Properties class in Java

Properties class is the child class of Hashtable. It works in conjunction with properties file which is a text file. A properties file contains key-value in text format.

Properties class get us access to properties file.

Usage of properties file

Some times we need some properties to be used which we want to provide in such a way that we don’t have to restart our system or service. In those situations, we use properties file.

1. First of all, create a file called test.properties in the root of the project
2. Write java class

Example

package com.javatrainingschool;

import java.io.FileReader;
import java.util.Properties;

public class PropertiesFileExample {

	public static void main(String[] args) throws Exception {
		FileReader reader = new FileReader("test.properties");

		Properties prop = new Properties();
		prop.load(reader);

		System.out.println(prop.getProperty("application.name"));
		System.out.println(prop.getProperty("year"));
	}
}

Output:
My Test Application
2021

Read properties file from resources folder in maven project

In this example, we will see how to read properties file if it resides in the resources folder in a maven project. Below is the directory structure for the same.

package com.javatrainingschool;

 
import java.io.File;
import java.io.FileReader;
import java.net.URL;
import java.util.Properties;

public class PropertiesFileExample {

	public static void main(String[] args) throws Exception {
		
		URL resource = new PropertiesFileExample().getClass().getClassLoader()
                                    .getResource("test.properties");
		File file = new File(resource.toURI());
		FileReader reader = new FileReader(file);

		Properties prop = new Properties();
		prop.load(reader);

		System.out.println(prop.getProperty("application.name"));
		System.out.println(prop.getProperty("year"));
	}
}

Output :
My Test Application
2021

System properties and how to iterate properties file

In this example, we will see how we can iterate over a properties file. For this, we will create a properties file by fetching system properties, and then iterate it.

package com.javatrainingschool;

import java.util.Map.Entry;
import java.util.Properties;

public class PropertiesFileExample {

	public static void main(String[] args) throws Exception {
		
		Properties prop = System.getProperties();
		for(Entry<Object, Object> entry : prop.entrySet()) {
			System.out.println(entry.getKey() + " : " + entry.getValue());
		}
	}
}

Read properties file from a hard-coded path

package com.javatrainingschool;

import java.io.FileReader;
import java.util.Properties;

public class PropertiesFileExample {

	public static void main(String[] args) throws Exception {
		FileReader reader = new FileReader("C:\\Users\\saras\\Desktop\\cutom-files\\test.properties");

		Properties prop = new Properties();
		prop.load(reader);

		System.out.println(prop.getProperty("application.name"));
		System.out.println(prop.getProperty("year"));
	}
}

Create and set values to a properties file

package com.javatrainingschool;

import java.io.FileWriter;
import java.util.Properties;

public class PropertiesFileExample {

	public static void main(String[] args) throws Exception {

		Properties prop = new Properties();
		prop.setProperty("application.name", "Test application");
		prop.setProperty("launch.year", "2021");
		prop.store(new FileWriter("C:\\Users\\saras\\Desktop\\cutom-files\\test.properties"),
				"Java Training School Properties File Example");

		System.out.println("File created successfully.");
	}
}

After the program is executed successfully, please check at the location specified, you will see a file created with name test.properties. The content of the file should be as shown below