android:persistent属性

为什么写这篇文章呢？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。

 

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
                            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

 

    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。

 

   

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; }

 

    下面介绍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);

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();

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。

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("");
            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(""); 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；

 

 

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);

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