本文通过时序图简要分析ContentProvider的获取过程，重点解释installProvider()函数及其参数holder的作用，强调在不同进程如何实例化ContentProvider，并指出holder.provider在进程间通信中的角色。
关于ContentProvider网上有很多文章进行了详细的分析，这里我初略地画了一个getContentProvider时序图，帮助整体上进行理解把握。其中ActivityThread_A代表进程A的ActivityThread，ActivityThread_B代表进程B的ActivityThread，这里是在进程A中去get进程B中的ContentProvider。有四个关键点，下面将一一详细说明下。
节点1、在getContentProviderImpl()函数中会判断
ContentProvider所在的目标进程B是否已经启动，如果未启动，那么会先启动目标进程B，然后当前线程会wait在一个ContentProviderRecord对象上，只有等到进程B启动并将ContentProvider publish到AMS中，通过notifyAll()唤醒，前面那个线程才会继续执行。
节点2、进程B启动过程中，在handleBindApplication()函数中进行
ContentProvider安装，安装函数便是installProvider()，我们可以看到关键点4也是调用installProvider()进行
ContentProvider安装，为什么会有两次ContentProvider安装呢？是因为同时需要在进程A和进程B中进行安装，那么进程A、B中安装有何不同？答案就在installProvider()函数中，installProvider()具体分析见下文。
节点3、当进程B本地安装完ContentProvider，然后夸Binder调用AMS.publishContentProviders()接口将ContentProvider中Transport 的Binder实体传递到AMS中，并调用notifyAll()进行唤醒上面所说的等待线程。
节点4、进程A中调用完AMS.getContentProvider()后立即调用installProvider()进行ContentProvider本地安装，这是第二次调用
installProvider()，具体分析见下面。
installProvider()函数分析：
在分析这个函数之前我们先要搞清楚ContentProviderHolder类的两个成员变量provider、connection。在进程B启动过程中，会实例化ContentProvider对象， ContentProvider中有一个Transport类型的Binder本地对象mTransport，进程A就是要拿到这个mTransport的代理来与B进程中的ContentProvider建立Binder通信。
那进程A是如何拿到进程B中ContentProvider.mTransport的代理的呢？mTransport是一个匿名Binder对象，我们知道匿名Binder对象是可以在已经建立起的Binder通信的链路中进行传递的，于是，mTransport便是这样传递到进程A并保存在ContentProviderHolder.provider中：
①进程B启动安装
ContentProvider时通过publishContentProviders()接口将mTransport传递到AMS，并保存在ContentProviderRecord.provider中；
②进程A调用AMS.getContentProvider()接口，AMS会将ContentProviderRecord.provider和新创建的ContentProviderConnection匿名Binder对象一并传递到进程A中，分别保存在ContentProviderHolder.provider和ContentProviderHolder.connection中。
注意，进程A中ContentProviderHolder.provider与AMS中的ContentProviderRecord.provider均是进程B中mTransport的Binder代理。进程A拿到了进程B中ContentProvider.mTransport代理，便建立Binder通信。更具体的ContentProvider操作在自定义的ContentProvider中实现，Transport只是通信的封装。
了解了上面这些信息我们再来看installProvider()函数。在上面的时序图中有两次调用，一次在进程B中，一次在进程A中。这里我们只关心installProvider(..，holder，..)函数的第二个参数holder。holder是一个ContentProviderHolder类型参数，如果holder.provider为null，那边需要在本次installProvider()函数调用中实例化ContentProvider对象，如果holder.provider不为null，则无需实例化ContentProvider对象。在进程B中installProvider()函数调用参数holder.provider=null，在进程A中installProvider()函数调用参数holder.provider！=null，因此进程B中会实例化ContentProvider对象，而进程A中holder.provider保存着进程B中ContentProvider.mTransport的代理，这一点读者稍微跟下代码便可发现。
private IActivityManager.ContentProviderHolder installProvider(Context context,
IActivityManager.ContentProviderHolder holder, ProviderInfo info,
boolean noisy, boolean noReleaseNeeded, boolean stable) {
ContentProvider localProvider = null;
IContentProvider provider;
if (holder == null || holder.provider == null) { //holder.provider=null，意味着需要在此实例化ContentProvider对象。进程B中调用该函数满足这个条件。
if (DEBUG_PROVIDER || noisy) {
Slog.d(TAG, "Loading provider " + info.authority + ": "
+ info.name);
}
Context c = null;
ApplicationInfo ai = info.applicationInfo;
Slog.d(TAG, "installProvider: context.getPackageName()=" + context.getPackageName());
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); //创建ContentProvider所在的package的Context；
} 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;
}
try {
final java.lang.ClassLoader cl = c.getClassLoader();
localProvider = (ContentProvider)cl.
loadClass(info.name).newInstance(); //实例化ContentProvider对象；
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.
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 { //holder.provider!=null，进程A中调用该函数时满足这个逻辑，holder.provider指向mTransport的代理；
provider = holder.provider;
if (DEBUG_PROVIDER) Slog.v(TAG, "Installing external provider " + info.authority + ": "
+ info.name);
}
IActivityManager.ContentProviderHolder retHolder;
//下面的逻辑主要是进行本地保存provider，方便第二次调用同一个ContentProvider时，无需重新到ＡＭＳ中去查询。
synchronized (mProviderMap) {
if (DEBUG_PROVIDER) Slog.v(TAG, "Checking to add " + provider
+ " / " + info.name);
IBinder jBinder = provider.asBinder();
if (localProvider != null) {
ComponentName cname = new ComponentName(info.packageName, info.name);
ProviderClientRecord pr = mLocalProvidersByName.get(cname);
if (pr != null) {
if (DEBUG_PROVIDER) {
Slog.v(TAG, "installProvider: lost the race, "
+ "using existing local provider");
}
provider = pr.mProvider;
} else {
holder = new IActivityManager.ContentProviderHolder(info);
holder.provider = provider;
holder.noReleaseNeeded = true;
pr = installProviderAuthoritiesLocked(provider, localProvider, holder);
mLocalProviders.put(jBinder, pr);
mLocalProvidersByName.put(cname, pr);
}
retHolder = pr.mHolder;
} else {
ProviderRefCount prc = mProviderRefCountMap.get(jBinder);
if (prc != null) {
if (DEBUG_PROVIDER) {
Slog.v(TAG, "installProvider: lost the race, updating ref count");
}
// We need to transfer our new reference to the existing
// ref count, releasing the old one... but only if
// release is needed (that is, it is not running in the
// system process).
if (!noReleaseNeeded) {
incProviderRefLocked(prc, stable);
try {
ActivityManagerNative.getDefault().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) {
prc = new ProviderRefCount(holder, client, 1000, 1000);
} else {
prc = stable
? new ProviderRefCount(holder, client, 1, 0)
: new ProviderRefCount(holder, client, 0, 1);
}
mProviderRefCountMap.put(jBinder, prc);
}
retHolder = prc.holder;
}
}
return retHolder;
}
附getContentProviderImpl()函数分析
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); //①获取调用进程记录信息块
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); //②根据authority获取ContentProviderRecord，如果之前publish过ContentProvider，那么返回值必然不为null。
boolean providerRunning = cpr != null;
if (providerRunning) { //②providerRunning=true表示ContentProvider已经启动过了；
cpi = cpr.info;
String msg;
if ((msg=checkContentProviderPermissionLocked(cpi, r)) != null) {
throw new SecurityException(msg);
}
if (r != null && cpr.canRunHere(r)) { //③如果ContentProvider允许多进程实例化或目标进程就是调用进程，那么直接返回。返回的时候将ContentProviderHolder. Provider置空，这样调用进程便会在自己的进程中实例化ContentProvider。
// 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);<span style="white-space:pre"> </span>//④创建一个ContentProviderConnection；
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, null);
}
}
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); //返回false表示cpr.proc被杀了
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) { // ⑤providerRunning=false，有两种情况：ContentProvider所在进程没有启动过、ContentProvider所在进程启动了，并publish了ContentProvider，但是挂掉了。
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) { //这是第一种情况，ContentProvider所在进程未启动过，那么new ContentProviderRecord
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)) { //⑤如果ContentProvider允许多进程实例化，那么直接返回，让调用进程自己实例化ContentProvider；
// 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);
}
// 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; i++) { //查询是否有其他进程正在加载该Provider
if (mLaunchingProviders.get(i) == cpr) {
break;
}
}
// If the provider is not already being launched, then get it
// started.
if (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);
}
// Use existing process if already started
ProcessRecord proc = getProcessRecordLocked(
cpi.processName, cpr.appInfo.uid, false);
if (proc != null && proc.thread != null) {
if (DEBUG_PROVIDER) {
Slog.d(TAG, "Installing in existing process " + proc);
}
proc.pubProviders.put(cpi.name, cpr);
try {
proc.thread.scheduleInstallProvider(cpi);
} catch (RemoteException e) {
}
} else { //⑥启动Provider所在的目标进程；
proc = startProcessLocked(cpi.processName,
cpr.appInfo, false, 0, "content provider",
new ComponentName(cpi.applicationInfo.packageName,
cpi.name), false, 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) { //⑦等待Provider被Publish，目标进程publish后会调用notifiyAll()接口，此时当前线程便可返回；
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;
}
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