A Vert.x 3 launcher which overrides root level JSON config properties with values from system properties and/or environment variables

PropertiesLauncher.java

import io.vertx.core.DeploymentOptions;
import io.vertx.core.Launcher;
import io.vertx.core.json.JsonObject;

import java.io.InputStream;
import java.lang.reflect.InvocationTargetException;
import java.util.Map;
import java.util.Scanner;

/**
 * A launcher which enables overriding properties in the deployment config
 * by using system properties and environment variables.
 * 
 * The launcher also looks for a default config in the classpath at /defaults-conf.json.
 * If you define sensible defaults in this file you get a nice setup where you aren't required
 * to send in any conf, but can if you want to. And you are able to override properties. 
 **/
public class PropertiesLauncher extends Launcher {

    public static void main(String[] args) {
        new PropertiesLauncher().dispatch(args);
    }
    
    @Override
    public void beforeDeployingVerticle(DeploymentOptions deploymentOptions) {
        JsonObject inputConf = deploymentOptions.getConfig();
        final JsonObject conf = inputConf == null ? readDefaultsConf() : inputConf;
        
        if (conf != null) {
            JsonObject moddedConf = conf.copy();
            for (Map.Entry<String, Object> e : moddedConf) {
                String key = e.getKey();
                Prop prop = getProp(key);
                if (prop != null) {
                    Class<?> type = e.getValue().getClass();
                    Object value = convertType(type, prop.value);
                    System.out.println(String.format(
                            "Overriding %s=%s of type %s with %s %s=%s. New converted value is '%s'",
                            key, e.getValue(), type.getName(), prop.source, prop.key, prop.value, value)
                    );
                    moddedConf.put(key, value);
                }
            }
            deploymentOptions.setConfig(moddedConf);
        }
    }

    private JsonObject readDefaultsConf() {
        InputStream defaultsConf = getClass().getResourceAsStream("/defaults-conf.json");
        return defaultsConf == null ? null: new JsonObject(toStr(defaultsConf));
    }

    private String toStr(InputStream is) {
        // http://stackoverflow.com/a/5445161/83741
        Scanner s = new Scanner(is).useDelimiter("\\A");
        return s.hasNext() ? s.next() : "";
    }

    private Object convertType(Class<?> type, String value) {
        // http://stackoverflow.com/a/13569408/83741
        try {
            return type.getConstructor( String.class ).newInstance(value);
        } catch (InstantiationException | IllegalAccessException | NoSuchMethodException | InvocationTargetException e) {
            throw new IllegalStateException(String.format("Could not convert value %s into type %s, tried finding a " +
                    "String constructor", value, type), e);
        }
    }

    static class Prop {
        String source;
        String key;
        String value;

        public Prop(String source, String key, String value) {
            this.source = source;
            this.key = key;
            this.value = value;
        }
    }

    static Prop getProp(final String name) {
        String sysPropValue = System.getProperty(name);
        if (sysPropValue != null) {
            return new Prop("system property", name, sysPropValue);
        }
        String envKey = name.toUpperCase().replaceAll("\\.", "_");
        String envValue = System.getenv(envKey);
        if (envValue != null) {
            return new Prop("environment variable", envKey, envValue);
        }
        return null;
    }
}