ResourceBundle Behavior in Java

Explore how ResourceBundle processes keys across multiple property files for different locales.

Introduction

The ResourceBundle class in Java is a powerful tool for managing locale-specific resources. When multiple property files are available, it merges and retrieves keys and values based on the specified locale. However, the order in which these keys are returned may sometimes surprise developers.

How ResourceBundle Works

When a ResourceBundle is created using the getBundle() method:

• It first looks for the most specific property file matching the requested locale. For example, mymsgs_en_UK.properties for the en_UK locale.
• If keys are missing in the locale-specific file, it falls back to the default property file (e.g., mymsgs.properties).

The merged result includes all keys from both files, prioritizing locale-specific values.

Code Example

import java.util.Enumeration;
import java.util.Locale;
import java.util.ResourceBundle;

public class LocalizationExample {
    public static void main(String[] args) {
        Locale myloc = new Locale.Builder().setLanguage("en").setRegion("UK").build();
        ResourceBundle msgs = ResourceBundle.getBundle("mymsgs", myloc);

        Enumeration<String> en = msgs.getKeys();
        while (en.hasMoreElements()) {
            String key = en.nextElement();
            String val = msgs.getString(key);
            System.out.println(key + " : " + val);
        }
    }
}

        

Sample Property Files

Default File: mymsgs.properties

okLabel=OK
cancelLabel=Cancel

        

Locale-Specific File: mymsgs_en_UK.properties

okLabel=YES
noLabel=NO

        

Output

When the above code is executed, the following output is observed:

noLabel : NO
okLabel : YES
cancelLabel : Cancel

        

Explanation

The output order is determined by how ResourceBundle merges and retrieves keys:

1. It processes keys from the locale-specific file first (mymsgs_en_UK.properties).
2. It then adds keys from the default file (mymsgs.properties) that are not already included.

This results in the following merged key order: noLabel, okLabel, and cancelLabel. Values are fetched from their respective sources based on priority.

Conclusion

The ResourceBundle lookup mechanism ensures seamless localization by combining resources from multiple files. Understanding this behavior helps in designing and debugging locale-specific applications.