How to read a file on the classpath in Java

Posted On July 21, 2021July 24, 2021

There are often programming situations when you need to read a file on the classpath via Java. In this article, I will show you how to write code for this.

Reading a file on the classpath

Let us first take a look at the code that you need to write to read a file on the classpath.

public class ReadFileFromClassPathDemo {

  public static void main(String[] args) throws IOException {
    // read in the file from the resources directory
    ClassLoader classLoader = ReadFileFromClassPathDemo.class.getClassLoader();
    URL url = classLoader.getResource("input.txt");
    String fileName = url.getFile();
    File file = new File(fileName);
    String str = new String(Files.readAllBytes(file.toPath()));
    System.out.println(str);
  }

}

Java has an in-built class called ClassLoader.  It is used to dynamically load classes. It can also be used to load resources from the classpath. The code above first obtains a ClassLoader. It then invokes the getResource method on the classLoader. This loads the resource (in this case a file named input.txt) from the file system and returns an URL instance. The URL class has a getFile method that returns the fully qualified filename.  A File object is then created using the filename. Finally, the Files.readAllBytes method is used to then read the contents of the file into a String.

Running code in Eclipse

In case you need to test the code above from an IDE like Eclipse, you need to do the following:

Step 1 – Right-click on the class name and click Run As > Run Configurations:

Step 2 – Click on the Dependencies Tab:

Step 3 – Click on Classpath entries. Click on Advanced > Add External Folder:

Step 4 – Navigate to the file system and select the folder which contains the file to be read (D:/Data/Temp in my case):

Step 5 – Verify that the folder is added to the classpath. Click on Apply and Run:

Step 6 – Verify that the file is read and its contents are displayed on the console:

Maven project

In the case of a Maven project, anything placed within the src/main/resources folder of the Maven project is automatically available on the classpath. So, if you place your input file in the src/main/resources folder, the code above will be able to read it.

Further Reading

• Java Programming Masterclass
• Core Java Made Easy

Conclusion

So, in this article, we saw how to read a file on the classpath via Java. We also saw how to run the code in Eclipse.