Access a File from the Classpath in a Spring Application

1. Introduction

In this tutorial, we’ll see various ways to access and load the contents of a file that’s on the classpath using Spring.

2. Using Resource

The Resource interface helps in abstracting access to low-level resources. In fact, it supports handling of all kinds of file resources in a uniform manner.

Let’s start by looking at various methods to obtain a Resource instance.

2.1. Manually

For accessing a resource from the classpath, we can simply use ClassPathResource:

public Resource loadEmployees() {
    return new ClassPathResource("data/employees.dat");
}

By default, ClassPathResource removes some boilerplate for selecting between the thread’s context classloader and the default system classloader.

However, we can also indicate the classloader to use either directly:

return new ClassPathResource("data/employees.dat", this.getClass().getClassLoader());

Or indirectly through a specified class:

return new ClassPathResource(
  "data/employees.dat", 
  Employee.class.getClassLoader());

Note that from Resource, we can easily jump to Java standard representations like InputStream or File.

2.2. Using @Value

We can also inject a Resource with @Value:

@Value("classpath:data/resource-data.txt")
Resource resourceFile;

And @Value supports other prefixes, too, like file: and url:.

2.3. Using ResourceLoader

Or, if we want to lazily load our resource, we can use ResourceLoader:

@Autowired
ResourceLoader resourceLoader;

And then we retrieve our resource with getResource:

public Resource loadEmployees() {
    return resourceLoader.getResource(
      "classpath:data/employees.dat");
}

Note, too that ResourceLoader is implemented by all concrete ApplicationContexts, which means that we can also simply depend on ApplicationContext if that suits our situation better:

ApplicationContext context;

public Resource loadEmployees() {
    return context.getResource("classpath:data/employees.dat");
}

3. Using ResourceUtils

As a caveat, there is another way to retrieve resources in Spring, but the ResourceUtils Javadoc is clear that the class is mainly for internal use.

If we see usages of ResourceUtils in our code:

public File loadEmployeesWithSpringInternalClass() 
  throws FileNotFoundException {
    return ResourceUtils.getFile(
      "classpath:data/employees.dat");
}

We should carefully consider the rationale as it’s probably better to use one of the standard approaches above.

4. Reading Resource Data

Once we have a Resource, it’s easy for us to read the contents. As we have already discussed, we can easily obtain a File or an InputStream reference from the Resource.

Let’s imagine we have the following file, data/employees.dat, on the classpath:

Joe Employee,Jan Employee,James T. Employee

4.1. Reading as a File

Now, we can read its contents by calling getFile:

@Test
public void whenResourceAsFile_thenReadSuccessful() 
  throws IOException {
 
    File resource = new ClassPathResource(
      "data/employees.dat").getFile();
    String employees = new String(
      Files.readAllBytes(resource.toPath()));
    assertEquals(
      "Joe Employee,Jan Employee,James T. Employee", 
      employees);
}

Although, note that this approach expects the resource to be present in the filesystem and not within a jar file.

4.2. Reading as an InputStream

Let’s say, though, that our resource is inside a jar.

Then, we can instead read a Resource as an InputStream:

@Test
public void whenResourceAsStream_thenReadSuccessful() 
  throws IOException {
    InputStream resource = new ClassPathResource(
      "data/employees.dat").getInputStream();
    try ( BufferedReader reader = new BufferedReader(
      new InputStreamReader(resource)) ) {
        String employees = reader.lines()
          .collect(Collectors.joining("\n"));
 
        assertEquals("Joe Employee,Jan Employee,James T. Employee", employees);
    }
}

5. Conclusion

In this quick article, we’ve seen a few ways to access and read a resource from the classpath using Spring including eager and lazy loading and on the filesystem or in a jar.

And, as always, I’ve posted all these examples over on GitHub.