class Member{
private long id;
private String name;
private Department department;
}
class Department{
private long id;
private String name;
}
join(list<Member>, "name", ",") = 人名字的字符串
join(list<Member>, "department.name", ",") = 部门名字的字符串
join(Collection, String, PageContext)雷同，只是分隔符在调用放指定，常用于id的分割