/*
 * Copyright 2010 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.gradle.internal.service;

import org.gradle.api.Action;
import org.gradle.api.Nullable;
import org.gradle.api.specs.Spec;
import org.gradle.internal.Factory;
import org.gradle.internal.concurrent.CompositeStoppable;
import org.gradle.internal.concurrent.Stoppable;
import org.gradle.internal.reflect.JavaReflectionUtil;

import java.io.Closeable;
import java.lang.reflect.*;
import java.util.*;

/**
 * A hierarchical {@link ServiceRegistry} implementation.
 *
 * <p>Subclasses can register services by:</p>
 *
 * <li>Calling {@link #add(Class, Object)} to register a service instance.</li>
 *
 * <li>Calling {@link #addProvider(Object)} to register a service provider bean. A provider bean may have factory, decorator and configuration methods as described below.</li>
 *
 * <li>Adding a factory method. A factory method should have a name that starts with 'create', and have a non-void return type. For example, <code>protected SomeService
 * createSomeService() { .... }</code>. Parameters are injected using services from this registry or its parents.</li>
 *
 * <li>Adding a decorator method. A decorator method should have a name that starts with 'decorate', take a single parameter, and a have a non-void return type. Before invoking the method, the
 * parameter is located in the parent service registry and then passed to the method.</li>
 *
 * <li>Adding a configure method. A configure method should be called 'configure', take a {@link ServiceRegistration} parameter, and a have a void return type. Additional parameters
 * are injected using services from this registry or its parents.</li>
 *
 * </ul>
 *
 * <p>Service instances are created on demand. {@link #getFactory(Class)} looks for a service instance which implements {@code Factory<T>} where {@code T} is the expected type.</p>
 *
 * <p>Service instances and factories are closed when the registry that created them is closed using {@link #close()}. If a service instance or factory implements
 * {@link java.io.Closeable} or {@link org.gradle.internal.concurrent.Stoppable} then the appropriate close() or stop() method is called. Instances are closed in
 * reverse dependency order.</p>
 *
 * <p>Service registries are arranged in a hierarchy. If a service of a given type cannot be located, the registry uses its parent registry, if any, to locate the service.</p>
 */
public class DefaultServiceRegistry implements ServiceRegistry, Closeable {
    private final Object lock = new Object();
    private final CompositeProvider allServices = new CompositeProvider();
    private final OwnServices ownServices;
    private final CompositeProvider parentServices;
    private final String displayName;
    private boolean closed;

    public DefaultServiceRegistry() {
        this(null, Collections.<ServiceRegistry>emptyList());
    }

    public DefaultServiceRegistry(String displayName) {
        this(displayName, Collections.<ServiceRegistry>emptyList());
    }

    public DefaultServiceRegistry(ServiceRegistry... parents) {
        this(null, parents);
    }

    public DefaultServiceRegistry(String displayName, ServiceRegistry... parents) {
        this(displayName, Arrays.asList(parents));
    }

    public DefaultServiceRegistry(String displayName, Collection<? extends ServiceRegistry> parents) {
        this.displayName = displayName != null ? displayName : getClass().getSimpleName();
        this.parentServices = parents.isEmpty() ? null : new CompositeProvider();
        this.ownServices = new OwnServices();
        allServices.providers.add(ownServices);
        if (parentServices != null) {
            allServices.providers.add(parentServices);
            for (ServiceRegistry parent : parents) {
                parentServices.providers.add(new ParentServices(parent));
            }
        }

        findProviderMethods(this);
    }

    /**
     * Creates a service registry that uses the given providers.
     */
    public static ServiceRegistry create(Object... providers) {
        DefaultServiceRegistry registry = new DefaultServiceRegistry();
        for (Object provider : providers) {
            registry.addProvider(provider);
        }
        return registry;
    }

    @Override
    public String toString() {
        return displayName;
    }

    private void findProviderMethods(Object target) {
        Set<String> methods = new HashSet<String>();
        for (Class<?> type = target.getClass(); type != Object.class; type = type.getSuperclass()) {
            findDecoratorMethods(target, type, methods, ownServices);
            findFactoryMethods(target, type, methods, ownServices);
        }
        findConfigureMethod(target);
    }

    private void findConfigureMethod(Object target) {
        for (Class<?> type = target.getClass(); type != Object.class; type = type.getSuperclass()) {
            for (Method method : type.getDeclaredMethods()) {
                if (!method.getName().equals("configure")) {
                    continue;
                }
                if (!method.getReturnType().equals(Void.TYPE)) {
                    throw new ServiceLookupException(String.format("Method %s.%s() must return void.", type.getSimpleName(), method.getName()));
                }
                Object[] params = new Object[method.getGenericParameterTypes().length];
                DefaultLookupContext context = new DefaultLookupContext();
                for (int i = 0; i < method.getGenericParameterTypes().length; i++) {
                    Type paramType = method.getGenericParameterTypes()[i];
                    if (paramType.equals(ServiceRegistration.class)) {
                        params[i] = newRegistration();
                    } else {
                        ServiceProvider paramProvider = context.find(paramType, allServices);
                        if (paramProvider == null) {
                            throw new ServiceLookupException(String.format("Cannot configure services using %s.%s() as required service of type %s is not available.",
                                    method.getDeclaringClass().getSimpleName(),
                                    method.getName(),
                                    format(paramType)));
                        }
                        params[i] = paramProvider.get();
                    }
                }
                try {
                    invoke(method, target, params);
                } catch (Exception e) {
                    throw new ServiceLookupException(String.format("Could not configure services using %s.%s().",
                            method.getDeclaringClass().getSimpleName(),
                            method.getName()), e);
                }
                return;
            }
        }
    }

    private void findFactoryMethods(Object target, Class<?> type, Set<String> factoryMethods, OwnServices ownServices) {
        for (Method method : type.getDeclaredMethods()) {
            if (method.getName().startsWith("create")
                    && !Modifier.isStatic(method.getModifiers())) {
                if (method.getReturnType().equals(Void.TYPE)) {
                    throw new ServiceLookupException(String.format("Method %s.%s() must not return void.", type.getSimpleName(), method.getName()));
                }
                if (factoryMethods.add(method.getName())) {
                    ownServices.add(new FactoryMethodService(target, method));
                }
            }
        }
    }

    private void findDecoratorMethods(Object target, Class<?> type, Set<String> decoratorMethods, OwnServices ownServices) {
        for (Method method : type.getDeclaredMethods()) {
            if (method.getName().startsWith("create")
                    && method.getParameterTypes().length == 1
                    && method.getParameterTypes()[0].equals(method.getReturnType())) {
                if (parentServices == null) {
                    throw new ServiceLookupException(String.format("Cannot use decorator method %s.%s() when no parent registry is provided.", type.getSimpleName(), method.getName()));
                }
                if (method.getReturnType().equals(Void.TYPE)) {
                    throw new ServiceLookupException(String.format("Method %s.%s() must not return void.", type.getSimpleName(), method.getName()));
                }
                if (decoratorMethods.add(method.getName())) {
                    ownServices.add(new DecoratorMethodService(target, method));
                }
            }
        }
    }

    /**
     * Adds services to this container using the given action.
     */
    public void register(Action<? super ServiceRegistration> action) {
        action.execute(newRegistration());
    }

    private ServiceRegistration newRegistration() {
        return new ServiceRegistration(){
            public <T> void add(Class<T> serviceType, T serviceInstance) {
                DefaultServiceRegistry.this.add(serviceType, serviceInstance);
            }

            public void add(Class<?> serviceType) {
                ownServices.add(new ConstructorService(serviceType));
            }

            public void addProvider(Object provider) {
                DefaultServiceRegistry.this.addProvider(provider);
            }
        };
    }

    /**
     * Adds a service to this registry. The given object is closed when this registry is closed.
     */
    public <T> DefaultServiceRegistry add(Class<T> serviceType, final T serviceInstance) {
        ownServices.add(new FixedInstanceService<T>(serviceType, serviceInstance));
        return this;
    }

    /**
     * Adds a service provider bean to this registry. This provider may define factory and decorator methods.
     */
    public DefaultServiceRegistry addProvider(Object provider) {
        findProviderMethods(provider);
        return this;
    }

    /**
     * Closes all services for this registry. For each service, if the service has a public void close() or stop() method, that method is called to close the service.
     */
    public void close() {
        synchronized (lock) {
            try {
                CompositeStoppable.stoppable(allServices).stop();
            } finally {
                closed = true;
            }
        }
    }

    public boolean isClosed() {
        return closed;
    }

    private static String format(Type type) {
        if (type instanceof Class) {
            Class<?> aClass = (Class) type;
            return aClass.getSimpleName();
        } else if (type instanceof ParameterizedType) {
            ParameterizedType parameterizedType = (ParameterizedType) type;
            StringBuilder builder = new StringBuilder();
            builder.append(format(parameterizedType.getRawType()));
            builder.append("<");
            for (int i = 0; i < parameterizedType.getActualTypeArguments().length; i++) {
                Type typeParam = parameterizedType.getActualTypeArguments()[i];
                if (i > 0) {
                    builder.append(", ");
                }
                builder.append(format(typeParam));
            }
            builder.append(">");
            return builder.toString();
        }

        return type.toString();
    }

    public <T> List<T> getAll(Class<T> serviceType) throws ServiceLookupException {
        synchronized (lock) {
            if (closed) {
                throw new IllegalStateException(String.format("Cannot locate service of type %s, as %s has been closed.", format(serviceType), displayName));
            }
            List<T> result = new ArrayList<T>();
            DefaultLookupContext context = new DefaultLookupContext();
            allServices.getAll(context, serviceType, result);
            return result;
        }
    }

    public <T> T get(Class<T> serviceType) throws UnknownServiceException, ServiceLookupException {
        return serviceType.cast(doGet(serviceType));
    }

    public Object get(Type serviceType) throws UnknownServiceException, ServiceLookupException {
        return doGet(serviceType);
    }

    private Object doGet(Type serviceType) throws IllegalArgumentException {
        synchronized (lock) {
            if (closed) {
                throw new IllegalStateException(String.format("Cannot locate service of type %s, as %s has been closed.", format(serviceType), displayName));
            }

            DefaultLookupContext context = new DefaultLookupContext();
            ServiceProvider provider = context.find(serviceType, allServices);
            if (provider != null) {
                return provider.get();
            }

            throw new UnknownServiceException(serviceType, String.format("No service of type %s available in %s.", format(serviceType), displayName));
        }
    }

    public <T> Factory<T> getFactory(Class<T> type) {
        synchronized (lock) {
            if (closed) {
                throw new IllegalStateException(String.format("Cannot locate factory for objects of type %s, as %s has been closed.", format(type), displayName));
            }

            DefaultLookupContext context = new DefaultLookupContext();
            ServiceProvider factory = allServices.getFactory(context, type);
            if (factory != null) {
                return (Factory<T>) factory.get();
            }

            throw new UnknownServiceException(type, String.format("No factory for objects of type %s available in %s.", format(type), displayName));
        }
    }

    public <T> T newInstance(Class<T> type) {
        return getFactory(type).create();
    }

    private static Object invoke(Method method, Object target, Object... args) {
        return JavaReflectionUtil.method(target, Object.class, method).invoke(target, args);
    }

    interface ServiceProvider {
        String getDisplayName();

        Object get();

        void requiredBy(Provider provider);
    }

    interface Provider extends Stoppable {
        /**
         * Locates a service instance of the given type. Returns null if this provider does not provide a service of this type.
         */
        ServiceProvider getService(LookupContext context, TypeSpec serviceType);

        /**
         * Locates a factory for services of the given type. Returns null if this provider does not provide any services of this type.
         */
        ServiceProvider getFactory(LookupContext context, Class<?> type);

        <T> void getAll(LookupContext context, Class<T> serviceType, List<T> result);
    }

    private class OwnServices implements Provider {
        private final List<Provider> providers = new ArrayList<Provider>();

        public ServiceProvider getFactory(LookupContext context, Class<?> type) {
            List<ServiceProvider> candidates = new ArrayList<ServiceProvider>();
            for (Provider provider : providers) {
                ServiceProvider factory = provider.getFactory(context, type);
                if (factory != null) {
                    candidates.add(factory);
                }
            }

            if (candidates.size() == 0) {
                return null;
            }
            if (candidates.size() == 1) {
                return candidates.get(0);
            }

            Set<String> descriptions = new TreeSet<String>();
            for (ServiceProvider candidate : candidates) {
                descriptions.add(candidate.getDisplayName());
            }

            Formatter formatter = new Formatter();
            formatter.format("Multiple factories for objects of type %s available in %s:", format(type), displayName);
            for (String description : descriptions) {
                formatter.format("%n   - %s", description);
            }
            throw new ServiceLookupException(formatter.toString());
        }

        public ServiceProvider getService(LookupContext context, TypeSpec serviceType) {
            List<ServiceProvider> candidates = new ArrayList<ServiceProvider>();
            for (Provider provider : providers) {
                ServiceProvider service = provider.getService(context, serviceType);
                if (service != null) {
                    candidates.add(service);
                }
            }

            if (candidates.size() == 0) {
                return null;
            }
            if (candidates.size() == 1) {
                return candidates.get(0);
            }

            Set<String> descriptions = new TreeSet<String>();
            for (ServiceProvider candidate : candidates) {
                descriptions.add(candidate.getDisplayName());
            }

            Formatter formatter = new Formatter();
            formatter.format("Multiple services of type %s available in %s:", format(serviceType.getType()), displayName);
            for (String description : descriptions) {
                formatter.format("%n   - %s", description);
            }
            throw new ServiceLookupException(formatter.toString());
        }

        public <T> void getAll(LookupContext context, Class<T> serviceType, List<T> result) {
            for (Provider provider : providers) {
                provider.getAll(context, serviceType, result);
            }
        }

        public void stop() {
            CompositeStoppable.stoppable(providers).stop();
        }

        public void add(Provider provider) {
            this.providers.add(provider);
        }
    }

    private static abstract class ManagedObjectProvider<T> implements Provider {
        private T instance;
        private final Set<Provider> dependents = new HashSet<Provider>();

        protected void setInstance(T instance) {
            this.instance = instance;
        }

        public T getInstance() {
            if (instance == null) {
                instance = create();
                assert instance != null : String.format("create() of %s returned null", toString());
            }
            return instance;
        }

        protected abstract T create();

        public void requiredBy(Provider provider) {
            dependents.add(provider);
        }

        public void stop() {
            try {
                if (instance != null) {
                    CompositeStoppable.stoppable(dependents).add(instance).stop();
                }
            } finally {
                dependents.clear();
                instance = null;
            }
        }
    }

    private static abstract class SingletonService extends ManagedObjectProvider<Object> implements ServiceProvider {
        final Type serviceType;
        final Class serviceClass;
        boolean bound;

        SingletonService(Type serviceType) {
            this.serviceType = serviceType;
            serviceClass = toClass(serviceType);
        }

        @Override
        public String toString() {
            return getDisplayName();
        }

        public Object get() {
            return getInstance();
        }

        private ServiceProvider prepare(LookupContext context) {
            if (!bound) {
                bind(context);
                bound = true;
            }
            return this;
        }

        protected void bind(LookupContext context) {
        }

        public ServiceProvider getService(LookupContext context, TypeSpec serviceType) {
            if (!serviceType.isSatisfiedBy(this.serviceType)) {
                return null;
            }
            return prepare(context);
        }

        public <T> void getAll(LookupContext context, Class<T> serviceType, List<T> result) {
            if (serviceType.isAssignableFrom(this.serviceClass)) {
                result.add(serviceType.cast(prepare(context).get()));
            }
        }

        public ServiceProvider getFactory(LookupContext context, Class<?> elementType) {
            if (!isFactory(serviceType, elementType)) {
                return null;
            }
            return prepare(context);
        }

        private boolean isFactory(Type type, Class<?> elementType) {
            Class c = toClass(type);
            if (!Factory.class.isAssignableFrom(c)) {
                return false;
            }

            if (type instanceof ParameterizedType) {
                // Check if type is Factory<? extends ElementType>
                ParameterizedType parameterizedType = (ParameterizedType) type;
                if (parameterizedType.getRawType().equals(Factory.class)) {
                    Type actualType = parameterizedType.getActualTypeArguments()[0];
                    if (actualType instanceof Class<?> && elementType.isAssignableFrom((Class<?>) actualType)) {
                        return true;
                    }
                }
            }

            // Check if type extends Factory<? extends ElementType>
            for (Type interfaceType : c.getGenericInterfaces()) {
                if (isFactory(interfaceType, elementType)) {
                    return true;
                }
            }

            return false;
        }

        private Class toClass(Type type) {
            if (type instanceof Class) {
                return (Class) type;
            } else {
                ParameterizedType parameterizedType = (ParameterizedType) type;
                return (Class) parameterizedType.getRawType();
            }
        }
    }

    private abstract class FactoryService extends SingletonService {
        private ServiceProvider[] paramProviders;

        protected FactoryService(Type serviceType) {
            super(serviceType);
        }

        protected abstract Type[] getParameterTypes();

        protected abstract Member getFactory();

        @Override
        protected void bind(LookupContext context) {
            Type[] parameterTypes = getParameterTypes();
            paramProviders = new ServiceProvider[parameterTypes.length];
            for (int i = 0; i < parameterTypes.length; i++) {
                Type paramType = parameterTypes[i];
                try {
                    if (paramType.equals(ServiceRegistry.class)) {
                        paramProviders[i] = getThisAsProvider();
                    } else {
                        ServiceProvider paramProvider = context.find(paramType, allServices);
                        if (paramProvider == null) {
                            throw new ServiceCreationException(String.format("Cannot create service of type %s using %s.%s() as required service of type %s is not available.",
                                    format(serviceType),
                                    getFactory().getDeclaringClass().getSimpleName(),
                                    getFactory().getName(),
                                    format(paramType)));

                        }
                        paramProviders[i] = paramProvider;
                        paramProvider.requiredBy(this);
                    }
                } catch (ServiceValidationException e) {
                    throw new ServiceCreationException(String.format("Cannot create service of type %s using %s.%s() as there is a problem with parameter #%s of type %s.",
                            format(serviceType),
                            getFactory().getDeclaringClass().getSimpleName(),
                            getFactory().getName(),
                            i+1,
                            format(paramType)), e);
                }
            }
        }

        @Override
        protected Object create() {
            Object[] params = assembleParameters();
            Object result = invokeMethod(params);
            // Can discard the state required to create instance
            paramProviders = null;
            return result;
        }

        private Object[] assembleParameters() {
            Object[] params = new Object[paramProviders.length];
            for (int i = 0; i < paramProviders.length; i++) {
                ServiceProvider paramProvider = paramProviders[i];
                params[i] = paramProvider.get();
            }
            return params;
        }

        protected abstract Object invokeMethod(Object[] params);
    }

    private class FactoryMethodService extends FactoryService {
        private final Method method;
        private Object target;

        public FactoryMethodService(Object target, Method method) {
            super(method.getGenericReturnType());
            this.target = target;
            this.method = method;
        }

        public String getDisplayName() {
            return String.format("Service %s at %s.%s()", format(method.getGenericReturnType()), method.getDeclaringClass().getSimpleName(), method.getName());
        }

        protected Type[] getParameterTypes() {
            return method.getGenericParameterTypes();
        }

        @Override
        protected Member getFactory() {
            return method;
        }

        protected Object invokeMethod(Object[] params) {
            Object result;
            try {
                result = invoke(method, target, params);
            } catch (Exception e) {
                throw new ServiceCreationException(String.format("Could not create service of type %s using %s.%s().",
                        format(serviceType),
                        method.getDeclaringClass().getSimpleName(),
                        method.getName()),
                        e);
            }
            try {
                if (result == null) {
                    throw new ServiceCreationException(String.format("Could not create service of type %s using %s.%s() as this method returned null.",
                            format(serviceType),
                            method.getDeclaringClass().getSimpleName(),
                            method.getName()));
                }
                return result;
            } finally {
                // Can discard the state required to create instance
                target = null;
            }
        }
    }

    private ServiceProvider getThisAsProvider() {
        return new ServiceProvider() {
            public String getDisplayName() {
                return String.format("ServiceRegistry %s", displayName);
            }

            public Object get() {
                return DefaultServiceRegistry.this;
            }

            public void requiredBy(Provider provider) {
            }
        };
    }

    private static class FixedInstanceService<T> extends SingletonService {
        public FixedInstanceService(Class<T> serviceType, T serviceInstance) {
            super(serviceType);
            setInstance(serviceInstance);
        }

        public String getDisplayName() {
            return String.format("Service %s with implementation %s", format(serviceType), getInstance());
        }

        @Override
        protected Object create() {
            throw new UnsupportedOperationException();
        }
    }

    private class ConstructorService extends FactoryService {
        private final Constructor<?> constructor;

        private ConstructorService(Class<?> serviceType) {
            super(serviceType);
            Constructor<?>[] constructors = serviceType.getDeclaredConstructors();
            if (constructors.length != 1) {
                throw new ServiceValidationException(String.format("Expected a single constructor for %s.", format(serviceType)));
            }
            this.constructor = constructors[0];
        }

        @Override
        protected Type[] getParameterTypes() {
            return constructor.getGenericParameterTypes();
        }

        @Override
        protected Member getFactory() {
            return constructor;
        }

        @Override
        protected Object invokeMethod(Object[] params) {
            try {
                return constructor.newInstance(params);
            } catch (InvocationTargetException e) {
                throw new ServiceCreationException(String.format("Could not create service of type %s.", format(serviceType)), e.getCause());
            } catch (Exception e) {
                throw new ServiceCreationException(String.format("Could not create service of type %s.", format(serviceType)), e);
            }
        }

        public String getDisplayName() {
            return String.format("Service %s", format(serviceType));
        }
    }

    private class DecoratorMethodService extends SingletonService {
        private final Method method;
        private Object target;
        private ServiceProvider paramProvider;

        public DecoratorMethodService(Object target, Method method) {
            super(method.getGenericReturnType());
            this.target = target;
            this.method = method;
        }

        public String getDisplayName() {
            return String.format("Service %s at %s.%s()", format(method.getGenericReturnType()), method.getDeclaringClass().getSimpleName(), method.getName());
        }

        @Override
        protected void bind(LookupContext context) {
            Type paramType = method.getGenericParameterTypes()[0];
            DefaultLookupContext parentLookupContext = new DefaultLookupContext();
            paramProvider = parentLookupContext.find(paramType, parentServices);
            if (paramProvider == null) {
                throw new ServiceCreationException(String.format("Cannot create service of type %s using %s.%s() as required service of type %s is not available in parent registries.",
                        format(method.getGenericReturnType()),
                        method.getDeclaringClass().getSimpleName(),
                        method.getName(),
                        format(paramType)));
            }
        }

        @Override
        protected Object create() {
            Object param = paramProvider.get();
            Object result;
            try {
                result = invoke(method, target, param);
            } catch (Exception e) {
                throw new ServiceCreationException(String.format("Could not create service of type %s using %s.%s().",
                        format(method.getGenericReturnType()),
                        method.getDeclaringClass().getSimpleName(),
                        method.getName()),
                        e);
            }
            try {
                if (result == null) {
                    throw new ServiceCreationException(String.format("Could not create service of type %s using %s.%s() as this method returned null.",
                            format(method.getGenericReturnType()),
                            method.getDeclaringClass().getSimpleName(),
                            method.getName()));
                }
                return result;
            } finally {
                // Can discard state required to create instance
                paramProvider = null;
                target = null;
            }
        }
    }

    private class CompositeProvider implements Provider {
        private final List<Provider> providers = new LinkedList<Provider>();

        public ServiceProvider getService(LookupContext context, TypeSpec serviceType) {
            for (Provider provider : providers) {
                ServiceProvider service = provider.getService(context, serviceType);
                if (service != null) {
                    return service;
                }
            }
            return null;
        }

        public ServiceProvider getFactory(LookupContext context, Class<?> type) {
            for (Provider provider : providers) {
                ServiceProvider factory = provider.getFactory(context, type);
                if (factory != null) {
                    return factory;
                }
            }
            return null;
        }

        public <T> void getAll(LookupContext context, Class<T> serviceType, List<T> result) {
            for (Provider provider : providers) {
                provider.getAll(context, serviceType, result);
            }
        }

        public void stop() {
            try {
                CompositeStoppable.stoppable(providers).stop();
            } finally {
                providers.clear();
            }
        }
    }

    private class ParentServices implements Provider {
        private final ServiceRegistry parent;

        private ParentServices(ServiceRegistry parent) {
            this.parent = parent;
        }

        public ServiceProvider getFactory(LookupContext context, Class<?> type) {
            try {
                Factory<?> factory = parent.getFactory(type);
                assert factory != null : String.format("parent returned null for factory type '%s'", type.getName());
                return wrap(factory);
            } catch (UnknownServiceException e) {
                if (!e.getType().equals(type)) {
                    throw e;
                }
            }
            return null;
        }

        public ServiceProvider getService(LookupContext context, TypeSpec serviceType) {
            try {
                Object service = parent.get(serviceType.getType());
                assert service != null : String.format("parent returned null for service type %s", format(serviceType.getType()));
                return wrap(service);
            } catch (UnknownServiceException e) {
                if (!e.getType().equals(serviceType.getType())) {
                    throw e;
                }
            }
            return null;
        }

        private ServiceProvider wrap(final Object instance) {
            return new ServiceProvider() {
                public String getDisplayName() {
                    return String.format("ServiceRegistry %s", parent);
                }

                public Object get() {
                    return instance;
                }

                public void requiredBy(Provider provider) {
                    // Ignore
                }
            };
        }

        public <T> void getAll(LookupContext context, Class<T> serviceType, List<T> result) {
            List<T> services = parent.getAll(serviceType);
            assert services != null : String.format("parent returned null for services of type %s", format(serviceType));
            result.addAll(services);
        }

        public void stop() {
        }
    }

    interface LookupContext {
        @Nullable
        ServiceProvider find(Type type, Provider provider);
    }

    interface TypeSpec extends Spec<Type> {
        Type getType();
    }

    private static class ClassSpec implements TypeSpec {
        private final Class<?> type;

        private ClassSpec(Class<?> type) {
            this.type = type;
        }

        public Type getType() {
            return type;
        }

        public boolean isSatisfiedBy(Type element) {
            if (element instanceof ParameterizedType) {
                ParameterizedType parameterizedType = (ParameterizedType) element;
                if (parameterizedType.getRawType() instanceof Class) {
                    return type.isAssignableFrom((Class) parameterizedType.getRawType());
                }
            } else if(element instanceof Class) {
                Class<?> other = (Class<?>) element;
                return type.isAssignableFrom(other);
            }
            return false;
        }
    }

    private static class ParameterizedTypeSpec implements TypeSpec {
        private final Type type;
        private final TypeSpec rawType;
        private final List<TypeSpec> paramSpecs;

        private ParameterizedTypeSpec(Type type, TypeSpec rawType, List<TypeSpec> paramSpecs) {
            this.type = type;
            this.rawType = rawType;
            this.paramSpecs = paramSpecs;
        }

        public Type getType() {
            return type;
        }

        public boolean isSatisfiedBy(Type element) {
            if (element instanceof ParameterizedType) {
                ParameterizedType parameterizedType = (ParameterizedType) element;
                if (!rawType.isSatisfiedBy(parameterizedType.getRawType())) {
                    return false;
                }
                for (int i = 0; i < parameterizedType.getActualTypeArguments().length; i++) {
                    Type type = parameterizedType.getActualTypeArguments()[i];
                    if (!paramSpecs.get(i).isSatisfiedBy(type)) {
                        return false;
                    }
                }
                return true;
            }
            return false;
        }
    }

    private static class DefaultLookupContext implements LookupContext {
        private final Set<Type> visiting = new HashSet<Type>();

        public ServiceProvider find(Type serviceType, Provider provider) {
            if (!visiting.add(serviceType)) {
                throw new ServiceValidationException(String.format("Cycle in dependencies of service of type %s.", format(serviceType)));
            }
            try {
                if (serviceType instanceof ParameterizedType) {
                    ParameterizedType parameterizedType = (ParameterizedType) serviceType;
                    if (parameterizedType.getRawType().equals(Factory.class)) {
                        Type typeArg = parameterizedType.getActualTypeArguments()[0];
                        if (typeArg instanceof Class) {
                            return provider.getFactory(this, (Class) typeArg);
                        }
                        if (typeArg instanceof WildcardType) {
                            WildcardType wildcardType = (WildcardType) typeArg;
                            if (wildcardType.getLowerBounds().length == 1 && wildcardType.getUpperBounds().length == 1) {
                                if (wildcardType.getLowerBounds()[0] instanceof Class && wildcardType.getUpperBounds()[0].equals(Object.class)) {
                                    return provider.getFactory(this, (Class<Object>) wildcardType.getLowerBounds()[0]);
                                }
                            }
                            if (wildcardType.getLowerBounds().length == 0 && wildcardType.getUpperBounds().length == 1) {
                                if (wildcardType.getUpperBounds()[0] instanceof Class) {
                                    return provider.getFactory(this, (Class<Object>) wildcardType.getUpperBounds()[0]);
                                }
                            }
                        }
                    }
                }

                return provider.getService(this, toSpec(serviceType));
            } finally {
                visiting.remove(serviceType);
            }
        }

        TypeSpec toSpec(Type serviceType) {
            if (serviceType instanceof ParameterizedType) {
                ParameterizedType parameterizedType = (ParameterizedType) serviceType;
                List<TypeSpec> paramSpecs = new ArrayList<TypeSpec>();
                for (Type paramType : parameterizedType.getActualTypeArguments()) {
                    paramSpecs.add(toSpec(paramType));
                }
                return new ParameterizedTypeSpec(serviceType, toSpec(parameterizedType.getRawType()), paramSpecs);
            } else if (serviceType instanceof Class) {
                Class<?> serviceClass = (Class<?>) serviceType;
                if (serviceClass.isArray()) {
                    throw new ServiceValidationException("Locating services with array type is not supported.");
                }
                if (serviceClass.isAnnotation()) {
                    throw new ServiceValidationException("Locating services with annotation type is not supported.");
                }
                return new ClassSpec(serviceClass);
            }

            throw new ServiceValidationException(String.format("Locating services with type %s is not supported.", format(serviceType)));
        }
    }
}
