ActivityManagerService（简称AMS）是Android系统的关键服务之中的一个。它的主要作用例如以下：

• 管理系统中全部应用进程的整个生命周期
• 管理应用进程中的Activity、Service、Broadcast和ContentProvider
• 内存管理，低内存释放等

AMS是一个服务端，定义了IBinder接口。其它的进程能够通过Binder机制与AMS进行通信。

AMS由system_server进程启动的，并作为一个独立线程执行在system_server进程中。
以下就简略分析一下AMS的启动过程

system_server启动AMS

PATH：frameworks/base/services/java/com/android/server/SystemServer.java
启动过程大致例如以下：
SystemServer.main()–>SystemServer.run()。

private void run() {
    ......
    startBootstrapServices();
    startOtherServices();
    ......
}

private void startBootstrapServices() {
    ......
    // 启动AMS服务
    mActivityManagerService = mSystemServiceManager.startService(
            ActivityManagerService.Lifecycle.class).getService();

    // 初始化AMS的成员变量mSystemServiceManager 
    mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
    // 初始化AMS的成员变量mInstaller
    mActivityManagerService.setInstaller(installer);     
    ......
    // 主要是创建system_server相应的ProcessRecord，并初始化 
    mActivityManagerService.setSystemProcess();
    ......
}

private void startOtherServices() {
    ......
    // 主要是创建并注冊SettingsProvider
    mActivityManagerService.installSystemProviders();
    ......
    // 系统启动前的准备工作，启动SystemUI和Home界面等
    mActivityManagerService.systemReady(new Runnable() {...});
}

重点看一下mSystemServiceManager.startService()：

public SystemService startService(String className) {
    final Class<SystemService> serviceClass;
    try {
        serviceClass = (Class<SystemService>)Class.forName(className);
    } catch (ClassNotFoundException ex) {
        ......
    }
    return startService(serviceClass);
}

public <T extends SystemService> T startService(Class<T> serviceClass) {
    final String name = serviceClass.getName();
    Slog.i(TAG, "Starting " + name);

    // Create the service.
    if (!SystemService.class.isAssignableFrom(serviceClass)) {
        throw new RuntimeException("Failed to create " + name
                + ": service must extend " + SystemService.class.getName());
    }
    final T service;
    try {
        Constructor<T> constructor = serviceClass.getConstructor(Context.class);
        //创建ActivityManagerService.Lifecycle对象
        service = constructor.newInstance(mContext);
    } catch (InstantiationException ex) {
        ......
    }

    // 加入到成员变量mServices中
    mServices.add(service);

    // Start it.
    try {
        // 执行ActivityManagerService.Lifecycle.onStart()
        service.onStart();
    } catch (RuntimeException ex) {
        throw new RuntimeException("Failed to start service " + name
                + ": onStart threw an exception", ex);
    }
    return service;
}

ActivityManagerService.Lifecycle

PATH：frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

public static final class Lifecycle extends SystemService {
    private final ActivityManagerService mService;

    public Lifecycle(Context context) {
        super(context);
        // 创建ActivityManagerServiceEx对象
        mService = new ActivityManagerServiceEx(context);
    }

    @Override
    public void onStart() {
        mService.start(); //执行ActivityManagerServiceEx.start()
    }

    public ActivityManagerService getService() {
        return mService;
    }
}

当中，ActivityManagerServiceEx是ActivityManagerService的子类，定义例如以下：

public final class ActivityManagerServiceEx extends ActivityManagerService {
    ......
    public ActivityManagerServiceEx(Context systemContext) {
       super(systemContext);
       mIsInHome = true;
    }
    ......
}

这样，在new ActivityManagerServiceEx()时候。会调用到其父类ActivityManagerService的构造函数。

ActivityManagerService构造函数

PATH：frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

public ActivityManagerService(Context systemContext) {
    mContext = systemContext;
    mFactoryTest = FactoryTest.getMode();
    mSystemThread = ActivityThread.currentActivityThread();

    Slog.i(TAG, "Memory class: " + ActivityManager.staticGetMemoryClass());

    //创建名称为“ActivityManager”的消息循环线程。
    mHandlerThread = new ServiceThread(TAG,
            android.os.Process.THREAD_PRIORITY_FOREGROUND, false /*allowIo*/);
    mHandlerThread.start();

    //将该线程绑定到MainHandler，由MainHandler完毕消息的处理
    mHandler = new MainHandler(mHandlerThread.getLooper());

    //创建前台广播接收器，执行超时为10s
    mFgBroadcastQueue = new BroadcastQueue(this, mHandler,
            "foreground", BROADCAST_FG_TIMEOUT, false);
    //创建后台广播接收器。执行超时为60s
    mBgBroadcastQueue = new BroadcastQueue(this, mHandler,
            "background", BROADCAST_BG_TIMEOUT, true);
    mBroadcastQueues[0] = mFgBroadcastQueue;
    mBroadcastQueues[1] = mBgBroadcastQueue;

    mServices = new ActiveServicesEx(this);
    mProviderMap = new ProviderMap(this);

    //新建/data/system文件夹
    File dataDir = Environment.getDataDirectory();
    File systemDir = new File(dataDir, "system");
    systemDir.mkdirs();

    //创建BatteryStatsService服务
    mBatteryStatsService = new BatteryStatsService(systemDir, mHandler);
    mBatteryStatsService.getActiveStatistics().readLocked();
    mBatteryStatsService.getActiveStatistics().writeAsyncLocked();
    mOnBattery = DEBUG_POWER ?
 true : mBatteryStatsService.getActiveStatistics().getIsOnBattery(); mBatteryStatsService.getActiveStatistics().setCallback(this); //创建ProcessStatsService服务 mProcessStats = new ProcessStatsService(this, new File(systemDir, "procstats")); //创建AppOpsService服务  mAppOpsService = new AppOpsService(new File(systemDir, "appops.xml"), mHandler); //创建AtomicFile文件  mGrantFile = new AtomicFile(new File(systemDir, "urigrants.xml")); // User 0 is the first and only user that runs at boot. mStartedUsers.put(0, new UserStartedState(new UserHandle(0), true)); mUserLru.add(Integer.valueOf(0)); updateStartedUserArrayLocked(); GL_ES_VERSION = SystemProperties.getInt("ro.opengles.version", ConfigurationInfo.GL_ES_VERSION_UNDEFINED); mTrackingAssociations = "1".equals(SystemProperties.get("debug.track-associations")); mConfiguration.setToDefaults(); mConfiguration.locale = Locale.getDefault(); mConfigurationSeq = mConfiguration.seq = 1; mProcessCpuTracker.init(); mCompatModePackages = new CompatModePackages(this, systemDir, mHandler); mIntentFirewall = new IntentFirewall(new IntentFirewallInterface(), mHandler); mStackSupervisor = new ActivityStackSupervisor(this); mTaskPersister = new TaskPersister(systemDir, mStackSupervisor); mProcessCpuThread = new Thread("CpuTracker") { @Override public void run() { while (true) { try { try { synchronized(this) { final long now = SystemClock.uptimeMillis(); long nextCpuDelay = (mLastCpuTime.get()+MONITOR_CPU_MAX_TIME)-now; long nextWriteDelay = (mLastWriteTime+BATTERY_STATS_TIME)-now; //Slog.i(TAG, "Cpu delay=" + nextCpuDelay // + ", write delay=" + nextWriteDelay); if (nextWriteDelay < nextCpuDelay) { nextCpuDelay = nextWriteDelay; } if (nextCpuDelay > 0) { mProcessCpuMutexFree.set(true); this.wait(nextCpuDelay); } } } catch (InterruptedException e) { } updateCpuStatsNow(); } catch (Exception e) { Slog.e(TAG, "Unexpected exception collecting process stats", e); } } } }; Watchdog.getInstance().addMonitor(this); Watchdog.getInstance().addThread(mHandler); }

首先，创建了一个名为“ActivityManager”的消息循环线程，不断地接收其它进程发给AMS的消息；并把该消息循环线程与MainHandler绑定，这样，由MainHandler完毕消息的详细处理。

然后，创建了一些服务，并在/data/system文件夹下创建该服务须要的文件或文件夹，详细例如以下：

另外，还创建了一个AtomicFile类型的文件mGrantFile ，文件路径为/data/system/urigrants.xml。
AtomicFile文件是通过创建一个备份文件来执行原子性操作的帮助类，保证文件读写的完整。
这样。system_server就完毕AMS的构造和启动。
回到system_server中，我们看到又调用了AMS的setSystemProcess方法。以下就分析一下该方法。

ActivityManagerService.setSystemProcess

public void setSystemProcess() {
    try {
        ServiceManager.addService(Context.ACTIVITY_SERVICE, this, true);
        ServiceManager.addService(ProcessStats.SERVICE_NAME, mProcessStats);
        ServiceManager.addService("meminfo", new MemBinder(this));
        ServiceManager.addService("gfxinfo", new GraphicsBinder(this));
        ServiceManager.addService("dbinfo", new DbBinder(this));
        if (MONITOR_CPU_USAGE) {
            ServiceManager.addService("cpuinfo", new CpuBinder(this));
        }
        ServiceManager.addService("permission", new PermissionController(this));

        ApplicationInfo info = mContext.getPackageManager().getApplicationInfo(
                "android", STOCK_PM_FLAGS);
        mSystemThread.installSystemApplicationInfo(info, getClass().getClassLoader());

        synchronized (this) {
            ProcessRecord app = newProcessRecordLocked(info, info.processName, false, 0);
            app.persistent = true;
            app.pid = MY_PID;
            app.maxAdj = ProcessList.SYSTEM_ADJ;
            app.makeActive(mSystemThread.getApplicationThread(), mProcessStats);
            mProcessNames.put(app.processName, app.uid, app);
            synchronized (mPidsSelfLocked) {
                mPidsSelfLocked.put(app.pid, app);
            }
            updateLruProcessLocked(app, false, null);
            updateOomAdjLocked();
        }
    } catch (PackageManager.NameNotFoundException e) {
        throw new RuntimeException(
                "Unable to find android system package", e);
    }
}

首先，向SystemServiceManager中加入了若干个服务：

然后。调用PMS的getApplicationInfo接口，获取名为”android”的应用程序信息。包名为”android”的apk即/system/framework/framework-res.apk。里面保存着系统GUI美化的相关文件，包含图标，弹出对话框的样式，动作特效，界面布局等。调用installSystemApplicationInfo载入framework-res.apk文件。
接着，调用newProcessRecordLocked新建一个ProcessRecord 对象app。ProcessRecord用来描写叙述一个进程的全部信息，包含该进程的全部activity和service等。在这里就是system_server（AMS就是在system_server进程中执行的）。

创建后，对app的一些成员变量进行初始化。包含设置为常驻内存执行；设置system_server的pid等。
最后。调用mProcessNames.put()将创建的ProcessRecord 对象app加入到ProcessMap< ProcessRecord >类型的成员变量mProcessNames中。

这里。app.processName=“system”。

这样，AMS就得到了system_server的ProcessRecord，以后AMS也能够管理system_server了。

继续回到system_server中。我们看到又调用了AMS的installSystemProviders方法，以下就分析一下该方法。

ActivityManagerService.installSystemProviders

public final void installSystemProviders() {
    List<ProviderInfo> providers;
    synchronized (this) {
        ProcessRecord app = mProcessNames.get("system", Process.SYSTEM_UID);
        providers = generateApplicationProvidersLocked(app);
        if (providers != null) {
            for (int i=providers.size()-1; i>=0; i--) {
                ProviderInfo pi = (ProviderInfo)providers.get(i);
                if ((pi.applicationInfo.flags&ApplicationInfo.FLAG_SYSTEM) == 0) {
                    Slog.w(TAG, "Not installing system proc provider " + pi.name
                            + ": not system .apk");
                    providers.remove(i);
                }
            }
        }
    }
    if (providers != null) {
        mSystemThread.installSystemProviders(providers);
    }

    mCoreSettingsObserver = new CoreSettingsObserver(this);

    //mUsageStatsService.monitorPackages();
}

首先，取出在setSystemProcess()中put到mProcessNames中的ProcessRecord对象app，即system_server的进程信息。

然后，调用generateApplicationProvidersLocked：

private final List<ProviderInfo> generateApplicationProvidersLocked(ProcessRecord app) {
    List<ProviderInfo> providers = null;
    try {
        providers = AppGlobals.getPackageManager().
            queryContentProviders(app.processName, app.uid,
                    STOCK_PM_FLAGS | PackageManager.GET_URI_PERMISSION_PATTERNS);
    } catch (RemoteException ex) {
    }
    if (DEBUG_MU)
        Slog.v(TAG_MU, "generateApplicationProvidersLocked, app.info.uid = " + app.uid);
    int userId = app.userId;
    if (providers != null) {
        int N = providers.size();
        app.pubProviders.ensureCapacity(N + app.pubProviders.size());
        for (int i=0; i<N; i++) {
            ProviderInfo cpi =
                (ProviderInfo)providers.get(i);
            boolean singleton = isSingleton(cpi.processName, cpi.applicationInfo,
                    cpi.name, cpi.flags);
            if (singleton && UserHandle.getUserId(app.uid) != 0) {
                providers.remove(i);
                N--;
                i--;
                continue;
            }

            ComponentName comp = new ComponentName(cpi.packageName, cpi.name);
            ContentProviderRecord cpr = mProviderMap.getProviderByClass(comp, userId);
            if (cpr == null) {
                cpr = new ContentProviderRecord(this, cpi, app.info, comp, singleton);
                mProviderMap.putProviderByClass(comp, cpr);
            }
            if (DEBUG_MU)
                Slog.v(TAG_MU, "generateApplicationProvidersLocked, cpi.uid = " + cpr.uid);
            app.pubProviders.put(cpi.name, cpr);
            if (!cpi.multiprocess || !"android".equals(cpi.packageName)) {
                app.addPackage(cpi.applicationInfo.packageName, cpi.applicationInfo.versionCode,
                        mProcessStats);
            }
            ensurePackageDexOpt(cpi.applicationInfo.packageName);
        }
    }
    return providers;
}

首先，调用PMS的queryContentProviders查找processName=“system”和uid=SYSTEM_UID的provider。即SettingsProvider。并获取其ProviderInfo 。
然后新建一个ContentProviderRecord（描写叙述一个ContentProvider，这里为SettingsProvider），并加入到AMS的成员变量mProviderMap和ProcessRecord对象的pubProviders中。

最后将SettingsProvider所在的package加入到ProcessRecord对象的pkglist中。

继续回到installSystemProviders()。调用installSystemProviders，先创建后注冊SettingsProvider。这样，其它进程就能够调用SettingsProvider。查询或改动一些系统设置了。

在installSystemProviders()的最后，注冊一个ContentObserver来监听SettingsProvider中的状态变化。

继续回到system_server中。我们看到又调用了AMS的systemReady方法。以下就分析一下该方法。

ActivityManagerService.systemReady

该函数主要完毕了以下几件事：

• 在当前执行的进程中，查找在systemReady之前不同意启动的进程。然后调用removeProcessLocked()终止该进程并释放资源。推断条件例如以下：

boolean isAllowedWhileBooting(ApplicationInfo ai) {
    return (ai.flags&ApplicationInfo.FLAG_PERSISTENT) != 0;
}

• 载入若干配置信息和资源信息，代码例如以下：

retrieveSettings();
loadResourcesOnSystemReady();

private void retrieveSettings() {
    final ContentResolver resolver = mContext.getContentResolver();
    String debugApp = Settings.Global.getString(
        resolver, Settings.Global.DEBUG_APP);
    boolean waitForDebugger = Settings.Global.getInt(
        resolver, Settings.Global.WAIT_FOR_DEBUGGER, 0) != 0;
    boolean alwaysFinishActivities = Settings.Global.getInt(
        resolver, Settings.Global.ALWAYS_FINISH_ACTIVITIES, 0) != 0;
    boolean forceRtl = Settings.Global.getInt(
            resolver, Settings.Global.DEVELOPMENT_FORCE_RTL, 0) != 0;
    ......
}

private void loadResourcesOnSystemReady() {
    final Resources res = mContext.getResources();
    mHasRecents = res.getBoolean(com.android.internal.R.bool.config_hasRecents);
    mThumbnailWidth = res.getDimensionPixelSize(com.android.internal.R.dimen.thumbnail_width);
    mThumbnailHeight = res.getDimensionPixelSize(com.android.internal.R.dimen.thumbnail_height);
}

• 调用回调函数goingCallback.run()。主要完毕两件事：执行其它系统服务的systemready()或systemRunning()；启动SystemUI。

  goingCallback.run()的实如今systemserver.java中：

mActivityManagerService.systemReady(new Runnable() {
    @Override
    public void run() {
        Slog.i(TAG, "Making services ready");
        mSystemServiceManager.startBootPhase(
                SystemService.PHASE_ACTIVITY_MANAGER_READY);

        try {
            mActivityManagerService.startObservingNativeCrashes();
        } catch (Throwable e) {
            reportWtf("observing native crashes", e);
        }

        Slog.i(TAG, "WebViewFactory preparation");
        WebViewFactory.prepareWebViewInSystemServer();

        try {
            startSystemUi(context); //启动SystemUI
        } catch (Throwable e) {
            reportWtf("starting System UI", e);
        }
        try {
            if (mountServiceF != null) mountServiceF.systemReady();
        } catch (Throwable e) {
            reportWtf("making Mount Service ready", e);
        }
        try {
            if (networkScoreF != null) networkScoreF.systemReady();
        } catch (Throwable e) {
            reportWtf("making Network Score Service ready", e);
        }
        try {
            if (networkManagementF != null) networkManagementF.systemReady();
        } catch (Throwable e) {
            reportWtf("making Network Managment Service ready", e);
        }
        try {
            if (networkStatsF != null) networkStatsF.systemReady();
        } catch (Throwable e) {
            reportWtf("making Network Stats Service ready", e);
        }
        try {
            if (networkPolicyF != null) networkPolicyF.systemReady();
        } catch (Throwable e) {
            reportWtf("making Network Policy Service ready", e);
        }
        try {
            if (connectivityF != null) connectivityF.systemReady();
        } catch (Throwable e) {
            reportWtf("making Connectivity Service ready", e);
        }
        try {
            if (audioServiceF != null) audioServiceF.systemReady();
        } catch (Throwable e) {
            reportWtf("Notifying AudioService running", e);
        }
        Watchdog.getInstance().start();

        // It is now okay to let the various system services start their
        // third party code...
        mSystemServiceManager.startBootPhase(
                SystemService.PHASE_THIRD_PARTY_APPS_CAN_START);

        try {
            if (wallpaperF != null) wallpaperF.systemRunning();
        } catch (Throwable e) {
            reportWtf("Notifying WallpaperService running", e);
        }
        try {
            if (immF != null) immF.systemRunning(statusBarF);
        } catch (Throwable e) {
            reportWtf("Notifying InputMethodService running", e);
        }
        try {
            if (locationF != null) locationF.systemRunning();
        } catch (Throwable e) {
            reportWtf("Notifying Location Service running", e);
        }
        try {
            if (countryDetectorF != null) countryDetectorF.systemRunning();
        } catch (Throwable e) {
            reportWtf("Notifying CountryDetectorService running", e);
        }
        try {
            if (networkTimeUpdaterF != null) networkTimeUpdaterF.systemRunning();
        } catch (Throwable e) {
            reportWtf("Notifying NetworkTimeService running", e);
        }
        try {
            if (commonTimeMgmtServiceF != null) {
                commonTimeMgmtServiceF.systemRunning();
            }
        } catch (Throwable e) {
            reportWtf("Notifying CommonTimeManagementService running", e);
        }
        try {
            if (textServiceManagerServiceF != null)
                textServiceManagerServiceF.systemRunning();
        } catch (Throwable e) {
            reportWtf("Notifying TextServicesManagerService running", e);
        }
        try {
            if (atlasF != null) atlasF.systemRunning();
        } catch (Throwable e) {
            reportWtf("Notifying AssetAtlasService running", e);
        }
        try {
            // TODO(BT) Pass parameter to input manager
            if (inputManagerF != null) inputManagerF.systemRunning();
        } catch (Throwable e) {
            reportWtf("Notifying InputManagerService running", e);
        }
        try {
            if (telephonyRegistryF != null) telephonyRegistryF.systemRunning();
        } catch (Throwable e) {
            reportWtf("Notifying TelephonyRegistry running", e);
        }

        try {
            if (mediaRouterF != null) mediaRouterF.systemRunning();
        } catch (Throwable e) {
            reportWtf("Notifying MediaRouterService running", e);
        }

        if (SystemProperties.get("persist.support.securetest").equals("1")) {
            if (securityF != null) {
                try {
                    securityF.systemReady(context);
                } catch (Throwable e) {
                    reportWtf("Security Service ready", e);
                }
            }
        }
    }
});

• 调用addAppLocked启动那些声明为“FLAG_SYSTEM|FLAG_PERSISTENT”的应用程序：

final ProcessRecord addAppLocked(ApplicationInfo info, boolean isolated,
            String abiOverride) {
    ProcessRecord app;
    if (!isolated) {
        app = getProcessRecordLocked(info.processName, info.uid, true);
    } else {
        ......
    }
    ......
    // This package really, really can not be stopped.
    try {
        AppGlobals.getPackageManager().setPackageStoppedState(
                info.packageName, false, UserHandle.getUserId(app.uid));
    } catch (RemoteException e) {
    } catch (IllegalArgumentException e) {
        Slog.w(TAG, "Failed trying to unstop package "
                + info.packageName + ": " + e);
    }
    //推断条件
    if ((info.flags&(ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT))
            == (ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT)) {
        app.persistent = true;
        app.maxAdj = ProcessList.PERSISTENT_PROC_ADJ;
    }
    //假设该app的IApplicationThread等于null，而且没有在ArrayList<ProcessRecord>类型
    //的成员变量mPersistentStartingProcesses中，
    //启动该app，并把它加入到mPersistentStartingProcesses中。
    if (app.thread == null && mPersistentStartingProcesses.indexOf(app) < 0) {
        mPersistentStartingProcesses.add(app);
        startProcessLocked(app, "added application", app.processName, abiOverride,
                null /* entryPoint */, null /* entryPointArgs */);
    }

    return app;
}

• 广播两个消息：ACTION_USER_STARTED和ACTION_USER_STARTING。标识用户已经started

Intent intent = new Intent(Intent.ACTION_USER_STARTED);
intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY
        | Intent.FLAG_RECEIVER_FOREGROUND);
intent.putExtra(Intent.EXTRA_USER_HANDLE, mCurrentUserId);
broadcastIntentLocked(null, null, intent,
        null, null, 0, null, null, null, AppOpsManager.OP_NONE,
        false, false, MY_PID, Process.SYSTEM_UID, mCurrentUserId);
intent = new Intent(Intent.ACTION_USER_STARTING);
intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY);
intent.putExtra(Intent.EXTRA_USER_HANDLE, mCurrentUserId);
broadcastIntentLocked(null, null, intent,
    null, new IIntentReceiver.Stub() {
       @Override
        public void performReceive(Intent intent, int resultCode, String data,
                Bundle extras, boolean ordered, boolean sticky, int sendingUser)
                throws RemoteException {
        }
        }, 0, null, null,
    INTERACT_ACROSS_USERS, AppOpsManager.OP_NONE,
    true, false, MY_PID, Process.SYSTEM_UID, UserHandle.USER_ALL);
} catch (Throwable t) {
......

• 调用startHomeActivityLocked()启动HOME界面。

这样，ActivityManagerService的启动和初始化就完毕了。