Java Read and Write Properties File Example

In this Java tutorial, learn to read properties file using Properties.load() method. Also we will use Properties.setProperty() method to write a new property into the .properties file.

1. Setup

Given below is a property file that we will use in our example.

firstName=Lokesh
lastName=Gupta
blog=howtodoinjava
technology=java

2. Reading Properties File

In most applications, the properties file is loaded during the application startup and is cached for future references. Whenever we need to get a property value by its key, we will refer to the properties cache and get the value from it.

The properties cache is usually a singleton static instance so that application does not require to read the property file multiple times; because the IO cost of reading the file again is very high for any time-critical application.

Example 1: Reading a .properties file in Java

In the given example, we are reading the properties from a file app.properties which is in the classpath. The class PropertiesCache acts as a cache for loaded properties.

The file loads the properties lazily, but only once.

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;
import java.util.Set;

public class PropertiesCache
{
   private final Properties configProp = new Properties();
   
   private PropertiesCache()
   {
      //Private constructor to restrict new instances
      InputStream in = this.getClass().getClassLoader().getResourceAsStream("application.properties");
      System.out.println("Reading all properties from the file");
      try {
          configProp.load(in);
      } catch (IOException e) {
          e.printStackTrace();
      }
   }

   //Bill Pugh Solution for singleton pattern
   private static class LazyHolder
   {
      private static final PropertiesCache INSTANCE = new PropertiesCache();
   }

   public static PropertiesCache getInstance()
   {
      return LazyHolder.INSTANCE;
   }
   
   public String getProperty(String key){
      return configProp.getProperty(key);
   }
   
   public Set<String> getAllPropertyNames(){
      return configProp.stringPropertyNames();
   }
   
   public boolean containsKey(String key){
      return configProp.containsKey(key);
   }
}

In the above code, we have used Bill Pugh technique for creating a singleton instance.

Let’s test the above code.

public static void main(String[] args)
{
  //Get individual properties
  System.out.println(PropertiesCache.getInstance().getProperty("firstName"));
  System.out.println(PropertiesCache.getInstance().getProperty("lastName"));
  
  //All property names
  System.out.println(PropertiesCache.getInstance().getAllPropertyNames());
}

Program Output:

Read all properties from file
Lokesh
Gupta
[lastName, technology, firstName, blog]

3. Writing into the Property File

Personally, I do not find any good reason for modifying a property file from the application code. Only time, it may make sense if you are preparing data for exporting to third party vendor/ or application that needs data in this format only.

Example 2: Java program to write a new key-value pair in properties file

So, if you are facing similar situation then create two more methods in PropertiesCache.java like this:

public void setProperty(String key, String value){
  configProp.setProperty(key, value);
}

public void flush() throws FileNotFoundException, IOException {
	try (final OutputStream outputstream 
				= new FileOutputStream("application.properties");) {
		configProp.store(outputstream,"File Updated");
		outputstream.close();
	}
}

• Use the setProperty(k, v) method to write new property to the properties file.
• Use the flush() method to write the updated properties back into the application.properties file.

PropertiesCache cache = PropertiesCache.getInstance();
if(cache.containsKey("country") == false){
 cache.setProperty("country", "INDIA");
}

//Verify property
System.out.println(cache.getProperty("country")); 

//Write to the file
PropertiesCache.getInstance().flush(); 

Program Output:

Reading all properties from the file
INDIA 

And the updated properties file is:

#File Updated
#Fri Aug 14 16:14:33 IST 2020
firstName=Lokesh
lastName=Gupta
technology=java
blog=howtodoinjava
country=INDIA

That’s all for this simple and easy tutorial related to reading and writing property files using java.

Happy Learning !!