跨多个类使用属性

Question

My question is the same as this one except it is for Java, specifically applied to properties. Ideally I would like to create one instance of Properties, and call the methods from all of the classes without creating new instances. I would also want to read from a single instance of properties so I only have a single source of the truth.

I have read the API for Properties and it doesn't answer my question.

This question indicates I need to include the reference in the class constructor. Is there a better way??

Answer 1

Let's take the system properties as example. In this implementation http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/lang/System.java#System.getProperty%28java.lang.String%29 , the properties are just stored in a static class attribute. Either make this attribute public or create public accessor methods. Short answer: just make it static.

You can initialize static data with static initializers, if things get a little bit more complex. ( https://docs.oracle.com/javase/tutorial/java/javaOO/initial.html )

Answer 2

The fisrt link, "this one" is a link to the Oracle documentation...

If you want to load your properties only once, you should use the singleton pattern. But be carefull that this pattern can be an anti-pattern and may make your unit tests more complex.

To avoid those drawbacks it is better to pass the reference to your properties via a constructor.

/* This is your singleton. It takes care of loading the properties only once and can delegate access method  to it */
public class Configuration {
   private static Configuration instance; // created only once
   public static getInstance() {
      instance = // Read the Singelton pattern to create it only once   
   }


   private Properties properties; // loaded only once

   public String get(String key) {
       return properties.getProperty(key);
   }

}

public class Component {
    private final Configuration cfg;
    public Component (Configuration cfg) {
        this.cfg = cfg;
    }
}

public class StarterOrDiContainer {
    // ..
    Component component = new Component(cfg.getInstance());
}