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A properties file is a configuration resource that stores information in the form of key/value pairs.

This code reads and writes a properties file:

	// Read the property file
	Properties props = new Properties();
	FileInputStream in = new FileInputStream("app.properties");
	props.load(in);
	String pool = props.getProperty("sys.pool");
	String alarm = props.getProperty("sys.alarm");
	in.close();
		
	// Show the properties
	System.out.printf("Pool:%s Alarm:%s", pool, alarm);
		
	// Write to the property file
	props.setProperty("net.roundrobin", "yes"); // New property
	props.store(new FileOutputStream("app.properties"),"Demonstration");

Where the properties file contains:

sys.alarm=no
sys.pool=5

If you need to iterate on all the keys, you can loop:

	for(String key : props.stringPropertyNames()) ...

The JVM has its own properties that describe the environment. Retrieving and adding system properties is quite easy:

	String version = System.getProperty("java.version");
	System.setProperty("test.props", "123");