Java compact1, compact2, compact3 java.util.spi Class LocaleServiceProvider（JDK8）

public abstract class LocaleServiceProvider
extends Object

This is the super class of all the locale sensitive service provider interfaces (SPIs).

Locale sensitive service provider interfaces are interfaces that correspond to locale sensitive classes in the java.text and java.util packages. 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 java.text and 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.

Packaging of Locale Sensitive Service Provider Implementations

Implementations of these locale sensitive services are packaged using the Java Extension Mechanism as installed extensions. A provider identifies itself with a provider-configuration file in the resource directory META-INF/services, using the fully qualified provider interface class name as the file 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 '#' ('#'); on each line all characters following the first comment character are ignored. The file must be encoded in UTF-8.

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.

For example, an implementation of the DateFormatProvider class should take the form of a jar file which contains the file:

 META-INF/services/java.text.spi.DateFormatProvider
 

And the file java.text.spi.DateFormatProvider should have a line such as:

 com.foo.DateFormatProviderImpl
 

which is the fully qualified class name of the class implementing DateFormatProvider.

Invocation of Locale Sensitive Services

Locale sensitive factory methods and methods for name retrieval in the java.text and 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 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 locale's extensions are ignored by default. (If locale's extensions should also be checked, the 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 ResourceBundle by default (see 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 Locale extensions. The Java runtime environment must support the root locale for all locale sensitive services in order to guarantee that this process terminates.

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 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.

Starting from JDK8, the search order of locale sensitive services can be configured by using the "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. It is only read at the Java runtime startup, so the later call to System.setProperty() won't affect the order.

For example, if the following is specified in the property:

 java.locale.providers=SPI,JRE
 

where "SPI" represents the locale sensitive services implemented in the installed SPI providers, and "JRE" represents the locale sensitive services in the Java Runtime Environment, the locale sensitive services in the SPI providers are looked up first.

There are two other possible locale sensitive service providers, i.e., "CLDR" which is a provider based on Unicode Consortium's CLDR Project, and "HOST" which is a provider that reflects the user's custom settings in the underlying operating system. These two providers may not be available, depending on the Java Runtime Environment implementation. Specifying "JRE,SPI" is identical to the default behavior, which is compatibile with the prior releases.