Spring MVC 3 中的表单提交 - 解释 [英] Form submit in Spring MVC 3 - explanation

问题描述

我在理解 Spring 3 MVC 中表单提交的工作方式时遇到问题.

I'm having problems understanding how does a form submit in Spring 3 MVC work.

我想要做的是创建一个控制器，该控制器将采用用户的姓名并将其显示给他.不知何故，我已经做到了，但我真的不明白它是如何工作的.所以..

What I want to do, is to create a controller which would take the user's name and display it to him. And somehow I have done it but I don't really understand how it works. So..

我有一个看起来像这样的表格:

I have a form which looks like this:

<form:form method="post" modelAttribute="person">
    <form:label path="firstName">First name</form:label>
    <form:input path="firstName" />
    <br />

    <form:label path="lastName">Last name</form:label>
    <form:input path="lastName" />
    <br />

    <input type="submit" value="Submit" />
</form:form>

我也有一个看起来像这样的控制器:

I also have a controller which looks like this:

@Controller
public class HomeController {

    @RequestMapping(value = "/", method = RequestMethod.GET)
    public String showHelloPage(Model model) {
        model.addAttribute("person", new Person());
        return "home";
    }

    @RequestMapping(value = "/", method = RequestMethod.POST)
    public String sayHello(Person person, Model model) {
        model.addAttribute("person", person);
        return "home";
    }
}

要向用户显示欢迎消息，我在 JSP 页面中使用以下代码:

To display a welcome message to a user I use the following code in the JSP page:

<c:if test="${not empty person.firstName and not empty person.lastName}">
    Hello ${person.firstName} ${person.lastName}!
</c:if>

它可以工作(我省略了 XML 配置文件，因为它们与问题无关).

And it works (I omit the XML configuration files because they are irrelevant to the problem).

我认为表单中的modelAttribute"属性指向 bean 变量，该变量应该填充输入的值(如在其路径"属性中设置的).但看起来，它的工作方式非常不同.如果我删除该行

I thought that the "modelAttribute" attribute in a form points to the bean variable which should be populated with inputs' values (as set in their "path" attributes). But looks, it works in a very different way. If I remove the line

model.addAttribute("person", new Person());

从showHelloPage"方法中，我得到一个(常见的)异常既不是 BindingResult 也不是...".

from "showHelloPage" method I get an (common) exception "Neither BindingResult nor...".

此外，一开始，sayHello"方法看起来像:

Also, on the beginning, the "sayHello" method looked like:

(...)
public String sayHello(@ModelAttribute("person") Person person, Model model) {
(...)

我的意思是，它有ModelAttribute"注释.我添加了它，因为在我阅读的教程中，它始终存在.但是在我删除它之后，一切都运行良好，就像以前一样.

I mean, it had the "ModelAttribute" annotation. I added it, because in the tutorials I have read, it was always present. But after I removed it, everything worked well, as it did before.

所以我的问题是 - ModelAttribute"注释的用途是什么?是否有某种方法可以省略表单中的modelAttribute"属性?第二部分，有什么方法(可能是一些注释)使表单自动将输入的值绑定到正确的 bean 属性(将声明为方法参数)?无需在发送表单之前添加空 bean(因为我现在必须这样做).

So my question is - what is the use of the "ModelAttribute" anonnatation? Is it some way to omit a "modelAttribute" attribute in a form? And the second part, what is the way (maybe some annotation) to make a form automatically bind inputs' values to the proper bean's properties (which would be declared as a method parameter)? Without a need of adding an empty bean before sending a form (as I have to do it now).

感谢您的回复(不是 Spring 文档的链接，因为我已经阅读了).

Thanks for your replies (which aren't links to the Spring's documentation, because I have already read it).

推荐答案

@ModelAttribute 注释在这种情况下用于标识 Spring 应添加为模型属性的对象.模型属性是 HttpServletRequest 属性的抽象.基本上，它们是由一些键标识的对象，这些键会进入 HttpServletRequest 属性.您可以通过使用 Model#addAttribute(String, Object) 手动添加属性、使用 @ModelAttribute 注释方法或通过使用 注释方法参数来实现此目的@ModelAttribute.

The @ModelAttribute annotation in this case is used to identify an object that Spring should add as a model attribute. Model attributes are an abstraction from the HttpServletRequest attributes. Basically, they are objects identified by some key that will find their way into the HttpServletRequest attributes. You can do this by manually adding an attribute with Model#addAttribute(String, Object), have a @ModelAttribute annotated method, or by annotating a method parameter with @ModelAttribute.

您需要了解的是 Spring 如何解析您的处理程序方法参数并注入参数.它使用 HandlerMethodArgumentResolver 接口来实现.有许多实现类(参见 javadoc)，每个类都有责任通过返回 Spring 将用于 invoke() 您的处理程序方法的参数来resolveArgument()反射.如果 HandlerMethodArgumentResolver supportsParameter() 方法为特定参数返回 true，Spring 只会调用 resolveArgument() 方法.

The thing you need to understand is how Spring resolves your handler method parameters and injects arguments. It uses the HandlerMethodArgumentResolver interface to do so. There are a number of implementing classes (see javadoc) and each has the responsibility to resolveArgument() by returning the argument that Spring will use to invoke() your handler method through reflection. Spring will only call the resolveArgument() method if the HandlerMethodArgumentResolver supportsParameter() method returns true for the specific parameter.

这里有问题的 HandlerMethodArgumentResolver 实现是 ServletModelAttributeMethodProcessor 扩展自 ModelAttributeMethodProcessor 声明

The HandlerMethodArgumentResolver implementation in question here is ServletModelAttributeMethodProcessor which extends from ModelAttributeMethodProcessor which states

解析用@ModelAttribute 注释的方法参数和句柄从用@ModelAttribute 注释的方法返回值.

Resolves method arguments annotated with @ModelAttribute and handles return values from methods annotated with @ModelAttribute.

Spring (3.2) 将 注册这个HandlerMethodArgumentResolver和其他

Spring (3.2) will register this HandlerMethodArgumentResolver and others

private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
        List<HandlerMethodArgumentResolver> resolvers = new ArrayList<HandlerMethodArgumentResolver>();

    // Annotation-based argument resolution
    resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
    resolvers.add(new RequestParamMapMethodArgumentResolver());
    resolvers.add(new PathVariableMethodArgumentResolver());
    resolvers.add(new ServletModelAttributeMethodProcessor(false));
    resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters()));
    resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters()));
    resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
    resolvers.add(new RequestHeaderMapMethodArgumentResolver());
    resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
    resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));

    // Type-based argument resolution
    resolvers.add(new ServletRequestMethodArgumentResolver());
    resolvers.add(new ServletResponseMethodArgumentResolver());
    resolvers.add(new HttpEntityMethodProcessor(getMessageConverters()));
    resolvers.add(new RedirectAttributesMethodArgumentResolver());
    resolvers.add(new ModelMethodProcessor());
    resolvers.add(new MapMethodProcessor());
    resolvers.add(new ErrorsMethodArgumentResolver());
    resolvers.add(new SessionStatusMethodArgumentResolver());
    resolvers.add(new UriComponentsBuilderMethodArgumentResolver());

    // Custom arguments
    if (getCustomArgumentResolvers() != null) {
        resolvers.addAll(getCustomArgumentResolvers());
    }

    // Catch-all
    resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
    resolvers.add(new ServletModelAttributeMethodProcessor(true));

    return resolvers;
}

当 Spring 需要调用您的处理程序方法时，它会遍历参数类型和上面的列表，并使用 supportsParameter() 的第一个.

When Spring needs to invoke your handler method, it'll iterate through the parameter types and through the above list and use the first one that supportsParameter().

请注意，添加了两个 ServletModelAttributeMethodProcessor 实例(一个在 //catch all 注释之后).ModelAttributeMethodProcessor 有一个 annotationNotRequired 字段，它告诉它是否应该查找 @ModelAttribute.第一个实例必须查找 @ModelAttribute，第二个不查找.Spring 这样做是为了让您可以注册自己的 HandlerMethodArgumentResolver 实例，请参阅 //自定义参数 注释.

Notice that two instances of ServletModelAttributeMethodProcessor are added (one after a //catch all comment). The ModelAttributeMethodProcessor has a annotationNotRequired field which tells it if it should look for the @ModelAttribute or not. The first instance must look for @ModelAttribute, the second one doesn't. Spring does this so that you can register your own HandlerMethodArgumentResolver instances, see the // Custom arguments comment.

特别地

@RequestMapping(value = "/", method = RequestMethod.POST)
public String sayHello(Person person, Model model) {
    model.addAttribute("person", person);
    return "home";
}

在这种情况下，您的 Person 参数是否被注释并不重要.ModelAttributeMethodProcessor 将解析它并绑定表单字段，即.请求参数，到实例的字段.您甚至不需要将它添加到 model 中，因为 ModelAttributeMethodProcessor 类会处理它.

In this case, it doesn't matter if your Person parameter is annotated or not. A ModelAttributeMethodProcessor will resolve it and bind form fields, ie. request parameters, to the fields of the instance. You shouldn't even need to add it to the model as the ModelAttributeMethodProcessor class will handle that.

在你的 showHelloPage() 方法中

model.addAttribute("person", new Person());

需要

标签库.这就是它解析 input 字段的方式.

is needed with the <form> taglib. That's how it resolves its input fields.

所以我的问题是 - ModelAttribute"的用途是什么注释?

So my question is - what is the use of the "ModelAttribute" anonnatation?

自动将指定的参数(或方法返回值)添加到模型中.

To automatically add the specified parameter (or method return value) to the model.

是否有某种方法可以省略表单中的modelAttribute"属性?

Is it some way to omit a "modelAttribute" attribute in a form?

不，form 绑定会在 Model 中查找一个对象，并将其字段绑定到 html input 元素.

No, the form binding looks for an object in the Model and binds its fields to html input elements.

第二部分，有什么方法(可能是一些注释)来做一个表单自动将输入的值绑定到正确的 bean 的属性(这将被声明为方法参数)?无需添加发送表单之前的空 bean(因为我现在必须这样做).

And the second part, what is the way (maybe some annotation) to make a form automatically bind inputs' values to the proper bean's properties (which would be declared as a method parameter)? Without a need of adding an empty bean before sending a form (as I have to do it now).

Spring 标记锁定到模型属性对象上，并使用其字段来创建 input 和 label 元素.对象如何在模型中结束并不重要，只要它这样做了.如果它找不到具有您指定的名称(键)的模型属性，则会抛出异常，如您所见.

A Spring <form> tag latches onto a model attribute object and uses its fields to create input and label elements. It doesn't matter how the object ended up in the model as long as it did. If it can't find a model attribute with the name (key) you specified, it throws exceptions, as you saw.

 <form:form method="post" modelAttribute="person">

提供空 bean 的替代方法是自己创建 html.Spring 的 所做的就是使用 bean 的字段名称来创建 input 元素.所以这个

The alternative to providing an empty bean is to create the html yourself. All Spring's <form> does is use the bean's field names to create an input element. So this

<form:form method="post" modelAttribute="person">
    <form:label path="firstName">First name</form:label>
    <form:input path="firstName" />

创建类似的东西

<form method="post" action="[some action url]">
    <label for="firstName">First name<label>
    <input type="text" name="firstName" value="[whatever value firstName field had]" />
    ...

Spring 使用 name 属性将请求参数绑定到实例字段.

Spring binds request parameters to instance fields using the name attribute.

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