public abstract Class ResourceBundle

This allows you to write programs that can:

• be easily localized, or translated, into different languages
• handle multiple locales at once
• be easily modified later to support even more locales

Resource bundles belong to families whose members share a common base name, but whose names also have additional components that identify their locales. For example, the base name of a family of resource bundles might be "MyResources". The family should have a default resource bundle which simply has the same name as its family - "MyResources" - and will be used as the bundle of last resort if a specific locale is not supported. The family can then provide as many locale-specific members as needed, for example a German one named "MyResources_de".

Each resource bundle in a family contains the same items, but the items have been translated for the locale represented by that resource bundle. For example, both "MyResources" and "MyResources_de" may have a String that's used on a button for canceling operations. In "MyResources" the String may contain "Cancel" and in "MyResources_de" it may contain "Abbrechen".

If there are different resources for different countries, you can make specializations: for example, "MyResources_de_CH" contains objects for the German language (de) in Switzerland (CH). If you want to only modify some of the resources in the specialization, you can do so.

When your program needs a locale-specific object, it loads the ResourceBundle class using the getBundle method:

ResourceBundle myResources =
     ResourceBundle.getBundle("MyResources", currentLocale);

Resource bundles contain key/value pairs. The keys uniquely identify a locale-specific object in the bundle. Here's an example of a ListResourceBundle that contains two key/value pairs:

public class MyResources extends ListResourceBundle {
    protected Object[][] getContents() {
        return new Object[][] {
            // LOCALIZE THE SECOND STRING OF EACH ARRAY (e.g., "OK")
            {"OkKey", "OK"},
            {"CancelKey", "Cancel"},
            // END OF MATERIAL TO LOCALIZE
       };
    }
}

You retrieve an object from resource bundle using the appropriate getter method. Because "OkKey" and "CancelKey" are both strings, you would use getString to retrieve them:

button1 = new Button(myResources.getString("OkKey"));
button2 = new Button(myResources.getString("CancelKey"));

Besides getString, ResourceBundle also provides a method for getting string arrays, getStringArray, as well as a generic getObject method for any other type of object. When using getObject, you'll have to cast the result to the appropriate type. For example:

int[] myIntegers = (int[]) myResources.getObject("intList");

The Java Platform provides two subclasses of ResourceBundle, ListResourceBundle and PropertyResourceBundle, that provide a fairly simple way to create resources. As you saw briefly in a previous example, ListResourceBundle manages its resource as a list of key/value pairs. PropertyResourceBundle uses a properties file to manage its resources.

If ListResourceBundle or PropertyResourceBundle do not suit your needs, you can write your own ResourceBundle subclass. Your subclasses must override two methods: handleGetObject and getKeys().

The implementation of a ResourceBundle subclass must be thread-safe if it's simultaneously used by multiple threads. The default implementations of the non-abstract methods in this class, and the methods in the direct known concrete subclasses ListResourceBundle and PropertyResourceBundle are thread-safe.

Resource Bundles and Named Modules

Resource bundles together with an application

Resource bundles as service providers

A resource bundle provider can provide resource bundles in any format such XML which replaces the need of ResourceBundle.Control.

Resource bundles in other modules and class path

The getBundle factory methods with no Control parameter locate and load resource bundles from service providers. It may continue the search as if calling Module#getResourceAsStream(String) to find the named resource from a given module and calling ClassLoader#getResourceAsStream(String); refer to the specification of the getBundle method for details. Only non-encapsulated resource bundles of "java.class" or "java.properties" format are searched.

If the caller module is a resource bundle provider, it does not fall back to the class loader search.

In cases where the getBundle factory method is called from a context where there is no caller frame on the stack (e.g. when called directly from a JNI attached thread), the caller module is default to the unnamed module for the system class loader.

Resource bundles in automatic modules

ResourceBundle.Control

ResourceBundle.Control is designed for an application deployed in an unnamed module, for example to support resource bundles in non-standard formats or package localized resources in a non-traditional convention. ResourceBundleProvider is the replacement for ResourceBundle.Control when migrating to modules. UnsupportedOperationException will be thrown when a factory method that takes the ResourceBundle.Control parameter is called.

For the getBundle factory methods that take no Control instance, their default behavior of resource bundle loading can be modified with custom ResourceBundleControlProvider implementations. If any of the providers provides a Control for the given base name, that Control will be used instead of the default Control. If there is more than one service provider for supporting the same base name, the first one returned from ServiceLoader will be used. A custom Control implementation is ignored by named modules.

Cache Management

Example

// default (English language, United States)
public class MyResources extends ResourceBundle {
    public Object handleGetObject(String key) {
        if (key.equals("okKey")) {
           return "Ok";
        }
        if (key.equals("cancelKey")) {
           return "Cancel";
        }
        return null;
    }

    public Enumeration<String> getKeys() {
        return Collections.enumeration(keySet());
    }

    // Overrides handleKeySet() so that the getKeys() implementation
    // can rely on the keySet() value.
    protected Set<String> handleKeySet() {
        return new HashSet<String>(Arrays.asList("okKey", "cancelKey"));
    }
}

// German language
public class MyResources_de extends MyResources {
    public Object handleGetObject(String key) {
        // don't need okKey, since parent level handles it.
        if (key.equals("cancelKey")) {
           return "Abbrechen";
        }
        return null;
    }

    protected Set<String> handleKeySet() {
        return new HashSet<String>(Arrays.asList("cancelKey"));
    }
}