Spring MVC：请解释@RequestParam和@ModelAttribute之间的区别 [英] Spring MVC: please explain difference between @RequestParam and @ModelAttribute

问题描述

我是Spring MVC的新手。请帮我解开文档。

I'm new to Spring MVC. Please help me unpack the documentation.

文档

Spring MVC文档声明（强调我的）：

• 方法参数上的@ModelAttribute 表示应从模型中检索参数。如果模型中不存在，则应首先实例化参数，然后将其添加到模型中。一旦出现在模型中，参数的字段应该从所有具有匹配名称的请求参数中填充。 WebDataBinder类将请求参数名称（包括查询字符串参数和表单字段）与名称模型属性字段进行匹配。

• @ModelAttribute on a method argument indicates the argument should be retrieved from the model. If not present in the model, the argument should be instantiated first and then added to the model. Once present in the model, the argument’s fields should be populated from all request parameters that have matching names. The WebDataBinder class matches request parameter names — including query string parameters and form fields — to model attribute fields by name.

@RequestParam 将请求参数绑定到控制器中的方法参数 。

免责声明/澄清者

我知道 @ModelAttribute 和 @RequestParam 不是一回事，不是互相排斥的，不要执行相同的角色，可以同时使用，如 this问题 - 实际上， @RequestParam 可用于填充 @ModelAttribute 的字段。我的问题更侧重于内部运作之间的差异。

I know that @ModelAttribute and @RequestParam are not the same thing, are not mutually exclusive, do not perform the same role, and can be used simultaneously, as in this question - indeed, @RequestParam can be used to populate fields of @ModelAttribute. My question is more geared towards the difference between their internal workings.

问题：

@ModelAttribute （用于方法参数，而不是方法）和 @RequestParam 之间有什么区别？具体来说：

What is the difference between @ModelAttribute (used on a method argument, not method) and @RequestParam? Specifically:

• 来源：执行 @RequestParam 和 @ModelAttribute 具有相同的
  信息/填充来源，即URL中的请求参数，这些参数可能是作为<$的表单/模型的元素提供的C $ C> POST 编？


• 用法：使用 @RequestParam 检索的变量是否正确（除非传入模型），而用 @ModelAttribute 检索的变量会自动输入到要返回的模型中吗？

• Source: Do @RequestParam and @ModelAttribute have the same source of information / population, i.e. request parameters in URL, which may have been supplied as elements of a form / model that was POSTed?
• Usage: Is it correct that variables retrieved with @RequestParam are thrown away (unless passed into a model), whereas variables retrieved with @ModelAttribute are automatically fed into the model to be returned?

或者在非常基本的编码示例中，这两个示例之间真正的工作差异是什么？

Or in very basic coding examples, what is the real working difference between these two examples?

示例1： @RequestParam ：

// foo and bar are thrown away, and are just used (e.g.) to control flow?
@RequestMapping(method = RequestMethod.POST)
public String testFooBar(@RequestParam("foo") String foo,
@RequestParam("bar") String bar, ModelMap model) {
    try {
     doStuff(foo, bar);
    }
    // other code
  }

示例2： @ModelAttribute ：

// FOOBAR CLASS
// Fields could of course be explicitly populated from parameters by @RequestParam
public class FooBar{
    private String foo;
    private String bar;
   // plus set() and get() methods
}

// CONTROLLER
// Foo and Bar become part of the model to be returned for the next view?
@RequestMapping(method = RequestMethod.POST)
public String setupForm(@ModelAttribute("fooBar") FooBar foobar) {
   String foo = fooBar.getFoo();
   String bar = fooBar.getBar();
   try {
      doStuff(foo, bar);
   }
   // other code
}

---

我目前的理解：

@ModelAttribute 和 @RequestParam 都询问请求参数以获取信息，但他们以不同方式使用此信息：

@ModelAttribute and @RequestParam both interrogate the request parameters for information, but they use this information differently:

• @RequestParam 只填充独立变量（当然可能是 @ModelAttribute 中的字段类）。完成控制器后，这些变量将被丢弃，除非它们已被输入模型。

• @RequestParam just populates stand-alone variables (which may of course be fields in a @ModelAttribute class). These variables will be thrown away when the Controller is done, unless they have been fed into the model.

@ModelAttribute 填充类的字段，然后填充要传递回视图的模型的属性

@ModelAttribute populates the fields of a class, which then populates an attribute of the model to be passed back to the view

这是正确的吗？

推荐答案

@RequestParam 只填充独立变量（当然可能是 @ModelAttribute 类中的字段）。完成控制器后，这些变量将被丢弃，除非它们已被送入模型。

@RequestParam just populates stand-alone variables (which may of course be fields in a @ModelAttribute class). These variables will be thrown away when the Controller is done, unless they have been fed into the model.

不要混淆这个词会话的模特。 http对话通常是： HTTP.GET ，服务器响应，然后 HTTP.POST 。当您使用 @ModelAttribute 注释时，您总是在构建一个已注释的实例，这就是让您认为向模型提供信息的原因变量坚持不懈。这是不正确的，一旦 HttpServletRequest 完成，这些变量就不应再成为浏览器/服务器对话的一部分，除非它们已保存在会话中。

Don't confuse the word "model" with session. The http conversation is generally: HTTP.GET, server response, then HTTP.POST. When you have the @ModelAttribute annotation in use you are always constructing an instance of whatever you have annotated, this is what makes you think that 'feeding things to the model' might make variables stick around. This isn't correct, once the HttpServletRequest has finished those variables should no longer be a part of the browser/server conversation unless they've been saved in a session.

@ModelAttribute 填充类的字段，然后填充模型的属性被传递回视图

@ModelAttribute populates the fields of a class, which then populates an attribute of the model to be passed back to the view

是的！为了正确， @ModelAttribute 告诉Spring使用其默认的Web数据绑定器来填充来自 HttpServletRequest 。选择将此数据传递回视图取决于程序员。如果有一个使用 @ModelAttribute 注释的方法，则每次代码命中该servlet时都会调用它。如果您将 @ModelAttribute 作为方法参数之一，我们将讨论传入的Http表单数据绑定。

Yes! To be correct, @ModelAttribute tells Spring to use its default web data binder to populate an instance of something with data from the HttpServletRequest. Choosing to pass this data back to the view is up to the programmer. When you have a method annotated with @ModelAttribute, it is being called every time code hits that servlet. When you have @ModelAttribute as one of the method's parameters, we are talking about incoming Http form data-binding.

调用 @RequestParam 是说出 request.getParameter（foo）的快捷方式;在引擎盖下，Java的 HttpServletRequest 允许您通过执行key-> value查找来从请求对象获取值。返回的值是Object类型。如果您没有在Web应用程序中使用Spring，那么这就是您要输入的内容。

Calling @RequestParam is a shortcut for saying request.getParameter("foo"); under the hood, Java's HttpServletRequest lets you get values from the request object by doing a key->value look up. The value returned is of type Object. This is what you would be typing a lot if you were not using Spring in your web application.

当您开始使用<$>时，Spring会将此抽象更进一步C $ C> @ModelAttribute 。该注释采用数据绑定的概念。数据绑定的目标是控制器中的代码不必为每个表单元素调用 request.getParameter（foo1）。想象一下，你有一个包含5个字段的Web表单。如果没有数据绑定，程序员必须手动检索并验证每个字段。程序员必须确保请求包含属性，属性的值存在，以及属性的值是每个字段的预期类型。使用 @ModelAttribute 告诉Spring为您完成这项工作。

Spring then takes this abstraction a step further when you start to use @ModelAttribute. This annotation employs the concept of data-binding. The goal of data-binding is that the code in your controller won't have to call request.getParameter("foo1"), for every form element. Imagine you have a web form with 5 fields. Without data-binding the programmer has to manually retrieve, and validate each of those fields. The programmer has to make sure that the request contains the property, that the property's value exists, and that the property's value is of the type expected for each field. Using @ModelAttribute tells Spring to do this work for you.

如果使用 @ModelAttribute（fooBar）注释控制器中的方法FooBar fooBar FooBar 总是由Spring构造，并提供给您的方法。数据绑定发挥作用的地方，就是在Method的参数中使用此注释时; Spring查看 HttpServletRequest 的实例，看看它是否可以将请求中的数据与 FooBar 。这是基于java属性约定，你有一个字段，如 foo 和公共getter和setter名为 getFoo 和 setFoo 。这可能看起来很神奇，但是如果要破坏约定，那么Spring数据绑定将停止工作，因为它无法知道 where 绑定来自 HttpServletRequest的数据您仍然会获得 FooBar 的实例，但不会将属性设置为请求中的任何值。

If you annotate a method in your controller with @ModelAttribute("fooBar") FooBar fooBar An instance of FooBar will always be constructed by Spring, and supplied to your method. Where the data binding comes into play, is when this annotation is used in a Method's parameters; Spring looks at the instance of HttpServletRequest and sees if it can match the data in the request to the right property on an instance of FooBar. This is based off the java properties convention, where you have a field such as foo and public getters and setters called getFoo and setFoo. This might seem magic but if you were to break convention, your Spring data binding would stop working because it wouldn't be able to know where to bind the data from your HttpServletRequest You would still get an instance of FooBar, but the properties would not be set to any values from the request.

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