android 判断手机是否插入SIM卡

TelephonyManager tm = (TelephonyManager) getSystemService(TELEPHONY_SERVICE);
String simSer = tm.getSimSerialNumber();
if(simSer == null || simSer.equals("")) {
//Toast.makeText(this, "插入SIM卡才能开启此应用", Toast.LENGTH_LONG).show();
Builder builder = new AlertDialog.Builder(this);
builder.setIcon(R.drawable.datasync);
builder.setTitle("插入SIM卡才能开启此应用");
builder.setPositiveButton("确认", new DialogInterface.OnClickListener() {

@Override
public void onClick(DialogInterface dialog, int which) {
System.exit(0);
}

});
builder.show();

}