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best ways to dynamically load config in java?

i am designing a web service in java where i need to do a sort of AB testing with requests in java.

Basically I’m looking for ways to easily configure parameters which will be dynamically loaded by a request handler to determine the code path based on config values.

for example, let’s say i need to get some data either from an external web service or from local DB. i want to have a way to configure parameters(criteria in this context) so that it would determine whether to go fetch data from external web service or from local DB.

if i go with a key-value pair config system above example might produce something like this.

locale=us
percentage=30
browser=firefox

which would mean that i would be fetching data from local DB for 30% of requests from US users whose user-agent is firefox. and i would like this config system to be dynamic so that server does not need to be restarted.

sorry about very high level description but any insights/leads would be appreciated. if this is a topic that is beaten to death in the past, please kindly let me know the links.

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Answer

I’ve used this in the past. It is the most common way in java to achieve what you are asking using java.util.Properties:

import java.io.FileInputStream;
import java.io.IOException;
import java.util.Properties;

/**
* Class that loads settings from config.properties
* @author Brant Unger
*
*/
public class Settings 
{
public static boolean DEBUG = false; 

/**
 * Load settings from config.properties
 */
public static void load()
{
    try
    {
        Properties appSettings = new Properties();
        FileInputStream fis = new FileInputStream("config.properties"); //put config properties file to buffer
        appSettings.load(fis); //load config.properties file

                   //This is where you add your config variables:
                   DEBUG = Boolean.parseBoolean((String)appSettings.get("DEBUG"));

        fis.close();
        if(DEBUG) System.out.println("Settings file successfuly loaded");

    }
    catch(IOException e)
    {
        System.out.println("Could not load settings file.");
        System.out.println(e.getMessage());
    }
}

}

Then in your main class you can do:

Settings.load(); //Load settings

Then you can check the values of those variables in every other class like:

if (Settings.DEBUG) System.out.println("The debug value is true");

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