Android persistent属性研究

来源：互联网发布：网络投票水军编辑：程序博客网时间：2024/06/08 07:50

前段时间在研究telephony时，一直没有在framework下发现对telephony的初始化(PhoneFactory.java中的makeDefaultPhones函数)的调用。结果全局搜索之后发现在application PhoneApp(packages/apps/Phone)中调用了。但是application PhoneApp既没有被Broadcast唤醒，也没有被其他service调用，那么是Android是通过什么方式来启动PhoneApp，所以就发现了属性android:persistent。

在AndroidManifest.xml定义中，application有这么一个属性android:persistent，根据字面意思来理解就是说该应用是可持久的，也即是常驻的应用。其实就是这么个理解，被android:persistent修饰的应用会在系统启动之后被AM启动。

AM首先去PM(PackageManagerService)中去查找设置了Android:persistent的应用

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public void systemReady(final Runnable goingCallback) {
if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {
try {
List apps = AppGlobals.getPackageManager().
getPersistentApplications(STOCK_PM_FLAGS);
if (apps != null) {
int N = apps.size();
int i;
for (i=0; i<N; i++) {
ApplicationInfo info
= (ApplicationInfo)apps.get(i);
if (info != null &&
!info.packageName.equals("Android")) {
addAppLocked(info);
}
}
}
} catch (RemoteException ex) {
// pm is in same process, this will never happen.
}
}
}

public void systemReady(final Runnable goingCallback) {              if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {                  try {                      List apps = AppGlobals.getPackageManager().                          getPersistentApplications(STOCK_PM_FLAGS);                      if (apps != null) {                          int N = apps.size();                          int i;                          for (i=0; i<N; i++) {                              ApplicationInfo info                                  = (ApplicationInfo)apps.get(i);                              if (info != null &&                                      !info.packageName.equals("Android")) {                                  addAppLocked(info);                              }                          }                      }                  } catch (RemoteException ex) {                      // pm is in same process, this will never happen.                   }              }     }

假如该被Android:persistent修饰的应用此时并未运行的话，那么AM将调用startProcessLocked启动该app，关于startProcessLocked不再描述，另外一篇文章《How to start a new process for Android?》中做了详细的介绍。

app的启动过程就是启动app所在的package对应的进程。

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final ProcessRecord addAppLocked(ApplicationInfo info) {
ProcessRecord app = getProcessRecordLocked(info.processName, info.uid);
if (app == null) {
app = newProcessRecordLocked(null, info, null);
mProcessNames.put(info.processName, info.uid, app);
updateLruProcessLocked(app, true, true);
}
if ((info.flags&(ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT))
== (ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT)) {
app.persistent = true;
app.maxAdj = CORE_SERVER_ADJ;
}
if (app.thread == null && mPersistentStartingProcesses.indexOf(app) < 0) {
mPersistentStartingProcesses.add(app);
startProcessLocked(app, "added application", app.processName);
}
return app;
}

final ProcessRecord addAppLocked(ApplicationInfo info) {          ProcessRecord app = getProcessRecordLocked(info.processName, info.uid);          if (app == null) {              app = newProcessRecordLocked(null, info, null);              mProcessNames.put(info.processName, info.uid, app);              updateLruProcessLocked(app, true, true);          }          if ((info.flags&(ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT))                  == (ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT)) {              app.persistent = true;              app.maxAdj = CORE_SERVER_ADJ;          }          if (app.thread == null && mPersistentStartingProcesses.indexOf(app) < 0) {              mPersistentStartingProcesses.add(app);              startProcessLocked(app, "added application", app.processName);          }          return app;      }

面介绍app所在的package对应的进程启动完成之后，app是如何被create的。

从文章《How to start a new process for Android?》中可知，zygote在创建新的进程均会启动它的mainThread android.app.ActivityThread，因此我们从ActivityThread的main函数中接着分析app的create过程。

在main中有下面这个操作

thread.attach(false);

在attach过程中，ActivityThread会将对应的application attach到AM中去，交与AM去管理。这里需要注意一个变量

final ApplicationThread mAppThread = new ApplicationThread();

mAppThread是一个ApplicationThread对象，mAppThread可以看作是当前进程主线程的核心，它负责处理本进程与其他进程(主要是AM)之间的通信，同时通过attachApplication将mAppThread的代理Binder传递给AM。

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private final void attach(boolean system) {
sThreadLocal.set(this);
mSystemThread = system;
if (!system) {
ViewRoot.addFirstDrawHandler(new Runnable() {
public void run() {
ensureJitEnabled();
}
});
Android.ddm.DdmHandleAppName.setAppName("<pre-initialized>");
RuntimeInit.setApplicationObject(mAppThread.asBinder());
IActivityManager mgr = ActivityManagerNative.getDefault();
try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
}
}
}

private final void attach(boolean system) {          sThreadLocal.set(this);          mSystemThread = system;          if (!system) {              ViewRoot.addFirstDrawHandler(new Runnable() {                  public void run() {                      ensureJitEnabled();                  }              });              Android.ddm.DdmHandleAppName.setAppName("<pre-initialized>");              RuntimeInit.setApplicationObject(mAppThread.asBinder());              IActivityManager mgr = ActivityManagerNative.getDefault();              try {                  mgr.attachApplication(mAppThread);              } catch (RemoteException ex) {              }          }      }

上面的attach代码中，我们顺着IPC调用AM的attachApplication过程再往下看。

在该过程中，AM调用到了IPC通信调用mAppThread的bindApplication；

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private final boolean attachApplicationLocked(IApplicationThread thread,
int pid) {
thread.bindApplication(processName, app.instrumentationInfo != null
? app.instrumentationInfo : app.info, providers,
app.instrumentationClass, app.instrumentationProfileFile,
app.instrumentationArguments, app.instrumentationWatcher, testMode,
isRestrictedBackupMode || !normalMode,
mConfiguration, getCommonServicesLocked());
updateLruProcessLocked(app, false, true);
app.lastRequestedGc = app.lastLowMemory = SystemClock.uptimeMillis();
}

private final boolean attachApplicationLocked(IApplicationThread thread,              int pid) {                    thread.bindApplication(processName, app.instrumentationInfo != null                      ? app.instrumentationInfo : app.info, providers,                      app.instrumentationClass, app.instrumentationProfileFile,                      app.instrumentationArguments, app.instrumentationWatcher, testMode,                       isRestrictedBackupMode || !normalMode,                      mConfiguration, getCommonServicesLocked());              updateLruProcessLocked(app, false, true);              app.lastRequestedGc = app.lastLowMemory = SystemClock.uptimeMillis();  }

mAppThread的bindApplication再通过消息机制向ActivityThread自身维护的handler发送BIND_APPLICATION消息。下面看看ActivityThread自身维护的handler对消息BIND_APPLICATION的处理，最终会调用到handleBindApplication函数

你会发现在handleBindApplication函数中有这么一句

mInstrumentation.callApplicationOnCreate(app);

我们最终在绕了好大一圈之后，调用了app的onCreate函数来启动这个application。