Hey there, let’s work through this 404 issue step by step—since you’re using JDK 1.8 with Spring Boot, here are the most common culprits and fixes:

1. Incomplete or Redundant Startup Class #

First, your provided DemoApplication code cuts off at publi...—a valid Spring Boot startup class must have a main method to bootstrap the application. Also, note that @SpringBootApplication already includes @ComponentScan and @EnableAutoConfiguration, so adding those explicitly is redundant (and can cause confusion if misconfigured). Here’s the corrected, complete startup class:

package org.demo;

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

// @SpringBootApplication = @Configuration + @EnableAutoConfiguration + @ComponentScan
@SpringBootApplication
public class DemoApplication {

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

2. Missing or Misconfigured REST Controller #

The #1 cause of 404s in Spring Boot REST apps is no valid controller being detected. You haven’t shared your controller code, so let’s confirm you have one set up correctly:

• Create a class annotated with @RestController (this combines @Controller and @ResponseBody for REST responses)
• Ensure the controller is in the org.demo package (or a subpackage like org.demo.controller)—this is critical because @SpringBootApplication scans the startup class’s package and all subpackages by default.

Example valid controller:

package org.demo.controller;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class DemoController {

    // Define a GET endpoint at /hello
    @GetMapping("/hello")
    public String getHelloMessage() {
        return "Hello from your Spring Boot REST API!";
    }
}

3. Incorrect Dependencies #

Since you’re using JDK 1.8, you must use Spring Boot 2.x (Spring Boot 3.x requires JDK 17+). Make sure your build file includes the spring-boot-starter-web dependency, which provides all the web/REST functionality:

Maven (pom.xml) #

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.7.17</version> <!-- Last Spring Boot 2.x version supporting JDK 8 -->
    <relativePath/>
</parent>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

Gradle (build.gradle) #

plugins {
    id 'org.springframework.boot' version '2.7.17'
    id 'io.spring.dependency-management' version '1.0.15.RELEASE'
    id 'java'
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-web'
}

4. Wrong Request Path/Port #

Double-check your request details:

• If your controller uses @GetMapping("/api/hello"), your request URL must be http://localhost:8080/api/hello (not just /hello)
• If you’ve modified the default port in application.properties (e.g., server.port=8081), use that port instead of 8080
• Ensure there are no typos in the endpoint path (case-sensitive in most setups)

5. Component Scan Scope Issues #

If your controller is in a package outside org.demo, you’ll need to explicitly include it in @ComponentScan. For example:

@SpringBootApplication(scanBasePackages = {"org.demo", "com.other.controller"})
public class DemoApplication { ... }

Quick Test #

Once you’ve fixed the above, start your app and send a GET request to http://localhost:8080/hello (or your endpoint path). If you get the expected response, the 404 should be resolved!

内容的提问来源于stack exchange，提问作者greenPadawan