BusinessRuleProviderFactory.java

package org.drools.compiler;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.util.Enumeration;

import org.drools.CheckedDroolsException;
import org.drools.util.ServiceRegistryImpl;

public class BusinessRuleProviderFactory {
	private static BusinessRuleProviderFactory instance = new BusinessRuleProviderFactory();
	private BusinessRuleProvider provider;

	private BusinessRuleProviderFactory() {
	}

	public static BusinessRuleProviderFactory getInstance() {
		return instance;
	}

	public BusinessRuleProvider getProvider() throws CheckedDroolsException {
		if (null == provider)
			provider = loadProvider();
		return provider;
	}

	private BusinessRuleProvider loadProvider() throws CheckedDroolsException {
		String interfaceName = BusinessRuleProvider.class.getName();
		try {
			URL systemResource = null;
			for (Enumeration<URL> systemResources = ClassLoader
					.getSystemResources("META-INF/services/" + interfaceName); systemResources
					.hasMoreElements();) {
				if (null != systemResource)
					throwMultipleImplementationsDetected();
				systemResource = systemResources.nextElement();
			}

			if (systemResource == null) {
				throwNoImplementationFound();
			}

        	BufferedReader reader = new BufferedReader(new InputStreamReader(systemResource.openStream()));
			String className = null;
			for (String currentName; (currentName = reader.readLine()) != null;) {
				if (className != null) {
					throwMultipleImplementationsDetected();
				}
				className = currentName;
			}

			if (null == className) {
				throwNoImplementationFound();
			}

        	ServiceRegistryImpl.getInstance().addDefault(BusinessRuleProvider.class, className);
			return ServiceRegistryImpl.getInstance().get(BusinessRuleProvider.class);
        } catch (IOException e) {
			throw new CheckedDroolsException("Error obtaining " + interfaceName, e);
        }
	}

	private void throwNoImplementationFound() throws CheckedDroolsException {
		throw new CheckedDroolsException("Unable to find implementation for BusinessRuleProvider");
	}

	private void throwMultipleImplementationsDetected() {
		throw new IllegalStateException("multiple BusinessRuleProvider implementations detected");
	}
}