Java 无法使用thymeleaf和Spring引导保存实体

我最近开始在JPA和thymeleaf中使用springboot（我主要来自python/flask和node背景），并尝试在我的数据库中创建一个项目。一切都很顺利，我可以添加、删除等等。。项目

我将变量集用户添加到我的项目实体中，如下所示：

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

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name = "project_id")
private int id;

@Column(name = "title")
@NotEmpty(message = "*Please provide a title")
private String title;

@Column(name = "description")
private String description;

@OneToOne(cascade = CascadeType.ALL)
@JoinTable(name = "project_lead", joinColumns = @JoinColumn(name = "project_id"), inverseJoinColumns = @JoinColumn(name = "user_id"))
private User projectLead;

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "project_user", joinColumns = @JoinColumn(name = "project_id"), inverseJoinColumns = @JoinColumn(name = "user_id"))
private Set<User> users;
...
}

@Entity
@Table(name = "user")
public class User {

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name = "user_id")
private int id;

@Column(name = "email")
@Email(message = "*Please provide a valid Email")
@NotEmpty(message = "*Please provide an email")
private String email;

@Column(name = "username")
@NotEmpty(message = "*Please provide a username")
private String username;

@Column(name = "password")
@Length(min = 5, message = "*Your password must have at least 5 characters")
@NotEmpty(message = "*Please provide your password")
@Transient
private String password;

@Column(name = "enabled")
private int enabled;

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "user_role", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "role_id"))
private Set<Role> roles;
...
}

@RequestMapping(value = "/projects", method=RequestMethod.GET)
public ModelAndView showProjectForm() {
    ModelAndView modelAndView = new ModelAndView();
    // Get authenticated user
    Authentication auth = SecurityContextHolder.getContext().getAuthentication();
    User user = userService.findByEmail(auth.getName());

    modelAndView.addObject("userName", "Welcome " + user.getUsername() + " (" + user.getEmail() + ")");
    modelAndView.addObject("project", new Project());
    modelAndView.addObject("allUsers", userService.findAll());
    modelAndView.setViewName("project_creation");
    return modelAndView;
}

@RequestMapping(value = "/projects", method=RequestMethod.POST)
public ModelAndView processProjectForm(@Valid Project project, BindingResult bindingResult) {
    ModelAndView modelAndView = new ModelAndView();
    // Get authenticated user
    Authentication auth = SecurityContextHolder.getContext().getAuthentication();
    User user = userService.findByEmail(auth.getName());

    // Get all projects
    List<Project> allProjects = projectService.findAll();

    // Check if project already exists
    Project projectExist = projectService.findByTitle(project.getTitle());
    if(projectExist != null) {
        bindingResult
        .rejectValue("title", "error.project",
                "There is already a project with this title");
    }


    // Get all users
    List<User> allUsers = userService.findAll();

    if(bindingResult.hasErrors()) {
        modelAndView.addObject("userName", "Welcome " + user.getUsername() + " (" + user.getEmail() + ")");
        modelAndView.addObject("project", new Project());
        modelAndView.addObject("allUsers", allUsers);
        modelAndView.setViewName("project_creation");
    } else {

        // Create project
        project.setProjectLead(user);
        projectService.saveProject(project);
        modelAndView.addObject("userName", "Welcome " + user.getUsername() + " (" + user.getEmail() + ")");
        modelAndView.addObject("success", "Project successfully created!");
        modelAndView.addObject("project", new Project());
        modelAndView.addObject("projects", allProjects);
        modelAndView.setViewName("redirect:/secure/dashboard");
    }
    return modelAndView;
} 

@实体
@表（name=“项目”）
公共类项目{
@身份证
@GeneratedValue（策略=GenerationType.AUTO）
@列（name=“project\u id”）
私有int-id；
@列（name=“title”）
@NotEmpty（message=“*请提供标题”）
私有字符串标题；
@列（name=“description”）
私有字符串描述；
@OneToOne（级联=级联类型.ALL）
@JoinTable（name=“project\u lead”，joinColumns=@JoinColumn（name=“project\u id”），inverseJoinColumns=@JoinColumn（name=“user\u id”））
私人用户项目领导；
@多个（级联=级联类型.ALL）
@JoinTable（name=“project\u user”，joinColumns=@JoinColumn（name=“project\u id”），inverseJoinColumns=@JoinColumn（name=“user\u id”））
私人用户；
...
}

用户类如下所示：

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

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name = "project_id")
private int id;

@Column(name = "title")
@NotEmpty(message = "*Please provide a title")
private String title;

@Column(name = "description")
private String description;

@OneToOne(cascade = CascadeType.ALL)
@JoinTable(name = "project_lead", joinColumns = @JoinColumn(name = "project_id"), inverseJoinColumns = @JoinColumn(name = "user_id"))
private User projectLead;

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "project_user", joinColumns = @JoinColumn(name = "project_id"), inverseJoinColumns = @JoinColumn(name = "user_id"))
private Set<User> users;
...
}

@Entity
@Table(name = "user")
public class User {

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name = "user_id")
private int id;

@Column(name = "email")
@Email(message = "*Please provide a valid Email")
@NotEmpty(message = "*Please provide an email")
private String email;

@Column(name = "username")
@NotEmpty(message = "*Please provide a username")
private String username;

@Column(name = "password")
@Length(min = 5, message = "*Your password must have at least 5 characters")
@NotEmpty(message = "*Please provide your password")
@Transient
private String password;

@Column(name = "enabled")
private int enabled;

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "user_role", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "role_id"))
private Set<Role> roles;
...
}

@RequestMapping(value = "/projects", method=RequestMethod.GET)
public ModelAndView showProjectForm() {
    ModelAndView modelAndView = new ModelAndView();
    // Get authenticated user
    Authentication auth = SecurityContextHolder.getContext().getAuthentication();
    User user = userService.findByEmail(auth.getName());

    modelAndView.addObject("userName", "Welcome " + user.getUsername() + " (" + user.getEmail() + ")");
    modelAndView.addObject("project", new Project());
    modelAndView.addObject("allUsers", userService.findAll());
    modelAndView.setViewName("project_creation");
    return modelAndView;
}

@RequestMapping(value = "/projects", method=RequestMethod.POST)
public ModelAndView processProjectForm(@Valid Project project, BindingResult bindingResult) {
    ModelAndView modelAndView = new ModelAndView();
    // Get authenticated user
    Authentication auth = SecurityContextHolder.getContext().getAuthentication();
    User user = userService.findByEmail(auth.getName());

    // Get all projects
    List<Project> allProjects = projectService.findAll();

    // Check if project already exists
    Project projectExist = projectService.findByTitle(project.getTitle());
    if(projectExist != null) {
        bindingResult
        .rejectValue("title", "error.project",
                "There is already a project with this title");
    }


    // Get all users
    List<User> allUsers = userService.findAll();

    if(bindingResult.hasErrors()) {
        modelAndView.addObject("userName", "Welcome " + user.getUsername() + " (" + user.getEmail() + ")");
        modelAndView.addObject("project", new Project());
        modelAndView.addObject("allUsers", allUsers);
        modelAndView.setViewName("project_creation");
    } else {

        // Create project
        project.setProjectLead(user);
        projectService.saveProject(project);
        modelAndView.addObject("userName", "Welcome " + user.getUsername() + " (" + user.getEmail() + ")");
        modelAndView.addObject("success", "Project successfully created!");
        modelAndView.addObject("project", new Project());
        modelAndView.addObject("projects", allProjects);
        modelAndView.setViewName("redirect:/secure/dashboard");
    }
    return modelAndView;
} 

@实体
@表（name=“user”）
公共类用户{
@身份证
@GeneratedValue（策略=GenerationType.AUTO）
@列（name=“user\u id”）
私有int-id；
@列（name=“email”）
@电子邮件（message=“*请提供有效电子邮件”）
@NotEmpty（message=“*请提供电子邮件”）
私人字符串电子邮件；
@列（name=“username”）
@NotEmpty（message=“*请提供用户名”）
私有字符串用户名；
@列（name=“password”）
@长度（最小值=5，message=“*您的密码必须至少包含5个字符”）
@NotEmpty（message=“*请提供您的密码”）
@短暂的
私有字符串密码；
@列（name=“enabled”）
启用私有int；
@多个（级联=级联类型.ALL）
@JoinTable（name=“user\u role”，joinColumns=@JoinColumn（name=“user\u id”），inverseJoinColumns=@JoinColumn（name=“role\u id”））
私人设定角色；
...
}

当我没有为项目指定用户时，我可以创建新项目。但是，当我使用多重选择指定用户时，我在创建项目时遇到了问题。在我的表格中，我有：

<form th:action="@{/secure/projects}" th:object="${project}" method="POST">
        <div class="form-group">
            <label th:if="${#fields.hasErrors('title')}" th:errors="*{title}" class="validation-message"></label>
            <input type="text" class="form-control" th:field="*{title}" id="title" th:placeholder="Title" /> 
        </div>

        <div class="form-group">
            <input type="text" class="form-control" th:field="*{description}" id="description" th:placeholder="Description" /> 
        </div>

        <div class="form-group">
            <select th:field="*{users}" class="users-select form-control" multiple="multiple">
              <option th:each="user : ${allUsers}" th:value="${user}" th:text="${user.username}"></option>
            </select>
        </div>


        <button name="Submit" value="Submit" type="Submit" th:text="Create"></button>
   </form>

对于th:field=“*{users}”的属性“users”，我不断获取“未能将类型为“java.lang.String”的属性值转换为所需类型“java.util.Set”。我不明白为什么th:value=“${user}”被认为是一个字符串，而它应该属于user类。有没有一种方法可以让我简单地获得select的结果，通过它循环并手动将它添加到控制器中到我的对象项目中

我的控制器如下所示：

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

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name = "project_id")
private int id;

@Column(name = "title")
@NotEmpty(message = "*Please provide a title")
private String title;

@Column(name = "description")
private String description;

@OneToOne(cascade = CascadeType.ALL)
@JoinTable(name = "project_lead", joinColumns = @JoinColumn(name = "project_id"), inverseJoinColumns = @JoinColumn(name = "user_id"))
private User projectLead;

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "project_user", joinColumns = @JoinColumn(name = "project_id"), inverseJoinColumns = @JoinColumn(name = "user_id"))
private Set<User> users;
...
}

@Entity
@Table(name = "user")
public class User {

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name = "user_id")
private int id;

@Column(name = "email")
@Email(message = "*Please provide a valid Email")
@NotEmpty(message = "*Please provide an email")
private String email;

@Column(name = "username")
@NotEmpty(message = "*Please provide a username")
private String username;

@Column(name = "password")
@Length(min = 5, message = "*Your password must have at least 5 characters")
@NotEmpty(message = "*Please provide your password")
@Transient
private String password;

@Column(name = "enabled")
private int enabled;

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "user_role", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "role_id"))
private Set<Role> roles;
...
}

@RequestMapping(value = "/projects", method=RequestMethod.GET)
public ModelAndView showProjectForm() {
    ModelAndView modelAndView = new ModelAndView();
    // Get authenticated user
    Authentication auth = SecurityContextHolder.getContext().getAuthentication();
    User user = userService.findByEmail(auth.getName());

    modelAndView.addObject("userName", "Welcome " + user.getUsername() + " (" + user.getEmail() + ")");
    modelAndView.addObject("project", new Project());
    modelAndView.addObject("allUsers", userService.findAll());
    modelAndView.setViewName("project_creation");
    return modelAndView;
}

@RequestMapping(value = "/projects", method=RequestMethod.POST)
public ModelAndView processProjectForm(@Valid Project project, BindingResult bindingResult) {
    ModelAndView modelAndView = new ModelAndView();
    // Get authenticated user
    Authentication auth = SecurityContextHolder.getContext().getAuthentication();
    User user = userService.findByEmail(auth.getName());

    // Get all projects
    List<Project> allProjects = projectService.findAll();

    // Check if project already exists
    Project projectExist = projectService.findByTitle(project.getTitle());
    if(projectExist != null) {
        bindingResult
        .rejectValue("title", "error.project",
                "There is already a project with this title");
    }


    // Get all users
    List<User> allUsers = userService.findAll();

    if(bindingResult.hasErrors()) {
        modelAndView.addObject("userName", "Welcome " + user.getUsername() + " (" + user.getEmail() + ")");
        modelAndView.addObject("project", new Project());
        modelAndView.addObject("allUsers", allUsers);
        modelAndView.setViewName("project_creation");
    } else {

        // Create project
        project.setProjectLead(user);
        projectService.saveProject(project);
        modelAndView.addObject("userName", "Welcome " + user.getUsername() + " (" + user.getEmail() + ")");
        modelAndView.addObject("success", "Project successfully created!");
        modelAndView.addObject("project", new Project());
        modelAndView.addObject("projects", allProjects);
        modelAndView.setViewName("redirect:/secure/dashboard");
    }
    return modelAndView;
} 

@RequestMapping（value=“/projects”，method=RequestMethod.GET）
公共模型和视图showProjectForm（）{
ModelAndView ModelAndView=新建ModelAndView（）；
//获取经过身份验证的用户
Authentication auth=SecurityContextHolder.getContext（）.getAuthentication（）；
User=userService.findByEmail（auth.getName（））；
modelAndView.addObject（“用户名”，“欢迎”+user.getUsername（）+”（“+user.getEmail（）+”））；
addObject（“project”，newproject（））；
modelAndView.addObject（“alluser”，userService.findAll（））；
setViewName（“项目创建”）；
返回模型和视图；
}
@RequestMapping（value=“/projects”，method=RequestMethod.POST）
公共模型和视图processProjectForm（@Valid Project，BindingResult BindingResult）{
ModelAndView ModelAndView=新建ModelAndView（）；
//获取经过身份验证的用户
Authentication auth=SecurityContextHolder.getContext（）.getAuthentication（）；
User=userService.findByEmail（auth.getName（））；
//获取所有项目
列出所有项目=projectService.findAll（）；
//检查项目是否已经存在
Project projectExist=projectService.findbytle（Project.getTitle（））；
if（projectExist！=null）{
绑定结果
.rejectValue（“标题”、“错误.项目”，
“已存在具有此标题的项目”）；
}
//获取所有用户
List alluser=userService.findAll（）；
if（bindingResult.hasErrors（））{
modelAndView.addObject（“用户名”，“欢迎”+user.getUsername（）+”（“+user.getEmail（）+”））；
addObject（“project”，newproject（））；
modelAndView.addObject（“allUsers”，allUsers）；
setViewName（“项目创建”）；
}否则{
//创建项目
project.setProjectLead（用户）；
projectService.saveProject（project）；
modelAndView.addObject（“用户名”，“欢迎”+user.getUsername（）+”（“+user.getEmail（）+”））；
addObject（“成功”，“项目成功创建！”）；
addObject（“project”，newproject（））；
添加对象（“项目”，所有项目）；
modelAndView.setViewName（“重定向：/secure/dashboard”）；
}
返回模型和视图；
} 

我能够修复属性“用户”的“未能将类型为“java.lang.String”的属性值转换为所需类型“java.util.Set”的问题。我只需要添加一个转换器类，告诉我如何从字符串转换到用户对象

@Component
public class StringToUser implements Converter<String, User> {

@Autowired
private UserService userService;

@Override
public User convert(String arg0) {
    Integer id = new Integer(arg0);
    return userService.findOne(id);
}
}

@组件
公共类StringToUser实现转换器{
@自动连线
私人用户服务；
@凌驾
公共用户转换（字符串arg0）{
整数id=新整数（arg0）；
返回userService.findOne（id）；
}
}

并将表单中的选项更改为user.id值，而不是user对象。转换器将处理其余部分：

<option th:each="user : ${allUsers}" th:value="${user.getId()}" th:text="${user.getUsername()}"></option>