项目中遇到的场景，亲测实用
表单添加时，select选中多个，编辑表单时，select多选回显，如图

代码：

// 新增代码
<label class="col-sm-3 control-label">通道：</label><div class="col-sm-8"><select name="channelIds" multiple id="channelIds" class="form-control"><option th:each="channel:${channels}" th:value="*{channel.id}" th:text="${channel.channelName}"></option></select></div>

// 编辑回显
<label class="col-sm-3 control-label">通道权限</label><div class="col-sm-8"><select name="channelIds" multiple id="channelIds" class="form-control"><option th:each="channel:${channels}" th:value="*{channel.id}" th:text="${channel.channelName}"th:selected="${selectChannels.contains(channel.id)}"></option></select></div>

编辑回显重点是th:selected=“${selectChannels.contains(channel.id)}”
selectChannels是后端返回的已绑定的通道列表，channels是通道列表