在Android 5.0中，新增了一个DisplayThread类，源码位置在frameworks/base/services/core/java/com/android/server/DisplayThread.java. 

这个类的注释是这么写的，

 * Shared singleton foreground thread for the system.  This is a thread for
 * operations that affect what's on the display, which needs to have a minimum
 * of latency.  This thread should pretty much only be used by the WindowManager,
 * DisplayManager, and InputManager to perform quick operations in real time.

也就是说，从Android 5.0开始，所有跟系统前景相关的操作都集中到这个线程类中。这个类用来处理需要低时延的显示相关的操作。这个线程只能由WindowManager，DisplayManager和InputManager用来实时显示相关的快速操作。

于是WindowManagerService运行的线程再次发生了变化。在Android 4.1的时代，WmS拥有一个自己的主线程WMThread。4.1之后到4.4，SystemServer提供了一个wmHandlerThread来替代它。到了5.0之后，WmS改成运行在DisplayThread的Handler中。

    public static WindowManagerService main(final Context context,
            final InputManagerService im,
            final boolean haveInputMethods, final boolean showBootMsgs,
            final boolean onlyCore) {
        final WindowManagerService[] holder = new WindowManagerService[1];
        DisplayThread.getHandler().runWithScissors(new Runnable() {
            @Override
            public void run() {
                holder[0] = new WindowManagerService(context, im,
                        haveInputMethods, showBootMsgs, onlyCore);
            }
        }, 0);
        return holder[0];
    }