Java SPI: Extend Java Functionality Without Modifying Code

Java SPI (Service Provider Interface) is a mechanism that allows different implementations of an interface to be loaded at runtime. In other words, SPI allows you to extend the functionality of a Java application without modifying its source code.

The SPI mechanism is based on the concept of service providers and consumers. A service provider is a class that implements an interface or abstract class and provides a concrete implementation of its methods. A service consumer is a class that uses the services provided by the service provider.

To use the SPI mechanism, you need to define an interface or abstract class that represents the service you want to provide. You also need to define a file called META-INF/services/ that contains the fully-qualified names of the service providers that implement the interface.

At runtime, the Java runtime environment will use the SPI mechanism to load the service providers and instantiate the appropriate implementations of the interface. This allows you to dynamically extend the functionality of your application without recompiling or modifying its source code.

Java SPI is commonly used in frameworks such as Hibernate and Log4j to allow developers to plug in custom implementations of certain functionality.