In the following code snippet we will learn how to load files from resource directory. Resource files can be in a form of image, audio, text, etc. Text resource file for example can be used to store application configurations, such as database configuration.

To load this resource file you can use a couple methods utilizing the java.lang.Class methods or the java.lang.ClassLoader methods. Both Class and ClassLoader provides getResource() and getResourceAsStream() methods to load resource file. The first method return a URL object while the second method return an InputStream.

When using the Class method, if the resource name started with “/” that identifies it is an absolute name. Absolute name means that it will load from the specified directory name or package name. While if it is not started with “/” then it is identified as a relative name. This means that it will look in the same package as the class that tries to load the resource.

App.class.getResource("database.conf");

The snippet will attempt to load the resource file from the same package as the App class. If the App class package is org.kodejava then the database.conf file must be located at /org/kodejava/. This is the relative resource name.

App.class.getResource("/org/kodejava/conf/database.conf"):

The snippet will attempt to load the resource file from the given package name. You should place the configuration file under /org/kodejava/conf/ to enable the application to load it. This is the absolute resource name.

Below is a snippet that use the Class method to load resources.

private void loadUsingClassMethod() throws IOException {
    System.out.println("LoadResourceFile.loadUsingClassMethod");
    Properties properties = new Properties();

    // Load resource relatively to the LoadResourceFile package.
    // This actually load resource from
    // "/org/kodejava/lang/database.conf".
    URL resource = getClass().getResource("database.conf");
    properties.load(new FileReader(Objects.requireNonNull(resource).getFile()));
    System.out.println("JDBC Driver: " + properties.get("jdbc.driver"));

    // Load resource using absolute name. This will read resource
    // from the root of the package. This will load "/database.conf".
    InputStream is = getClass().getResourceAsStream("/database.conf");
    properties.load(is);
    System.out.println("JDBC Driver: " + properties.get("jdbc.driver"));
}

When we use the ClassLoader method the resource name should not begins with a “/“. This method will not apply any absolute / relative transformation to the resource name like the Class method. Here a snippet of a method that use the ClassLoader method.

private void loadUsingClassLoaderMethod() throws IOException {
    System.out.println("LoadResourceFile.loadUsingClassLoaderMethod");
    Properties properties = new Properties();

    // When using the ClassLoader method, the resource name should
    // not be started with "/". This method will not apply any
    // absolute/relative transformation to the resource name.
    ClassLoader classLoader = getClass().getClassLoader();
    URL resource = classLoader.getResource("database.conf");
    properties.load(new FileReader(Objects.requireNonNull(resource).getFile()));
    System.out.println("JDBC URL: " + properties.get("jdbc.url"));

    InputStream is = classLoader.getResourceAsStream("database.conf");
    properties.load(is);
    System.out.println("JDBC URL: " + properties.get("jdbc.url"));
}

Below is the main program that calls the methods above.

package org.kodejava.lang;

import java.io.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.util.Objects;
import java.util.Properties;

public class LoadResourceFile {
    public static void main(String[] args) throws Exception {
        LoadResourceFile demo = new LoadResourceFile();
        demo.loadUsingClassMethod();
        demo.loadUsingClassLoaderMethod();
    }
}

In the snippet above we load two difference resources. One contains Oracle database configuration and the other is MySQL database configuration.

/resources/org/kodejava/lang/database.conf

jdbc.driver=oracle.jdbc.driver.OracleDriver
jdbc.url=jdbc:oracle:thin:@localhost:1521:xe
jdbc.username=kodejava
jdbc.password=kodejava123

/resources/database.conf

jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost/kodejava
jdbc.username=kodejava
jdbc.password=kodejava123

The result of this code snippet are:

LoadResourceFile.loadUsingClassMethod
JDBC Driver: oracle.jdbc.driver.OracleDriver
JDBC Driver: com.mysql.jdbc.Driver

LoadResourceFile.loadUsingClassLoaderMethod
JDBC URL: jdbc:mysql://localhost/kodejava
JDBC URL: jdbc:mysql://localhost/kodejava