Load and Store Properties File

Properties files are usually used to externalize configuration from applications to text files. This article introduces the syntax and usage of properties files.

java.util.Properties class allows developers to load, change, store properties easily while providing a way to optionally save it in a filesystem.

Syntax Rules

1. Since a property file is just a text file and read as an input stream, it could have any name and any extension.
2. it could have two styles of commenting: ! comments... or # comments... (space after the initial character is optional).
3. A backslash could be used to break up a single line into multiple lines.
4. Properties files contain key-value pairs. Key-value pair could be defined as key=value, key:value, key value.
5. space around the value is ignored when reading the file.

Usage

System Properties

import java.util.Properties;

class SysProps {
  public static void main(String... args) {
    Properties sysProps = System.getProperties();
    sysProps.setProperty("foo", "bar");
    sysProps.list(System.out);
  }
}

This outputs:

...
java.vm.info=mixed mode
java.version=1.8.0_181
foo=bar
java.ext.dirs=/Users/hua/Library/Java/Extensions:/L...
...

Store as a Properties File

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

class Props1 {
  public static void main(String... args) {
    Properties p = new Properties();
    p.setProperty("k1", "v1");
    p.setProperty("k2", "v2");
    p.list(System.out);
    try(FileOutputStream out = 
        new FileOutputStream("myProps1.properties")) {
      p.store(out, "The header comment");
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}

This outputs:

-- listing properties --
k2=v2
k1=v1

and the myProps1.properties looks like:

#The header comment
#Sun Aug 26 16:06:31 EDT 2018
k2=v2
k1=v1

Reading the Saved Properties File

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

class Props2 {
  public static void main(String... args) {
    Properties p = new Properties();
    try(FileInputStream in = new FileInputStream("myProps1.properties");
      FileOutputStream out = new FileOutputStream("myProps2.properties");) {
      p.load(in);
      p.list(System.out);
      p.setProperty("k3", "v3");
      p.list(System.out);
      p.store(out, "A different header comment");
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}

This outputs:

-- listing properties --
k2=v2
k1=v1
-- listing properties --
k3=v3
k2=v2
k1=v1

and the myProps2.properties file looks like:

#A different header comment
#Sun Aug 26 16:07:34 EDT 2018
k3=v3
k2=v2
k1=v1

Useful methods

String getProperty(String key)

void list(PrintStream out)

void load(InputStream inStream)

Object setProperty(String key, String value)

void store(OutputStream outStream, String headerComment)