将 JSP 转换为 Thymleaf：属性名称不能为 Null 或 Empty答案

【问题标题】：Converting JSP to Thymleaf: Attribute Name cannot be Null or Empty将 JSP 转换为 Thymleaf：属性名称不能为 Null 或 Empty
【发布时间】：2020-01-11 05:55:13
【问题描述】：

我目前正在使用Thymeleaf 将许多.jsp 页面转换为HTML。我已经成功转换了其中的一些，但是在检查任何带有表单标签的页面时，我收到以下错误：

Attribute name cannot be null or empty during the initial page load.

我一直在关注以下指南：https://spring.io/guides/gs/handling-form-submission/

查看代码

    <!-- Registration Form -->
            <form action="#" th:action="@{/register/processRegistrationForm}" th:object="${user}" method="POST" class="form-horizontal">

    <!-- User name -->
                    <div style="margin-bottom: 25px" class="input-group">
                        <span class="input-group-addon"><i class="glyphicon glyphicon-user"></i></span> 
                        <input type="text" th:field:="*{userName}" placeholder="Username (*)" class="form-control" />
                    </div>

                    <!-- Password -->
                    <div style="margin-bottom: 25px" class="input-group">
                        <span class="input-group-addon"><i class="glyphicon glyphicon-lock"></i></span> 
                        <input type="text" th:field:="*{password}" placeholder="First Name (*)" class="form-control" />
                    </div>

控制器代码

@GetMapping("/showRegistrationForm")
public String showMyRegistrationPage(
        Model theModel) {

    theModel.addAttribute("user", new UserRegistrationDto());

    return "Login/registration-form";
}

@PostMapping("/processRegistrationForm")
public String processRegistrationForm(
            @Valid @ModelAttribute ("user") UserRegistrationDto userDto, 
            BindingResult theBindingResult, 
            Model theModel) {

    String userName = userDto.getUserName();
    logger.info("Processing registration form for: " + userName);

    // form validation
     if (theBindingResult.hasErrors()){
         return "Login/registration-form";
        }

    // check the database if user already exists
    User existing = userService.findByUserName(userName);
    if (existing != null){
        theModel.addAttribute("user", new UserRegistrationDto());
        theModel.addAttribute("registrationError", "User name already exists.");

        logger.warning("User name already exists.");
        return "Login/registration-form";
    }
 // create user account                             
    userService.save(userDto);

    logger.info("Successfully created user: " + userName);

    return "redirect:/loginPage?rSuccess";      
}

DTO 代码

public class UserRegistrationDto {

    @NotNull(message = "is required")
    @Size(min = 1, message = "is required")
    private String userName;

    @NotNull(message = "is required")
    @Size(min = 1, message = "is required")
    private String password;

实体代码

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

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "id")
private Long id;

@Column(name = "username")
private String userName;

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

@Column(name = "first_name")
private String firstName;

@Column(name = "last_name")
private String lastName;

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

@ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
@JoinTable(name = "users_roles", 
joinColumns = @JoinColumn(name = "user_id"), 
inverseJoinColumns = @JoinColumn(name = "role_id"))
private Collection<Role> roles;

public User() {
}

public User(String userName, String password, String firstName, String lastName, String email) {
    this.userName = userName;
    this.password = password;
    this.firstName = firstName;
    this.lastName = lastName;
    this.email = email;
}

public User(String userName, String password, String firstName, String lastName, String email,
        Collection<Role> roles) {
    this.userName = userName;
    this.password = password;
    this.firstName = firstName;
    this.lastName = lastName;
    this.email = email;
    this.roles = roles;
}

public Long getId() {
    return id;
}

public void setId(Long id) {
    this.id = id;
}

public String getUserName() {
    return userName;
}

public void setUserName(String userName) {
    this.userName = userName;
}

public String getPassword() {
    return password;
}

public void setPassword(String password) {
    this.password = password;
}

堆栈跟踪指向方法周围的问题：

@GetMapping("/showRegistrationForm")
public String showMyRegistrationPage(

& 用

读取

An error happened during template parsing (template: "class path resource [templates/Login/registration-form.html]")
Caused by: java.lang.IllegalArgumentException: Attribute name cannot be null or empty

只有在使用 th:field:="{}"* 时才会出现这种情况，并且在以 .jsp 离开页面时效果很好。

我看不出代码有问题。有人知道是什么原因造成的吗？

我已经尝试从DTO 中删除验证，因为它是由它引起的，但错误消息没有改变。

【问题讨论】：

你可以试试th:field=（删除colon，即:在=之前）？还有一个建议，将existing 变量而不是new UserRegistrationDto() 放入theModel.addAttribute("user", new UserRegistrationDto()); 行中
嗨乔希尔！这是第二个：不敢相信我错过了！非常感谢:)

标签： java spring-boot jsp thymeleaf

【解决方案1】：

我认为问题在于这两行：

<input type="text" th:field:="*{userName}" placeholder="Username (*)" class="form-control" />

<input type="text" th:field:="*{password}" placeholder="First Name (*)" class="form-control" />

由于您在th:field 之后添加了“:”列，因此可能会出现错误。所以这两行应该是这样的：

<input type="text" th:field="*{userName}" placeholder="Username (*)" class="form-control" />

<input type="text" th:field="*{password}" placeholder="First Name (*)" class="form-control" />

由于 thymeleaf 没有很好地解释问题或有问题的行号，所以找到问题真的很头疼。它只是说“属性名称不能为空或空”，而您只是在搜索空属性。

【讨论】：

【解决方案2】：

我遇到了同样的问题，我只是将错误 1 减 1。所以我对每个输入和每个表单都进行了评论。而问题就在这里

<form action="#" th:action="@{/register/processRegistrationForm}" th:object="${user}" method="POST" class="form-horizontal">

th:object="${user}"，也许这个技巧可以帮助你一点，因为这个问题还没有解决。

【讨论】：