How to overwrite one property in .properties without overwriting the whole file?

Basically, I have to overwrite a certain property in a .properties file through a Java app, but when I use Properties.setProperty() and Properties.Store() it overwrites the whole file rather than just that one property.

I've tried constructing the FileOutputStream with append = true, but with that it adds another property and doesn't delete/overwrite the existing property.

How can I code it so that setting one property overwrites that specific property, without overwriting the whole file?

Edit: I tried reading the file and adding to it. Here's my updated code:

FileOutputStream out = new FileOutputStream("file.properties");
FileInputStream in开发者_开发技巧 = new FileInputStream("file.properties");
Properties props = new Properties();

props.load(in);
in.close();

props.setProperty("somekey", "somevalue");
props.store(out, null);
out.close();

The Properties API doesn't provide any methods for adding/replacing/removing a property in the properties file. The model that the API supports is to load all of the properties from a file, make changes to the in-memory Properties object, and then store all of the properties to a file (the same one or a different one).

But the Properties API is not unusual in the respect. In reality, in-place updating of a text file is difficult to implement without rewriting the entire file. This difficulty is a direct consequence of the way that files / file systems are implemented by a modern operating system.

If you really need to do incremental updates, then you need to use some kind of database to hold the properties, not a ".properties" file.

Other Answers have suggested the following approach in various guises:

1. Load properties from file into Properties object.
2. Update Properties object.
3. Save Properties object on top of existing file.

This works for some use-cases. However the load / save is liable to reorder the properties, remove embedded comments and white space. These things may matter¹.

The other point is that this involves rewriting the entire properties file, which the OP is explicitly trying² to avoid.

^{1 - If the API is used as the designers intended, property order, embedded comments, and so on wouldn't matter. But lets assume that the OP is doing this for "pragmatic reasons".
2 - Not that it is practical to avoid this; see earlier.}

You can use PropertiesConfiguration from Apache Commons Configuration.

In version 1.X:

PropertiesConfiguration config = new PropertiesConfiguration("file.properties");
config.setProperty("somekey", "somevalue");
config.save();

From version 2.0:

Parameters params = new Parameters();
FileBasedConfigurationBuilder<FileBasedConfiguration> builder =
    new FileBasedConfigurationBuilder<FileBasedConfiguration>(PropertiesConfiguration.class)
    .configure(params.properties()
        .setFileName("file.properties"));
Configuration config = builder.getConfiguration();
config.setProperty("somekey", "somevalue");
builder.save();

Properties files are an easy way to provide configuration for an application, but not necessarily a good way to do programmatic, user-specific customization, for just the reason that you've found.

For that, I'd use the Preferences API.

I do the following method:-

1. Read the file and load the properties object
2. Update or add new properties by using ".setProperty" method. (setProperty method is better than .put method as it can be used for inserting as well as updating the property object)
3. Write the property object back to file to keep the file in sync with the change.

public class PropertiesXMLExample {
    public static void main(String[] args) throws IOException {

    // get properties object
    Properties props = new Properties();

    // get path of the file that you want
    String filepath = System.getProperty("user.home")
            + System.getProperty("file.separator") +"email-configuration.xml";

    // get file object
    File file = new File(filepath);

    // check whether the file exists
    if (file.exists()) {
        // get inpustream of the file
        InputStream is = new FileInputStream(filepath);

        // load the xml file into properties format
        props.loadFromXML(is);

        // store all the property keys in a set 
        Set<String> names = props.stringPropertyNames();

        // iterate over all the property names
        for (Iterator<String> i = names.iterator(); i.hasNext();) {
            // store each propertyname that you get
            String propname = i.next();

            // set all the properties (since these properties are not automatically stored when you update the file). All these properties will be rewritten. You also set some new value for the property names that you read
            props.setProperty(propname, props.getProperty(propname));
        }

        // add some new properties to the props object
        props.setProperty("email.support", "donot-spam-me@nospam.com");
        props.setProperty("email.support_2", "donot-spam-me@nospam.com");

       // get outputstream object to for storing the properties into the same xml file that you read
        OutputStream os = new FileOutputStream(
                System.getProperty("user.home")
                        + "/email-configuration.xml");

        // store the properties detail into a pre-defined XML file
        props.storeToXML(os, "Support Email", "UTF-8");

        // an earlier stored property
        String email = props.getProperty("email.support_1");

        System.out.println(email);
      }
   }
}

The output of the program would be:

support@stackoverflow.com

Please use just update file line instead using Properies e.g.

public static void updateProperty(String key, String oldValue, String newValue)
{
    File f = new File(CONFIG_FILE);
    try {
        List<String> fileContent = new ArrayList<>(Files.readAllLines(f.toPath(), StandardCharsets.UTF_8));

        for (int i = 0; i < fileContent.size(); i++) {
            if (fileContent.get(i).replaceAll("\\s+","").equals(key + "=" + oldValue)) {
                fileContent.set(i, key + " = " + newValue);
                break;
            }
        }
        Files.write(f.toPath(), fileContent, StandardCharsets.UTF_8);
    } catch (Exception e) {
        
    }
}

import java.io.*;

import java.util.*;

class WritePropertiesFile

{

         public static void main(String[] args) {
    try {
        Properties p = new Properties();
        p.setProperty("1", "one");
        p.setProperty("2", "two");
        p.setProperty("3", "three");

        File file = new File("task.properties");
        FileOutputStream fOut = new FileOutputStream(file);
        p.store(fOut, "Favorite Things");
        fOut.close();
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

}

}

If you just want to override 1 prop why not just add parameter to your java command. Whatever you provide in your properties file they will be overrided with properties args.

java -Dyour.prop.to.be.overrided="value" -jar  your.jar