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service-loader-demo

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 main that uses the CodecLocator defined in the service API to locate codecs for two different character encodings