提前谢谢
我有添加员工表格的问题，用户设置了这么多信息姓名，电话。。。。直到下拉多选（我的问题）用户必须从下拉列表中选择一个或多个，但问题是我无法得到这个用户已经选择的列表我尝试了许多方法，但我找到了这两种方法，这两种方法更接近解决我的问题下来你会发现2个版本的控制器和视图+类信息，如果你需要的话

controller class+html视图“version one”：问题是如果我从html页面的下拉列表中选择了3个条目，那么我在controller中得到的正确的所选列表大小是3，但是如果我尝试得到这个条目的项目或雇员，那么它会显示为null

@Controller
        @RequestMapping("/employee")
        public class EmployeeController {

            // more codes

            @PostMapping("/save")
            public String saveEmployee(
            @ModelAttribute("employee") Employee employee,
            Model model) {

            System.out.println("size ="+ employee.getWorkOns.size()); // its work well 
System.out.println("size ="+ employee.getWorkOns.get(0).getEmployee()); // nullpointerexception
System.out.println("size ="+ employee.getWorkOns.get(0).getProject()); // nullpointerexception

            // workOns is list of workOn

            model.addAttribute("workOnList", workOns);

            // more code
         }

html选择选项版本1：

<form th:action="@{/employee/save}" th:object="${employee}" method="POST">

        <!-- more code -->

        <select th:field="*{workOns}" multiple="multiple"> <!-- workOns is field from employee and its a list<WorkOn> -->
                <option th:each="workon: ${workOnList}"
                    th:value="${{workon}}"
                    th:text="${workon.project.name}"/>      
        </select>

        <!-- more code -->
</form>

controller class+html视图“version tow”：在这种情况下，我尝试获取工作项的项目名称selecte the problem如果我从html页面的下拉列表中选择3个项，那么我在controller中得到的selected list的大小是错误的，大小总是1

@Controller
    @RequestMapping("/employee")
    public class EmployeeController {

        // more codes

        @PostMapping("/save")
        public String saveEmployee(
        @ModelAttribute("employee") Employee employee,
        @ModelAttribute("workOns") List<String> workOnList,
        Model model) {

        System.out.println("size ="+ employee.getWorkOns.size()); // always is print 1 

        // workOns is list of workOn

        model.addAttribute("workOnList", workOns);

        // more code
     }

html选择选项版本2：

<form th:action="@{/employee/save}" th:object="${employee}" method="POST">

            <!-- more code -->

                    <select name="workOns" multiple="multiple">
                            <option th:each="workon: ${workOnList}"
                                th:value="${workon.project.projId}"
                                th:text="${workon.project.name}"/> 
                    </select>
 <!-- here i try get list names of the project of workOn whish the user get selected -->
            <!-- more code -->
    </form>

员工类别

@Entity
@Table(name = "employee") 
public class Employee {

    // more vars ....

    @OneToMany(mappedBy = "employee", cascade = { CascadeType.DETACH, CascadeType.MERGE, CascadeType.REFRESH,CascadeType.REMOVE })
    private List<WorkOn> workOns;

    // constructors and setters and getters ....

}

项目类

@Entity
@Table(name="project")
public class Project { 

   // more vars ....

   @OneToMany(mappedBy = "project", cascade = {CascadeType.DETACH,CascadeType.MERGE,CascadeType.REFRESH,CascadeType.REMOVE })
   private List<WorkOn> WorkOns;

   // constructors and setters and getters ....

 }

修井班

@Entity
@Table(name = "workon")
public class WorkOn {

    // more vars ....

    @ManyToOne(cascade = {CascadeType.DETACH , CascadeType.MERGE , CascadeType.REFRESH})
    @JoinColumn(name = "emp_id")
    private Employee employee;

    @ManyToOne(cascade = {CascadeType.DETACH , CascadeType.MERGE , CascadeType.REFRESH})
    @JoinColumn(name = "proj_id")
    private Project project;

    // constructors and setters and getters ....
}

部门级

@Entity
@Table(name="department" )//,schema = "company_management")
public class Department {

    // more vars ....

    @OneToMany(mappedBy = "department" , cascade = {CascadeType.DETACH,CascadeType.MERGE,CascadeType.REFRESH,CascadeType.REMOVE})
    private List<Employee> employees;

    @OneToMany(mappedBy = "department" , cascade = {CascadeType.DETACH,CascadeType.MERGE,CascadeType.REFRESH,CascadeType.REMOVE})
    private List<Project> projects; 

    // constructors and setters and getters ....

}