How to Build a Plugin‑Based Service Framework in Java Using SPI

Background: When customizing business logic, different sites require different interface implementations, so the service must be provided in a plug‑in manner.

Solution analysis:

Considered OSGi, but abandoned due to its heavyweight nature.

Considered dynamic loading via a custom class loader, but it was complex.

Adopted Java Service Provider Interface (SPI), which is lightweight and simple, and chosen as the preferred solution.

How to implement the plugin framework with Java SPI:

Create the service interface project SPI-Service that defines an abstract class or interface for all custom services.

Create the implementation project SPI-CN-Service that provides concrete implementations, packaging them as JARs with the META-INF/services file.

Create a client project SPI-Service-Client that uses ServiceLoader to discover and invoke the services. Add the JARs from steps 1 and 2 to its classpath.

Code examples:

package hxb.spi.service;

public abstract class ITimeService {
    private String serviceName;
    public abstract String getCurrentTime();
    public String getServiceName() { return serviceName; }
    public void setServiceName(String serviceName) { this.serviceName = serviceName; }
}

package hxb.spi.service;

import java.text.SimpleDateFormat;
import java.util.Date;

public class CNTimeService extends ITimeService {
    public CNTimeService() { super.setServiceName("CN-Time-Service"); }
    @Override
    public String getCurrentTime() {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        return getServiceName() + ":" + sdf.format(new Date());
    }
}

package hxb.spi.service;

import java.text.SimpleDateFormat;
import java.util.Date;

public class USTimeService extends ITimeService {
    public USTimeService() { super.setServiceName("US-Time-Service"); }
    @Override
    public String getCurrentTime() {
        SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");
        return getServiceName() + ":" + sdf.format(new Date());
    }
}

// META-INF/services/hxb.spi.service.ITimeService
hxb.spi.service.CNTimeService
hxb.spi.service.USTimeService

package hxb.spi.test;

import hxb.spi.service.ITimeService;
import java.util.ServiceLoader;

public class SPIClient {
    public static void main(String[] args) {
        ServiceLoader<ITimeService> sloader = ServiceLoader.load(ITimeService.class);
        for (ITimeService iTimeService : sloader) {
            System.out.println(iTimeService.getCurrentTime());
        }
    }
}

The diagram below illustrates the project relationships.