Resolving Spring Boot Resource Loading Errors When Running from a JAR

When reading files from the resources directory, the code works locally but throws an error in the test environment: class path resource [xxxx] cannot be resolved to absolute file path because it does not reside in the file system: jar:file:xxxx.jar!/BOOT-INF/classes!xxxx .

1. Project Code

GetResourceTest :

public class GetResourceTest {
    public InputStream getResource1() throws IOException {
        File file = new DefaultResourceLoader().getResource("/template/qiankuan.ftl").getFile();
        return Files.newInputStream(file.toPath());
    }

    public InputStream getResource2() throws IOException {
        ResourcePatternResolver resolver = new PathMatchingResourcePatternResolver();
        Resource[] resources = resolver.getResources("/template/qiankuan.ftl");
        Resource resource = resources[0];
        return resource.getInputStream();
    }
}

TestController :

@RestController
@RequestMapping("/test")
public class TestController {
    @GetMapping(value = "/getResource")
    @ResponseBody
    public void getResource() throws IOException {
        GetResourceTest getResourceTest = new GetResourceTest();
        getResourceTest.getResource1();
    }
}

2. Investigation Process

1) Build the project into a JAR with Maven

The mvn install command creates the JAR in the target directory.

2) Run the JAR with remote debugging

java -jar -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=9992 api-test-1.0.0-SNAPSHOT.jar

The application starts successfully.

3) Add remote debugging configuration

In the IDE, add a Remote JVM Debug configuration and set the port to the one used above (e.g., 9992).

4) Call the test endpoint

Request: http://127.0.0.1:9991/test/getResource

5) Observe the error

The response shows the same FileNotFoundException as in the test environment.

6) Debug with breakpoints

Setting a breakpoint at the controller entry reveals that the resource path is resolved to a JAR URL like jar:file:/.../api-test-1.0.0-SNAPSHOT.jar!/BOOT-INF/classes!/template/qiankuan.ftl , and resourceUrl.getProtocol() returns jar instead of file , causing the exception.

3. Solution

The ResourceUtils.getFile() method only works for non‑compressed files and cannot read resources inside a JAR. Use a method that reads the resource as a stream, such as ClassPathResource or PathMatchingResourcePatternResolver :

ClassPathResource classPathResource = new ClassPathResource("/template/qiankuan.ftl");
InputStream inputStream = classPathResource.getInputStream();

or

ResourcePatternResolver resolver = new PathMatchingResourcePatternResolver();
Resource[] resources = resolver.getResources("/template/qiankuan.ftl");
Resource resource = resources[0];
InputStream inputStream = resource.getInputStream();

These approaches correctly load the template file whether the application runs from the IDE or from a packaged JAR.