方法一：

android4.0/frameworks/base/services/java/com/android/server/wm/WindowManagerService.java
int computeForcedAppOrientationLocked() {
int req = getOrientationFromWindowsLocked();
if (req == ActivityInfo.SCREEN_ORIENTATION_UNSPECIFIED) {
req = getOrientationFromAppTokensLocked();
}
// req = ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE; //强制横屏0度
// req = ActivityInfo.SCREEN_ORIENTATION_PORTRAIT; //强制竖屏左转90度
return req;
}

方法二：

android4.0/frameworks/base/services/surfaceflinger/SurfaceFlinger.cpp
void GraphicPlane::setDisplayHardware(DisplayHardware *hw)
{
mHw = hw;
// initialize the display orientation transform.
// it's a constant that should come from the display driver.
int displayOrientation = ISurfaceComposer::eOrientationDefault;
char property[PROPERTY_VALUE_MAX];
if (property_get("ro.sf.hwrotation", property, NULL) > 0) {
//displayOrientation
switch (atoi(property)) {
case 0:
displayOrientation = ISurfaceComposer::eOrientationDefault;
break;
case 90:
displayOrientation = ISurfaceComposer::eOrientation90;
break;
case 180:
displayOrientation = ISurfaceComposer::eOrientation180;
break;
case 270:
displayOrientation = ISurfaceComposer::eOrientation270;
break;
}
}
/* 强制旋转90度 */
displayOrientation = ISurfaceComposer::eOrientation90;
const float w = hw->getWidth();
const float h = hw->getHeight();
GraphicPlane::orientationToTransfrom(displayOrientation, w, h,
&mDisplayTransform);
}