How to store Java Properties? Properties class has methods to store/save to different outputs – OutputStream, Writer or even XML file. Here’s an example of Properties.store(OutputStream).

java.util.Properties class has a few methods that allow to save properties (key-value pairs) in a format suitable for further load using Properties.load() methods. The methods to save properties are:

• void save(OutputStream out, String comments)
  This one is Deprecated. so we won’t describe it – use below methods instead.
• void store(OutputStream out, String comments)
  Writes property list into OutputStream and precedes with comments (see below example and it become clear ;-) )
• void store(Writer writer, String comments)
  Writes property list into Writer and precedes with comments (as the above method, but using Writer).

Here’s an example of Properties.store(OutputStream) in action:

package com.farenda.java.util;

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

public class PropertiesStore {

    public static void main(String[] args) throws IOException {
        Properties config = createAppProperties();

        printToSystemOutput(config);

        Path configFile = saveToFile(config);

        displayFile(configFile);
    }

    private static Properties createAppProperties() {
        Properties config = new Properties();
        config.setProperty("minThreads", "5");
        config.setProperty("maxThreads", "40");
        return config;
    }

    private static void displayFile(Path path) throws IOException {
        System.out.printf("%nContents of %s:%n", path.toString());
        Files.readAllLines(path).forEach(System.out::println);
    }

    private static void printToSystemOutput(Properties config) throws IOException {
        config.store(System.out, "Writing to System.out:");
    }

    private static Path saveToFile(Properties config) throws IOException {
        Path file = Files.createTempFile("app-config", ".properties");
        // To use store(Writer) change to Files.newBufferedWriter(file)
        try (OutputStream stream = Files.newOutputStream(file)) {
            config.store(stream, "Writing to a file using OutputStream:");
        }
        return file;
    }
}

In the example we’ve used a few features from Java 7 and Java 8 – java.Path, Files, try-with-resources, and forEach to which we pass static method as parameter.

And here is the result of running the above code:

#Writing to System.out:
#Sat Sep 05 10:25:24 CEST 2015
maxThreads=40
minThreads=5

Contents of /tmp/app-config8047556758778700402.properties:
#Writing to a file using OutputStream:
#Sat Sep 05 10:25:24 CEST 2015
maxThreads=40
minThreads=5

Note that the properties have not been written in the same order they were added into the Properties object. Another thing is that the timestamp is added automatically – at wasn’t in the comments.