[置顶] ContentProvider 原理分析一

本文目标：以MediaProvider为例，想搞清楚调用ContentResolver访问各个ContentProvider的调用过程。

Java code:

getContentResolver().query(MediaStore.Images.Thumbnails.EXTERNAL_CONTENT_URI,null,null)

具体调用过程是

1.通过ContentResolver先查找对应给定Uri的ContentProvider，返回对应的BinderProxy

如果该Provider尚未被调用进程使用过:

a.通过ServiceManager查找activity service得到ActivityManagerService对应BinderProxy

b.调用BinderProxy的transcat方法发送GET_CONTENT_PROVIDER_TRANSACTION命令，得到对应ContentProvider的BinderProxy。

如果该Provider已被调用进程使用过，则调用进程会保留使用过provider的HashMap。此时直接从此表查询即得。

2.调用BinderProxy的query()

通过下图，可以清楚的看到，getContentResolver().query调用时首先得到Actiivity服务，再次查询Activity服务中记录的对应ContentProviderRecord.如果发现此ContentProvider尚未publish则引发publish该ContentProvider，详见分析二一文。查询到ContentProviderRecord后返回对应MediaProvider的IBinder并返回给调用者。

整个调用过程中需要经过两次Binder调用以实现跨进程访问，即:

Calling Process -> ActivityManagerService Process -> MediaProvider process

Detailed call sequence(If calling process doesn't ever used the Provider):

源代码调用路径：

第1,2步：

frameworks/base/core/java/android/app/ContextImpl.java

    private static final class ApplicationContentResolver extends ContentResolver {
        public ApplicationContentResolver(Context context, ActivityThread mainThread) {
            super(context);
            mMainThread = mainThread;
        }

        @Override
        protected IContentProvider acquireProvider(Context context, String name) {
            return mMainThread.acquireProvider(context, name);
        }

ActivityThread

    public final IContentProvider acquireProvider(Context c, String name) {
        IContentProvider provider = getProvider(c, name);
        if(provider == null)
            return null;
        IBinder jBinder = provider.asBinder();
        synchronized(mProviderMap) {
            ProviderRefCount prc = mProviderRefCountMap.get(jBinder);
            if(prc == null) {
                mProviderRefCountMap.put(jBinder, new ProviderRefCount(1)); //创建对此Provider的引用计数
            } else {
                prc.count++; //计数+1
            } //end else
        } //end synchronized
        return provider;
    }

得到名字为name的Provider

    private final IContentProvider getProvider(Context context, String name) {
        IContentProvider existing = getExistingProvider(context, name);
        if (existing != null) { 
            return existing; //Provider已经publish,直接返回
        }

        IActivityManager.ContentProviderHolder holder = null;
        try {
            holder = ActivityManagerNative.getDefault().getContentProvider(
                getApplicationThread(), name);
        } catch (RemoteException ex) {
        }
        if (holder == null) {
            Slog.e(TAG, "Failed to find provider info for " + name);
            return null;
        }

        IContentProvider prov = installProvider(context, holder.provider,
                holder.info, true);
        if (holder.noReleaseNeeded || holder.provider == null) {
            // We are not going to release the provider if it is an external
            // provider that doesn't care about being released, or if it is
            // a local provider running in this process.
            //Slog.i(TAG, "*** NO RELEASE NEEDED");
            synchronized(mProviderMap) {
                mProviderRefCountMap.put(prov.asBinder(), new ProviderRefCount(10000)); //为何holder.provider == null??
            }
        }
        return prov;
    }

得到ActivityManagerService。

frameworks/base/core/java/android/app/ActivityManagerNative.java

    static public IActivityManager getDefault()
    {
        if (gDefault != null) {
            return gDefault;
        }
        IBinder b = ServiceManager.getService("activity");
        gDefault = asInterface(b);
        return gDefault;
    }

frameworks/base/services/java/com/android/server/am/ActivityManagerService.java

    public final ContentProviderHolder getContentProvider(
            IApplicationThread caller, String name) {
        if (caller == null) {
            String msg = "null IApplicationThread when getting content provider "
                    + name;
             throw new SecurityException(msg);
        }

        return getContentProviderImpl(caller, name);
    }

    private final ContentProviderHolder getContentProviderImpl(
        IApplicationThread caller, String name) {
        ContentProviderRecord cpr;
        ProviderInfo cpi = null;

        synchronized(this) {
            ProcessRecord r = null;
            if (caller != null) {
                r = getRecordForAppLocked(caller); //caller app must be registered
                if (r == null) {
                    throw new SecurityException(
                            "Unable to find app for caller " + caller
                          + " (pid=" + Binder.getCallingPid()
                          + ") when getting content provider " + name);
                }
            }

            // First check if this content provider has been published...
            cpr = mProvidersByName.get(name);
            if (cpr != null) {
                cpi = cpr.info;
                String msg;
                if ((msg=checkContentProviderPermissionLocked(cpi, r)) != null) { //检查app是否有访问权限
                    throw new SecurityException(msg);
                }

                if (r != null && cpr.canRunHere(r)) {
                    // This provider has been published or is in the process
                    // of being published...  but it is also allowed to run
                    // in the caller's process, so don't make a connection
                    // and just let the caller instantiate its own instance.
                    if (cpr.provider != null) {
                        // don't give caller the provider object, it needs
                        // to make its own.
                        cpr = new ContentProviderRecord(cpr);
                    }
                    return cpr;
                }

                final long origId = Binder.clearCallingIdentity();

                // In this case the provider instance already exists, so we can
                // return it right away.
                if (r != null) {
                    if (DEBUG_PROVIDER) Slog.v(TAG,
                            "Adding provider requested by "
                            + r.processName + " from process "
                            + cpr.info.processName);
                    Integer cnt = r.conProviders.get(cpr);
                    if (cnt == null) {
                        r.conProviders.put(cpr, new Integer(1));
                    } else {
                        r.conProviders.put(cpr, new Integer(cnt.intValue()+1));
                    }
                    cpr.clients.add(r);
                    if (cpr.app != null && r.setAdj <= PERCEPTIBLE_APP_ADJ) {
                        // If this is a perceptible app accessing the provider,
                        // make sure to count it as being accessed and thus
                        // back up on the LRU list.  This is good because
                        // content providers are often expensive to start.
                        updateLruProcessLocked(cpr.app, false, true);
                    }
                } else {
                    cpr.externals++;
                }

                if (cpr.app != null) {
                    updateOomAdjLocked(cpr.app);
                }

                Binder.restoreCallingIdentity(origId);

            } 
...
        }

        // Wait for the provider to be published...
        synchronized (cpr) {
            while (cpr.provider == null) {
                if (cpr.launchingApp == null) {
                    Slog.w(TAG, "Unable to launch app "
                            + cpi.applicationInfo.packageName + "/"
                            + cpi.applicationInfo.uid + " for provider "
                            + name + ": launching app became null");
                    EventLog.writeEvent(EventLogTags.AM_PROVIDER_LOST_PROCESS,
                            cpi.applicationInfo.packageName,
                            cpi.applicationInfo.uid, name);
                    return null;
                }
                try {
                    cpr.wait(); //publishContentProvider结束后会notify
                } catch (InterruptedException ex) {
                }
            }
        }
        return cpr;
    }

frameworks/base/core/java/android/content/ContentProviderNative.java

final class ContentProviderProxy implements IContentProvider
{
    public Cursor query(Uri url, String[] projection, String selection,
            String[] selectionArgs, String sortOrder) throws RemoteException {
        //TODO make a pool of windows so we can reuse memory dealers
        CursorWindow window = new CursorWindow(false /* window will be used remotely */);
        BulkCursorToCursorAdaptor adaptor = new BulkCursorToCursorAdaptor();
        IBulkCursor bulkCursor = bulkQueryInternal(
            url, projection, selection, selectionArgs, sortOrder,
            adaptor.getObserver(), window,
            adaptor);
        return adaptor;
    }
    private IBulkCursor bulkQueryInternal(
        Uri url, String[] projection,
        String selection, String[] selectionArgs, String sortOrder,
        IContentObserver observer, CursorWindow window,
        BulkCursorToCursorAdaptor adaptor) throws RemoteException {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IContentProvider.descriptor);

        url.writeToParcel(data, 0);
        int length = 0;
        if (projection != null) {
            length = projection.length;
        }
        data.writeInt(length);
        for (int i = 0; i < length; i++) {
            data.writeString(projection[i]);
        }
        data.writeString(selection);
        if (selectionArgs != null) {
            length = selectionArgs.length;
        } else {
            length = 0;
        }
        data.writeInt(length);
        for (int i = 0; i < length; i++) {
            data.writeString(selectionArgs[i]);
        }
        data.writeString(sortOrder);
        data.writeStrongBinder(observer.asBinder());
        window.writeToParcel(data, 0);

        // Flag for whether or not we want the number of rows in the
        // cursor and the position of the "_id" column index (or -1 if
        // non-existent).  Only to be returned if binder != null.
        final boolean wantsCursorMetadata = (adaptor != null);
        data.writeInt(wantsCursorMetadata ? 1 : 0);
        mRemote.transact(IContentProvider.QUERY_TRANSACTION, data, reply, 0);

        DatabaseUtils.readExceptionFromParcel(reply);

        IBulkCursor bulkCursor = null;
        IBinder bulkCursorBinder = reply.readStrongBinder();
        if (bulkCursorBinder != null) {
            bulkCursor = BulkCursorNative.asInterface(bulkCursorBinder);

            if (wantsCursorMetadata) {
                int rowCount = reply.readInt();
                int idColumnPosition = reply.readInt();
                if (bulkCursor != null) {
                    adaptor.set(bulkCursor, rowCount, idColumnPosition);
                }
            }
        }

        data.recycle();
        reply.recycle();

        return bulkCursor;
    }

frameworks/base/core/java/android/content/ContentProvider.java

    class Transport extends ContentProviderNative {
...
        public Cursor query(Uri uri, String[] projection,
                String selection, String[] selectionArgs, String sortOrder) {
            enforceReadPermission(uri);
            return ContentProvider.this.query(uri, projection, selection,
                    selectionArgs, sortOrder);
        }

MediaProvider.java

    public Cursor query(Uri uri, String[] projectionIn, String selection,
            String[] selectionArgs, String sort) {   //调用到真正做事情的地方