Java Programs›File Handling›Read a Properties File

Read a Properties File in Java

beginner· File Handling · File I/O

Problem

A .properties file stores configuration as key=value lines, with # starting a comment; java.util.Properties reads and writes this format.

Load an application's name, port and feature flag from a properties file, using a default for a setting that's missing.

Input

app.name=DevStack Demo, app.port=8080, feature.dark=true

Output

Port: 8080, Timeout: 30 (default)

Java Program

Java

import java.io.IOException;
import java.io.Reader;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Properties;

public class ReadPropertiesFile {
    public static void main(String[] args) throws IOException {
        Path file = Paths.get("app.properties");
        Files.writeString(file, "# application settings\napp.name=DevStack Demo\napp.port=8080\nfeature.dark=true\n");

        Properties props = new Properties();
        try (Reader reader = Files.newBufferedReader(file)) {
            props.load(reader); // parses key=value lines and skips # comments
        }

        String name = props.getProperty("app.name");
        int port = Integer.parseInt(props.getProperty("app.port"));
        boolean dark = Boolean.parseBoolean(props.getProperty("feature.dark"));
        String timeout = props.getProperty("app.timeout", "30"); // default when the key is missing

        System.out.println("Name: " + name);
        System.out.println("Port: " + port);
        System.out.println("Dark mode: " + dark);
        System.out.println("Timeout: " + timeout + " (default)");
        Files.delete(file);
    }
}

▶ Run Code

Output

Name: DevStack Demo
Port: 8080
Dark mode: true
Timeout: 30 (default)

Core Logic

Properties.load parses the whole file into string pairs, and the program converts each value to the type it needs.

How It Works

1The program writes a sample app.properties containing a comment and three settings.
2props.load(reader) parses every key=value line and skips # comments.
3getProperty returns strings, which are converted with Integer.parseInt and Boolean.parseBoolean.
4getProperty("app.timeout", "30") returns the default because the key is missing.

The missing timeout falls back to 30 without any null check.

💡

Key Point: All values come back as text — forgetting to convert "8080" to an int is a common source of bugs.

Key Concepts

java.util.Propertiesload()getProperty(key, default)

Approach 2: Writing a Properties File

Java

import java.io.IOException;
import java.io.Reader;
import java.io.Writer;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Properties;
import java.util.TreeSet;

public class WritePropertiesFile {
    public static void main(String[] args) throws IOException {
        Path file = Paths.get("user.properties");
        Properties out = new Properties();
        out.setProperty("theme", "dark");
        out.setProperty("language", "en");
        out.setProperty("fontSize", "14");

        try (Writer w = Files.newBufferedWriter(file)) {
            out.store(w, "User preferences"); // writes a comment line, a timestamp line, then key=value pairs
        }

        Properties in = new Properties();
        try (Reader r = Files.newBufferedReader(file)) {
            in.load(r);
        }
        for (String key : new TreeSet<>(in.stringPropertyNames())) { // sorted for stable output
            System.out.println(key + " = " + in.getProperty(key));
        }
        Files.delete(file);
    }
}

▶ Run Code

Output

fontSize = 14
language = en
theme = dark

Core Logic

store writes the settings back out in the same format, with a comment header, so they can be reloaded later.

How It Works

1setProperty adds three user preferences.
2store(writer, "User preferences") writes a comment line, a timestamp and the key=value pairs.
3The file is loaded again, and the keys are printed in sorted order.

The reloaded values match what was stored: theme, language and font size.

💡

Key Point: Properties doesn't preserve key order, so sort the keys when printing or comparing.