BuilderConfigurationWrapperFactory

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 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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
 *
 *     https://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
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 */
package org.apache.commons.configuration2.builder;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

import org.apache.commons.configuration2.ConfigurationUtils;
import org.apache.commons.configuration2.ImmutableConfiguration;
import org.apache.commons.configuration2.event.EventSource;
import org.apache.commons.configuration2.ex.ConfigurationException;

/**
 * <p>
 * A class that allows the creation of configuration objects wrapping a {@link ConfigurationBuilder}.
 * </p>
 * <p>
 * Using this class special {@code ImmutableConfiguration} proxies can be created that delegate all method invocations
 * to another {@code ImmutableConfiguration} obtained from a {@code ConfigurationBuilder}. For instance, if there is a
 * configuration {@code c} wrapping the builder {@code builder}, the call {@code c.getString(myKey)} is transformed to
 * {@code builder.getConfiguration().getString(myKey)}.
 * </p>
 * <p>
 * There are multiple use cases for such a constellation. One example is that client code can continue working with
 * {@code ImmutableConfiguration} objects while under the hood builders are used. Another example is that dynamic
 * configurations can be realized in a transparent way: a client holds a single configuration (proxy) object, but the
 * underlying builder may return a different data object on each call.
 * </p>
 *
 * @since 2.0
 */
public class BuilderConfigurationWrapperFactory {

    /**
     * A specialized {@code InvocationHandler} implementation for wrapper configurations. Here the logic of accessing a
     * wrapped builder is implemented.
     */
    private static final class BuilderConfigurationWrapperInvocationHandler implements InvocationHandler {

        /** The wrapped builder. */
        private final ConfigurationBuilder<? extends ImmutableConfiguration> builder;

        /** The level of {@code EventSource} support. */
        private final EventSourceSupport eventSourceSupport;

        /**
         * Creates a new instance of {@code BuilderConfigurationWrapperInvocationHandler}.
         *
         * @param wrappedBuilder the wrapped builder
         * @param evSrcSupport the level of {@code EventSource} support
         */
        public BuilderConfigurationWrapperInvocationHandler(final ConfigurationBuilder<? extends ImmutableConfiguration> wrappedBuilder,
            final EventSourceSupport evSrcSupport) {
            builder = wrappedBuilder;
            eventSourceSupport = evSrcSupport;
        }

        /**
         * Handles a method invocation on the associated builder's configuration object.
         *
         * @param method the method to be invoked
         * @param args method arguments
         * @return the return value of the method
         * @throws Exception if an error occurs
         */
        private Object handleConfigurationInvocation(final Method method, final Object[] args) throws ReflectiveOperationException, ConfigurationException {
            return method.invoke(builder.getConfiguration(), args);
        }

        /**
         * Handles a method invocation on the {@code EventSource} interface. This method evaluates the current
         * {@code EventSourceSupport} object in order to find the appropriate target for the invocation.
         *
         * @param method the method to be invoked
         * @param args method arguments
         * @return the return value of the method
         * @throws ReflectiveOperationException if an error occurs
         */
        private Object handleEventSourceInvocation(final Method method, final Object... args) throws ReflectiveOperationException {
            return method.invoke(EventSourceSupport.DUMMY == eventSourceSupport ? ConfigurationUtils.asEventSource(this, true) : builder, args);
        }

        /**
         * Handles method invocations. This implementation handles methods of two different interfaces:
         * <ul>
         * <li>Methods from the {@code EventSource} interface are handled according to the current support level.</li>
         * <li>Other method calls are delegated to the builder's configuration object.</li>
         * </ul>
         *
         * @param proxy the proxy object
         * @param method the method to be invoked
         * @param args method arguments
         * @return the return value of the method
         * @throws ReflectiveOperationException if an error occurs
         * @throws ConfigurationException if an error occurs
         */
        @Override
        public Object invoke(final Object proxy, final Method method, final Object[] args) throws ReflectiveOperationException, ConfigurationException {
            return EventSource.class.equals(method.getDeclaringClass()) ? handleEventSourceInvocation(method, args)
                : handleConfigurationInvocation(method, args);
        }
    }

    /**
     * Enumerates options for supporting the {@code EventSource} interface in generated
     * {@code ImmutableConfiguration} proxies.
     * <p>
     * Using literals of this class it is possible to specify that a {@code ImmutableConfiguration} object returned by
     * {@code BuilderConfigurationWrapperFactory} also implements the {@code EventSource} interface and how this
     * implementation should work. See the documentation of the single constants for more details.
     * </p>
     */
    public enum EventSourceSupport {

        /**
         * No support of the {@code EventSource} interface. If this option is set, {@code ImmutableConfiguration} objects
         * generated by {@code BuilderConfigurationWrapperFactory} do not implement the {@code EventSource} interface.
         */
        NONE,

        /**
         * Dummy support of the {@code EventSource} interface. This option causes {@code ImmutableConfiguration} objects
         * generated by {@code BuilderConfigurationWrapperFactory} to implement the {@code EventSource} interface, however, this
         * implementation consists only of empty dummy methods without real functionality.
         */
        DUMMY,

        /**
         * {@code EventSource} support is implemented by delegating to the associated {@code ConfigurationBuilder} object. If
         * this option is used, generated {@code ImmutableConfiguration} objects provide a fully functional implementation of
         * {@code EventSource} by delegating to the builder. Because the {@code ConfigurationBuilder} interface extends
         * {@code EventSource} this delegation is always possible.
         */
        BUILDER
    }

    /**
     * Creates a {@code ImmutableConfiguration} object which wraps the specified {@code ConfigurationBuilder}. Each access
     * of the configuration is delegated to a corresponding call on the {@code ImmutableConfiguration} object managed by the
     * builder. This is a convenience method which allows creating wrapper configurations without having to instantiate this
     * class.
     *
     * @param <T> the type of the configuration objects returned by this method
     * @param ifcClass the class of the configuration objects returned by this method; this must be an interface class and
     *        must not be <strong>null</strong>
     * @param builder the wrapped {@code ConfigurationBuilder} (must not be <strong>null</strong>)
     * @param evSrcSupport the level of {@code EventSource} support
     * @return the wrapper configuration
     * @throws IllegalArgumentException if a required parameter is missing
     * @throws org.apache.commons.configuration2.ex.ConfigurationRuntimeException if an error occurs when creating the
     *         result {@code ImmutableConfiguration}
     */
    public static <T extends ImmutableConfiguration> T createBuilderConfigurationWrapper(final Class<T> ifcClass,
        final ConfigurationBuilder<? extends T> builder, final EventSourceSupport evSrcSupport) {
        if (ifcClass == null) {
            throw new IllegalArgumentException("Interface class must not be null.");
        }
        if (builder == null) {
            throw new IllegalArgumentException("Builder must not be null.");
        }

        return ifcClass.cast(Proxy.newProxyInstance(BuilderConfigurationWrapperFactory.class.getClassLoader(), getSupportedInterfaces(ifcClass, evSrcSupport),
            new BuilderConfigurationWrapperInvocationHandler(builder, evSrcSupport)));
    }

    /**
     * Gets an array with the classes the generated proxy has to support.
     *
     * @param ifcClass the class of the configuration objects returned by this method; this must be an interface class and
     *        must not be <strong>null</strong>
     * @param evSrcSupport the level of {@code EventSource} support
     * @return an array with the interface classes to implement
     */
    private static Class<?>[] getSupportedInterfaces(final Class<?> ifcClass, final EventSourceSupport evSrcSupport) {
        return EventSourceSupport.NONE == evSrcSupport ? new Class<?>[] {ifcClass} : new Class<?>[] {EventSource.class, ifcClass};
    }

    /** The current {@code EventSourceSupport} value. */
    private final EventSourceSupport eventSourceSupport;

    /**
     * Creates a new instance of {@code BuilderConfigurationWrapperFactory} setting the default {@code EventSourceSupport}
     * <em>NONE</em>.
     */
    public BuilderConfigurationWrapperFactory() {
        this(EventSourceSupport.NONE);
    }

    /**
     * Creates a new instance of {@code BuilderConfigurationWrapperFactory} and sets the property for supporting the
     * {@code EventSource} interface.
     *
     * @param evSrcSupport the level of {@code EventSource} support
     */
    public BuilderConfigurationWrapperFactory(final EventSourceSupport evSrcSupport) {
        eventSourceSupport = evSrcSupport;
    }

    /**
     * Creates a wrapper {@code ImmutableConfiguration} on top of the specified {@code ConfigurationBuilder}. This
     * implementation delegates to
     * {@link #createBuilderConfigurationWrapper(Class, ConfigurationBuilder, EventSourceSupport)}.
     *
     * @param <T> the type of the configuration objects returned by this method
     * @param ifcClass the class of the configuration objects returned by this method; this must be an interface class and
     *        must not be <strong>null</strong>
     * @param builder the wrapped {@code ConfigurationBuilder} (must not be <strong>null</strong>)
     * @return the wrapper configuration
     * @throws IllegalArgumentException if a required parameter is missing
     * @throws org.apache.commons.configuration2.ex.ConfigurationRuntimeException if an error occurs when creating the
     *         result {@code ImmutableConfiguration}
     */
    public <T extends ImmutableConfiguration> T createBuilderConfigurationWrapper(final Class<T> ifcClass, final ConfigurationBuilder<? extends T> builder) {
        return createBuilderConfigurationWrapper(ifcClass, builder, getEventSourceSupport());
    }

    /**
     * Gets the level of {@code EventSource} support used when generating {@code ImmutableConfiguration} objects.
     *
     * @return the level of {@code EventSource} support
     */
    public EventSourceSupport getEventSourceSupport() {
        return eventSourceSupport;
    }
}