Loading Configuration Data

Configuration files can be accessed using the class loader to obtain an input stream directly:

username=admin
password=secret123

public class ConfigReader {
    public static void main(String[] args) throws IOException {
        ConfigReader app = new ConfigReader();
        InputStream configStream = app.getClass()
                                     .getClassLoader()
                                     .getResourceAsStream("app.config");
        
        Properties config = new Properties();
        config.load(configStream);
        
        System.out.println(config.getProperty("username"));
        System.out.println(config.getProperty("password"));
    }
}

Alternatively, file paths can be resolved first before opening streams:

public class ConfigReaderAlt {
    public static void main(String[] args) throws IOException {
        ConfigReaderAlt app = new ConfigReaderAlt();
        String configPath = app.getClass()
                              .getClassLoader()
                              .getResource("app.config")
                              .getPath();
        
        FileInputStream fileStream = new FileInputStream(configPath);
        Properties config = new Properties();
        config.load(fileStream);
        
        System.out.println(config.getProperty("username"));
        System.out.println(config.getProperty("password"));
    }
}

Modifying Configuration Values

To update configuration entries, use the setProperty method followed by storing changes:

config.setProperty("key", "value");
config.store(outputStream, null);

Compleet example for updating configurations:

Properties settings = loadConfig(filePath);
settings.setProperty("timeout", "30");

FileOutputStream output = null;
try {
    output = new FileOutputStream(
        Thread.currentThread()
               .getContextClassLoader()
               .getResource(filePath)
               .getPath()
    );
    settings.store(output, null);
} catch (IOException error) {
    error.printStackTrace();
} finally {
    if (output != null) {
        output.close();
    }
}

Utility Implementation

A reusable utility simplifies configuraiton management operations:

package com.example.utils;

import java.io.*;
import java.util.Properties;

public class PropertyManager {
    
    public static Properties loadFromFile(String filePath) {
        Properties props = new Properties();
        try {
            props.load(Thread.currentThread()
                           .getContextClassLoader()
                           .getResourceAsStream(filePath));
        } catch (IOException exception) {
            exception.printStackTrace();
        }
        return props;
    }
    
    public static String getValue(String filePath, String propertyKey) {
        Properties props = loadFromFile(filePath);
        return props.getProperty(propertyKey);
    }
    
    public static void updateValue(String filePath, String key, String newValue) {
        Properties props = loadFromFile(filePath);
        props.setProperty(key, newValue);
        
        FileOutputStream fileOut = null;
        try {
            fileOut = new FileOutputStream(
                Thread.currentThread()
                       .getContextClassLoader()
                       .getResource(filePath)
                       .getPath()
            );
            props.store(fileOut, null);
        } catch (IOException exception) {
            exception.printStackTrace();
        } finally {
            if (fileOut != null) {
                try {
                    fileOut.close();
                } catch (IOException ignored) {}
            }
        }
    }
}

Example usage of the utility:

public static void main(String[] args) throws IOException {
    String currentValue = PropertyManager.getValue("application.settings", "mode");
    System.out.println(currentValue);
    
    PropertyManager.updateValue("application.settings", "mode", "production");
    
    String updatedValue = PropertyManager.getValue("application.settings", "mode");
    System.out.println(updatedValue);
}