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AddThis Social Bookmark Button O'Reilly Book Excerpts: Java Examples in a Nutshell, 3rd Edition

Internationalization, Part 2

Editor's note: Having your Java apps run correctly both down the street and across the globe presents some hefty challenges. Part one of this two-part excerpt from Java Examples in a Nutshell, 3rd Edition covered the first two steps to internationalization in Java: using Unicode character encoding and handling local customs. This week deals with the third step in the process: localizing user-visible messages.

Localizing User-Visible Messages

The third task of internationalization involves ensuring that there are no user-visible strings that are hardcoded in an application; instead, strings should be looked up based on the locale. In Example 8-3, for example, the strings "Portfolio value", "Symbol", "Shares", and others are hardcoded in the application and appear in English, even when the program is run in the French locale. The only way to prevent this is to fetch all user-visible messages at runtime and to translate every message into each language your application must support.

Related Reading

Java Examples in a Nutshell
By David Flanagan

Java helps you handle this task with the ResourceBundle class of the java.util package. This class represents a bundle of resources that can be looked up by name. You define a localized resource bundle for each locale you want to support, and Java loads the correct bundle for the default (or specified) locale. With the correct bundle loaded, you can look up the resources (typically strings) your program needs at runtime.

Working with Resource Bundles

To define a bundle of localized resources, you create a subclass of ResourceBundle and provide definitions for the handleGetObject( ) and getKeys( ) methods. handleGetObject( ) is passed the name of a resource; it should return an appropriate localized version of that resource. getKeys( ) should return an Enumeration object that gives the user a list of all resource names defined in the ResourceBundle. Instead of subclassing ResourceBundle directly, however, it is often easier to subclass ListResourceBundle. You can also simply provide a property file (see the java.util.Properties class) that ResourceBundle.getBundle( ) uses to create an instance of PropertyResourceBundle.

To use localized resources from a ResourceBundle in a program, you should first call the static getBundle( ) method, which dynamically loads and instantiates a ResourceBundle, as described shortly. The returned ResourceBundle has the name you specify and is appropriate for the specified locale (or for the default locale if no locale is explicitly specified). Once you have obtained a ResourceBundle object with getBundle( ), use the getObject( ) method to look up resources by name. Note that there is a convenience method, getString( ), that simply casts the value returned by getObject( ) to be a String object.

When you call getBundle( ), you specify the base name of the desired ResourceBundle and a desired locale (if you don't want to rely on the default locale). Recall that a Locale is specified with a two-letter language code, an optional two-letter country code, and an optional variant string. getBundle( ) looks for an appropriate ResourceBundle class for the locale by appending this locale information to the base name for the bundle. The method looks for an appropriate class with the following algorithm:

  1. Search for a class with the following name:

    basename_language_country_variant

    If no such class is found or no variant string is specified for the locale, it goes to the next step.

  2. Search for a class with the following name:

    basename_language_country

    If no such class is found or no country code is specified for the locale, it goes to the next step.

  3. Search for a class with the following name:

    basename_language

    If no such class is found, it goes to the final step.

  4. Search for a class that has the same name as the basename, or, in other words, search for a class with the following name:

    basename

    This represents a default resource bundle used by any locale that is not explicitly supported.

At each step in this process, getBundle( ) checks first for a class file with the given name. If no class file is found, it uses the getResourceAsStream( ) method of ClassLoader to look for a Properties file with the same name as the class and a .properties extension. If such a properties file is found, its contents are used to create a Properties object, and getBundle( ) instantiates and returns a PropertyResourceBundle that exports the properties in the Properties file through the ResourceBundle API.

If getBundle( ) cannot find a class or properties file for the specified locale in any of the four previous search steps, it repeats the search using the default locale instead of the specified locale. If no appropriate ResourceBundle is found in this search either, getBundle( ) throws a MissingResourceException.

Any ResourceBundle object can have a parent ResourceBundle specified for it. When you look up a named resource in a ResourceBundle, getObject( ) first looks in the specified bundle, but if the named resource is not defined in that bundle, it recursively looks in the parent bundle. Thus, every ResourceBundle inherits the resources of its parent and may choose to override some, or all, of these resources. (Note that we are using the terms "inherit" and "override" in a different sense than we do when talking about classes that inherit and override methods in their superclass.) What this means is that every ResourceBundle you define does not have to define every resource required by your application. For example, you might define a ResourceBundle of messages to display to French-speaking users. Then you might define a smaller and more specialized ResourceBundle that overrides a few of these messages so that they are appropriate for French-speaking users who live in Canada.

Your application is not required to find and set up the parent objects for the ResourceBundle objects it uses. The getBundle( ) method actually does this for you. When getBundle( ) finds an appropriate class or properties file as described previously, it does not immediately return the ResourceBundle it has found. Instead, it continues through the remaining steps in the previous search process, looking for less-specific class or properties files from which the ResourceBundle may inherit resources. If and when getBundle( ) finds these less-specific resource bundles, it sets them up as the appropriate ancestors of the original bundle. Only once it has checked all possibilities does it return the original ResourceBundle object that it created.

To continue the example begun earlier, when a program runs in Quebec, getBundle( ) might first find a small specialized ResourceBundle class that has only a few specific Quebecois resources. Next, it looks for a more general ResourceBundle that contains French messages, and it sets this bundle as the parent of the original Quebecois bundle. Finally, getBundle( ) looks for (and probably finds) a class that defines a default set of resources, probably written in English (assuming that English is the native tongue of the original programmer). This default bundle is set as the parent of the French bundle (which makes it the grandparent of the Quebecois bundle). When the application looks up a named resource, the Quebecois bundle is searched first. If the resource isn't defined there, the French bundle is searched, and any named resource not found in the French bundle is looked up in the default bundle.

ResourceBundle Example

Examining some code makes this discussion of resource bundles much clearer. Example 8-4 is a convenience routine for creating Swing menu panes. Given a list of menu item names, it looks up labels and menu shortcuts for those named menu items in a resource bundle and creates a localized menu pane. The example has a simple test program attached.

Figure 8-3 shows the menus it creates in the American, British, and French locales. This program cannot run, of course, without localized resource bundles from which the localized menu labels are looked up.

Figure 8-3. Localized menu panes
Figure 8-3. Localized menu panes

Example 8-4. SimpleMenu.java
package je3.i18n;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.Locale;
import java.util.ResourceBundle;
import java.util.MissingResourceException;

/** A convenience class to automatically create localized menu panes */
public class SimpleMenu {
  /** The convenience method that creates menu panes */
  public static JMenu create(ResourceBundle bundle,
                String menuname, String[] itemnames,
                ActionListener listener)
  {
    // Get the menu title from the bundle. Use name as default label.
    String menulabel;
    try { menulabel = bundle.getString(menuname + ".label"); }
    catch(MissingResourceException e) { menulabel = menuname; }

    // Create the menu pane.
    JMenu menu = new JMenu(menulabel);

    // For each named item in the menu.
    for(int i = 0; i < itemnames.length; i++) {
      // Look up the label for the item, using name as default.
      String itemlabel;
      try {
        itemlabel =
          bundle.getString(menuname+"."+itemnames[i]+".label");
      }
      catch (MissingResourceException e) { itemlabel = itemnames[i]; }

      JMenuItem item = new JMenuItem(itemlabel);

      // Look up an accelerator for the menu item
      try {
        String acceleratorText = bundle.getString(menuname+"."+itemnames[i]+".accelerator");
        item.setAccelerator(KeyStroke.getKeyStroke(acceleratorText));
      }
      catch (MissingResourceException e) {}

      // Register an action listener and command for the item.
      if (listener != null) {
        item.addActionListener(listener);
        item.setActionCommand(itemnames[i]);
      }

      // Add the item to the menu.
      menu.add(item);
    }

    // Return the automatically created localized menu.
    return menu;
  }

  /** A simple test program for the above code */
  public static void main(String[] args) {
    // Get the locale: default, or specified on command-line
    Locale locale;
    if (args.length == 2) locale = new Locale(args[0], args[1]);
    else locale = Locale.getDefault( );

    // Get the resource bundle for that Locale. This will throw an
    // (unchecked) MissingResourceException if no bundle is found.
    ResourceBundle bundle =
      ResourceBundle.getBundle("com.davidflanagan.examples.i18n.Menus",
                   locale);

    // Create a simple GUI window to display the menu with
    final JFrame f = new JFrame("SimpleMenu: " +  // Window title
                locale.getDisplayName(Locale.getDefault( )));
    JMenuBar menubar = new JMenuBar( );     // Create a menubar.
    f.setJMenuBar(menubar);          // Add menubar to window

    // Define an action listener that our menu will use.
    ActionListener listener = new ActionListener( ) {
        public void actionPerformed(ActionEvent e) {
          String s = e.getActionCommand( );
          Component c = f.getContentPane( );
          if (s.equals("red")) c.setBackground(Color.red);
          else if (s.equals("green")) c.setBackground(Color.green);
          else if (s.equals("blue")) c.setBackground(Color.blue);
        }
      };

    // Now create a menu using our convenience routine with the resource
    // bundle and action listener we've created
    JMenu menu = SimpleMenu.create(bundle, "colors",
                    new String[] {"red", "green", "blue"},
                    listener);

    // Finally add the menu to the GUI, and pop it up
    menubar.add(menu);      // Add the menu to the menubar
    f.setSize(300, 150);     // Set the window size.
    f.setVisible(true);     // Pop the window up.
  }
}

As I've already said, this example does not stand alone. It relies on resource bundles to localize the menu. The following listing shows three property files that serve as resource bundles for this example. Note that this single listing contains the bodies of three separate files:

# The file Menus.properties is the default "Menus" resource bundle.
# As an American programmer, I made my own locale the default.
colors.label=Colors
colors.red.label=Red
colors.red.accelerator=alt R
colors.green.label=Green
colors.green.accelerator=alt G
colors.blue.label=Blue
colors.blue.accelerator=alt B

# This is the file Menus_en_GB.properties.  It is the resource bundle for
# British English.  Note that it overrides only a single resource definition
# and simply inherits the rest from the default (American) bundle.
colors.label=Colours

# This is the file Menus_fr.properties.  It is the resource bundle for all
# French-speaking locales.  It overrides most, but not all, of the resources
# in the default bundle.
colors.label=Couleurs
colors.red.label=Rouge
colors.green.label=Vert
colors.green.accelerator=control shift V
colors.blue.label=Bleu

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