本文深入解析Android Framework 8.0中ContentProvider的工作原理，重点介绍了客户端应用程序如何通过系统服务获取ContentProvider接口，涵盖数据对象介绍、获取流程及关键步骤。
本篇基于Android Framework 8.0的源码
对ContentProvider的使用从根本上来说也是围绕着Binder IPC。跟使用其他系统服务类似，APP对ContentProvider的使用可以分成三部分：
- Client APP如何从系统服务中获取到ContentProvider的调用接口
- ContentProvider如何将自己的调用接口发布到系统服务当中。
- Client APP调用ContentProvider接口的具体流程。
本文的核心便是Client APP如何从系统服务中获取到ContentProvider的调用接口。
重要的数据对象
1. ContentProviderRecord
在记录ContentProvider重要信息的数据对象中，毫无疑问ContentProviderRecord是最重要的。无论是在ProcessRecord中记录本APP内已经发布的ContentProvider，还是在AMS中记录所有APP发布的ContentProvider。
ContentProviderRecord中记录的重要内容：
2. ProviderClientRecord
另外一个重要的则是ProviderClientRecord，用于在Client APP中缓存已经获得的ContentProvider信息（包括本地的ContentProvider）。在使用ContentProvider时，如果发现缓存中已经存在对应的ContentProvider信息，则不再向AMS请求获取ContentProvider接口，而是直接使用缓存中记录的IContentProvider接口。
获取ContentProvider的流程解析
在Client APP使用过程中，一般不会直接与ContentProvider进行通信，使用ContentProvider的请求都是交由ContentResolver来执行。平时获取ContentResolver都是通过Context的getContentResolver()来执行，最终都会执行的是ContextImpl中的getContentResolver():
//ContextImpl.java
public ContentResolver getContentResolver() {
return mContentResolver;
}
复制代码返回的是已经保存在Context中的mContentResolver，这一实例的初始化在ContextImpl的相关初始化函数当中：( 8.0在私有构造器中，4.2在init函数当中 )
//ContextImpl.java
...
mContentResolver = new ApplicationContentResolver(this, mainThread, user);
...
复制代码这里赋值的实例ApplicationContentResolver类型，这是一个ContentResolver接口的实现类，里边重写了获取和释放ContentProvider的逻辑。
@Override
protected IContentProvider acquireProvider(Context context, String auth) {
return mMainThread.acquireProvider(context,
ContentProvider.getAuthorityWithoutUserId(auth),
resolveUserIdFromAuthority(auth), true);
}
@Override
protected IContentProvider acquireExistingProvider(Context context, String auth) {
return mMainThread.acquireExistingProvider(context,
ContentProvider.getAuthorityWithoutUserId(auth),
resolveUserIdFromAuthority(auth), true);
}
@Override
public boolean releaseProvider(IContentProvider provider) {
return mMainThread.releaseProvider(provider, true);
}
@Override
protected IContentProvider acquireUnstableProvider(Context c, String auth) {
return mMainThread.acquireProvider(c,
ContentProvider.getAuthorityWithoutUserId(auth),
resolveUserIdFromAuthority(auth), false);
}
@Override
public boolean releaseUnstableProvider(IContentProvider icp) {
return mMainThread.releaseProvider(icp, false);
}
@Override
public void unstableProviderDied(IContentProvider icp) {
mMainThread.handleUnstableProviderDied(icp.asBinder(), true);
}
@Override
public void appNotRespondingViaProvider(IContentProvider icp) {
mMainThread.appNotRespondingViaProvider(icp.asBinder());
}
复制代码具体方法实现的内容只是相当于转发了一下请求到ActivityThread当中去执行具体的逻辑，由ActivityThread来负责同AMS进行交互。
acquireExistingProvider、acquireProvider和acquireUnstableProvider这三个方法最终都是转发执行的同一个方法ActivityThread.acquireProvider()。区别在于最后一个标识符，true代表stable provider，false代表unstable provider. 在ActivityThread中的实现：
final IContentProvider provider = acquireExistingProvider(c, auth, userId, stable);
if (provider != null) {
return provider;
}
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;
}
holder = installProvider(c, holder, holder.info,
true /*noisy*/, holder.noReleaseNeeded, stable);
return holder.provider;
复制代码acquireProvider() 分为三部分：
- 1.首先尝试获取缓存的IContentProvider。
- 2.获取不到才通过AMS获取IContentProvider。
- 3.获取成功后先执行ActivityThread.installProvider再返回provider给ContentResolver。
第一部分：获取缓存中的IContentProvider的逻辑：
synchronized (mProviderMap) {
final ProviderKey key = new ProviderKey(auth, userId);
final ProviderClientRecord pr = mProviderMap.get(key);
if (pr == null) {
return null;
}
IContentProvider provider = pr.mProvider;
IBinder jBinder = provider.asBinder();
if (!jBinder.isBinderAlive()) {
Log.i(TAG, "Acquiring provider " + auth + " for user " + userId
+ ": existing object's process dead");
handleUnstableProviderDiedLocked(jBinder, true);
return null;
}
ProviderRefCount prc = mProviderRefCountMap.get(jBinder);
if (prc != null) {
incProviderRefLocked(prc, stable);
}
return provider;
}
复制代码- 由于接口可能会被多线程调用，因此这里先加了锁。
- 首先通过auth、userId从mProviderMap中获取provider。
- 如果该provider所在的进程已经死亡则调用handleUnstableProviderDiedLocked，并返回null。
- 在handleUnstableProviderDiedLocked()里，如果死亡的provider保存在需要释放的Provider列表里，则对其进行释放清理；并通知AMS该unstable provider已经死亡。
- 如果保存在需要释放的Provider列表里，则对其引用计数+1。
第二部分：通过AMS.getContentProvider()获取IContentProvider接口
最后都在 AMS.getContentProviderImpl() 中执行。(这里是Blocking执行，lock是AMS。这把锁很多地方都用到，比方注册广播接收器时也是用的这把锁)
ActivityManagerService's getContentProviderImpl()
long startTime = SystemClock.uptimeMillis();
//获取Caller的进程信息
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);
}
}
boolean checkCrossUser = true;
checkTime(startTime, "getContentProviderImpl: getProviderByName");
//检查对应的ContentProvider是否已经存在
cpr = mProviderMap.getProviderByName(name, userId);
if (cpr == null && userId != UserHandle.USER_SYSTEM) {
cpr = mProviderMap.getProviderByName(name, UserHandle.USER_SYSTEM);
if (cpr != null) {
cpi = cpr.info;
if (isSingleton(cpi.processName, cpi.applicationInfo,
cpi.name, cpi.flags)
&& isValidSingletonCall(r.uid, cpi.applicationInfo.uid)) {
userId = UserHandle.USER_SYSTEM;
checkCrossUser = false;
} else {
//不是单例则清空继续执行获取
cpr = null;
cpi = null;
}
}
}
// ContentProvider存在同时所在进程存在&该进程没被杀
boolean providerRunning = cpr != null && cpr.proc != null && !cpr.proc.killed;
// 后续的执行的内容分段讲解
...
...
复制代码AMS会先尝试获取缓存的ContentProvider信息，并判断该ContentProvider是否保持运行，并以此为根据来进行后续的执行。后续方法体的执行分为三部分：
1.ContentProvider保持运行：
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");
//如果provider已经发布或者正在发布则直接返回。
if (r != null && cpr.canRunHere(r)) {
ContentProviderHolder holder = cpr.newHolder(null);
holder.provider = null;
return holder;
}
try {
if (AppGlobals.getPackageManager()
.resolveContentProvider(name, 0 /*flags*/, userId) == null) {
return null;
}
} catch (RemoteException e) {
}
final long origId = Binder.clearCallingIdentity();
checkTime(startTime, "getContentProviderImpl: incProviderCountLocked");
// 增加引用计数
conn = incProviderCountLocked(r, cpr, token, stable);
if (conn != null && (conn.stableCount+conn.unstableCount) == 1) {
if (cpr.proc != null && r.setAdj <= ProcessList.PERCEPTIBLE_APP_ADJ) {
//更新LRU
checkTime(startTime, "getContentProviderImpl: before updateLruProcess");
updateLruProcessLocked(cpr.proc, false, null);
checkTime(startTime, "getContentProviderImpl: after updateLruProcess");
}
}
checkTime(startTime, "getContentProviderImpl: before updateOomAdj");
final int verifiedAdj = cpr.proc.verifiedAdj;
//更新进程adj
boolean success = updateOomAdjLocked(cpr.proc, true);
if (success && verifiedAdj != cpr.proc.setAdj && !isProcessAliveLocked(cpr.proc)) {
success = false;
}
maybeUpdateProviderUsageStatsLocked(r, cpr.info.packageName, name);
checkTime(startTime, "getContentProviderImpl: after updateOomAdj");
if (DEBUG_PROVIDER) Slog.i(TAG_PROVIDER, "Adjust success: " + success);
//进程已经被杀
if (!success) {
//确保自己不为同时杀掉
Slog.i(TAG, "Existing provider " + cpr.name.flattenToShortString()
+ " is crashing; detaching " + r);
//减少引用计数
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;
} else {
cpr.proc.verifiedAdj = cpr.proc.setAdj;
}
Binder.restoreCallingIdentity(origId);
}
复制代码关键的步骤都在上面有中文注释，值得注意的是如果减少了引用计数之后，Client和ContentProider之间还有Connection，会导致Client进程被杀掉，所以直接返回空。
2.ContentProvider没有在运行
if (!providerRunning) {
try {
checkTime(startTime, "getContentProviderImpl: before resolveContentProvider");
//先尝试从PackageManager中获取ContentProviderInfo
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;
}
boolean singleton = isSingleton(cpi.processName, cpi.applicationInfo,
cpi.name, cpi.flags)
&& isValidSingletonCall(r.uid, cpi.applicationInfo.uid);
if (singleton) {
userId = UserHandle.USER_SYSTEM;
}
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
&& !cpi.processName.equals("system")) {
throw new IllegalArgumentException(
"Attempt to launch content provider before system ready");
}
//如果provider的提供用户没有在运行则直接返回。
if (!mUserController.isUserRunningLocked(userId, 0)) {
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");
//尝试从通过包名从缓存中获取ContentProvider
cpr = mProviderMap.getProviderByClass(comp, userId);
checkTime(startTime, "getContentProviderImpl: after getProviderByClass");
final boolean firstClass = cpr == null;
//没有缓存在进入处理，尝试获取新的ContentProviderRecord.
if (firstClass) {
final long ident = Binder.clearCallingIdentity();
if (mPermissionReviewRequired) {
if (!requestTargetProviderPermissionsReviewIfNeededLocked(cpi, r, userId)) {
return null;
}
}
try {
checkTime(startTime, "getContentProviderImpl: before getApplicationInfo");
//根据包名和userId尝试获取新的ApplicationInfo.
ApplicationInfo ai =
AppGlobals.getPackageManager().
getApplicationInfo(
cpi.applicationInfo.packageName,
STOCK_PM_FLAGS, userId);
checkTime(startTime, "getContentProviderImpl: after getApplicationInfo");
//安装包中获取不到对应的信息，直接返回
if (ai == null) {
Slog.w(TAG, "No package info for content provider "
+ cpi.name);
return null;
}
ai = getAppInfoForUser(ai, userId);
//封装在新的ContentProviderRecord
cpr = new ContentProviderRecord(this, cpi, ai, comp, singleton);
} catch (RemoteException ex) {
// pm is in same process, this will never happen.
} finally {
Binder.restoreCallingIdentity(ident);
}
}
checkTime(startTime, "getContentProviderImpl: now have ContentProviderRecord");
if (r != null && cpr.canRunHere(r)) {
//如果ContentProvider声明了多进程运行或者处于同一个进程或者拥有同样的uid, 则直接返回。
return cpr.newHolder(null);
}
if (DEBUG_PROVIDER) Slog.w(TAG_PROVIDER, "LAUNCHING REMOTE PROVIDER (myuid "
+ (r != null ? r.uid : null) + " pruid " + cpr.appInfo.uid + "): "
+ cpr.info.name + " callers=" + Debug.getCallers(6));
//如果该ContentProvider已经正在启动中，则i < 正在启动的ContentProvider数。
final int N = mLaunchingProviders.size();
int i;
for (i = 0; i < N; i++) {
if (mLaunchingProviders.get(i) == cpr) {
break;
}
}
//没有在启动中，启动它。
if (i >= N) {
final long origId = Binder.clearCallingIdentity();
try {
//设置包状态
try {
checkTime(startTime, "getContentProviderImpl: before set stopped state");
AppGlobals.getPackageManager().setPackageStoppedState(
cpr.appInfo.packageName, false, userId);
checkTime(startTime, "getContentProviderImpl: after set stopped state");
} catch (RemoteException e) {
} catch (IllegalArgumentException e) {
Slog.w(TAG, "Failed trying to unstop package "
+ cpr.appInfo.packageName + ": " + e);
}
//获取对应的进程信息
checkTime(startTime, "getContentProviderImpl: looking for process record");
ProcessRecord proc = getProcessRecordLocked(
cpi.processName, cpr.appInfo.uid, false);
if (proc != null && proc.thread != null && !proc.killed) {
if (DEBUG_PROVIDER) Slog.d(TAG_PROVIDER,
"Installing in existing process " + proc);
if (!proc.pubProviders.containsKey(cpi.name)) {
checkTime(startTime, "getContentProviderImpl: scheduling install");
//provider对应的进程已经启动，但是该provider还没有被publish
//因此发送消息触发publish provider的操作。
proc.pubProviders.put(cpi.name, cpr);
try {
proc.thread.scheduleInstallProvider(cpi);
} catch (RemoteException e) {
}
}
} else {
//provider所在的进程没有启动，或者已经被杀
//启动provider对应的进程
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);
checkTime(startTime, "getContentProviderImpl: after start process");
if (proc == null) {
//无法启动，直接返回空
Slog.w(TAG, "Unable to launch app "
+ cpi.applicationInfo.packageName + "/"
+ cpi.applicationInfo.uid + " for provider "
+ name + ": process is bad");
return null;
}
}
//将ContentProviderRecord中保存的launchingApp设置为所在的进程，并将provider
//保存到正在启动的ContentProvider列表
cpr.launchingApp = proc;
mLaunchingProviders.add(cpr);
} finally {
Binder.restoreCallingIdentity(origId);
}
}
checkTime(startTime, "getContentProviderImpl: updating data structures");
//保存新的ContentProviderRecord到mProviderMap中。
if (firstClass) {
mProviderMap.putProviderByClass(comp, cpr);
}
mProviderMap.putProviderByName(name, cpr);
//增加Client与ContentProvider之间的引用计数。
conn = incProviderCountLocked(r, cpr, token, stable);
if (conn != null) {
conn.waiting = true;
}
}
复制代码关键步骤在文中有注释，主要流程是:
- 权限检查
- 通过PM获取对应provider信息
- 检查provider是否能够正常运行
- 启动对应的进程，然后publish provider
- 更新包状态、缓存、引用计数。
3. 轮询确保provider被publish完成
synchronized (cpr) {
while (cpr.provider == null) {
//provider所在进程已经不存在则直接返回
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);
//Client与ContentProvider之间有引用时才执行
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;
复制代码概要总结getContentProviderImpl是如何实现获取ContentProvider： 总是先尝试从缓存中获取已经publish的provider，并返回；如果缓存中不存在才尝试创建新的provider并启动对应的进程。
第三部分：执行ActivityThread.installProvider()
private ContentProviderHolder installProvider(Context context,
ContentProviderHolder holder, ProviderInfo info,
boolean noisy, boolean noReleaseNeeded, boolean stable) {
ContentProvider localProvider = null;
IContentProvider provider;
if (holder == null || holder.provider == null) {
//本地provider
if (DEBUG_PROVIDER || noisy) {
Slog.d(TAG, "Loading provider " + info.authority + ": "
+ info.name);
}
Context c = null;
ApplicationInfo ai = info.applicationInfo;
//provider与caller在同一个包内
if (context.getPackageName().equals(ai.packageName)) {
c = context;
} else if (mInitialApplication != null &&
mInitialApplication.getPackageName().equals(ai.packageName)) {
c = mInitialApplication;
} else {
try {
c = context.createPackageContext(ai.packageName,
Context.CONTEXT_INCLUDE_CODE);
} catch (PackageManager.NameNotFoundException e) {
// Ignore
}
}
if (c == null) {
Slog.w(TAG, "Unable to get context for package " +
ai.packageName +
" while loading content provider " +
info.name);
return null;
}
if (info.splitName != null) {
try {
c = c.createContextForSplit(info.splitName);
} catch (NameNotFoundException e) {
throw new RuntimeException(e);
}
}
//创建本地provider
try {
final java.lang.ClassLoader cl = c.getClassLoader();
localProvider = (ContentProvider)cl.
loadClass(info.name).newInstance();
provider = localProvider.getIContentProvider();
if (provider == null) {
Slog.e(TAG, "Failed to instantiate class " +
info.name + " from sourceDir " +
info.applicationInfo.sourceDir);
return null;
}
if (DEBUG_PROVIDER) Slog.v(
TAG, "Instantiating local provider " + info.name);
// XXX Need to create the correct context for this provider.
//创建本地ContentProvider的Context
localProvider.attachInfo(c, info);
} catch (java.lang.Exception e) {
if (!mInstrumentation.onException(null, e)) {
throw new RuntimeException(
"Unable to get provider " + info.name
+ ": " + e.toString(), e);
}
return null;
}
} else {
//来自本App外的ContentProvider
provider = holder.provider;
if (DEBUG_PROVIDER) Slog.v(TAG, "Installing external provider " + info.authority + ": "
+ info.name);
}
ContentProviderHolder retHolder;
synchronized (mProviderMap) {
if (DEBUG_PROVIDER) Slog.v(TAG, "Checking to add " + provider
+ " / " + info.name);
IBinder jBinder = provider.asBinder();
if (localProvider != null) {
//本地provider分支
ComponentName cname = new ComponentName(info.packageName, info.name);
ProviderClientRecord pr = mLocalProvidersByName.get(cname);
if (pr != null) {
//缓存中已经存在本地provider实例。
if (DEBUG_PROVIDER) {
Slog.v(TAG, "installProvider: lost the race, "
+ "using existing local provider");
}
provider = pr.mProvider;
} else {
//不存在，创建新的Client provider record并install，
//然后保存到本地的缓存中
holder = new ContentProviderHolder(info);
holder.provider = provider;
holder.noReleaseNeeded = true;
pr = installProviderAuthoritiesLocked(provider, localProvider, holder);
mLocalProviders.put(jBinder, pr);
mLocalProvidersByName.put(cname, pr);
}
//返回值为install过的本地Content Provider Holder
retHolder = pr.mHolder;
} else {
ProviderRefCount prc = mProviderRefCountMap.get(jBinder);
if (prc != null) {
//不为空，说明本地provider 引用缓存中已经保存有该provider的引用计数。
if (DEBUG_PROVIDER) {
Slog.v(TAG, "installProvider: lost the race, updating ref count");
}
//外部provider则增加引用计数。
if (!noReleaseNeeded) {
incProviderRefLocked(prc, stable);
try {
ActivityManager.getService().removeContentProvider(
holder.connection, stable);
} catch (RemoteException e) {
//do nothing content provider object is dead any way
}
}
} else {
ProviderClientRecord client = installProviderAuthoritiesLocked(
provider, localProvider, holder);
if (noReleaseNeeded) {
//本地provider或者来自系统的provider
prc = new ProviderRefCount(holder, client, 1000, 1000);
} else {
//其他App的provider
prc = stable
? new ProviderRefCount(holder, client, 1, 0)
: new ProviderRefCount(holder, client, 0, 1);
}
//在本App的缓存中保存provider的引用计数。
mProviderRefCountMap.put(jBinder, prc);
}
retHolder = prc.holder;
}
}
return retHolder;
}
复制代码对于本地ContentProvider, installProvider做的事情是先创建一个本地provider，并将相应的内容（IContentProvider接口、本地ContentProvider实例等）保存到本App的缓存当中；对于外部的ContentProvider，installProvider做的事情则是在本App的缓存当中保存provider的引用计数。
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