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); 
    }