Locale and Resource Bundles

locale-and-resource-bundles

Resource bundles allow you to move locale-specific information out from your main source code to a properties file or a java class. In this article, we will introduce the usage of both implementations of the ResourceBundle interface – PropertyResourceBundle and ListResourceBundle class.

Using a Properties File

PropertyResourceBundle takes an InputStream, so you could point it to a properties file for where the locale-specific information is.

A locale properties file must have the extension of .properties, and optionally suffixed with _language or _language_COUNTRY. e.g. Labels_zh.properties, Labels_zh_CN.properties, Foo_fr_CA.properties, Foo_de.properties, Foo.properties are all legal locale properties files. The ResourceBundle always searches from the most specific properties first and will stop search when there is a match.

A locale properties file just looks like a normal properties file, e.g. Labels_en_US.properties

greetings=How you doing?

while Labels_en_UK.properties might be:

greetings=How do you do?

Using a Java Class

You could extend the ListResourceBundle abstract class and override the abstract method Object[][] getContents() to returns an array of key-value pairs. Because the return type is an array of array objects, that means the value in the key-value pair could be of any type. The class name is similar to the locale properties file name other than instead of .properties it uses .java as the extension. e.g. Labels_fr_CA.java is a legal name for this use, and the class might look like this:

import java.util.ListResourceBundle;

public class Labels_en_CA extends ListResourceBundle {
  @Override
  protected Object[][] getContents() {
    return new Object[][] {
      {"hello", "Bonjour"},
      {"thank", new StringBuilder("merci")}
    };
  }
}

Using the ResourceBundle

There are three steps to follow to use the ResourceBundle with Locale.

  1. Create/get a Locale object;
  2. Calling a static method to get the resource bundle with the locale object.
  3. get locale-specific information from the bundle.

Example

Labels_en.properties

thank=Thank you

Labels_en_UK.properties

greeting=How do you do

Labels_en_UK.java

import java.util.ListResourceBundle;

public class Labels_en_UK extends ListResourceBundle {
  @Override
  protected Object[][] getContents() {
    return new Object[][] {
      {"elevator", "lift"},
      {"apartment", new StringBuilder("flat")}
    };
  }
}

LocaleLanguge.java

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

class LocaleLanguage {
  public static void main(String... args) {

    Locale loc = args.length == 1 ? 
      new Locale(args[0]) : new Locale(args[0], args[1]);
    ResourceBundle rb = ResourceBundle.getBundle("Labels", loc);
    System.out.println(rb.getObject("apartment"));
    System.out.println(rb.getString("elevator"));
    System.out.println(rb.getString("greeting"));
    System.out.println(rb.getString("thank"));
  }
}

When invoked with java LocaleLanguage, the output follows:

flat
lift
How do you do
Thank you

The key things here are:

  1. The ResourceBundle will search resources by their hierarchy. Begins with the most specific locale. The rules give an inheritance of the locale information from its less specific ones within its hierarchy.
  2. The ResourceBundle will prefer .java over .properties resources within the same hierarchy.
  3. When the key searched could not return any resources, a java.util.MissingResourceException is thrown.

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