急Xposed框架 XposedHelpers.findAndHookMethod找不到系统服务类

Xposed框架 XposedHelpers.findAndHookMethod 找系统服务类如下（PackageManagerService）拦截报java.lang.ClassNotFoundException: com.android.server.pm.PackageManagerService，有哪位大神知道具体原因 万分感谢
@Overrid
    public void handleLoadPackage(final LoadPackageParam lpparam) throws Throwable {        LogUtils.i(lpparam.packageName + "      " + lpparam.processName);
        if(("android").equals(lpparam.packageName)){
            XposedHelpers.findAndHookMethod("com.android.server.pm.PackageManagerService", lpparam.classLoader, "checkSignatures", String.class, String.class,new XC_MethodHook(){                @Override
                protected void beforeHookedMethod(MethodHookParam param) throws Throwable {
                    super.beforeHookedMethod(param);
                    LogUtils.i( "beforeHookedMethod com.android.server.pm.PackageManagerService$checkSignatures") ;
                }
                @Override
                protected void afterHookedMethod(MethodHookParam param) throws Throwable {
                    super.afterHookedMethod(param);
                    LogUtils.i( "afterHookedMethod com.android.server.pm.PackageManagerService$checkSignatures") ;
                }
            });        }
}