Creating A Singleton Using Java

Posted by tech on July 12, 2009

In software engineering, the singleton pattern is a design pattern that is used to restrict instantiation of a class to one object. While this concept is also sometimes generalized to restrict the instance to a specific number of objects, in most cases, I tend to have it instantiated only once and used by the entire system. A good use for this would be loading property values and retrieving the class that holds those properties. Here is a sample code for a singleton class.

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public class MailProperties {
 
  private static MailProperties _instance = null;
  private Properties props = new Properties();
 
  private MailProperties() throws Exception {
    InputStream is = Thread.currentThread().getContextClassLoader().getResourceAsStream("com/property/mail.properties");
    if (is != null) props.load(is);
  }
 
    public static MailProperties getInstance() throws Exception {
    if (_instance == null) _instance = new MailProperties();
    return _instance;
  }
 
  public String getProperty(String name) throws Exception {
    return props.getProperty(name);
  }
 
  public int getIntProperty(String name) throws Exception {
    return Integer.parseInt(getProperty(name));
  }
 
}

This class is hooked into a jar file and loads all properties found in the package com/property/. Once we call the class’ getInstance() method, a static object will be called that holds all properties found in the mail.properties file. This way, whenever we call it to retrieve a property, the object will only be instantiated once and called as many times.

To retrieve a property, you can call it like this:

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System.out.println(MailProperties.getInstance().get("my_property"));