src/java.base/share/classes/java/util/spi/LocaleServiceProvider.java

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package java.util.spi;

import java.util.Locale;

/**
 * <p>
 * This is the super class of all the locale sensitive service provider
 * interfaces (SPIs).
 * <p>
 * Locale sensitive service provider interfaces are interfaces that
 * correspond to locale sensitive classes in the {@code java.text}
 * and {@code java.util} packages in order to provide the locale
 * data used for each service. The interfaces enable the
 * construction of locale sensitive objects and the retrieval of
 * localized names for these packages. Locale sensitive factory methods
 * and methods for name retrieval in the {@code java.text} and
 * {@code java.util} packages use implementations of the provider
 * interfaces to offer support for locales beyond the set of locales
 * supported by the Java runtime environment itself. Locale sensitive service
 * providers are deployed on the application module path or the application class
 * path. In order to be looked up, providers must be visible to the {@link
 * ClassLoader#getSystemClassLoader() system class loader}.
 * See {@link java.util.ServiceLoader##developing-service-providers Deploying
 * Service Providers} for further detail on deploying a locale sensitive service
 * provider as a module or on the class path.
 *
 * <h2>Packaging of Locale Sensitive Service Provider Implementations</h2>
 *
 * <p> For a locale sensitive service provider deployed in a module, the <i>provides</i>
 * directive must be specified in the module declaration. The <i>provides</i>
 * directive specifies both the service and the service provider.
 *
 * <p> For example, an implementation of the {@link java.text.spi.DateFormatProvider
 * DateFormatProvider} class deployed as a module might specify the following directive:
 * <pre>{@code
 *     provides java.text.spi.DateFormatProvider with com.example.ExternalDateFormatProvider;
 * }</pre>
 *
 * <p> For a Locale Service Provider deployed on the class path, the provider
 * identifies itself with a provider-configuration file in the resource directory
 * META-INF/services. The file name should be the fully fully qualified provider
 * interface class name. The file should contain a list of fully-qualified concrete
 * provider class names, one per line. A line is terminated by any one of a line
 * feed ('\n'), a carriage return ('\r'), or a carriage return followed immediately
 * by a line feed. Space and tab characters surrounding each name, as well as
 * blank lines, are ignored. The comment character is '#' ('\u0023'); on each line
 * all characters following the first comment character are ignored. The file must
 * be encoded in UTF-8.
 * <p>
 * If a particular concrete provider class is named in more than one configuration
 * file, or is named in the same configuration file more than once, then the
 * duplicates will be ignored. The configuration file naming a particular provider
 * need not be in the same jar file or other distribution unit as the provider itself.
 * The provider must be accessible from the same class loader that was initially
 * queried to locate the configuration file; this is not necessarily the class loader
 * that loaded the file.
 * <p>
 * For example, an implementation of the
 * {@link java.text.spi.DateFormatProvider DateFormatProvider} class should
 * take the form of a jar file which contains the file:
 * <pre>
 * META-INF/services/java.text.spi.DateFormatProvider
 * </pre>
 * And the file {@code java.text.spi.DateFormatProvider} should have
 * a line such as:
 * <pre>
 * {@code com.foo.DateFormatProviderImpl}
 * </pre>
 * which is the fully qualified class name of the class implementing
 * {@code DateFormatProvider}.
 * <h2>Invocation of Locale Sensitive Services</h2>
 * <p>
 * Locale sensitive factory methods and methods for name retrieval in the
 * {@code java.text} and {@code java.util} packages invoke
 * service provider methods when needed to support the requested locale.
 * The methods first check whether the Java runtime environment itself
 * supports the requested locale, and use its support if available.
 * Otherwise, they call the {@link #isSupportedLocale(Locale) isSupportedLocale}
 * methods of installed providers for the appropriate interface to find one that
 * supports the requested locale. If such a provider is found, its other
 * methods are called to obtain the requested object or name.  When checking
 * whether a locale is supported, the {@linkplain Locale##def_extensions
 * locale's extensions} are ignored by default. (If a locale's extensions should
 * also be checked, the {@code isSupportedLocale} method must be overridden).
 * If neither the Java runtime environment itself nor an installed provider
 * supports the requested locale, the methods go through a list of candidate
 * locales and repeat the availability check for each until a match is found.
 * The algorithm used for creating a list of candidate locales is same as
 * the one used by {@code ResourceBundle} by default (see
 * {@link java.util.ResourceBundle.Control#getCandidateLocales getCandidateLocales}
 * for the details).  Even if a locale is resolved from the candidate list,
 * methods that return requested objects or names are invoked with the original
 * requested locale including {@code Locale} extensions. The Java runtime
 * environment must support the root locale for all locale sensitive services in
 * order to guarantee that this process terminates.
 * <p>
 * Providers of names (but not providers of other objects) are allowed to
 * return null for some name requests even for locales that they claim to
 * support by including them in their return value for
 * {@code getAvailableLocales}. Similarly, the Java runtime
 * environment itself may not have all names for all locales that it
 * supports. This is because the sets of objects for which names are
 * requested can be large and vary over time, so that it's not always
 * feasible to cover them completely. If the Java runtime environment or a
 * provider returns null instead of a name, the lookup will proceed as
 * described above as if the locale was not supported.
 * <p>
 * The search order of locale sensitive services can
 * be configured by using the {@systemProperty java.locale.providers} system property.
 * This system property declares the user's preferred order for looking up
 * the locale sensitive services separated by a comma. As this property value is
 * read and cached only at the initialization of this class, users should specify the
 * property on the java launcher command line. Setting it at runtime with
 * {@link System#setProperty(String, String)} is discouraged and it may not affect
 * the order.
 * JDK Reference Implementation provides the following three
 * locale data providers:
 * <ul>
 * <li> "CLDR": A locale data provider based on the Unicode Consortium's
 * <a href="http://cldr.unicode.org/">Common Locale Data Repository (CLDR)</a>.
 * <li> "SPI": represents the locale sensitive services implementing the subclasses of
 * this {@code LocaleServiceProvider} class.
 * <li> "HOST": A locale data provider that reflects the user's custom settings in the
 * underlying operating system. This provider may not be available, depending
 * on the JDK Reference Implementation.
 * </ul>
 * <p>
 * For example, if the following is specified in the property:
 * <pre>
 * java.locale.providers=SPI,CLDR
 * </pre>
 * the locale sensitive services in the SPI providers are looked up first. If the
 * desired locale sensitive service is not available, then the runtime looks for CLDR.
 * <p>
 * The default value for looking up the preferred locale data providers is "CLDR",
 * so specifying only "CLDR" is identical to the default behavior. Applications which
 * require implementations of the locale sensitive services must explicitly specify
 * "SPI" in order for the Java runtime to load them from the classpath.
 *
 * @implNote The JDK uses locale data from the Unicode Consortium's
 * <a href="http://cldr.unicode.org/">Common Locale Data Repository (CLDR)</a>
 * to implement locale-sensitive APIs in the {@code java.util} and
 * {@code java.text} packages. This locale data derives the set of locales
 * supported by the Java runtime environment. The following table lists the
 * version of CLDR used in each JDK release. Unless otherwise specified, all
 * update releases in a given JDK release family use the same CLDR version.
 * Note that the CLDR locale data are subject to change. Users should not assume
 * that the locale data remain the same across CLDR versions. Otherwise, unexpected
 * incompatible behaviors may occur, such as an exception on parsing a date.
 * Refer to <a href="https://cldr.unicode.org/index/downloads">CLDR Releases</a>
 * for the deltas between their releases.
 * <table class="striped">
 * <caption style="display:none">JDK releases and supported CLDR versions</caption>
 * <thead>
 * <tr><th scope="col">JDK release</th>
 *     <th scope="col">CLDR version</th></tr>
 * </thead>
 * <tbody>
 * <tr><th scope="row" style="text-align:left">JDK 25</th>
 *     <td>CLDR 47</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 24</th>
 *     <td>CLDR 46</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 23</th>
 *     <td>CLDR 45</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 22</th>
 *     <td>CLDR 44</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 21</th>
 *     <td>CLDR 43</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 20</th>
 *     <td>CLDR 42</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 19</th>
 *     <td>CLDR 41</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 18</th>
 *     <td>CLDR 39</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 17</th>
 *     <td>CLDR 39</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 16</th>
 *     <td>CLDR 38</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 15</th>
 *     <td>CLDR 37</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 14</th>
 *     <td>CLDR 36</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 13</th>
 *     <td>CLDR 35.1</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 12</th>
 *     <td>CLDR 33</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 11</th>
 *     <td>CLDR 33</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 10</th>
 *     <td>CLDR 29</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 9</th>
 *     <td>CLDR 29</td></tr>
 * <tr><th scope="row" style="text-align:left">JDK 8</th>
 *     <td>CLDR 21.0.1</td></tr>
 * </tbody>
 * </table>
 *
 * @since        1.6
 */
public abstract class LocaleServiceProvider {

    /**
     * Initializes a new locale service provider.
     */
    protected LocaleServiceProvider() {}

    /**
     * {@return an array of all locales for which this locale service provider
     * can provide localized objects or names}
     *
     * This information is used to compose {@code getAvailableLocales()}
     * values of the locale-dependent services, such as
     * {@code DateFormat.getAvailableLocales()}.
     *
     * <p>The array returned by this method should not include two or more
     * {@code Locale} objects only differing in their extensions.
     */
    public abstract Locale[] getAvailableLocales();

    /**
     * Returns {@code true} if the given {@code locale} is supported by
     * this locale service provider. The given {@code locale} may contain
     * {@linkplain Locale##def_extensions extensions} that should be
     * taken into account for the support determination.
     *
     * <p>The default implementation returns {@code true} if the given {@code locale}
     * is equal to any of the available {@code Locale}s returned by
     * {@link #getAvailableLocales()} with ignoring any extensions in both the
     * given {@code locale} and the available locales. Concrete locale service
     * provider implementations should override this method if those
     * implementations are {@code Locale} extensions-aware. For example,
     * {@code DecimalFormatSymbolsProvider} implementations will need to check
     * extensions in the given {@code locale} to see if any numbering system is
     * specified and can be supported. However, {@code CollatorProvider}
     * implementations may not be affected by any particular numbering systems,
     * and in that case, extensions for numbering systems should be ignored.
     *
     * @param locale a {@code Locale} to be tested
     * @return {@code true} if the given {@code locale} is supported by this
     *         provider; {@code false} otherwise.
     * @throws NullPointerException
     *         if the given {@code locale} is {@code null}
     * @see Locale#hasExtensions()
     * @see Locale#stripExtensions()
     * @since 1.8
     */
    public boolean isSupportedLocale(Locale locale) {
        locale = locale.stripExtensions(); // throws NPE if locale == null
        for (Locale available : getAvailableLocales()) {
            if (locale.equals(available.stripExtensions())) {
                return true;
            }
        }
        return false;
    }
}