Spring新项目构建失败求助：执行mvn spring-boot:run报错

Hey there, let's break down your issues step by step since you're new to Spring Boot—no worries, these are common gotchas for beginners!

1. Java Version Compatibility Problem #

Your system is running JDK 9.0.4, but your pom.xml specifies <java.version>1.8</java.version>, and Spring Boot 1.5.9.RELEASE (the version you're using) has limited support for JDK 9—it's primarily built for JDK 8. This mismatch is likely causing low-level compatibility errors during startup.

Solutions: #

• Option 1 (Most Stable): Switch your system's JDK to version 8. Since Spring Boot 1.5.x is heavily optimized for JDK 8, this will avoid most compatibility headaches.
• Option 2: Upgrade your Spring Boot version to 2.x (e.g., 2.1.0.RELEASE or later), which fully supports JDK 9. Remember to update the <java.version> property in your pom.xml to 9 as well.

2. Controller Return Value Misconfiguration #

You've added the spring-boot-starter-thymeleaf dependency, which tells Spring Boot to treat your controller's returned string as a Thymeleaf template filename, not plain text. Your code returns "Hello World.", so Spring tries to find a template file named Hello World..html in src/main/resources/templates—which doesn't exist, leading to an error.

Solutions: #

• If you want to return plain text: Add the @ResponseBody annotation to your controller method, or replace @Controller with @RestController (which automatically applies @ResponseBody to all methods):

  package com.demo.spring.controller;
  import org.springframework.web.bind.annotation.RequestMapping;
  import org.springframework.web.bind.annotation.RequestMethod;
  import org.springframework.web.bind.annotation.RestController;
  
  @RestController
  public class HomeController {
      @RequestMapping(value = "/", method = RequestMethod.GET)
      public String index(){
          return "Hello World.";
      }
  }
  

• If you want to use Thymeleaf: Create a template file named index.html in src/main/resources/templates, then modify your controller to return "index" instead:

  @RequestMapping(value = "/", method = RequestMethod.GET)
  public String index(){
      return "index";
  }
  

3. Missing Spring Boot Application Class #

Spring Boot requires a root application class annotated with @SpringBootApplication to bootstrap the application. This class needs to be in a parent package of your controller (e.g., com.demo.spring for your com.demo.spring.controller package) so it can scan and register your controller bean. You haven't mentioned this class, so it's probably missing.

Solution: #

Create a file named DemoSpringApplication.java in src/main/java/com/demo/spring:

package com.demo.spring;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoSpringApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoSpringApplication.class, args);
    }
}

Final Steps #

After fixing these issues, run mvn spring-boot:run again. If you still see errors, re-run Maven with the -e flag (mvn spring-boot:run -e) to get the full stack trace—it will give more specific details about any remaining problems.

内容的提问来源于stack exchange，提问作者Dan