안드로이드 메인클래스

안드로이드 메인클래스

android.app.ActivityThread 에서 main() 메서드를 호출 하면서 애플리케이션이 실행된다. main() 메서드에서는 Looper.prepareMainLooper(), Looper.loop() 로 Looper를 준비하고 구동 시킨다. sMainThreadHandler 에서 애플리케이션의 다양한 이벤트를 처리한다. loop() 메서드는 무한루프와 blocking 으로 동작 하므로 main() 메서드는 프로세스가 종료 되기 전까지 종료되지 않는다.

public static void main(String[] args) {
   ...

    Looper.prepareMainLooper();

    ActivityThread thread = new ActivityThread();
    thread.attach(false);

    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }

   ...
    Looper.loop();

    throw new RuntimeException("Main thread loop unexpectedly exited");
}

sMainThreadHandler는 private class H extends Handler 로 구현되어 있으며, Activity, Service, Broadcase 등 애플리케이션에서 발생하는 이벤트를 처리한다.

H 클래스의 정의 된 Message.what에 관한 상수 들과 handleMessage 메서드의 내용을 보면 어떤 일들을 처리 하는지 알 수 있다

private class H extends Handler {
    public static final int LAUNCH_ACTIVITY         = 100;
    public static final int PAUSE_ACTIVITY          = 101;
    public static final int PAUSE_ACTIVITY_FINISHING= 102;
    public static final int STOP_ACTIVITY_SHOW      = 103;
    public static final int STOP_ACTIVITY_HIDE      = 104;
    public static final int SHOW_WINDOW             = 105;
    public static final int HIDE_WINDOW             = 106;
    public static final int RESUME_ACTIVITY         = 107;
    public static final int SEND_RESULT             = 108;
    public static final int DESTROY_ACTIVITY        = 109;
    public static final int BIND_APPLICATION        = 110;
    public static final int EXIT_APPLICATION        = 111;
    public static final int NEW_INTENT              = 112;
    public static final int RECEIVER                = 113;
    public static final int CREATE_SERVICE          = 114;
    public static final int SERVICE_ARGS            = 115;
    public static final int STOP_SERVICE            = 116;

    public static final int CONFIGURATION_CHANGED   = 118;
    public static final int CLEAN_UP_CONTEXT        = 119;
    public static final int GC_WHEN_IDLE            = 120;
    public static final int BIND_SERVICE            = 121;
    public static final int UNBIND_SERVICE          = 122;
    public static final int DUMP_SERVICE            = 123;
    public static final int LOW_MEMORY              = 124;
    public static final int ACTIVITY_CONFIGURATION_CHANGED = 125;
    public static final int RELAUNCH_ACTIVITY       = 126;
    public static final int PROFILER_CONTROL        = 127;
    public static final int CREATE_BACKUP_AGENT     = 128;
    public static final int DESTROY_BACKUP_AGENT    = 129;
    public static final int SUICIDE                 = 130;
    public static final int REMOVE_PROVIDER         = 131;
    public static final int ENABLE_JIT              = 132;
    public static final int DISPATCH_PACKAGE_BROADCAST = 133;
    public static final int SCHEDULE_CRASH          = 134;
    public static final int DUMP_HEAP               = 135;
    public static final int DUMP_ACTIVITY           = 136;
    public static final int SLEEPING                = 137;
    public static final int SET_CORE_SETTINGS       = 138;
    public static final int UPDATE_PACKAGE_COMPATIBILITY_INFO = 139;
    public static final int TRIM_MEMORY             = 140;
    public static final int DUMP_PROVIDER           = 141;
    public static final int UNSTABLE_PROVIDER_DIED  = 142;
    public static final int REQUEST_ASSIST_CONTEXT_EXTRAS = 143;
    public static final int TRANSLUCENT_CONVERSION_COMPLETE = 144;
    public static final int INSTALL_PROVIDER        = 145;
    public static final int ON_NEW_ACTIVITY_OPTIONS = 146;
    public static final int ENTER_ANIMATION_COMPLETE = 149;
    public static final int START_BINDER_TRACKING = 150;
    public static final int STOP_BINDER_TRACKING_AND_DUMP = 151;
    public static final int MULTI_WINDOW_MODE_CHANGED = 152;
    public static final int PICTURE_IN_PICTURE_MODE_CHANGED = 153;
    public static final int LOCAL_VOICE_INTERACTION_STARTED = 154;
    public static final int ATTACH_AGENT = 155;
    public static final int APPLICATION_INFO_CHANGED = 156;
    public static final int ACTIVITY_MOVED_TO_DISPLAY = 157;

public void handleMessage(Message msg) {
    if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
    switch (msg.what) {
        case LAUNCH_ACTIVITY: {
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
            final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

            r.packageInfo = getPackageInfoNoCheck(
                    r.activityInfo.applicationInfo, r.compatInfo);
            handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        } break;
        case RELAUNCH_ACTIVITY: {
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
            ActivityClientRecord r = (ActivityClientRecord)msg.obj;
            handleRelaunchActivity(r);
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        } break;
        case PAUSE_ACTIVITY: {
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
            SomeArgs args = (SomeArgs) msg.obj;
            handlePauseActivity((IBinder) args.arg1, false,
                    (args.argi1 & USER_LEAVING) != 0, args.argi2,
                    (args.argi1 & DONT_REPORT) != 0, args.argi3);
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        } break;
        case PAUSE_ACTIVITY_FINISHING: {
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
            SomeArgs args = (SomeArgs) msg.obj;
            handlePauseActivity((IBinder) args.arg1, true, (args.argi1 & USER_LEAVING) != 0,
                    args.argi2, (args.argi1 & DONT_REPORT) != 0, args.argi3);
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        } break;
...

https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/app/ActivityThread.java