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Wednesday, April 14, 2021
How to Read Properties File in Java
In this tutorial you will see how to read a properties file in Java. If you have any configurable data in your application like DB configuration, user settings its better to keep it in a properties file and read it from there. A properties file stores data in the form of key/value pair.
- Loading properties file from the file system. See example.
- Loading properties file from classpath. See example.
Project structure
For this example we’ll have a properties file named app.properties file in a folder called resource. The resource folder is at the same level at the src folder in the Java project.
Steps for reading a properties file in Java
- Create an instance of Properties class.
- Create a FileInputStream by opening a connection to the properties file.
- Read property list (key and element pairs) from the input stream using load() method of the Properties class.
Content of the properties file
Here the properties file used is named app.properties file with it’s content as-
user=TestUser url=https://www.netjstech.com
Loading properties file from the file system
One way to read properties file in Java is to load it from the file system.
import java.io.FileInputStream; import java.io.IOException; import java.io.InputStream; import java.util.Properties; public class PropDemo < private Properties properties = new Properties(); public static void main(String[] args) < PropDemo pDemo = new PropDemo(); pDemo.loadPropertiesFile(); pDemo.readProperties(); >// This method is used to load the properties file private void loadPropertiesFile() < InputStream iStream = null; try < // Loading properties file from the path (relative path given here) iStream = new FileInputStream("resource/app.properties"); properties.load(iStream); >catch (IOException e) < // TODO Auto-generated catch block e.printStackTrace(); >finally < try < if(iStream != null)< iStream.close(); >> catch (IOException e) < // TODO Auto-generated catch block e.printStackTrace(); >> > /** * Method to read the properties from a * loaded property file */ private void readProperties() < System.out.println("User name - " + properties.getProperty("user")); System.out.println("URL - " + properties.getProperty("url")); // reading property which is not there System.out.println("City - " + properties.getProperty("city")); >> User name - TestUser URL - https://www.netjstech.com City - null
Here you can see that in the code there is an attempt to read the property “city” which doesn’t exist in the app.properties file that’s why it is retrieved as null.
Loading properties file from classpath
If you have properties file in the project classpath then you can load it by using the getResourceAsStream method. That is another way to read properties file in Java.
import java.io.FileInputStream; import java.io.IOException; import java.io.InputStream; import java.util.Properties; public class PropDemo < private Properties properties = new Properties(); public static void main(String[] args) < PropDemo pDemo = new PropDemo(); pDemo.loadProperties(); pDemo.readProperties(); >// This method is used to load the properties file private void loadProperties() < InputStream iStream = null; try < // Loading properties file from the classpath iStream = this.getClass().getClassLoader() .getResourceAsStream("app.properties"); if(iStream == null)< throw new IOException("File not found"); >properties.load(iStream); > catch (IOException e) < e.printStackTrace(); >finally < try < if(iStream != null)< iStream.close(); >> catch (IOException e) < // TODO Auto-generated catch block e.printStackTrace(); >> > /** * Method to read the properties from a * loaded property file */ private void readProperties() < System.out.println("User name - " + properties.getProperty("user")); System.out.println("URL - " + properties.getProperty("url")); >> User name - TestUser URL - https://www.netjstech.com
That’s all for this topic How to Read Properties File in Java. If you have any doubt or any suggestions to make please drop a comment. Thanks!
Java Properties file examples
Normally, Java properties file is used to store project configuration data or settings. In this tutorial, we will show you how to read and write to/from a .properties file.
Properties prop = new Properties(); // set key and value prop.setProperty("db.url", "localhost"); prop.setProperty("db.user", "mkyong"); prop.setProperty("db.password", "password"); // save a properties file prop.store(outputStream, ""); // load a properties file prop.load(inputStream) // get value by key prop.getProperty("db.url"); prop.getProperty("db.user"); prop.getProperty("db.password"); // get all keys prop.keySet(); // print everything prop.forEach((k, v) -> System.out.println("Key : " + k + ", Value : " + v)); A simple Maven project structure for testing.
1. Write to the properties file
Set the property key and value, and save it somewhere.
package com.mkyong; import java.io.FileOutputStream; import java.io.IOException; import java.io.OutputStream; import java.util.Properties; public class App1 < public static void main(String[] args) < try (OutputStream output = new FileOutputStream("path/to/config.properties")) < Properties prop = new Properties(); // set the properties value prop.setProperty("db.url", "localhost"); prop.setProperty("db.user", "mkyong"); prop.setProperty("db.password", "password"); // save properties to project root folder prop.store(output, null); System.out.println(prop); >catch (IOException io) < io.printStackTrace(); >> > The path/to/config.properties is created.
#Thu Apr 11 17:37:58 SRET 2019 db.user=mkyong db.password=password db.url=localhost 2. Load a properties file
Load a properties file from the file system and retrieved the property value.
package com.mkyong; import java.io.FileInputStream; import java.io.IOException; import java.io.InputStream; import java.util.Properties; public class App2 < public static void main(String[] args) < try (InputStream input = new FileInputStream("path/to/config.properties")) < Properties prop = new Properties(); // load a properties file prop.load(input); // get the property value and print it out System.out.println(prop.getProperty("db.url")); System.out.println(prop.getProperty("db.user")); System.out.println(prop.getProperty("db.password")); >catch (IOException ex) < ex.printStackTrace(); >> > localhost mkyong password 3. Load a properties file from classpath
Load a properties file config.properties from project classpath, and retrieved the property value.
db.url=localhost db.user=mkyong db.password=password package com.mkyong; import java.io.IOException; import java.io.InputStream; import java.util.Properties; public class App3 < public static void main(String[] args) < try (InputStream input = App3.class.getClassLoader().getResourceAsStream("config.properties")) < Properties prop = new Properties(); if (input == null) < System.out.println("Sorry, unable to find config.properties"); return; >//load a properties file from class path, inside static method prop.load(input); //get the property value and print it out System.out.println(prop.getProperty("db.url")); System.out.println(prop.getProperty("db.user")); System.out.println(prop.getProperty("db.password")); > catch (IOException ex) < ex.printStackTrace(); >> > localhost mkyong password 4. Prints everything from a properties file
Load a properties file config.properties from project classpath, and print out the keys and values.
package com.mkyong; import java.io.IOException; import java.io.InputStream; import java.util.Properties; import java.util.Set; public class App4 < public static void main(String[] args) < App4 app = new App4(); app.printAll("config.properties"); >private void printAll(String filename) < try (InputStream input = getClass().getClassLoader().getResourceAsStream(filename)) < Properties prop = new Properties(); if (input == null) < System.out.println("Sorry, unable to find " + filename); return; >prop.load(input); // Java 8 , print key and values prop.forEach((key, value) -> System.out.println("Key : " + key + ", Value : " + value)); // Get all keys prop.keySet().forEach(x -> System.out.println(x)); Set objects = prop.keySet(); /*Enumeration e = prop.propertyNames(); while (e.hasMoreElements()) < String key = (String) e.nextElement(); String value = prop.getProperty(key); System.out.println("Key : " + key + ", Value : " + value); >*/ > catch (IOException ex) < ex.printStackTrace(); >> > Key : db.user, Value : mkyong Key : db.password, Value : password Key : db.url, Value : localhost db.user db.password db.url Download Source Code
References
mkyong
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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
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.