ContentProvider的工作过程

向右看齐 2023-10-17 10:35 6阅读 0赞

## ContentProvider的工作过程 ##

### ContentProvider的调用过程 ###

![ContentProvider的调用流程图][ContentProvider]

ContentProvider所在的进程启动时，ContentProvider会同时启动并发布到AMS中。**ContentProvider的onCreate要先于Application的onCreate而执行。**

访问ContentProvider需要通过ContentResolver，ContentResolver是一个抽象类，实际实现者是ContentImpl中的ApplicationContentResolver

private static final class ApplicationContentResolver extends ContentResolver {
    
        @Override
        protected IContentProvider acquireProvider(Context context, String auth) {
            return mMainThread.acquireProvider(context,
                    ContentProvider.getAuthorityWithoutUserId(auth),
                    resolveUserIdFromAuthority(auth), true);
        }
    
        //...
    }

通过ContentResolver的四个方法的任意一个，都可以触发ContentProvider的启动(如果还没有启动的话)

这里以query()示例

public final Cursor query(final Uri uri, String[] projection,
            String selection, String[] selectionArgs, String sortOrder,
            CancellationSignal cancellationSignal) {
        IContentProvider unstableProvider = acquireUnstableProvider(uri);
        IContentProvider stableProvider = null;
        //...
        try {
            qCursor = unstableProvider.query(mPackageName, uri, projection,
                    selection, selectionArgs, sortOrder, remoteCancellationSignal);
        } catch (DeadObjectException e) {
            stableProvider = acquireProvider(uri);
            qCursor = stableProvider.query(mPackageName, uri, projection,
                    selection, selectionArgs, sortOrder, remoteCancellationSignal);
        }
         //...
    }

可以看到，先调用了acquireUnstableProvider()，如果失败，则会调用acquireProvider()，但最终都是通过acquireProvider()来获取ContentProvider

ApplicationContentResolver的acquireProvider方法并没有处理任何逻辑，它直接调用了ActivityThread的acquireProvider()

来看ActivityThread\#acquireProvider()

public final IContentProvider acquireProvider(
            Context c, String auth, int userId, boolean stable) {
        //查找是否已经存在目标ContentProvider，如果存在就直接返回
        final IContentProvider provider = acquireExistingProvider(c, auth, userId, stable);
    
        //启动ContentProvider
        holder = ActivityManagerNative.getDefault().getContentProvider(
                getApplicationThread(), auth, userId, stable);
    
        //安装Provider，增加引用的数量
        holder = installProvider(c, holder, holder.info,
                true /*noisy*/, holder.noReleaseNeeded, stable);
        return holder.provider;
    }

### 启动ContentProvider的过程 ###

![启动ContentProvider的过程][ContentProvider 1]

ContentProvider启动之前，会前启动ContentProvider的进程

启动进程由ActivityManagerService的startProcessLocked()来完成，其内部主要通过Process的start方法来完成一个新进程的启动。

新进程启动后其入口为ActivityThread的main方法。

public static void main(String[] args) {
        //...
        //Z-创建主线程的消息队列
        Looper.prepareMainLooper();
    
        //Z-创建ActivityThread实例
        ActivityThread thread = new ActivityThread();
        //Z-远程调用AMS的attachApplication方法，进行初始化，并将ApplicationThread对象提供给AMS
        thread.attach(false);
    
        //...
        Looper.loop();
    }

来看ActivityThread\#attach()

private void attach(boolean system) {
        final IActivityManager mgr = ActivityManagerNative.getDefault();
        mgr.attachApplication(mAppThread);  
        //...
    }

再来看ActivityManagerService.attachApplication()

public final void attachApplication(IApplicationThread thread) {
        synchronized (this) {
            int callingPid = Binder.getCallingPid();
            final long origId = Binder.clearCallingIdentity();
            attachApplicationLocked(thread, callingPid);
            Binder.restoreCallingIdentity(origId);
        }
    }

再来看ActivityManagerService.attachApplicationLocked()

private final boolean attachApplicationLocked(IApplicationThread thread,
                                                  int pid) {
    
        //...
        //Z- ApplicationThread.bindApplication
        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());
    
        //...
        return true;
    }

再来看ApplicationThread.bindApplication()

public final void bindApplication(String processName, ApplicationInfo appInfo,
                List<ProviderInfo> providers, ComponentName instrumentationName,
                ProfilerInfo profilerInfo, Bundle instrumentationArgs,
                IInstrumentationWatcher instrumentationWatcher,
                IUiAutomationConnection instrumentationUiConnection, int debugMode,
                boolean enableOpenGlTrace, boolean isRestrictedBackupMode, boolean persistent,
                Configuration config, CompatibilityInfo compatInfo, Map<String, IBinder> services,
                Bundle coreSettings) {
    
            //...
            //Z- 通过ActivityThread的mH Handler切换到ActivityThread中去执行
            sendMessage(H.BIND_APPLICATION, data);
        }

Message的发送通过handleBindApplication()来处理

private void handleBindApplication(AppBindData data) {
        //... 
        //1.创建ContextImpl和Instrumentation
        final ContextImpl appContext = ContextImpl.createAppContext(this, data.info);
        //...
    
        //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并调用其onCreate方法
        List<ProviderInfo> providers = data.providers;
        if (providers != null) {
            installContentProviders(app, providers);
            mH.sendEmptyMessageDelayed(H.ENABLE_JIT, 10*1000);
        }
    
        //4.调用Application的onCreate方法
        mInstrumentation.callApplicationOnCreate(app);
    }

其中会调用 installContentProviders()

private void installContentProviders(
            Context context, List<ProviderInfo> providers) {
    
        //...
        //通过类加载器完成ContentProvider的创建
        IActivityManager.ContentProviderHolder cph = installProvider(context, 
        //...
        //通过AMS存储到Map中
        ActivityManagerNative.getDefault().publishContentProviders(
            getApplicationThread(), results);
    }

又会调用installProvider()

private IActivityManager.ContentProviderHolder installProvider(Context context,
        IActivityManager.ContentProviderHolder holder, ProviderInfo info,
        boolean noisy, boolean noReleaseNeeded, boolean stable) {
    
        //通过类加载器完成ContentProvider的创建
        final java.lang.ClassLoader cl = c.getClassLoader();
        localProvider = (ContentProvider)cl.
            loadClass(info.name).newInstance();
        provider = localProvider.getIContentProvider();
    
        //该方法会调用ContentProvider的onCrate方法
        localProvider.attachInfo(c, info);
    
        return retHolder;
    }

### 通过调用提供的接口来访问ContentProvider ###

ContentProvider\#query()

public Cursor query(String callingPkg, Uri uri, String[] projection,
        String selection, String[] selectionArgs, String sortOrder,
        ICancellationSignal cancellationSignal) {
        //...
        return ContentProvider.this.query(
                uri, projection, selection, selectionArgs, sortOrder,
                CancellationSignal.fromTransport(cancellationSignal));
        //...
    }

[ContentProvider]: https://img-blog.csdn.net/20170215223248307?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvRXRoYW5Dbw==/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[ContentProvider 1]: https://img-blog.csdn.net/20170215223324820?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvRXRoYW5Dbw==/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast