Master Java Plugin Architecture: From ServiceLoader to Spring Factories

1. Introduction

Plugins help decouple modules, improve extensibility, and simplify third‑party integration.

1.1 Benefits of using plugins

Module decoupling

Enhanced extensibility and openness

Easy third‑party integration

2. Common implementation approaches in Java

2.1 ServiceLoader (SPI)

Java's ServiceLoader loads implementations declared in META‑INF/services files.

public interface MessagePlugin {
    String sendMsg(Map msgMap);
}

public class AliyunMsg implements MessagePlugin {
    @Override
    public String sendMsg(Map msgMap) {
        System.out.println("aliyun sendMsg");
        return "aliyun sendMsg";
    }
}

public class TencentMsg implements MessagePlugin {
    @Override
    public String sendMsg(Map msgMap) {
        System.out.println("tencent sendMsg");
        return "tencent sendMsg";
    }
}

The configuration file META-INF/services/com.example.MessagePlugin lists the implementation class names.

2.2 Custom configuration + reflection

Define a properties file that lists implementation class names, load the file, and instantiate classes via Class.forName.

@ConfigurationProperties("impl")
public class ClassImpl {
    private String name;
    private String[] clazz;
}

2.3 Loading JARs from a directory

Read JAR files from a designated lib folder, create a URLClassLoader, and invoke methods reflectively.

URL url = path.toURI().toURL();
URLClassLoader loader = (URLClassLoader) ClassLoader.getSystemClassLoader();
Method method = URLClassLoader.class.getMethod("sendMsg", Map.class);
method.invoke(loader, url);

3. Spring Boot plugin mechanism

3.1 Spring’s SPI via META-INF/spring.factories

SpringFactoriesLoader scans META-INF/spring.factories to obtain implementations of a given interface.

List<SmsPlugin> plugins = SpringFactoriesLoader.loadFactories(SmsPlugin.class, null);
for (SmsPlugin p : plugins) {
    p.sendMessage(msg);
}

3.2 Example

Define SmsPlugin interface, two implementations ( BizSmsImpl, SystemSmsImpl), and register them in spring.factories:

com.example.SmsPlugin=\
com.example.SystemSmsImpl,\
com.example.BizSmsImpl

A controller can invoke all plugins with a single endpoint.

4. Practical case

A micro‑service ( biz‑pp) defines MessagePlugin. Two separate modules ( bitpt and miz‑pt) implement the interface for Aliyun and Tencent SMS. The main service loads the appropriate plugin based on a configuration property and falls back to a default implementation if none is found.

MessagePlugin plugin = PluginFactory.getTargetPlugin(msgType);
if (plugin != null) {
    return plugin.sendMsg(params);
}
return defaultSmsService.sendMsg(params);