Java Properties file

Properties class represents a persistent set of properties. These properties can be saved to a stream or loaded from a stream. Java properties file is used to store project configuration data or settings.

Java Properties class provides below constructors, to instantiate Properties class.

Properties()

Creates an empty property list with no default values.

Properties(Properties defaults)

Creates an empty property list with the specified defaults.

How to write to properties file

Step 1: Instantiate Properties class.

Ex: Properties properties = new Properties();

Step 2: Create properties file

Ex: FileOutputStream out = new FileOutputStream("project.properties");

Step 3: Set properties

Ex: properties.setProperty("width", "120");

Step 4: Store the properties to file.

Ex: properties.store(out, "Project configurations");

Example

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.Properties;

public class WriteToPropertyFile {
    public static void main(String args[]) throws FileNotFoundException, IOException{
        Properties properties = new Properties();
        FileOutputStream out = new FileOutputStream("project.properties");

        properties.setProperty("width", "120");
        properties.setProperty("height", "55");
        properties.setProperty("maxicons", "9");

        properties.store(out, "Project configurations");
        
        out.close();
    }
}

When you run above program, “project.properties” file will be created with below content.

project.properties

#Project configurations

#Thu Oct 16 17:35:41 IST 2014

height=55

width=120

maxicons=9

How to read from properties file

Step 1: Instantiate Properties class.

Ex: Properties properties = new Properties();

Step 2: Create FileInputStream instance to read from properties file

Ex: FileInputStream fin = new FileInputStream("project.properties");

Step 3: Load properties file

Ex: properties.load(fin);

Step 4: Read the properties.

Ex: properties.getProperty("height")

Example

import java.io.FileInputStream;
import java.util.Properties;

public class ReadFromPropertyFile {
    public static void main(String args[]) throws Exception{
        Properties properties = new Properties();
        FileInputStream fin = new FileInputStream("project.properties");

        properties.load(fin);

        System.out.println(properties.getProperty("height"));
        System.out.println(properties.getProperty("width"));
        System.out.println(properties.getProperty("maxicons"));

        fin.close();
    }
}

Output

55
120
9