My config code

gistfile1.java

import com.google.common.base.Joiner;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableMap;
import com.google.common.io.Closeables;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.annotation.concurrent.ThreadSafe;
import java.io.*;
import java.util.Date;
import java.util.Properties;

/**
 * @author eshioji@gmail.com
 */
@ThreadSafe
public class Config {
    private static final Logger log = LoggerFactory.getLogger(Config.class);
    private static final Joiner joiner = Joiner.on("\n\t");
    public static final String CONFIG_FILE_NAME = "some.stuff";
    private volatile ImmutableMap<Prop, Object> store;


    public void load(String resourceName){
        Properties properties = new Properties();
        InputStream is = ClassLoader.getSystemResourceAsStream(resourceName);
        try {
            Preconditions.checkNotNull(is,"No config file found. You must include the config file "+CONFIG_FILE_NAME+" in the classpath");
            properties.load(is);
            ImmutableMap.Builder<Prop, Object> ret = ImmutableMap.builder();
            for (Prop prop : Prop.values()) {
                String val = properties.getProperty(prop.name());
                if (val == null) {
                    val = prop._default;
                    if (val == null) {
                        throw new PropertiesException("No configured value nor a default value is available for " + prop + ". Please provide a configuration in " + CONFIG_FILE_NAME);
                    }
                }
                ret.put(prop, prop.parse(val));
            }
            ImmutableMap<Prop, Object> built = ret.build();
            store = built;
            log.info("Read configuration:\n\t" + joiner.join(built.entrySet()));
        } catch (IOException e) {
            log.error("Unable to read config file", e);
            throw new PropertiesException(e);
        } finally {
            Closeables.closeQuietly(is);
        }
    }

    public <T> T value(Prop prop, Class<T> type) {
        Preconditions.checkState(store!=null,"Coding error; Please call load method before using this method.");
        Object ret = store.get(prop);
        Preconditions.checkArgument(ret.getClass() == type,"Coding error. You are specifying the wrong class. You should give:" + prop.type);
        return (T)ret;
    }

    public static void main(String[] args) throws IOException {
        if(args.length!=1){
            throw new RuntimeException("Please provide output directory");
        }
        PrintWriter pw = new PrintWriter(new FileWriter(args[0] + File.separator + CONFIG_FILE_NAME + ".template"));
        try {
            pw.println("#Configuration file template for " + CONFIG_FILE_NAME + " generated on " + new Date());
            for (Prop prop : Prop.values()) {
                pw.println("# " + prop);
                pw.println("# " + prop.name() + "=" + prop._default);
                pw.println();
            }
        } finally {
            Closeables.closeQuietly(pw);
        }
    }
}




import com.google.common.base.Splitter;
import com.google.common.collect.ImmutableList;
import com.ning.http.client.ProxyServer;
import com.ning.http.client.oauth.ConsumerKey;

import java.io.File;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.List;

/**
 * Entity class for managing properties.</br>
 * THE HARD CODED VALUES ARE DEFAULTS! VALUE IN THE PROPERTY FILE WILL OVERWRITE THEM</br>
 *
 * <h3>How to add a new property</h3>
 * basic example</br>
 <pre>
 {@code
 // name(default_value, type)
 MAX_FAILURE("3",Integer.class),
 }
 * </pre>

 * advanced example:</br>
 * <pre>
  {@code
      //name                  // default value in string                      // type       // description (optional)
      WORKER_THREAD_NUM(""+Runtime.getRuntime().availableProcessors(), Integer.class, "Number of workers threads. Default value is number of available cores."),
  }
 * </pre>
 *
 * If your type has "valueOf" method, this is all you need.</br>
 * If you want to use a type that has no "valueOf" method, you have to implement the {@link #doParse} method, like so:</br>
 * <pre>
   {@code
       TWITTER_TOKEN_FILE("src/test/resources/dummy.dummy", File.class){
           @Override
           protected Object doParse(String value) {
               File tokenFile = new File(value);
               if(tokenFile.canRead()){
                   return tokenFile;
               }else{
                   throw new PropertiesException("Configuration for " + this + " invalid. File "+tokenFile.getAbsolutePath() +" cannot be read.");
               }
           }
   },
 * </pre>
 *
 * If you provide null as the default value, the initialization fails unless you provide a configuration value in the property file.
 * @author Enno Shioji (eshioji@gmail.com)
 */
public enum Prop {

    // Simplest example
    THREAD_NUM("10", Integer.class),

    // Requires user to provide config
    SECRET_KEY(null, String.class),

    // Dynamic values
    WORKER_THREAD_NUM(""+Runtime.getRuntime().availableProcessors(),Integer.class, "Number of workers. Default value is number of available cores."),

    // In-build validation
    TWITTER_TOKEN_FILE("src/test/resources/dummy.tokens", File.class){
        @Override
        protected Object doParse(String value) {
            File tokenFile = new File(value);
            if(tokenFile.canRead()){
                return tokenFile;
            }else{
                throw new PropertiesException("Configuration for " + this + " invalid. File "+tokenFile.getAbsolutePath() +" cannot be read.");
            }
        }
    },

    // Some other examples
    FOLLOW_REDIRECTS("true",Boolean.class),
    HTTP_CONN_TIMEOUT_MS("12000",Integer.class),
    HTTP_COMPRESSION_ENABLED("true",Boolean.class),

    PROXY_ENABLED("false", Boolean.class),
    PROXY_SERVER("secret.proxy:8080", ProxyServer.class){
        @Override
        protected Object doParse(String value) {
            try{
                new URI("http://" + value);
            } catch (URISyntaxException e) {
                throw new PropertiesException("Invalid config for " + this +" Was given:"+value +" was expecting something like " +_default+" (host:port)");
            }
            Splitter splitter = Splitter.on(":");
            List<String> split = ImmutableList.copyOf(splitter.split(value));
            if(split.size()!=2){
                throw new PropertiesException("Invalid config for " + this +" Was given:"+value +" was expecting something like " +_default+" (host:port) (did you forgot to give port?)");
            }
            String host = split.get(0);
            int port = Integer.valueOf(split.get(1));
            return new ProxyServer(ProxyServer.Protocol.HTTP,host,port);
        }
    };





    final String _default;
    final Class<?> type;
    final String desc;

    /**
     *
     * @param _default
     * @param type
     */
    Prop(String _default, Class<?> type) {
        this(_default, type, null);
    }

    Prop(String _default, Class<?> type, String desc) {
        this._default = _default;
        this.type = type;
        this.desc = desc;
    }


    final Object parse(String value) {
        return doParse(value);
    }

    protected Object doParse(String value) {
        return valof(value);
    }

    private Object valof(String value) {
        try {
            if (this.type == String.class) {
                return value;
            }
            Method valof = this.type.getDeclaredMethod("valueOf", String.class);
            return valof.invoke(null, value);
        } catch (InvocationTargetException e) {
            throw new PropertiesException("Invalid input for " +this + ". Was given " + value + ". Please check your config. Was expecting something like " + _default);
        }catch (NoSuchMethodException e) {
            throw new AssertionError("Please implement a custom doParse method on " + this);
        } catch (IllegalAccessException e) {
            throw new AssertionError("Please implement a custom doParse method on " + this);
        }catch(Exception e){
            throw new PropertiesException("Invalid input for " +this + ". Was given " + value + ". Please check your config. Was expecting something like " + _default);
        }
    }


    @Override
    public String toString() {
        return "Name:" + this.name() + " Type:" + this.type.getSimpleName() + " Default value=" + (this._default != null ? this._default : "No default value") +(this.desc != null ? " Description:" + this.desc : "");
    }
}