Android内容提供者（ContentProvider）浅析（二）

前文提到了，在ActivityThread中有三个map保存了当前的provider信息，mLocalProviders、mLocalProvidersByName和mProviderMap。事实上mLocalProviders是作为收集组件和卸载provider时候用的。mLocalProvidersByName中保存的key是组件名，它与mProviderMap的区别也就是在这，mProviderMap的key值是ProviderKey一个与authority和uid有关的东西，这就决定了我们在获取已经存在的provider组件时都是从mProviderMap获取的，因为我们一般都是通过authority来进行操作的。

继续分析我们的流程，上一步调用了一个重要的方法用来将创建的provider信息保存到Service中，为什么要保存呢？因为我们的provider是一种支持跨进程数据接口，要让其他应用也能够使用最好的办法就是找一个中间机构把所有已经就绪的provider保存起来想要的时候直接去找。

ActivityManagerNative.getDefault().publishContentProviders(  
            getApplicationThread(), results);  

在ActivityManagerService中来看看这个方法

    public final void publishContentProviders(IApplicationThread caller,
            List providers) {
        if (providers == null) {
            return;
        }

        enforceNotIsolatedCaller("publishContentProviders");
        synchronized (this) {
            final ProcessRecord r = getRecordForAppLocked(caller);
            if (r == null) {
                throw new SecurityException(
                        "Unable to find app for caller " + caller
                      + " (pid=" + Binder.getCallingPid()
                      + ") when publishing content providers");
            }

            final long origId = Binder.clearCallingIdentity();

            final int N = providers.size();
            for (int i=0; i

其实一看也没做什么事情，主要就是将provider信息保存到了mProviderMap里面。到这里加载provider的过程就完了。下面继续看看第二个大问题：ContentProvider的调用。

大家都知道是contentprovider是这么用的

getContentResolver().query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder);对了，那我们就从getContentResolver()说起

简单给大家列一下这里边的方法堆栈调用，然后就其他几个重要的点特别说明一下。

getContentResolver()获取到的就是ContextImpl中的mContentResolver，它是一个ApplicationContentResolver继承于ContentResolver。看看执行query发生了什么事情，ContentResolver又是如何与对应的ContentProvider发生关系的呢，要说清楚这个还得先说说ContentProvider是如何具备跨进程通信能力的。这里有三个类功不可没，IContentProvider、ContentProviderNative和ContentProviderProxy。这几个名字有木有很熟悉，还是一个代理模式还是Binder机制。获取到的IContentProvider接口实际上是一个在ContentProvider类的内部所创建的一个Transport对象的远程接口。这个Transport类继承了ContentProviderNative类，是一个Binder对象的Stub类，因此，接下来就会进入到这个Binder对象的Proxy类ContentProviderProxy中执行对应操作。

    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;
        try {
            long startTime = SystemClock.uptimeMillis();

            ICancellationSignal remoteCancellationSignal = null;
            if (cancellationSignal != null) {
                cancellationSignal.throwIfCanceled();
                remoteCancellationSignal = unstableProvider.createCancellationSignal();
                cancellationSignal.setRemote(remoteCancellationSignal);
            }
            Cursor qCursor;
            try {
                qCursor = unstableProvider.query(uri, projection,
                        selection, selectionArgs, sortOrder, remoteCancellationSignal);
            } catch (DeadObjectException e) { 
                // The remote process has died...  but we only hold an unstable
                // reference though, so we might recover!!!  Let's try!!!!
                // This is exciting!!1!!1!!!!1
                unstableProviderDied(unstableProvider);
                stableProvider = acquireProvider(uri);
                if (stableProvider == null) {
                    return null;
                }
                qCursor = stableProvider.query(uri, projection,
                        selection, selectionArgs, sortOrder, remoteCancellationSignal);
            }
            if (qCursor == null) {
                return null;
            }
            // force query execution
            qCursor.getCount();
            long durationMillis = SystemClock.uptimeMillis() - startTime;
            maybeLogQueryToEventLog(durationMillis, uri, projection, selection, sortOrder);
            // Wrap the cursor object into CursorWrapperInner object
            CursorWrapperInner wrapper = new CursorWrapperInner(qCursor,
                    stableProvider != null ? stableProvider : acquireProvider(uri));
            stableProvider = null;
            return wrapper;
        } catch (RemoteException e) {
            // Arbitrary and not worth documenting, as Activity
            // Manager will kill this process shortly anyway.
            return null;
        } finally { 
            if (unstableProvider != null) {
                releaseUnstableProvider(unstableProvider);
            }
            if (stableProvider != null) {
                releaseProvider(stableProvider);
            }
        }
    }

这就是ContentResolver的query方法，这里最核心就是几个acquireProvider的方法，也就是获取ContentProvider的地方。它们最后都会调到ActivityThread的acquireProvider方法

    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;
}

这里有三步，首先会调用acquireExistingProvider在本进程中去查找其实就是在前面提到的mProviderMap去找，如果没有就会执行第二步去ActivityManagerService中去getContentProvider，找到之后继续第三不将这个对象保存到当前进程中。SO，同一个进程中使用的是同一个ContentProvider对象。如果要去服务中找这个过程就是进程间的问题咯，下面继续分析

    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);
        }

        userId = handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(), userId,
                false, true, "getContentProvider", null);
        return getContentProviderImpl(caller, name, null, stable, userId);
    }

    private final ContentProviderHolder getContentProviderImpl(IApplicationThread caller,
            String name, IBinder token, boolean stable, int userId) {
        ContentProviderRecord cpr;
        ContentProviderConnection conn = null;
        ProviderInfo cpi = null;

        synchronized(this) {
            ProcessRecord r = null;
            if (caller != null) {
                r = getRecordForAppLocked(caller);
                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 = mProviderMap.getProviderByName(name, userId);
            boolean providerRunning = cpr != null;
            if (providerRunning) {
                cpi = cpr.info;
                String msg; 
                if ((msg=checkContentProviderPermissionLocked(cpi, r)) != null) {
                    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.
                    ContentProviderHolder holder = cpr.newHolder(null);
                    // don't give caller the provider object, it needs
                    // to make its own.
                    holder.provider = null;
                    return holder;
                }

                final long origId = Binder.clearCallingIdentity();

                // In this case the provider instance already exists, so we can
                // return it right away.
                conn = incProviderCountLocked(r, cpr, token, stable); 
                if (conn != null && (conn.stableCount+conn.unstableCount) == 1) {
                    if (cpr.proc != null && r.setAdj <= ProcessList.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.proc, false);
                    }
                }

                if (cpr.proc != null) {
                    if (false) {
                        if (cpr.name.flattenToShortString().equals(
                                "com.android.providers.calendar/.CalendarProvider2")) {
                            Slog.v(TAG, "****************** KILLING "
                                + cpr.name.flattenToShortString());
                            Process.killProcess(cpr.proc.pid);
                        }
                    }
                    boolean success = updateOomAdjLocked(cpr.proc);
                    if (DEBUG_PROVIDER) Slog.i(TAG, "Adjust success: " + success); 
                    // NOTE: there is still a race here where a signal could be
                    // pending on the process even though we managed to update its
                    // adj level.  Not sure what to do about this, but at least
                    // the race is now smaller.
                    if (!success) {
                        // Uh oh...  it looks like the provider's process
                        // has been killed on us.  We need to wait for a new
                        // process to be started, and make sure its death
                        // doesn't kill our process.
                        Slog.i(TAG,
                                "Existing provider " + cpr.name.flattenToShortString()
                                + " is crashing; detaching " + r);
                        boolean lastRef = decProviderCountLocked(conn, cpr, token, stable);
                        appDiedLocked(cpr.proc, cpr.proc.pid, cpr.proc.thread);
                        if (!lastRef) {
                            // This wasn't the last ref our process had on
                            // the provider...  we have now been killed, bail.
                            return null;
                        }
                        providerRunning = false; 
                        conn = null;
                    }
                }

                Binder.restoreCallingIdentity(origId);
            }

            boolean singleton;
            if (!providerRunning) {
                try {
                    cpi = AppGlobals.getPackageManager().
                        resolveContentProvider(name,
                            STOCK_PM_FLAGS | PackageManager.GET_URI_PERMISSION_PATTERNS, userId);
                } catch (RemoteException ex) {
                }
                if (cpi == null) {
                    return null;
                }
                singleton = isSingleton(cpi.processName, cpi.applicationInfo,
                        cpi.name, cpi.flags); 
                if (singleton) {
                    userId = 0;
                } 
                cpi.applicationInfo = getAppInfoForUser(cpi.applicationInfo, userId);

                String msg;
                if ((msg=checkContentProviderPermissionLocked(cpi, r)) != null) {
                    throw new SecurityException(msg);
                }

                if (!mProcessesReady && !mDidUpdate && !mWaitingUpdate
                        && !cpi.processName.equals("system")) {
                    // If this content provider does not run in the system
                    // process, and the system is not yet ready to run other
                    // processes, then fail fast instead of hanging.
                    throw new IllegalArgumentException(
                            "Attempt to launch content provider before system ready");
                }

                // Make sure that the user who owns this provider is started.  If not,
                // we don't want to allow it to run. 
                if (mStartedUsers.get(userId) == null) {
                    Slog.w(TAG, "Unable to launch app "
                            + cpi.applicationInfo.packageName + "/"
                            + cpi.applicationInfo.uid + " for provider "
                            + name + ": user " + userId + " is stopped");
                    return null;
                }

                ComponentName comp = new ComponentName(cpi.packageName, cpi.name);
                cpr = mProviderMap.getProviderByClass(comp, userId);
                final boolean firstClass = cpr == null;
                if (firstClass) {
                    try {
                        ApplicationInfo ai =
                            AppGlobals.getPackageManager().
                                getApplicationInfo(
                                        cpi.applicationInfo.packageName,
                                        STOCK_PM_FLAGS, userId); 
                        if (ai == null) {
                            Slog.w(TAG, "No package info for content provider "
                                    + cpi.name);
                            return null;
                        }
                        ai = getAppInfoForUser(ai, userId);
                        cpr = new ContentProviderRecord(this, cpi, ai, comp, singleton);
                    } catch (RemoteException ex) {
                        // pm is in same process, this will never happen.
                    }
                }

                if (r != null && cpr.canRunHere(r)) { 
                    // If this is a multiprocess provider, then just return its
                    // info and allow the caller to instantiate it.  Only do
                    // this if the provider is the same user as the caller's
                    // process, or can run as root (so can be in any process).
                    return cpr.newHolder(null);
                }

                if (DEBUG_PROVIDER) {
                    RuntimeException e = new RuntimeException("here");
                    Slog.w(TAG, "LAUNCHING REMOTE PROVIDER (myuid " + r.uid
                          + " pruid " + cpr.appInfo.uid + "): " + cpr.info.name, e);
                }

                // This is single process, and our app is now connecting to it.
                // See if we are already in the process of launching this
                // provider.
                final int N = mLaunchingProviders.size();
                int i; 
                for (i=0; i= N) {
                    final long origId = Binder.clearCallingIdentity();

                    try {
                        // Content provider is now in use, its package can't be stopped.
                        try {
                            AppGlobals.getPackageManager().setPackageStoppedState(
                                    cpr.appInfo.packageName, false, userId);
                        } catch (RemoteException e) {
                        } catch (IllegalArgumentException e) {
                            Slog.w(TAG, "Failed trying to unstop package "
                                    + cpr.appInfo.packageName + ": " + e);
                        } 

                        ProcessRecord proc = startProcessLocked(cpi.processName,
                                cpr.appInfo, false, 0, "content provider",
                                new ComponentName(cpi.applicationInfo.packageName,
                                        cpi.name), false, false);
                        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);
                    } finally {
                        Binder.restoreCallingIdentity(origId);
                    }
                } 

                // Make sure the provider is published (the same provider class
                // may be published under multiple names).
                if (firstClass) {
                    mProviderMap.putProviderByClass(comp, cpr);
                }

                mProviderMap.putProviderByName(name, cpr);
                conn = incProviderCountLocked(r, cpr, token, stable);
                if (conn != null) {
                    conn.waiting = true;
                }
            }
        } 

        // 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,
                            UserHandle.getUserId(cpi.applicationInfo.uid),
                            cpi.applicationInfo.packageName,
                            cpi.applicationInfo.uid, name);
                    return null;
                }
                try {
                    if (DEBUG_MU) {
                        Slog.v(TAG_MU, "Waiting to start provider " + cpr + " launchingApp="
                                + cpr.launchingApp);
                    }
                    if (conn != null) {
                        conn.waiting = true;
                    }
                    cpr.wait();
                } catch (InterruptedException ex) { 
                } finally {
                    if (conn != null) {
                        conn.waiting = false;
                    }
                }
            }
        }
        return cpr != null ? cpr.newHolder(conn) : null;
    }

上面这个方法需要大家细细的看一下，里面有一些需要注意的地方我已经将原有注释标红了。在AndroidManifest中注册ContentProvider时可以设置一个android:multiprocess="true"，意思是说配置当前Provider组件对象是否可以运行在多个进程（配置进程之间是否是单例）。同时这里还可以很明显的看到关于进程优先级的概念，在某些生命周期时设置不运行结束当前进程等维护LRU的操作。

大家注意到了上面的方法中还有一个ContentProviderConnection的东东，这个作用还没有说到，另外还有第三个问题，内容监听机制也会在后文中说明。