Android 如何在service和BroadCastReceiver中 启动activity和alertdialog？

一.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(),SecondActivity.class); 
startActivity(intent); 

二.在BroadcastReceiver当中启动alertDialog总是会报错
首先加上权限：

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

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

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