处的installProvider方法用来将注释2处返回的ContentProvider相关的数据存储在mProviderMap中，起到缓存的作用，这样使用相同的Content Provider时，就不需要每次都要调用AMS的getContentProvider方法。使用我们接着查看AMS的getContentProvider方法，代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
@Override
public final ContentProviderHolder getContentProvider(
IApplicationThread caller, String name, int userId, boolean stable) {
...
return getContentProviderImpl(caller, name, null, stable, userId);
}getContentProvider方法return了getContentProviderImpl方法：
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
private ContentProviderHolder getContentProviderImpl(IApplicationThread caller,
String name, IBinder token, boolean stable, int userId) {
...
ProcessRecord proc = getProcessRecordLocked(
cpi.processName, cpr.appInfo.uid, false);//1
if (proc != null && proc.thread != null && !proc.killed) {
...
if (!proc.pubProviders.containsKey(cpi.name)) {
checkTime(startTime, "getContentProviderImpl: scheduling install");
proc.pubProviders.put(cpi.name, cpr);
try {
proc.thread.scheduleInstallProvider(cpi);//2
} 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);//3
checkTime(startTime, "getContentProviderImpl: after start process");
...
}
...
}getContentProviderImpl方法的代码很多，这里截取了关键的部分。注释1处通过getProcessRecordLocked方法来获取目标ContentProvider的应用程序进程信息，这些信息用ProcessRecord类型的proc来表示，如果该应用进程已经启动就会调用注释2处的代码，否则就会调用注释3的startProcessLocked方法来启动进程。这里我们假设ContentProvider的应用进程还没有启动，关于应用进程启动过程，我在Android应用程序进程启动过程（前篇）已经讲过，最终会调用ActivityThread的main方法，代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
public static void main(String[] args) {
...
Looper.prepareMainLooper();//1
ActivityThread thread = new ActivityThread();//2
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();//3
throw new RuntimeException("Main thread loop unexpectedly exited");
}注释1处通过prepareMainLooper方法在ThreadLocal中获取Looper，并在注释3处开启消息循环。在注释2处创建了ActivityThread并调用了它的attach方法：
frameworks/base/core/java/android/app/ActivityThread.java
private void attach(boolean system) {
...
final IActivityManager mgr = ActivityManagerNative.getDefault();//1
try {
mgr.attachApplication(mAppThread);//2
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
...
}注释1处最终会得到AMS，在注释2处调用AMS的attachApplication方法，并将ApplicationThread类型的mAppThread对象传进去。
query方法到AMS的调用过程，如下面时序图所示（省略应用程序进程启动过程）。
2.AMS启动Content Provider的过程
我们接着来查看AMS的attachApplication方法，如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
@Override
public final void attachApplication(IApplicationThread thread) {
synchronized (this) {
int callingPid = Binder.getCallingPid();
final long origId = Binder.clearCallingIdentity();
attachApplicationLocked(thread, callingPid);
Binder.restoreCallingIdentity(origId);
}
}attachApplication方法中又调用了attachApplicationLocked方法：
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
private final boolean attachApplicationLocked(IApplicationThread thread,
int pid) {
...
thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
app.instrumentationUiAutomationConnection, testMode,
mBinderTransactionTrackingEnabled, enableTrackAllocation,
isRestrictedBackupMode || !normalMode, app.persistent,
new Configuration(mConfiguration), app.compat,
getCommonServicesLocked(app.isolated),
mCoreSettingsObserver.getCoreSettingsLocked());
...
}attachApplicationLocked方法中调用了thread的bindApplication方法，thread是IApplicationThread类型的，从类型名字就可以看出来是用于进程间通信，这里实现bindApplication方法的是ApplicationThreadProxy类，它实现了IApplicationThread接口。
frameworks/base/core/java/android/app/ApplicationThreadNative.java
class ApplicationThreadProxy implements IApplicationThread {
...
@Override
public final void bindApplication(String packageName, ApplicationInfo info,
List<ProviderInfo> providers, ComponentName testName, ProfilerInfo profilerInfo,
Bundle testArgs, IInstrumentationWatcher testWatcher,
IUiAutomationConnection uiAutomationConnection, int debugMode,
boolean enableBinderTracking, boolean trackAllocation, boolean restrictedBackupMode,
boolean persistent, Configuration config, CompatibilityInfo compatInfo,
Map<String, IBinder> services, Bundle coreSettings) throws RemoteException {
...
mRemote.transact(BIND_APPLICATION_TRANSACTION, data, null,
IBinder.FLAG_ONEWAY);
data.recycle();
}
...
}到目前为止，上面的调用过程还是在AMS进程中执行的，因此，需要通过IBinder类型的mRemote对象向新创建的应用程序进程（目标Content Provider所在的进程）发送BIND_APPLICATION_TRANSACTION类型的通信请求。处理这个通信请求的是在新创建的应用程序进程中执行的ApplicationThread的bindApplication方法，如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
public final void bindApplication(String processName, ApplicationInfo appInfo,
List<ProviderInfo> providers, ComponentName instrumentationName,
ProfilerInfo profilerInfo, Bundle instrumentationArgs,
IInstrumentationWatcher instrumentationWatcher,
IUiAutomationConnection instrumentationUiConnection, int debugMode,
boolean enableBinderTracking, boolean trackAllocation,
boolean isRestrictedBackupMode, boolean persistent, Configuration config,
CompatibilityInfo compatInfo, Map<String, IBinder> services, Bundle coreSettings) {
...
sendMessage(H.BIND_APPLICATION, data);
}调用sendMessage方法像H发送BIND_APPLICATION类型消息，H的handleMessage方法如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
...
case BIND_APPLICATION:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
AppBindData data = (AppBindData)msg.obj;
handleBindApplication(data);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
...
}
...
}我们接着查看handleBindApplication方法：
frameworks/base/core/java/android/app/ActivityThread.java
private void handleBindApplication(AppBindData data) {
...
final ContextImpl appContext = ContextImpl.createAppContext(this, data.info);//1
try {
final ClassLoader cl = instrContext.getClassLoader();
mInstrumentation = (Instrumentation)
cl.loadClass(data.instrumentationName.getClassName()).newInstance();//2
} catch (Exception e) {
...
}
final ComponentName component = new ComponentName(ii.packageName, ii.name);
mInstrumentation.init(this, instrContext, appContext, component,
data.instrumentationWatcher, data.instrumentationUiAutomationConnection);//3
...
Application app = data.info.makeApplication(data.restrictedBackupMode, null);//4
mInitialApplication = app;
if (!data.restrictedBackupMode) {
if (!ArrayUtils.isEmpty(data.providers)) {
installContentProviders(app, data.providers);//5
mH.sendEmptyMessageDelayed(H.ENABLE_JIT, 10*1000);
}
}
...
mInstrumentation.callApplicationOnCreate(app);//6
...
}handleBindApplication方法的代码很长，这里截取了主要的部分。注释1处创建了ContextImpl 。注释2处通过反射创建Instrumentation并在注释3处初始化Instrumentation。注释4处创建Application并且在注释6处调用Application的onCreate方法，这意味着Content Provider所在的应用程序进程已经启动完毕，在这之前，注释5处调用installContentProviders方法来启动Content Provider，代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
private void installContentProviders(
Context context, List<ProviderInfo> providers) {
final ArrayList<IActivityManager.ContentProviderHolder> results =
new ArrayList<IActivityManager.ContentProviderHolder>();
for (ProviderInfo cpi : providers) {//1
...
IActivityManager.ContentProviderHolder cph = installProvider(context, null, cpi,
false /*noisy*/, true /*noReleaseNeeded*/, true /*stable*/);//2
...
}
try {
ActivityManagerNative.getDefault().publishContentProviders(
getApplicationThread(), results);//3
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}注释1处遍历当前应用程序进程的ProviderInfo列表，得到每个Content Provider的ProviderInfo（存储Content Provider的信息），并在注释2处调用installProvider方法来启动这些Content Provider。在注释3处通过AMS的publishContentProviders方法将这些Content Provider存储在AMS的mProviderMap中，这个mProviderMap在前面提到过，起到缓存的作用，防止每次使用相同的Content Provider时都会调用AMS的getContentProvider方法。来查看installProvider方法时如何启动Content Provider的，installProvider方法如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
private IActivityManager.ContentProviderHolder installProvider(Context context,
IActivityManager.ContentProviderHolder holder, ProviderInfo info,
boolean noisy, boolean noReleaseNeeded, boolean stable) {
ContentProvider localProvider = null;
...
final java.lang.ClassLoader cl = c.getClassLoader();
localProvider = (ContentProvider)cl.
loadClass(info.name).newInstance();//1
provider = localProvider.getIContentProvider();
if (provider == null) {
...
return null;
}
if (DEBUG_PROVIDER) Slog.v(
TAG, "Instantiating local provider " + info.name);
localProvider.attachInfo(c, info);//2
} catch (java.lang.Exception e) {
...
}
return null;
}
}
...
return retHolder;
}在注释1处通过反射来创建ContentProvider类型的localProvider对象，并在注释2处调用了它的attachInfo方法：
frameworks/base/core/java/android/content/ContentProvider.java
private void attachInfo(Context context, ProviderInfo info, boolean testing) {
...
ContentProvider.this.onCreate();
}
}在attachInfo方法中调用了onCreate方法，它是一个抽象方法。这样Content Provider就启动完毕。
最后给出AMS启动Content Provider的时序图。
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