Collections.singletonMap()用于返回单集合
具体用法：
List<User> list = new ArrayList();
User user1 = User.builder().id("aaaaa").username("test1").build();
User user2 = User.builder().id("bbbbb").username("test2").build();
list.add(user1);
list.add(user2);
Map<String, List<User>> singletonMap = Collections.singletonMap( "list",list);
System.out.println(singletonMap);输出结果：
{list=[User(id=aaaaa, username=test1, password=null, phone=null, email=null, status=null, createTime=null, updateTime=null), User(id=bbbbb, username=test2, password=null, phone=null, email=null, status=null, createTime=null, updateTime=null)]}