System

The Java platform itself uses a Properties object to maintain its own configuration.

See the property method of the system class.

Access to system properties can be restricted by the Security Manager.

To set a system property when starting the jvm

java -D<name>=<value>

Application

Properties extends java.util.Hashtable. Some of the methods inherited from Hashtable support the following actions:

• testing to see if a particular key or value is in the Properties object,
• getting the current number of key/value pairs,
• removing a key and its value,
• adding a key/value pair to the Properties list,
• enumerating over the values or the keys,
• retrieving a value by its key, and
• finding out if the Properties object is empty.

Load Property Files

// create and load default properties
Properties defaultProps = new Properties();
FileInputStream in = new FileInputStream("defaultProperties");
defaultProps.load(in);
in.close();

// create application properties with default
Properties applicationProps = new Properties(defaultProps);

// now load properties 
// from last invocation
in = new FileInputStream("appProperties");
applicationProps.load(in);
in.close();

Get

applicationProps.getProperty(String key, String default)

Loop

for (String key: properties.stringPropertyNames()){
     System.out.println ("The key ("+key+") has the value ("+properties.getProperty(key)+")";
}