Loading services with different fields using ServiceLoader in Java

Question

I am trying to use Service loader to load all the implementations of an Interface. However I am not unable get it working when Service implementation has private fields.

Here are my service and implementation setup.

public interface OrderProcessor {
  
    void process()
            
}

And in implentation, I need this field to be initialised.

public class OrderProcessorImpl implements OrderProcessor {

    private final int maxOrders;
    

    public OrderStreamProcessorImpl(int maxOrders) {
        this.maxOrders = maxOrders;
    }


    @Override
    public void process() throws IOException {
       // Logic which involves maxOrders.

    }
}

Here's how I am trying to load this service using ServiceLoaderAPI.

public OrderProcessor createProcessor(int maxOrders) {
        var factories =
                ServiceLoader.load(OrderProcessor.class);

        if(factories.hasNext()){
            return factories.next();
        } else {
            throw new IllegalStateException("No OrderProcessor found");
        }

    }

I also tried using

public static OrderProcessorImpl provider()

but this doesnt work with params.

How do I solve this issue ? Or ServiceLoader API is not the right option in this use case?

Thanks

Answer 1

You have missed out a factory interface which enables the service to create instances of an order processor. See details of ServiceLoader.

You would need two interfaces defining the service, for the factory and a processor. The ServiceLoader will provide instances of all registered Factory implementations:

public interface OrderProcessor {
    void process() throws IOException;
}

public interface OrderProcessorFactory {
    OrderProcessor createProcessor(int maxOrders);
}

Then you can implement 2 classes supporting the factory and processor:

public class OrderProcessorFactoryImpl implements OrderProcessorFactory {

    public OrderProcessorFactoryImpl() {
        System.out.println("OrderProcessorFactoryImpl()");
    }

    public OrderProcessor createProcessor(int maxOrders)
    {
        System.out.println("OrderProcessorFactoryImpl.createProcessor("+maxOrders+")");
        return new OrderProcessorImpl(maxOrders);
    }
}

public class OrderProcessorImpl implements OrderProcessor {

    private final int maxOrders;

    public OrderProcessorImpl(int maxOrders) {
        System.out.println("OrderProcessorImpl("+maxOrders+")");
        this.maxOrders = maxOrders;
    }

    @Override
    public void process() throws IOException {
        System.out.println("OrderProcessorImpl.process() where maxOrders is "+maxOrders);
    }
}

Add a file META-INF/services/packagenameof.theinterface.OrderProcessorFactory to register your implementation which contains one line:

yourpackage.OrderProcessorFactoryImpl

Then you can lookup a factory and instantiate an order processor:

ServiceLoader<OrderProcessorFactory> loader = ServiceLoader.load(OrderProcessorFactory.class);
System.out.println("loader="+loader);

for (OrderProcessorFactory factory : loader) {
    OrderProcessor max999 = factory.createProcessor(999);
    System.out.println("max999="+max999);
    if (max999 != null)
        max999.process();
}

Should print:

loader=java.util.ServiceLoader[stackoverflow.service.OrderFactory]
OrderFactoryImpl()
OrderFactoryImpl.createProcessor(999)
OrderProcessorImpl(999)
max999=yourpackage.OrderProcessorImpl@7699a589
OrderProcessorImpl.process maxOrders=999