Reading files from the resources folder in Spring Boot

Spring Boot is a popular Java-based framework for building web applications. One of the features it provides is easy access to resources, such as files, that are packaged within the application. In this blog post, we will discuss how to read files from the resources folder in Spring Boot.

First, let’s take a look at the structure of a Spring Boot project. By default, any files located in the src/main/resources folder will be included in the final packaged application. This means that these resources can be accessed at runtime without the need to load them from the file system.



Using the ClassPathResource class

There are a few different ways to read a file from the resources folder in Spring Boot, but one of the most common is to use the ClassPathResource class. This class provides a convenient way to access resources on the classpath. To use it, we simply need to pass in the path of the resource we want to access, relative to the resources folder.

Here’s an example of how to read a file from the resources folder in Spring Boot using the ClassPathResource class:

import org.springframework.core.io.ClassPathResource;

public class FileReader {
    public static void main(String[] args) throws IOException {
        // Create a ClassPathResource object, passing in the path of the file to be read
        ClassPathResource resource = new ClassPathResource("file.txt");

        // Open an InputStream to the file
        InputStream inputStream = resource.getInputStream();

        // Read the contents of the file into a String
        String content = new String(inputStream.readAllBytes(), StandardCharsets.UTF_8);

        // Print the contents of the file
        System.out.println(content);

        // Close the InputStream
        inputStream.close();
    }
}

In this example, we first create a ClassPathResource object by passing in the path of the file we want to read, “file.txt”. We then open an InputStream to the file using the getInputStream() method, and read the contents of the file into a String. Finally, we print the contents of the file and close the InputStream.



Using @Value annotation

Another way to read a file from the resources folder in Spring Boot is to use the @Value annotation. This annotation allows us to inject the contents of a file into a String variable. Here’s an example of how to use the @Value annotation to read a file from the resources folder:

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class FileReader {
    @Value("classpath:file.txt")
    private String fileContent;

    public void readFile() {
        System.out.println(fileContent);
    }
}

In this example, we use the @Value annotation to inject the contents of the “file.txt” file into the fileContent variable. We then simply print out the contents of the file. Note that the “classpath:” prefix is required when using the @Value annotation to read a file from the resources folder.



Using the ResourceLoader interface

Finally, there is another way to read a file from the resources folder in Spring Boot using the ResourceLoader interface. This interface provides a generic way to access resources, and can be used to read a file from the resources folder as follows:

import org.springframework.core.io.Resource;
import org.springframework.core.io.ResourceLoader;
import org.springframework.stereotype.Component;

@Component
public class FileReader {
private final ResourceLoader resourceLoader;


public FileReader(ResourceLoader resourceLoader) {
    this.resourceLoader = resourceLoader;
}

public void readFile() throws IOException {
    Resource resource = resourceLoader.getResource("classpath:file.txt");
    InputStream inputStream = resource.getInputStream();
    String content = new String(inputStream.readAllBytes(), StandardCharsets.UTF_8);
    System.out.println(content);
    inputStream.close();
}

}

In this example, we first inject an instance of the ResourceLoader interface into our FileReader class using constructor injection. We can then use the getResource() method to load the “file.txt” file from the resources folder. We open an InputStream to the file using the getInputStream() method and read the contents of the file into a String, just like in the previous examples.

It is also important to note that when using the ResourceLoader interface, you can also specify absolute file paths or even external URLs as the resource location. This can be useful if you need to read a file that is not located in the resources folder.



Using try catch to handle exceptions

Another important thing to consider when reading files from the resources folder in Spring Boot is handling exceptions. As you can see in the previous examples, some methods like getInputStream() and readAllBytes() can throw IOExceptions, which should be handled properly.

One way to handle these exceptions is to use try-catch blocks. For example:

public void readFile() {
    try {
        ClassPathResource resource = new ClassPathResource("file.txt");
        InputStream inputStream = resource.getInputStream();
        String content = new String(inputStream.readAllBytes(), StandardCharsets.UTF_8);
        System.out.println(content);
        inputStream.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

In this example, we are using a try-catch block to handle any IOExceptions that may be thrown while trying to read the file. The e.printStackTrace() method is used to print the stack trace of the exception, which can be helpful for debugging.



Conclusion

In conclusion, reading files from the resources folder in Spring Boot is a simple process. Whether you choose to use the ClassPathResource class, the @Value annotation, or the ResourceLoader interface, the process of reading a file is relatively the same. Each method has its own advantages and use cases, so it is important to choose the one that best fits your project’s requirements.