Java read property values

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.

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 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.

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()); >

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();

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.

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Properties File — Java Read & Write

This tutorial explains step by step to read and write a properties file in java with example You learned to read properties file with key and values as well as line by line and also write key and values to the properties file.

In this tutorial, How to read and write a properties file content in Java

Java properties file reader example

In this example, you will learn how to read a key and its values from a properties file and display it to console

Let’s declare the properties file

Create a properties object, Properties is a data structure to store keys and values in java

Create URL object using ClassLoader . getSystemResource method

Create a InputStream using url.openStream method

Load the properties content into the java properties object using the load method

Add try and catch block for IOExceptions and FIleNotFoundException

Oneway, Print the value using getProperty with the key of a properties object

Another way is to iterate properties object for the loop

First, get all keys using stringPropertyNames

using for loop print the key and values.

 catch (FileNotFoundException fie) < fie.printStackTrace(); >catch (IOException e) < e.printStackTrace(); >System.out.println(properties.getProperty("hostname")); Set keys = properties.stringPropertyNames(); for (String key : keys) < System.out.println(key + " - " + properties.getProperty(key)); >> > 

How to read a properties file line by line in java

• created a File object with an absolute path
• Create BufferedReader using FileReader object
• get the First line of the properties file using readLine of BufferedReader
• Loop using while loop until the end of the line reached
• Print each line

 > catch (FileNotFoundException e1) < e1.printStackTrace(); >catch (IOException e) < e.printStackTrace(); >finally < try < br.close(); >catch (IOException e) < e.printStackTrace(); >> > > 

How to write a key and values to a properties file in java

In this example, You can read and write a property using

• First create a File object
• Create a writer object using FileWriter
• Create properties object and add new properties or update existing properties if the properties file exists
• setProperties method do update or add key and values
• store method of properties object writes to the properties file, You have to add the comment which appends to the properties file as a comment

Here is a complete example read and write a properties file

 catch (FileNotFoundException e) < e.printStackTrace(); >catch (IOException e) < e.printStackTrace(); >> > 

Conclusion

You learned to read a properties file with keys and values as well as line by line and also write keys and values to the properties file

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Java Properties File: How to Read config.properties Values in Java?

.properties is a file extension for files mainly used in Java related technologies to store the configurable parameters of an application. They can also be used for storing strings for Internationalization and localization; these are known as Property Resource Bundles.

Each parameter is stored as a pair of strings, one storing the name of the parameter (called the key/map ), and the other storing the value.

Below is a sample Java program which demonstrate you how to retrieve/read config.properties values in Java. For update follow this tutorial.

We will create 3 files:

1. CrunchifyReadConfigMain.java
2. CrunchifyGetPropertyValues.java
3. config.properties file

Main Class (CrunchifyReadConfigMain.java) which will call getPropValues() method from class CrunchifyGetPropertyValues.java .

Let’s get started:

Step-1: Create config.properties file.

1. Create Folder “ resources ” under Java Resources folder if your project doesn’t have it.
2. create config.properties file with below value.

/Java Resources/config.properties file content:

#Crunchify Properties user=Crunchify company1=Google company2=eBay company3=Yahoo

Step-2

Create file CrunchifyReadConfigMain.java

package crunchify.com.tutorial; import java.io.IOException; /** * @author Crunchify.com * */ public class CrunchifyReadConfigMain < public static void main(String[] args) throws IOException < CrunchifyGetPropertyValues properties = new CrunchifyGetPropertyValues(); properties.getPropValues(); >>

Step-3

Create file CrunchifyGetPropertyValues.java

package crunchify.com.tutorial; import java.io.FileNotFoundException; import java.io.IOException; import java.io.InputStream; import java.util.Date; import java.util.Properties; /** * @author Crunchify.com * */ public class CrunchifyGetPropertyValues < String result = ""; InputStream inputStream; public String getPropValues() throws IOException < try < Properties prop = new Properties(); String propFileName = "config.properties"; inputStream = getClass().getClassLoader().getResourceAsStream(propFileName); if (inputStream != null) < prop.load(inputStream); >else < throw new FileNotFoundException("property file '" + propFileName + "' not found in the classpath"); >Date time = new Date(System.currentTimeMillis()); // get the property value and print it out String user = prop.getProperty("user"); String company1 = prop.getProperty("company1"); String company2 = prop.getProperty("company2"); String company3 = prop.getProperty("company3"); result = "Company List = " + company1 + ", " + company2 + ", " + company3; System.out.println(result + "\nProgram Ran on " + time + " by user=" + user); > catch (Exception e) < System.out.println("Exception: " + e); >finally < inputStream.close(); >return result; > >

Step-4

Run CrunchifyReadConfigMain and checkout result.

Company List = Google, eBay, Yahoo Program Ran on Mon May 13 21:54:55 PDT 2013 by user=Crunchify

As usually happy coding and enjoy. Do let me know if you see any exception. List of all Java Tutorials.

Are you running above program in IntelliJ IDE and getting NullPointerException?

Please follow below tutorial for fix.

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