A registry for service provider instances.
A service is a well-known set of interfaces and (usually abstract) classes. A service provider is a specific implementation of a service. The classes in a provider typically implement the interface or subclass the class defined by the service itself.
Service providers are stored in one or more categories, each of which is defined by a class of interface (described by a Class object) that all of its members must implement. The set of categories may be changed dynamically.
Only a single instance of a given leaf class (that is, the actual class returned by getClass(), as opposed to any inherited classes or interfaces) may be registered. That is, suppose that the com.mycompany.mypkg.GreenServiceProvider class implements the com.mycompany.mypkg.MyService interface. If a GreenServiceProvider instance is registered, it will be stored in the category defined by the MyService class. If a new instance of GreenServiceProvider is registered, it will replace the previous instance. In practice, service provider objects are usually singletons so this behavior is appropriate.
To declare a service provider, a services subdirectory is placed within the META-INF directory that is present in every JAR file. This directory contains a file for each service provider interface that has one or more implementation classes present in the JAR file. For example, if the JAR file contained a class named com.mycompany.mypkg.MyServiceImpl which implements the javax.someapi.SomeService interface, the JAR file would contain a file named:
META-INF/services/javax.someapi.SomeService
containing the line:
com.mycompany.mypkg.MyService
The service provider classes should be to be lightweight and quick to load. Implementations of these interfaces should avoid complex dependencies on other classes and on native code. The usual pattern for more complex services is to register a lightweight proxy for the heavyweight service.
An application may customize the contents of a registry as it sees fit, so long as it has the appropriate runtime permission.
For more details on declaring service providers, and the JAR format in general, see the JAR File Specification.
@see RegisterableService