ActivityThread管理application进程中主线程的执行、调度，执行activities、broadcasts和activity manager请求的其他操作。
AMS中持有的ActivityThread mSystemThead实际上是SystemServer进程的主线程，它的的生命周期如下图所示：
目录
1 systemServer.createSystemContext()
2 ActivityThread.systemMain()
3 ActivityThread.attach()
4. AMS成员函数ActivityThread mSystemThread
5. AMS.setSystemProcess()
1 systemServer.createSystemContext()
systemserver进程在创建系统上下文的时候调用了ActivityThread.systemMain()来创建该进程的主线程ActivityThread。
///frameworks/base/services/java/com/android/server/SystemServer.javapublic final class SystemServer implements Dumpable { private Context mSystemContext; public static void main(String[] args) { new SystemServer().run(); } private void run() { createSystemContext(); } private void createSystemContext() { //创建了systemserver的主线程 ActivityThread activityThread = ActivityThread.systemMain(); mSystemContext = activityThread.getSystemContext(); mSystemContext.setTheme(DEFAULT_SYSTEM_THEME); final Context systemUiContext = activityThread.getSystemUiContext(); systemUiContext.setTheme(DEFAULT_SYSTEM_THEME); }} 2 ActivityThread.systemMain()
该方法中new ActivityThread()，并调用了其attach()方法。
/frameworks/base/core/java/android/app/ActivityThread.javapublic final class ActivityThread extends ClientTransactionHandler implements ActivityThreadInternal { @UnsupportedAppUsage public static ActivityThread systemMain() { ThreadedRenderer.initForSystemProcess(); ActivityThread thread = new ActivityThread(); thread.attach(true, 0); return thread; }} 3 ActivityThread.attach()
attach()会给ActivityThread的成员变量sCurrentActivityThread赋值，this表示当前进程创建的ActivityThread实例，从前边分析我们知道此处的this是systemserver进程的主线程。
/frameworks/base/core/java/android/app/ActivityThread.javapublic final class ActivityThread extends ClientTransactionHandler implements ActivityThreadInternal { @UnsupportedAppUsage private static volatile ActivityThread sCurrentActivityThread; @UnsupportedAppUsage private void attach(boolean system, long startSeq) { //被赋值当前实例的ActivityThread sCurrentActivityThread = this; mSystemThread = system; ... }} 4. AMS成员函数ActivityThread mSystemThread
mSystemThread在AMS的构造函数中调用ActivityThread.currentActivityThread()被赋值。ActivityThread.currentActivityThread()返回的是他的成员变量sCurrentActivityThread，而该成员变量由systemserver进程启动时被赋值，返回的是systemserver进程的主线程。因此AMS中持有的mSystemThread是systemserver进程的主线程。
// /frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.javapublic class ActivityManagerService extends IActivityManager.Stub implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback, ActivityManagerGlobalLock { final ActivityThread mSystemThread; public ActivityManagerService(Context systemContext, ActivityTaskManagerService atm) { // 当前进程的 mSystemThread = ActivityThread.currentActivityThread(); mUiContext = mSystemThread.getSystemUiContext(); }} ActivityThread.currentActivityThread() 方法如下：
///frameworks/base/core/java/android/app/ActivityThread.javapublic final class ActivityThread extends ClientTransactionHandler implements ActivityThreadInternal { @UnsupportedAppUsage private static volatile ActivityThread sCurrentActivityThread; @UnsupportedAppUsage public static ActivityThread currentActivityThread() { return sCurrentActivityThread; }} 5. AMS.setSystemProcess()
在该方法中使用AMS持有的mSystemThread来安装package=“android”系统application，所以该系统应用运行在systemserver进程中。
public void setSystemProcess() { try { //为ActivityThread安装相关system application信息 mSystemThread.installSystemApplicationInfo(info, getClass().getClassLoader()); synchronized (this) { //4.为systemserver 主进程开辟一个ProcessRecord来维护进程的相关信息 ProcessRecord app = mProcessList.newProcessRecordLocked(info, info.processName, false, 0, new HostingRecord("system")); app.setPersistent(true); app.setPid(MY_PID); //PID是进程ID，AMS在systemserver进程中 app.mState.setMaxAdj(ProcessList.SYSTEM_ADJ); app.makeActive(mSystemThread.getApplicationThread(), mProcessStats); addPidLocked(app); updateLruProcessLocked(app, false, null); updateOomAdjLocked(OomAdjuster.OOM_ADJ_REASON_NONE); } } catch (PackageManager.NameNotFoundException e) { throw new RuntimeException( "Unable to find android system package", e); } ... }
