Lernen Dynamic Configuration at Runtime | System Properties and Environment Variables

When you need to change application behavior on the fly, dynamic configuration becomes essential. Java applications often rely on properties files for configuration, but by default, these files are read only once at startup. To enable dynamic updates, you can use techniques like polling, file watchers, or manual reload triggers.

Polling

Polling involves periodically checking the last modified time of the properties file. If the file has changed, the application reloads the configuration. This method is simple to implement and works across all platforms; however, there may be a delay between a change and its detection.

File watchers

File watchers use operating system notifications to detect changes instantly. Java's WatchService API allows you to monitor a directory for modifications, so you can reload the properties file as soon as it is updated. This approach is more efficient and responsive than polling.

Manual reload triggers

Manual reload triggers let users or administrators instruct the application to reload its configuration, for example, through a special command, button, or signal. This gives you full control over when changes take effect, which can be important for critical systems.

DynamicConfigWatcher.java

app.properties


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package com.example;

import java.io.IOException;
import java.nio.file.*;
import java.util.Properties;

public class DynamicConfigWatcher {
    private static final String CONFIG_FILE = "app.properties";
    private static Properties properties = new Properties();

    public static void main(String[] args) throws Exception {
        loadProperties();
        Thread watcherThread = new Thread(DynamicConfigWatcher::watchConfigFile);
        watcherThread.setDaemon(true);
        watcherThread.start();

        // Simulate application running and using config
        for (int i = 0; i < 60; i++) {
            System.out.println("Current config value: " + properties.getProperty("message", "default"));
            Thread.sleep(2000);
        }
    }

    private static void loadProperties() {
        try {
            properties.clear();
            properties.load(Files.newInputStream(Paths.get(CONFIG_FILE)));
            System.out.println("Reloaded properties: " + properties);
        } catch (IOException e) {
            System.err.println("Failed to load properties: " + e.getMessage());
        }
    }

    private static void watchConfigFile() {
        try {
            WatchService watchService = FileSystems.getDefault().newWatchService();
            Path configDir = Paths.get(".").toAbsolutePath();
            configDir.register(watchService, StandardWatchEventKinds.ENTRY_MODIFY);

            while (true) {
                WatchKey key = watchService.take();
                for (WatchEvent<?> event : key.pollEvents()) {
                    Path changed = (Path) event.context();
                    if (changed.toString().equals(CONFIG_FILE)) {
                        System.out.println("Config file changed, reloading...");
                        loadProperties();
                    }
                }
                key.reset();
            }
        } catch (Exception e) {
            System.err.println("Watcher error: " + e.getMessage());
        }
    }
}

Supporting runtime configuration changes brings new challenges. Ensuring consistency is crucial—if one part of the application reloads the configuration while another is still using the old values, unpredictable behavior can occur. Thread safety is another concern; you must coordinate access to the shared configuration to prevent race conditions or partial updates.

Rollback strategies are important if a configuration change introduces errors. You might want to keep a backup of the previous configuration and restore it if a reload fails, ensuring the application remains stable. Designing your reload logic to handle failures gracefully is key to robust dynamic configuration.

ManualReloadConfig.java

app.properties


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package com.example;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Properties;
import java.util.Scanner;

public class ManualReloadConfig {
    private static final String CONFIG_FILE = "app.properties";
    private static Properties properties = new Properties();

    public static void main(String[] args) {
        loadProperties();
        Scanner scanner = new Scanner(System.in);

        while (true) {
            System.out.println("Current config value: " + properties.getProperty("message", "default"));
            System.out.println("Type 'reload' to reload configuration or 'exit' to quit:");
            String input = scanner.nextLine().trim();
            if ("reload".equalsIgnoreCase(input)) {
                loadProperties();
            } else if ("exit".equalsIgnoreCase(input)) {
                break;
            }
        }
        scanner.close();
    }

    private static void loadProperties() {
        try {
            properties.clear();
            properties.load(Files.newInputStream(Paths.get(CONFIG_FILE)));
            System.out.println("Reloaded properties: " + properties);
        } catch (IOException e) {
            System.err.println("Failed to load properties: " + e.getMessage());
        }
    }
}

1. What is a potential benefit of supporting dynamic configuration reloads?

2. What challenge must be addressed when updating configuration at runtime in a multi-threaded application?

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Abschnitt 2. Kapitel 5

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Polling

File watchers

Manual reload triggers

DynamicConfigWatcher.java

app.properties


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package com.example;

import java.io.IOException;
import java.nio.file.*;
import java.util.Properties;

public class DynamicConfigWatcher {
    private static final String CONFIG_FILE = "app.properties";
    private static Properties properties = new Properties();

    public static void main(String[] args) throws Exception {
        loadProperties();
        Thread watcherThread = new Thread(DynamicConfigWatcher::watchConfigFile);
        watcherThread.setDaemon(true);
        watcherThread.start();

        // Simulate application running and using config
        for (int i = 0; i < 60; i++) {
            System.out.println("Current config value: " + properties.getProperty("message", "default"));
            Thread.sleep(2000);
        }
    }

    private static void loadProperties() {
        try {
            properties.clear();
            properties.load(Files.newInputStream(Paths.get(CONFIG_FILE)));
            System.out.println("Reloaded properties: " + properties);
        } catch (IOException e) {
            System.err.println("Failed to load properties: " + e.getMessage());
        }
    }

    private static void watchConfigFile() {
        try {
            WatchService watchService = FileSystems.getDefault().newWatchService();
            Path configDir = Paths.get(".").toAbsolutePath();
            configDir.register(watchService, StandardWatchEventKinds.ENTRY_MODIFY);

            while (true) {
                WatchKey key = watchService.take();
                for (WatchEvent<?> event : key.pollEvents()) {
                    Path changed = (Path) event.context();
                    if (changed.toString().equals(CONFIG_FILE)) {
                        System.out.println("Config file changed, reloading...");
                        loadProperties();
                    }
                }
                key.reset();
            }
        } catch (Exception e) {
            System.err.println("Watcher error: " + e.getMessage());
        }
    }
}

ManualReloadConfig.java

app.properties


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package com.example;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Properties;
import java.util.Scanner;

public class ManualReloadConfig {
    private static final String CONFIG_FILE = "app.properties";
    private static Properties properties = new Properties();

    public static void main(String[] args) {
        loadProperties();
        Scanner scanner = new Scanner(System.in);

        while (true) {
            System.out.println("Current config value: " + properties.getProperty("message", "default"));
            System.out.println("Type 'reload' to reload configuration or 'exit' to quit:");
            String input = scanner.nextLine().trim();
            if ("reload".equalsIgnoreCase(input)) {
                loadProperties();
            } else if ("exit".equalsIgnoreCase(input)) {
                break;
            }
        }
        scanner.close();
    }

    private static void loadProperties() {
        try {
            properties.clear();
            properties.load(Files.newInputStream(Paths.get(CONFIG_FILE)));
            System.out.println("Reloaded properties: " + properties);
        } catch (IOException e) {
            System.err.println("Failed to load properties: " + e.getMessage());
        }
    }
}

1. What is a potential benefit of supporting dynamic configuration reloads?

2. What challenge must be addressed when updating configuration at runtime in a multi-threaded application?

War alles klar?

Danke für Ihr Feedback!

Abschnitt 2. Kapitel 5