如何使当点击AlertDialog中的按钮时，AlertDialog不被dismiss掉

//通过反射来实现
@Override
protected Dialog onCreateDialog(int id) {
final AlertDialog.Builder builder=new AlertDialog.Builder(this);
if(id==1){
builder.setTitle("标题");
builder.setMessage("消息");
final EditText et = new EditText(this);
et.setWidth(LayoutParams.FILL_PARENT);
et.setPadding(5, 5 ,5, 5);
et.setTextSize(18);
et.setHeight(160);
et.setGravity(Gravity.TOP);
builder.setView(et);
final AlertDialog ad = builder.create();
ad.setButton("确定", new DialogInterface.OnClickListener(){
@Override
public void onClick(DialogInterface dialog, int which) {
if(et.getText().toString().trim().length()==0){
try {
Field field = ad.getClass().getSuperclass()
.getDeclaredField("mShowing");
field.setAccessible(true);
field.set(ad, false);
} catch (Exception e) {
e.printStackTrace();
}
}
}});
ad.setButton2("取消",new DialogInterface.OnClickListener(){
@Override
public void onClick(DialogInterface dialog, int which) {
try {
Field field = ad.getClass().getSuperclass()
.getDeclaredField("mShowing");
field.setAccessible(true);
field.set(ad, true);
} catch (Exception e) {
e.printStackTrace();
}
}});

return ad;
}
return super.onCreateDialog(id);
}