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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

Formatted Messages

We've seen that in order to internationalize programs, you must place all user-visible messages into resource bundles. This is straightforward when the text to be localized consists of simple labels such as those on buttons and menu items. It is trickier, however, with messages that are composed partially of static text and partially of dynamic values. For example, a compiler might have to display a message such as "Error at line 5 of file "hello.java"", in which the line number and filename are dynamic and locale-independent, while the rest of the message is static and needs to be localized.

The MessageFormat class of the java.text package helps tremendously with these types of messages. To use it, you store only the static parts of a message in the ResourceBundle and include special characters that indicate where the dynamic parts of the message are to be placed. For example, one resource bundle might contain the message: "Error at line {0} of file {1}". And another resource bundle might contain a "translation" that looks like this: "Erreur: {1}: {0}".

To use such a localized message, you create a MessageFormat object from the static part of the message and then call its format( ) method, passing in an array of the values to be substituted. In this case, the array contains an Integer object that specifies the line number and a String object that specifies the filename. The MessageFormat class knows about other Format classes defined in java.text. It creates and uses NumberFormat objects to format numbers and DateFormat objects to format dates and times. In addition, you can design messages that create ChoiceFormat objects to convert from numbers to strings. This is useful when working with enumerated types, such as numbers that correspond to month names, or when you need to use the singular or plural form of a word based on the value of some number.

Example 8-5 demonstrates this kind of MessageFormat usage. It is a convenience class with a single static method for the localized display of exception and error messages. When invoked, the code attempts to load a ResourceBundle with the basename "Errors". If found, it looks up a message resource using the class name of the exception object that was passed. If such a resource is found, it displays the error message. An array of five values is passed to the format( ) method. The localized error message can include any or all of these arguments.

The LocalizedError.display( ) method defined in this example was used in Example 8-2 at the beginning of this chapter. The default Errors.properties resource bundle used in conjunction with this example is shown following the code listing. Error message display for the program is nicely internationalized. Porting the program's error message to a new locale is simply a matter of translating (localizing) the Errors.properties file.

Example 8-5. LocalizedError.java
package je3.i18n;
import java.text.*;
import java.io.*;
import java.util.*;

/**
 * A convenience class that can display a localized exception message
 * depending on the class of the exception.  It uses a MessageFormat,
 * and passes five arguments that the localized message may include:
 *   {0}: the message included in the exception or error.
 *   {1}: the full class name of the exception or error.
 *   {2}: the file the exception occurred in
 *   {3}: a line number in that file.
 *   {4}: the current date and time.
 * Messages are looked up in a ResourceBundle with the basename
 * "Errors", using a the full class name of the exception object as
 * the resource name.  If no resource is found for a given exception
 * class, the superclasses are checked.
 **/
public class LocalizedError {
    public static void display(Throwable error) {
        ResourceBundle bundle;
        // Try to get the resource bundle.
        // If none, print the error in a nonlocalized way.
        try {
            String bundleName = "com.davidflanagan.examples.i18n.Errors";
            bundle = ResourceBundle.getBundle(bundleName);
        }
        catch (MissingResourceException e) {
            error.printStackTrace(System.err);
            return;
        }
        
        // Look up a localized message resource in that bundle, using the
        // classname of the error (or its superclasses) as the resource name.
        // If no resource was found, display the error without localization.
        String message = null;
        Class c = error.getClass( );
        while((message == null) && (c != Object.class)) {
            try { message = bundle.getString(c.getName( )); }
            catch (MissingResourceException e) { c = c.getSuperclass( ); }
        }
        if (message == null) { error.printStackTrace(System.err);  return; }
        
        // Get the filename and linenumber for the exception
        // In Java 1.4, this is easy, but in prior releases, we had to try
        // parsing the output Throwable.printStackTrace( );
        StackTraceElement frame = error.getStackTrace( )[0];  // Java 1.4
        String filename = frame.getFileName( );
        int linenum = frame.getLineNumber( );

        // Set up an array of arguments to use with the message
        String errmsg = error.getMessage( );
        Object[  ] args = {
            ((errmsg!= null)?errmsg:""), error.getClass( ).getName( ),
            filename, new Integer(linenum), new Date( )
        };

        // Finally, display the localized error message, using
        // MessageFormat.format( ) to substitute the arguments into the message.
        System.err.println(MessageFormat.format(message, args));
    }

    /** 
     * This is a simple test program that demonstrates the display( ) method.
     * You can use it to generate and display a FileNotFoundException or an
     * ArrayIndexOutOfBoundsException
     **/
    public static void main(String[  ] args) {
        try { FileReader in = new FileReader(args[0]); }
        catch(Exception e) { LocalizedError.display(e); }
    }
}

The following listing shows the resource bundle properties file used to localize the set of possible error messages that can be thrown by the ConvertEncoding class of Example 8-2:

#
# This is the file Errors.properties
# One property for each class of exceptions that our program might
# report.  Note the use of backslashes to continue long lines onto the
# next.  Also note the use of \n and \t for newlines and tabs
#
java.io.FileNotFoundException: \
Error: File "{0}" not found\n\t\
Error occurred at line {3} of file "{2}"\n\tat {4}

java.io.UnsupportedEncodingException: \
Error: Specified encoding not supported\n\t\
Error occurred at line {3} of file "{2}"\n\tat {4,time} on {4,date}

java.io.CharConversionException:\
Error: Character conversion failure.  Input data is not in specified format.

# A generic resource.  Display a message for any error or exception that
# is not handled by a more specific resource.
java.lang.Throwable:\
Error: {1}: {0}\n\t\
Error occurred at line {3} of file "{2}"\n\t{4,time,long} {4,date,long}

With a resource bundle like this, ConvertEncoding produces error messages like the following:

Error: File "myfile (No such file or directory)" not found
        Error occurred at line 64 of file "FileInputStream.java"
        at 7/9/00 9:28 PM

Or, if the current locale is fr_FR:

Error: File "myfile (Aucun fichier ou repertoire de ce type)" not found
        Error occurred at line 64 of file "FileInputStream.java"
        at 09/07/00 21:28

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