Understanding Java Service Provider Interface (SPI): Introduction, Example, and Implementation Details

Java's Service Provider Interface (SPI) is a mechanism that decouples an interface from its implementations, allowing the system to discover and load providers at runtime without hard‑coded dependencies.

The tutorial builds four Maven projects: spi-interface (defines Person ), spi-impl1 (implements Teacher ), spi-impl2 (implements Student ), and spi-test (loads implementations via ServiceLoader ).

The interface is defined as:

package com.jd.spi;
public interface Person {
    String favorite();
}

Implementation classes:

package com.jd.spi;
public class Teacher implements Person {
    public String favorite() { return "老师喜欢给学生上课"; }
}

package com.jd.spi;
public class Student implements Person {
    public String favorite() { return "学生喜欢努力学习"; }
}

Each implementation module contains a META-INF/services/com.jd.spi.Person file listing the fully‑qualified class name of its provider, enabling ServiceLoader to locate them.

The test module adds Maven dependencies on the three modules and includes a test class:

package com.jd.spi;
import java.util.Iterator;
import java.util.ServiceLoader;
public class SPITest {
    public static void main(String[] args) {
        ServiceLoader
loader = ServiceLoader.load(Person.class);
        for (Iterator
it = loader.iterator(); it.hasNext(); ) {
            Person p = it.next();
            System.out.println(p.favorite());
        }
    }
}

Running the test prints the favorite strings from both Teacher and Student , confirming that SPI successfully discovered both providers.

The article then examines the core of ServiceLoader : the static PREFIX = "META-INF/services/" , the providers cache, and the lookupIterator . It shows the iterator implementation, the lazy loading logic, and the reflective instantiation performed in nextService() , including security checks via AccessController .

Finally, the workflow is summarized: create the service interface, provide implementations, add configuration files under META-INF/services , and use ServiceLoader.load() to iterate over providers. Key points include the requirement for a no‑arg constructor in providers and the necessity of placing the fully‑qualified interface name file in the classpath.