How to override the value of a properties file through command line arguments?

Question

I have a properties file that looks like this:

hostName=machineA.domain.host.com emailFrom=[e-mail] emailTo=[e-mail] emailCc=[e-mail] 

And now I read the above properties file from my Java program as

 public class FileReaderTask { private static String hostName; private static String emailFrom; private static String emailTo; private static String emailCc; private static final String configFileName = "config.properties"; private static final Properties prop = new Properties(); public static void main(String[] args) { readConfig(arguments); } private static void readConfig(String[] args) throws FileNotFoundException, IOException { if (!TestUtils.isEmpty(args) && args.length != 0) { prop.load(new FileInputStream(args[0])); } else { prop.load(FileReaderTask.class.getClassLoader().getResourceAsStream(configFileName)); } hostName = prop.getProperty("hostName").trim(); emailFrom = prop.getProperty("emailFrom").trim(); emailTo = prop.getProperty("emailTo").trim(); emailCc = prop.getProperty("emailCc").trim(); } } 

In most cases, I will run my above program through the command line as an executable jar like this -

 java -jar abc.jar config.properties 

My questions -

• Is there any way to override the above attributes in the properties file via the command line without touching the file, if necessary? Since I do not want to change the config.properties file whenever I need to change the value of attributes? Can this be done?

Something like this should override the hostName value in the file?

 java -jar abc.jar config.properties hostName=machineB.domain.host.com 

• And is there also a way to add --help while abc.jar , which can tell us more about how to run the jar file and what each property means and how to use them? I saw --help while running most of the C ++ or Unix executable, so not sure how we can do the same in Java?

Do I need to use CommandLine parser for this in Java to achieve both things?

Answer 1

If your command line has the only things: hostName=machineB.domain.host.com , and not any other types of arguments, you can simplify the processing of your command line quite a lot:

First, attach all the command line arguments with new lines, as if they were a new configuration file:

 StringBuilder sb = new StringBuilder(); for (String arg : args) { sb.append(arg).append("\n"); } String commandlineProperties = sb.toString(); 

Now you have two sources of sources, your file and this line. You can load both of them into one instance of Properties, with one version overwriting the other:

 if (...the config file exists...) { try (FileReader fromFile = new FileReader("config.properties")) { prop.load(fromFile); } } if (!commandlineProperties.isEmpty()) { // read, and overwrite, properties from the commandline... prop.load(new StringReader(commandlineProperties)); }