ContentProvider学习笔记

Posted 2020-07-31

一、什么ContentProvider

二、如何使用ContentProvider

三、沙场练兵-实例操练

四、深入理解ContentProvider原理

为什么使用ContentProvider可以实现跨进程的通讯，第一反应肯定是这货和binder有关，因为android中只有稍微跟跨进程搭上边的，必定想到binder。

下面就来分析ContentProvider是怎么一步一步利用binder实现跨进程通信的：

1、首先你得创建一个ContentProvider运行在进程A,如上篇博客        

  AndroidManifest.xml中定义provider：

<provider
    android:name=".MyContentProvider"
    android:authorities="telefk"//这个很重要，Uri中的主机名，查询数据就靠它
    android:enabled="true"
    android:exported="true">
</provider>

 

2、进程B中通过如下接口来访问进程A的数据：

    getContentResolver().query(uri,columns,null,null,null);

getContentResolver（）实际调用的ContextImpl.java的方法返回mContentResolver对象

此对象在ContextImpl的构造方法中实例化

    mContentResolver = new ApplicationContentResolver(this, mainThread, user);

现在来看query方法，其在ContentResolver.java中定义：

public final @Nullable Cursor query(final @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);//这个函数很关键，通过指定的Uri找到我们刚刚定义的provider
    if (unstableProvider == null) {
           return null;
    }
    IContentProvider stableProvider = null;
      ......

接下来看acquireUnstableProvider()函数：

百转千回，最终走到了刚刚返回的ApplicationContentResolver.java中

@Override
protected IContentProvider acquireUnstableProvider(Context c, String auth) {
    return mMainThread.acquireProvider(c,
            ContentProvider.getAuthorityWithoutUserId(auth),
            resolveUserIdFromAuthority(auth), false);
}

这里又调用了ActivityThread.java 的acquireProvider()方法，我们继续往下跟：

    public final IContentProvider acquireProvider(
            Context c, String auth, int userId, boolean stable) {
        
        final IContentProvider provider = acquireExistingProvider(c, auth, userId, stable);//刚开始这里很定为空
        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);//很关键，请注意这里跨进程调到ActivityManagerService.java的方法
        } 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;
    }

　　我们继续来看ActivityManagerService.java 的getContentProvider()方法:

    @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);
    }

    肾呐～还没到头！又继续往下走，大家继续坚持：

    
//这个方法很长，我们挑选重点的分析，方法第二个参数name就是我们刚刚query传进来的uir的authority
private final ContentProviderHolder getContentProviderImpl(IApplicationThread caller,
            String name, IBinder token, boolean stable, int userId) {
    ContentProviderRecord cpr;//AMS中用来记录provider的，还有其他三大组件AcitityRecord、ServiceRecord、BroadcastRecord
    ContentProviderConnection conn = null;
    ProviderInfo cpi = null;//这个应该是PMS解析应用的AndroidManifest得来的信息
    ......

     // First check if this content provider has been published...
     cpr = mProviderMap.getProviderByName(name, userId);

     ......

如果发现对应的provider 已经运行，这个我们之后再分析，先分析没有运行的情况：

  boolean providerRunning = cpr != null;
            if (providerRunning) {
                cpi = cpr.info;
                String msg;
                checkTime(startTime, "getContentProviderImpl: before checkContentProviderPermission");
                if ((msg = checkContentProviderPermissionLocked(cpi, r, userId, checkCrossUser))
                        != null) {
                    throw new SecurityException(msg);
                }
                checkTime(startTime, "getContentProviderImpl: after checkContentProviderPermission");

                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();

                checkTime(startTime, "getContentProviderImpl: incProviderCountLocked");

                // 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.
                        checkTime(startTime, "getContentProviderImpl: before updateLruProcess");
                        updateLruProcessLocked(cpr.proc, false, null);
                        checkTime(startTime, "getContentProviderImpl: after updateLruProcess");
                    }
                }

                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);
                        }
                    }
                    checkTime(startTime, "getContentProviderImpl: before updateOomAdj");
                    boolean success = updateOomAdjLocked(cpr.proc);
                    maybeUpdateProviderUsageStatsLocked(r, cpr.info.packageName, name);
                    checkTime(startTime, "getContentProviderImpl: after updateOomAdj");
                    if (DEBUG_PROVIDER) Slog.i(TAG_PROVIDER, "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);
                        checkTime(startTime, "getContentProviderImpl: before appDied");
                        appDiedLocked(cpr.proc);
                        checkTime(startTime, "getContentProviderImpl: after appDied");
                        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);
            }

　　

            if (!providerRunning) {
                try {
                    checkTime(startTime, "getContentProviderImpl: before resolveContentProvider");
                    cpi = AppGlobals.getPackageManager().
                        resolveContentProvider(name,
                            STOCK_PM_FLAGS | PackageManager.GET_URI_PERMISSION_PATTERNS, userId);
                    checkTime(startTime, "getContentProviderImpl: after resolveContentProvider");
                } catch (RemoteException ex) {
                }
                if (cpi == null) {
                    return null;
                }
                // If the provider is a singleton AND
                // (it‘s a call within the same user || the provider is a
                // privileged app)
                // Then allow connecting to the singleton provider
                singleton = isSingleton(cpi.processName, cpi.applicationInfo,
                        cpi.name, cpi.flags)
                        && isValidSingletonCall(r.uid, cpi.applicationInfo.uid);
                if (singleton) {
                    userId = UserHandle.USER_OWNER;
                }
                cpi.applicationInfo = getAppInfoForUser(cpi.applicationInfo, userId);
                checkTime(startTime, "getContentProviderImpl: got app info for user");

                String msg;
                checkTime(startTime, "getContentProviderImpl: before checkContentProviderPermission");
                if ((msg = checkContentProviderPermissionLocked(cpi, r, userId, !singleton))
                        != null) {
                    throw new SecurityException(msg);
                }
                checkTime(startTime, "getContentProviderImpl: after checkContentProviderPermission");

                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 running.  If not,
                // we don‘t want to allow it to run.
                if (!isUserRunningLocked(userId, false)) {
                    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);
                checkTime(startTime, "getContentProviderImpl: before getProviderByClass");
                cpr = mProviderMap.getProviderByClass(comp, userId);
                checkTime(startTime, "getContentProviderImpl: after getProviderByClass");
                final boolean firstClass = cpr == null;
                if (firstClass) {
                    final long ident = Binder.clearCallingIdentity();
                    try {
                        checkTime(startTime, "getContentProviderImpl: before getApplicationInfo");
                        ApplicationInfo ai =
                            AppGlobals.getPackageManager().
                                getApplicationInfo(
                                        cpi.applicationInfo.packageName,
                                        STOCK_PM_FLAGS, userId);
                        checkTime(startTime, "getContentProviderImpl: after getApplicationInfo");
                        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);//很关键的，首先创建cpr,然后再启动目标进程