A small demo of the JDK ServiceLoader mechanism.
The java.util.ServiceLoader class was added in JDK 6 to provide support
for the de-facto service provider loading mechanism that has been used by
Java extensions for many years.
A service is defined as a set of interfaces and (often abstract) classes;
i.e. a service provides an API for some functionality. An example of a
service is the JavaMail (javax.mail) API. A service can have one or
more providers that implement all or part of the API, perhaps in different
ways.
A service provider specifies some metadata in plain text files in
the META-INF/services class path container. The name of each text
file specifies the fully-qualified class name of an interface or class
specified in the service API. The file contents consist of a single line
of text that specifies the fully-qualified class name of a class that
implements or extends the type specified in the file name. Each
implementation class must have a public no-arg constructor that is used
by the ServiceLoader to construct an instance of the class.
An application that wishes to use a service needs a mechanism to locate
a suitable provider for the service. The ServiceLoader mechanism provides
underlying support for a service locator object. The locator is used to
find a suitable provider for a given service. The ServiceLoader class
provides methods that can locate implementations of a given interface or
class in the service API on the application class path, using the metadata
specified by one or more service providers. The service locator is
responsible for determining which provider to select when multiple providers
are available.
This demo is Maven project that consists of several submodules:
- service-loader-demo -- is the top-level project
- service-loader-api -- provides the service API for a simple service that provides some sort of character encoding/decoding capability
- service-provider-standard -- is a service provider implementation
- service-provider-alternate -- is a service provider implementation
- service-provider-main -- is a simple
mainthat uses theCodecLocatordefined in the service API to locate codecs for two different character encodings