Constants and properties in java

Question

Java best practices recommends read properties as constants. So, what do you think is the best approach to reach it? My approach is: A Configuration class to read the properties file only one time (singleton pattern) and use this class to read properties when needed as constants. And a Constants class to store:

• The properties name to find them in the properties file (eg app.database.url).
• Static constants (those that I don't want the user to config eg CONSTANT_URL="myurl.com").

public final class Configurations {

private Properties properties = null;
private static Configurations instance = null;

/** Private constructor */
private Configurations (){
    this.properties = new Properties();
    try{
        properties.load(Thread.currentThread().getContextClassLoader().getResourceAsStream(Constants.PATH_CONFFILE));
    }catch(Exception ex){
        ex.printStackTrace();
    }
}   

/** Creates the instance is synchronized to avoid multithreads problems */
private synchronized static void createInstance () {
    if (instance == null) { 
        instance = new Configurations ();
    }
}

/** Get the properties instance. Uses singleton pattern */
public static Configurations getInstance(){
    // Uses singleton pattern to guarantee the creation of only one instance
    if(instance == null) {
        createInstance();
    }
    return instance;
}

/** Get a property of the property file */
public String getProperty(String key){
    String result = null;
    if(key !=null && !key.trim().isEmpty()){
        result = this.properties.getProperty(key);
    }
    return result;
}

/** Override the clone method to ensure the "unique instance" requeriment of this class */
public Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException();
}}

The Constant class contains the references to the properties and the Constants.

public class Constants {
// Properties (user configurable)
public static final String DB_URL = "db.url";
public static final String DB_DRIVER = "db.driver";

// Constants (not user configurable)
public static final String PATH_CONFFILE = "config/config.properties";
public static final int MYCONSTANT_ONE = 1;
}

And the properties files would be:

db.url=www.myurl.com
db.driver=mysql

To read the properties and constants would be:

// Constants
int i = Constants.MYCONSTANT_ONE;
// Properties
String url = Configurations.getInstance().getProperty(Constants.DB_URL);

Do you think this is a good approach? What is your way to read properties and constants in Java?

Thanks in advance.

Answer 1

I found a better solution to homogenize the code and has everything as constants. With the same Configurations class and .properties file: Since getInstance() method is static it is possible to init the constants in the class Constants.

A class to store the name to the property in .properties file:

public class Properties {
// Properties (user configurable)
public static final String DB_URL = "db.url";
public static final String DB_DRIVER = "db.driver";
}

And then the Constant class will be:

public class Constants {
// Properties (user configurable)
public static final String DB_URL = Configurations.getInstance().getProperty(Properties.DB_URL);
public static final String DB_DRIVER = Configurations.getInstance().getProperty(Properties.DB_DRIVER );

// Constants (not user configurable)
public static final String PATH_CONFFILE = "config/config.properties";
public static final int MYCONSTANT_ONE = 1;
}

And finally to use them:

// Constants
int i = Constants.MYCONSTANT_ONE;
// Properties
String url = Constants.DB_URL;

I think it is a clean and elegant solution that fix constants and properties problems in tests.