AMS中ActivityThread的生命周期

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：

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目录

1 systemServer.createSystemContext()

2 ActivityThread.systemMain()

3 ActivityThread.attach() 

4. AMS成员函数ActivityThread mSystemThread

5. AMS.setSystemProcess()

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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.java
public 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);
    }
}

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2 ActivityThread.systemMain()

该方法中new ActivityThread()，并调用了其attach()方法。

/frameworks/base/core/java/android/app/ActivityThread.java
public final class ActivityThread extends ClientTransactionHandler
        implements ActivityThreadInternal {
    @UnsupportedAppUsage
    public static ActivityThread systemMain() {
        ThreadedRenderer.initForSystemProcess();
        ActivityThread thread = new ActivityThread();
        thread.attach(true, 0);
        return thread;
    }
}

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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.java
public 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;
        ...
    }
}

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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.java
public 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.java
public 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);
        }
        ...
    }