Android service里面启动activity和alertdialog .

启动activity源码:(记得要加上Intent.FLAG_ACTIVITY_NEW_TASK)

Intent intent = new Intent();
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
intent.setClass(getApplicationContext(),FileBrowserActivity.class);
startActivity(intent);

启动alertDialog源码:

 AlertDialog.Builder builder = new AlertDialog.Builder(this);
 builder.setMessage("是否接受文件?")
 .setPositiveButton("是", new DialogInterface.OnClickListener() {
@Override
publicvoid onClick(DialogInterface dialog, int which) {

 }
 }).setNegativeButton("否", new OnClickListener() {
@Override
publicvoid onClick(DialogInterface dialog, int which) {
 }
 });
 AlertDialog ad = builder.create();
// ad.getWindow().setType(WindowManager.LayoutParams.TYPE_SYSTEM_DIALOG); //系统中关机对话框就是这个属性
 ad.getWindow().setType(WindowManager.LayoutParams.TYPE_SYSTEM_ALERT);
 ad.setCanceledOnTouchOutside(false); //点击外面区域不会让dialog消失
 ad.show();

还要加上权限 

<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />