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I'm trying to write a Spring MVC web application where users can add movie names to an in-memory collection. It's configured like so

public class Application extends AbstractAnnotationConfigDispatcherServletInitializer {
    protected Class<?>[] getRootConfigClasses() {
        return new Class<?>[] {};
    }
    protected Class<?>[] getServletConfigClasses() {
        return new Class<?>[] { SpringServletConfig.class };
    }
    protected String[] getServletMappings() {
        return new String[] { "/" };
    }
}

and

@Configuration
@ComponentScan("com.example")
public class SpringServletConfig extends WebMvcConfigurationSupport {
    @Bean
    public InternalResourceViewResolver resolver() {
        InternalResourceViewResolver vr = new InternalResourceViewResolver();
        vr.setPrefix("WEB-INF/jsps/");
        vr.setSuffix(".jsp");
        return vr;
    }
}

There's a single @Controller class in the com.example package

@Controller
public class MovieController {
    private final CopyOnWriteArrayList<Movie> movies = new CopyOnWriteArrayList<>();
    @RequestMapping(path = "/movies", method = RequestMethod.GET)
    public String homePage(Model model) {
        model.addAttribute("movies", movies);
        return "index";
    }
    @RequestMapping(path = "/movies", method = RequestMethod.POST)
    public String upload(@ModelAttribute("movie") Movie movie, BindingResult errors) {
        if (!errors.hasErrors()) {
            movies.add(movie);
        }
        return "redirect:/movies";
    }
    public static class Movie {
        private String filmName;
        public String getFilmName() {
            return filmName;
        }
        public void setFilmName(String filmName) {
            this.filmName = filmName;
        }
    }
}

WEB-INF/jsps/index.jsp contains

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<%@ taglib prefix="form" uri="http://www.springframework.org/tags/form"%>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Movies</title>
</head>
<body>
    Current Movies:
    <c:forEach items="${movies}" var="movieItem">
        <ul>
            <li>${movieItem.filmName}</li>
        </ul>
    </c:forEach>
    <form:form>
        <div>Movie name:</div>
        <form:input path="filmName" type="text" id="name" />
        <input type="submit" value="Upload">
    </form:form>
</body>
</html>

The application is configured with context path /Example. When I send a GET request to

http://localhost:8080/Example/movies

the request fails, Spring MVC responds with a 500 status code, and reports the following exception and stack trace

java.lang.IllegalStateException: Neither BindingResult nor plain target object for bean name 'command' available as request attribute
    org.springframework.web.servlet.support.BindStatus.<init>(BindStatus.java:144)
    org.springframework.web.servlet.tags.form.AbstractDataBoundFormElementTag.getBindStatus(AbstractDataBoundFormElementTag.java:168)
    org.springframework.web.servlet.tags.form.AbstractDataBoundFormElementTag.getPropertyPath(AbstractDataBoundFormElementTag.java:188)
    org.springframework.web.servlet.tags.form.AbstractDataBoundFormElementTag.getName(AbstractDataBoundFormElementTag.java:154)
    org.springframework.web.servlet.tags.form.AbstractDataBoundFormElementTag.writeDefaultAttributes(AbstractDataBoundFormElementTag.java:117)
    org.springframework.web.servlet.tags.form.AbstractHtmlElementTag.writeDefaultAttributes(AbstractHtmlElementTag.java:422)
    org.springframework.web.servlet.tags.form.InputTag.writeTagContent(InputTag.java:142)
    org.springframework.web.servlet.tags.form.AbstractFormTag.doStartTagInternal(AbstractFormTag.java:84)
    org.springframework.web.servlet.tags.RequestContextAwareTag.doStartTag(RequestContextAwareTag.java:80)
    org.apache.jsp.WEB_002dINF.jsps.index_jsp._jspx_meth_form_005finput_005f0(index_jsp.java:267)
    org.apache.jsp.WEB_002dINF.jsps.index_jsp._jspx_meth_form_005fform_005f0(index_jsp.java:227)
    org.apache.jsp.WEB_002dINF.jsps.index_jsp._jspService(index_jsp.java:142)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:438)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:168)
    org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:303)
    org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1257)
    org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1037)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:980)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

I expected the JSP to generate an HTML <form> with a single text input, for a Movie name, and a submit button, that I can use to send a POST request with a new Movie. Why does the JSP servlet instead fail to render Spring's <form:form> tag?

标签： java spring jsp spring-mvc

5条回答

姐姐魅力值爆表

2楼-- · 2018-12-31 06:17

You're trying to use Spring MVC's form tag.

This tag renders an HTML form tag and exposes a binding path to inner tags for binding. It puts the command object in the PageContext so that the command object can be accessed by inner tags. [..]

Let’s assume we have a domain object called User. It is a JavaBean with properties such as firstName and lastName. We will use it as the form backing object of our form controller which returns form.jsp.

In other words, Spring MVC will extract a command object and use its type as a blueprint for binding path expressions for form's inner tags, like input or checkbox, to render an HTML form element.

This command object is also called a model attribute and its name is specified in the form tag's modelAttribute or commandName attributes. You've omitted it in your JSP

<form:form>

You could've specified a name explicitly. Both of these are equivalent.

<form:form modelAttribute="some-example-name">
<form:form commandName="some-example-name">

The default attribute name is command (what you see in error message). A model attribute is an object, typically a POJO or collection of POJOs, that your application supplies to the Spring MVC stack and which the Spring MVC stack exposes to your view (ie. the M to the V in MVC).

Spring MVC collects all model attributes in a ModelMap (they all have names) and, in the case of JSPs, transfers them to the HttpServletRequest attributes, where JSP tags and EL expressions have access to them.

In your example, your @Controller handler method which handles a GET to the path /movies adds a single model attribute

model.addAttribute("movies", movies); // not named 'command'

and then forwards to the index.jsp. This JSP then tries to render

<form:form>
    ...
    <form:input path="name" type="text" id="name" />
    ...
</form:form>

While rendering this, FormTag (in reality, the InputTag) tries to find a model attribute named command (the default attribute name) so that it can produce an HTML <input> element with a name attribute constructed from the path expression and the corresponding property value, ie. the result of Movie#getFilmName().

Since it cannot find it, it throws the exception you see

java.lang.IllegalStateException: Neither BindingResult nor plain target object for bean name 'command' available as request attribute

The JSP engine catches it and responds with a 500 status code. If you want to take advantage of a Movie POJO to simply construct your form correctly, you can add a model attribute explicitly with

model.addAttribute("movie", new Movie());

or have Spring MVC create and add one for you (must have an accessible parameterless constructor)

@RequestMapping(path = "/movies", method = RequestMethod.GET)
public String homePage(@ModelAttribute("command") Movie movie, Model model) {...}

Alternatively, include a @ModelAttribute annotated method in your @Controller class

@ModelAttribute("command")
public Movie defaultInstance() {
    Movie movie = new Movie();
    movie.setFilmName("Rocky II");
    return movie;
}

Note that Spring MVC will call this method and implicitly add the object returned to its model attributes for each request handled by the enclosing @Controller.

You may have guessed from this description that Spring's form tag is more suited for rendering an HTML <form> from an existing object, with actual values. If you want to simply create a blank <form>, it may be more appropriate to construct it yourself and not rely on any model attributes.

<form method="post" action="${pageContext.request.contextPath}/movies">
    <input name="filmName" type="text" />
    <input type="submit" value="Upload" />
</form>

On the receiving side, your POST handler method, will still be able to extract the filmName input value and use it to initialize a Movie object.

Common Errors

As we've seen, FormTag looks for a model attribute named command by default or with the name specified in either modelAttribute or commandName. Make sure you're using the right name.

ModelMap has a addAttribute(Object) method which adds

the supplied attribute to this Map using a generated name.

where the general convention is to

return the uncapitalized short name of the [attribute's] Class, according to JavaBeans property naming rules: So, com.myapp.Product becomes product; com.myapp.MyProduct becomes myProduct; com.myapp.UKProduct becomes UKProduct

If you're using this (or a similar) method or if you're using one of the @RequestMapping supported return types that represents a model attribute, make sure the generated name is what you expect.

Another common error is to bypass your @Controller method altogether. A typical Spring MVC application follows this pattern:

Send HTTP GET request
DispatcherServlet selects @RequestMapping method to handle request
Handler method generates some model attributes and returns view name
DispatcherServlet adds model attributes to HttpServletRequest and forwards request to JSP corresponding to view name
JSP renders response

If, by some misconfiguration, you skip the @RequestMapping method altogether, the attributes will not have been added. This can happen

if your HTTP request URI accesses your JSP resources directly, eg. because they are accessible, ie. outside WEB-INF, or
if the welcome-list of your web.xml contains your JSP resource, the Servlet container will render it directly, bypassing the Spring MVC stack entirely

One way or another, you want your @Controller to be invoked so that the model attributes are added appropriately.

What does `BindingResult` have to do with this?

A BindingResult is a container for initialization or validation of model attributes. The Spring MVC documentation states

The Errors or BindingResult parameters have to follow the model object that is being bound immediately as the method signature might have more than one model object and Spring will create a separate BindingResult instance for each of them [...]

In other words, if you want to use BindingResult, it has to follow the corresponding model attribute parameter in a @RequestMapping method

@RequestMapping(path = "/movies", method = RequestMethod.POST)
public String upload(@ModelAttribute("movie") Movie movie, BindingResult errors) {

BindingResult objects are also considered model attributes. Spring MVC uses a simple naming convention to manage them, making it easy to find a corresponding regular model attribute. Since the BindingResult contains more data about the model attribute (eg. validation errors), the FormTag attempts to bind to it first. However, since they go hand in hand, it's unlikely one will exist without the other.

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像晚风撩人

3楼-- · 2018-12-31 06:26

In my case, it worked by adding modelAttribute="movie" to the form tag, and prepending the model name to the attribute, something like <form:input path="filmName" type="text" id="movie.name" />

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倾城一夜雪

4楼-- · 2018-12-31 06:32

Add Model Attribute (name of model you specified in controller) to relate your form. You must add following attributes to your form

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

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闭嘴吧你

5楼-- · 2018-12-31 06:33

To make things simple with the form tag just add a "commandName" which is a horrible name for what it is actually looking for...it wants the object you named in the MdelAttribute annotation. So in this case commandName="movie".

That'll save you reading long winded explanations friend.

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妖精总统

6楼-- · 2018-12-31 06:40

Updating from Spring version 3 to Spring version 5, produces the same error. All answers were satisfied already in my code. Adding the annotation @ControllerAdvice solved the problem for me.

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