Read a file from the ‘resources’ folder

2.1. ClassLoader.getResourceAsStream()

Use the getResourceAsStream() method to get the InputStream when reading a file from inside a jar file. Always use this method on the ClassLoader instance.

private InputStream getFileAsIOStream(final String fileName) 
{
    InputStream ioStream = this.getClass()
        .getClassLoader()
        .getResourceAsStream(fileName);

    if (ioStream == null) {
        throw new IllegalArgumentException(fileName + " is not found");
    }
    return ioStream;
}

2.2. Complete Example Once we have the InputStream reference, we can use it to read the file content or pass it to any resource handler class.

package com.howtodoinjava.io;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

public class ReadFileFromResourcesUsingGetResourceAsStream 
{
    public static void main(final String[] args) throws IOException 
    {
        //Creating instance to avoid static member methods
        ReadFileFromResourcesUsingGetResourceAsStream instance 
            = new ReadFileFromResourcesUsingGetResourceAsStream();

        InputStream is = instance.getFileAsIOStream("demo.txt");
        instance.printFileContent(is);

        is = instance.getFileAsIOStream("data/demo.txt");
        instance.printFileContent(is);
    }

    private InputStream getFileAsIOStream(final String fileName) 
    {
        InputStream ioStream = this.getClass()
            .getClassLoader()
            .getResourceAsStream(fileName);

        if (ioStream == null) {
            throw new IllegalArgumentException(fileName + " is not found");
        }
        return ioStream;
    }

    private void printFileContent(InputStream is) throws IOException 
    {
        try (InputStreamReader isr = new InputStreamReader(is); 
                BufferedReader br = new BufferedReader(isr);) 
        {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
            is.close();
        }
    }
}