Android阻止AlertDialog关闭

AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
builder.setTitle("测试");

LayoutInflater inflater = getActivity().getLayoutInflater();
View view = inflater.inflate(R.layout.dialogfragment_num_input, null);
builder.setView(view);

builder.setPositiveButton("确定",
        new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int id) {

                Field field = null;

                try {
                    //通过反射获取dialog中的私有属性mShowing
                    field = dialog.getClass().getSuperclass().getDeclaredField("mShowing");
                    field.setAccessible(true);//设置该属性可以访问
                } catch (Exception ex) {

                }

                String inputValue = String.valueOf(mEdit.getText());
                if (inputValue == null || "".equals(inputValue)) {
                    try {
                        //设置dialog不可关闭
                        field.set(dialog, false);
                        dialog.dismiss();
                    } catch (Exception ex) {
                    }
                } else {

                   //
                   //做自己的事
                   //
                    try {
                        //关闭
                        field.set(dialog, true);
                        dialog.dismiss();
                    } catch (Exception ex) {
                    }
                }
            }
        });
builder.setNegativeButton("取消",
        new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int id) {

                Field field = null;

                try {
                    //通过反射获取dialog中的私有属性mShowing
                    field = dialog.getClass().getSuperclass().getDeclaredField("mShowing");
                    field.setAccessible(true);//设置该属性可以访问
                } catch (Exception ex) {
                }

                try {
                    field.set(dialog, true);
                    dialog.dismiss();
                } catch (Exception ex) {
                }

            }
        });


builder.create();