AMS是Android系统服务中很重要的一个，他负责四大组件的启动、切换、调度、生命周期的管理等等，接下来我们根据AMS的启动来分析AMS的源码
ActivityManagerService是在SystemServer.java中启动并注册的：
private void startBootstrapServices() { ...... // 启动AMS，见小节2.1 mActivityManagerService = mSystemServiceManager.startService( ActivityManagerService.Lifecycle.class).getService(); // 设置AMS mActivityManagerService.setSystemServiceManager(mSystemServiceManager); //安装App安装器 mActivityManagerService.setInstaller(installer); ...... // 电源管理已经开启，在AMS中初始化PM，见小节3 Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "InitPowerManagement"); mActivityManagerService.initPowerManagement(); Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER); ...... // 设置应用实例，在系统进程开始的时候，见小节4 mActivityManagerService.setSystemProcess(); ...... }看源码可以看到（我没有贴出来），startBootstrapServices中启动并注册了很多其他的服务，比如：PowerManagerService，DisplayManagerService，LightsService，PackageManagerService，UserManagerService，SensorService(native)，这写服务彼此之间有依赖，所以都放在startBootstrapServices方法里面
文件：SystemServiceManager.java
public <T extends SystemService> T startService(Class<T> serviceClass) { try { final String name = serviceClass.getName(); Slog.i(TAG, "Starting " + name); Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartService " + name); // 创建服务 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); service = constructor.newInstance(mContext); } catch (InstantiationException ex) { throw new RuntimeException("Failed to create service " + name + ": service could not be instantiated", ex); } catch (IllegalAccessException ex) { throw new RuntimeException("Failed to create service " + name + ": service must have a public constructor with a Context argument", ex); } catch (NoSuchMethodException ex) { throw new RuntimeException("Failed to create service " + name + ": service must have a public constructor with a Context argument", ex); } catch (InvocationTargetException ex) { throw new RuntimeException("Failed to create service " + name + ": service constructor threw an exception", ex); } // 注册 mServices.add(service); // 开始服务 try { service.onStart(); } catch (RuntimeException ex) { throw new RuntimeException("Failed to start service " + name + ": onStart threw an exception", ex); } return service; } finally { Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER); } }可以看出这方法的作用是创建并且开始一个服务，但是这个服务的类必须是SystemService的子类，于是我们需要传入带有ActivityManagerService的Lifecycle
文件：SystemServiceManager.java
public static final class Lifecycle extends SystemService { private final ActivityManagerService mService; public Lifecycle(Context context) { super(context); // 构建一个新的AMS，见小节2.3 mService = new ActivityManagerService(context); } @Override public void onStart() { // 开始服务，2.1中的service.onStart()调用的就是它，见小节2.4 mService.start(); } public ActivityManagerService getService() { return mService; } }可以看出Lifecycle是继承SystemService的，并且在构造里构建了AMS，接下来我们来看看AMS的构建函数
这个方法会在main thread中被唤醒，但是它需要通过各个handers和其他thread通信，所以要注意明确looper。该构造函数，里面是初始化一些变量，及创建了一些线程，大部分我都进行了注释。
文件：ActivityManagerService.java
private void start() { // 移除所有的进程组 Process.removeAllProcessGroups(); // 开始监控进程的CPU使用情况 mProcessCpuThread.start(); // 注册电池统计服务 mBatteryStatsService.publish(mContext); // 注册应用权限检测服务 mAppOpsService.publish(mContext); Slog.d("AppOps", "AppOpsService published"); // 注册LocalService服务 LocalServices.addService(ActivityManagerInternal.class, new LocalService()); }启动ProcessCpuThread，注册电池统计服务，应用权限检测服务和LocalService，其中LocalService继承了ActivityManagerInternal。 小结：创建AMS，启动AMS
文件：ActivityManagerService.java
public void initPowerManagement() { // Activity堆栈管理器和电池统计服务初始化PM mStackSupervisor.initPowerManagement(); mBatteryStatsService.initPowerManagement(); mLocalPowerManager = LocalServices.getService(PowerManagerInternal.class); PowerManager pm = (PowerManager)mContext.getSystemService(Context.POWER_SERVICE); mVoiceWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, "*voice*"); // 该唤醒锁为不计数锁，即无论acquire()多少次，一次release()就可以解锁 mVoiceWakeLock.setReferenceCounted(false); }小结：这主要是在AMS中初始化PM
文件：ActivityManagerService.java
public void setSystemProcess() { try { // 以下都是想ServiceManager注册服务 ServiceManager.addService(Context.ACTIVITY_SERVICE, this, true); // 注册AMS自己 ServiceManager.addService(ProcessStats.SERVICE_NAME, mProcessStats); // 注册进程统计服务 ServiceManager.addService("meminfo", new MemBinder(this)); // 注册内存信息的服务 ServiceManager.addService("gfxinfo", new GraphicsBinder(this)); // 注册输出渲染信息的服务 ServiceManager.addService("dbinfo", new DbBinder(this)); // 注册输出数据库信息的服务 // MONITOR_CPU_USAGE默认为true if (MONITOR_CPU_USAGE) { ServiceManager.addService("cpuinfo", new CpuBinder(this)); // 输出进程使用CPU的情况 } ServiceManager.addService("permission", new PermissionController(this)); // 注册权限管理 ServiceManager.addService("processinfo", new ProcessInfoService(this)); // 注册进程信息 // 查询名为android的应用信息 ApplicationInfo info = mContext.getPackageManager().getApplicationInfo( "android", STOCK_PM_FLAGS | MATCH_SYSTEM_ONLY); // 调用installSystemApplicationInfo ，见小节4.2 mSystemThread.installSystemApplicationInfo(info, getClass().getClassLoader()); // synchronized (this) { // 创建一个ProcessRecord对象 ，见小节4.5 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); 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); } }文件：ActivityThread.java
public void installSystemApplicationInfo(ApplicationInfo info, ClassLoader classLoader) { synchronized (this) { // 看SystemService中创建的ContextIml的installSystemApplicationInfo，见小节4.3 getSystemContext().installSystemApplicationInfo(info, classLoader); // give ourselves a default profiler mProfiler = new Profiler(); } }文件：ContextImpl.java
void installSystemApplicationInfo(ApplicationInfo info, ClassLoader classLoader) { // 调用的是LoadeApk里面的installSystemApplicationInfo，见小节4.4 mPackageInfo.installSystemApplicationInfo(info, classLoader); }文件：LoadeApk.java
void installSystemApplicationInfo(ApplicationInfo info, ClassLoader classLoader) { // 断言只有packageName为android才能使用 assert info.packageName.equals("android"); mApplicationInfo = info; mClassLoader = classLoader; }将ApplicationInfo加入到LoadeApk中，因为SystemService创建LoadeApk时，PKMS并没有完成对手机中文件的解析
文件：ActivityManagerService.java
...... synchronized (this) { // 调用进程管理函数，见4.6 ProcessRecord app = newProcessRecordLocked(info, info.processName, false, 0); app.persistent = true; app.pid = MY_PID; app.maxAdj = ProcessList.SYSTEM_ADJ; // 将SystemServer对应的ApplicationThread保存到ProcessRecord中 app.makeActive(mSystemThread.getApplicationThread(), mProcessStats); synchronized (mPidsSelfLocked) { // 根据ProcessRecord的pid，将ProcessRecord存在mPidsSelfLocked中 mPidsSelfLocked.put(app.pid, app); } updateLruProcessLocked(app, false, null); updateOomAdjLocked(); } ......文件：ActivityManagerService.java
final ProcessRecord newProcessRecordLocked(ApplicationInfo info, String customProcess, boolean isolated, int isolatedUid) { String proc = customProcess != null ? customProcess : info.processName; BatteryStatsImpl stats = mBatteryStatsService.getActiveStatistics(); final int userId = UserHandle.getUserId(info.uid); int uid = info.uid; // isolated为false if (isolated) { ...... } // 创建一个进程记录对象，见小节4.7 final ProcessRecord r = new ProcessRecord(stats, info, proc, uid); // 判断是否为常驻的进程 if (!mBooted && !mBooting && userId == UserHandle.USER_SYSTEM && (info.flags & PERSISTENT_MASK) == PERSISTENT_MASK) { r.persistent = true; } // 将ProcessRecord保存在AMS里的mProcessNames里 addProcessNameLocked(r); return r; }文件：ProcessRecord.java
ProcessRecord(BatteryStatsImpl _batteryStats, ApplicationInfo _info, String _processName, int _uid) { mBatteryStats = _batteryStats; info = _info; isolated = _info.uid != _uid; uid = _uid; userId = UserHandle.getUserId(_uid); processName = _processName; pkgList.put(_info.packageName, new ProcessStats.ProcessStateHolder(_info.versionCode)); maxAdj = ProcessList.UNKNOWN_ADJ; curRawAdj = setRawAdj = ProcessList.INVALID_ADJ; curAdj = setAdj = verifiedAdj = ProcessList.INVALID_ADJ; persistent = false; removed = false; lastStateTime = lastPssTime = nextPssTime = SystemClock.uptimeMillis(); }这主要是保存一些ProcessRecord里面的属性。 小结：第四节的主要作用就是将一些服务注册到ServiceManger中，包括AMS自己；然后将framework-res-.apk中applicationInfo信息加入到SystemServeice生成的LoadedApk中，同时构建SystemService对应的ProcessRecord，最后通过addProcessNameLocked(r)来把SystemService加入AMS的管理中来。
