为什么当我将样式表链接到 Thymeleaf 页面时 Spring Boot 会丢失？答案

【问题标题】：Why SpringBoot drops when I link a stylesheet to a Thymleaf page?为什么当我将样式表链接到 Thymeleaf 页面时 Spring Boot 会丢失？
【发布时间】：2019-06-17 01:47:35
【问题描述】：

我有一个带有 Thymleaf 页面的 Spring Boot 服务器。当我不使用style.css 时它工作正常。但是当我链接样式表 (<link rel="stylesheet" type="text/css" href="../static/css/style.css" th:href="@{/css/style.css}"/>) 并尝试访问该页面时，样式表未链接，服务器会丢弃堆栈跟踪（见下文）。

项目结构：

├─ pom.xml
├─ src
│  ├─ main
│  │  ├── java
│  │  │   └── com.alexesmet.example
│  │  │       ├── Application.java
│  │  │       └── controller
│  │  │           └── RootController.java
│  │  └── resources
│  │      ├── application.properties
│  │      ├── static
│  │      │   └── css
│  │      │       └── style.css
│  │      └── templates
│  │          └── index.html
│  └─ test
│     └── java

我的pom.xml：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.alexesmet.example</groupId>
    <artifactId>webshop</artifactId>
    <version>1.0-SNAPSHOT</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.0.RELEASE</version>
    </parent>

    <dependencies>

        <!-- https://mvnrepository.com/artifact/org.springframework.boot/spring-boot-starter-web -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <version>2.1.1.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-thymeleaf</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>5.1.3.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>5.1.3.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.4</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
            <optional>true</optional>
        </dependency>    
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

我的index.html：

<!DOCTYPE html>
<html lang="en" xmlns:th="http://www.thymeleaf.org">
<head>
    <meta charset="UTF-8">
    <title>Web Shop</title>
    <link rel="stylesheet" type="text/css" href="../static/css/style.css" th:href="@{/css/style.css}"/>
</head>
<body>
    <p th:text="thymleaf">html</p>
</body>
</html>

我的控制器：

@Controller
public class RootController {

    @RequestMapping("/")
    public String index(){
        return "index";
    }

}

当尝试访问localhost:8080 上的页面时，浏览器会收到以下信息：

<html lang="en"><head>
    <meta charset="UTF-8">
    <title>Web Shop</title>
    <link rel="stylesheet" type="text/css" href="/css/style.css">
</head>
<body>
    <p>thymleaf</p>
</body>
</html>

Spring Boot 控制台显示如下：

2019-01-23 15:16:17.966 ERROR 5035 --- [nio-8080-exec-2] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause

java.lang.NullPointerException: null
    at org.springframework.web.servlet.DispatcherServlet.getHandler(DispatcherServlet.java:1231) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1014) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1005) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:897) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:635) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:882) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.28.jar:8.5.28]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:496) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1135) [na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) [na:na]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.28.jar:8.5.28]
    at java.base/java.lang.Thread.run(Thread.java:844) [na:na]

请记住，只有在我将这一行添加到 index.html 的头部时才会发生这种情况。

<link rel="stylesheet" type="text/css" href="../static/css/style.css" th:href="@{/css/style.css}"/>

为什么？ 我该如何解决这个问题？

附：为避免其他问题，我在GitHub 上有这个项目。

更新： 我更新到 Spring Boot 版本 2.1.2.RELEASE（如您所见，我使用的是 2.0.0）。控制台输出已更改：

2019-01-23 18:04:46.707  WARN 21852 --- [nio-8080-exec-2] o.s.web.servlet.PageNotFound             : No mapping for GET /css/style.css
2019-01-23 18:04:46.757 ERROR 21852 --- [nio-8080-exec-2] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] threw exception

java.lang.NoSuchMethodError: org.springframework.http.MediaType.equalsTypeAndSubtype(Lorg/springframework/util/MimeType;)Z
    at org.springframework.http.converter.AbstractHttpMessageConverter.canWrite(AbstractHttpMessageConverter.java:180) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.canWrite(AbstractJackson2HttpMessageConverter.java:172) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.http.converter.AbstractGenericHttpMessageConverter.canWrite(AbstractGenericHttpMessageConverter.java:76) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:278) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor.handleReturnValue(HttpEntityMethodProcessor.java:223) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:82) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at... (omitted)

2019-01-23 18:04:46.758 ERROR 21852 --- [nio-8080-exec-2] o.a.c.c.C.[Tomcat].[localhost]           : Exception Processing ErrorPage[errorCode=0, location=/error]

org.springframework.web.util.NestedServletException: Handler dispatch failed; nested exception is java.lang.NoSuchMethodError: org.springframework.http.MediaType.equalsTypeAndSubtype(Lorg/springframework/util/MimeType;)Z
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1053) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1005) ~[spring-webmvc-5.1.3.RELEASE.jar:5.1.3.RELEASE]
    at... (omitted)

【问题讨论】：

这……太奇怪了。如果这是一个准确的堆栈跟踪（并且您没有省略包装器或其他东西），那么应该不可能让 Spring 从其内部解析器中抛出 NPE。
预防性说明：这不是规范 NPE 问题的副本。

标签： java spring-boot spring-mvc tomcat thymeleaf

【解决方案1】：

去掉@EnableWebMvc注解，css可能会加载。

我在成功的路上做了什么：

我用Spring Initializr 中的一个替换了我的pom.xml。堆栈跟踪变得更具可读性。
现在我确定我的浏览器请求 style.css。在更新服务器响应 500 之前，但在更新之后它响应 404。
更新后我注意到@EnableWebMvc 已被弃用。 我删除了它。它奏效了。

【讨论】：

是的，除了 我在 Spring 的当前版本中没有看到弃用通知 docs.spring.io/spring-framework/docs/current/javadoc-api/org/…
@Antoniossss，是的，这很奇怪，我找不到@Deprecated 注释，甚至无法重现这个问题。