src/main/java/org/apache/commons/configuration2/builder/combined/MultiFileConfigurationBuilder.java

/*
 * 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
 *
 *     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.apache.commons.configuration2.builder.combined;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicReference;

import org.apache.commons.configuration2.ConfigurationUtils;
import org.apache.commons.configuration2.FileBasedConfiguration;
import org.apache.commons.configuration2.builder.BasicBuilderParameters;
import org.apache.commons.configuration2.builder.BasicConfigurationBuilder;
import org.apache.commons.configuration2.builder.BuilderParameters;
import org.apache.commons.configuration2.builder.ConfigurationBuilderEvent;
import org.apache.commons.configuration2.builder.ConfigurationBuilderResultCreatedEvent;
import org.apache.commons.configuration2.builder.FileBasedConfigurationBuilder;
import org.apache.commons.configuration2.event.Event;
import org.apache.commons.configuration2.event.EventListener;
import org.apache.commons.configuration2.event.EventListenerList;
import org.apache.commons.configuration2.event.EventType;
import org.apache.commons.configuration2.ex.ConfigurationException;
import org.apache.commons.configuration2.interpol.ConfigurationInterpolator;
import org.apache.commons.configuration2.interpol.InterpolatorSpecification;
import org.apache.commons.lang3.concurrent.ConcurrentUtils;

/**
 * <p>
 * A specialized {@code ConfigurationBuilder} implementation providing access to multiple file-based configurations
 * based on a file name pattern.
 * </p>
 * <p>
 * This builder class is initialized with a pattern string and a {@link ConfigurationInterpolator} object. Each time a
 * configuration is requested, the pattern is evaluated against the {@code ConfigurationInterpolator} (so all variables
 * are replaced by their current values). The resulting string is interpreted as a file name for a configuration file to
 * be loaded. For example, providing a pattern of <em>file:///opt/config/${product}/${client}/config.xml</em> will
 * result in <em>product</em> and <em>client</em> being resolved on every call. By storing configuration files in a
 * corresponding directory structure, specialized configuration files associated with a specific product and client can
 * be loaded. Thus an application can be made multi-tenant in a transparent way.
 * </p>
 * <p>
 * This builder class keeps a map with configuration builders for configurations already loaded. The
 * {@code getConfiguration()} method first evaluates the pattern string and checks whether a builder for the resulting
 * file name is available. If yes, it is queried for its configuration. Otherwise, a new file-based configuration
 * builder is created now and initialized.
 * </p>
 * <p>
 * Configuration of an instance happens in the usual way for configuration builders. A
 * {@link MultiFileBuilderParametersImpl} parameters object is expected which must contain a file name pattern string
 * and a {@code ConfigurationInterpolator}. Other properties of this parameters object are used to initialize the
 * builders for managed configurations.
 * </p>
 *
 * @since 2.0
 * @param <T> the concrete type of {@code Configuration} objects created by this builder
 */
public class MultiFileConfigurationBuilder<T extends FileBasedConfiguration> extends BasicConfigurationBuilder<T> {
    /**
     * Constant for the name of the key referencing the {@code ConfigurationInterpolator} in this builder's parameters.
     */
    private static final String KEY_INTERPOLATOR = "interpolator";

    /**
     * Creates a map with parameters for a new managed configuration builder. This method merges the basic parameters set
     * for this builder with the specific parameters object for managed builders (if provided).
     *
     * @param params the parameters of this builder
     * @param multiParams the parameters object for this builder
     * @return the parameters for a new managed builder
     */
    private static Map<String, Object> createManagedBuilderParameters(final Map<String, Object> params, final MultiFileBuilderParametersImpl multiParams) {
        final Map<String, Object> newParams = new HashMap<>(params);
        newParams.remove(KEY_INTERPOLATOR);
        final BuilderParameters managedBuilderParameters = multiParams.getManagedBuilderParameters();
        if (managedBuilderParameters != null) {
            // clone parameters as they are applied to multiple builders
            final BuilderParameters copy = (BuilderParameters) ConfigurationUtils.cloneIfPossible(managedBuilderParameters);
            newParams.putAll(copy.getParameters());
        }
        return newParams;
    }

    /**
     * Checks whether the given event type is of interest for the managed configuration builders. This method is called by
     * the methods for managing event listeners to find out whether a listener should be passed to the managed builders,
     * too.
     *
     * @param eventType the event type object
     * @return a flag whether this event type is of interest for managed builders
     */
    private static boolean isEventTypeForManagedBuilders(final EventType<?> eventType) {
        return !EventType.isInstanceOf(eventType, ConfigurationBuilderEvent.ANY);
    }

    /** A cache for already created managed builders. */
    private final ConcurrentMap<String, FileBasedConfigurationBuilder<T>> managedBuilders = new ConcurrentHashMap<>();

    /** Stores the {@code ConfigurationInterpolator} object. */
    private final AtomicReference<ConfigurationInterpolator> interpolator = new AtomicReference<>();

    /**
     * A flag for preventing reentrant access to managed builders on interpolation of the file name pattern.
     */
    private final ThreadLocal<Boolean> inInterpolation = new ThreadLocal<>();

    /** A list for the event listeners to be passed to managed builders. */
    private final EventListenerList configurationListeners = new EventListenerList();

    /**
     * A specialized event listener which gets registered at all managed builders. This listener just propagates
     * notifications from managed builders to the listeners registered at this {@code MultiFileConfigurationBuilder}.
     */
    private final EventListener<ConfigurationBuilderEvent> managedBuilderDelegationListener = this::handleManagedBuilderEvent;

    /**
     * Creates a new instance of {@code MultiFileConfigurationBuilder} without setting initialization parameters.
     *
     * @param resCls the result configuration class
     * @throws IllegalArgumentException if the result class is <b>null</b>
     */
    public MultiFileConfigurationBuilder(final Class<? extends T> resCls) {
        super(resCls);
    }

    /**
     * Creates a new instance of {@code MultiFileConfigurationBuilder} and sets initialization parameters.
     *
     * @param resCls the result configuration class
     * @param params a map with initialization parameters
     * @throws IllegalArgumentException if the result class is <b>null</b>
     */
    public MultiFileConfigurationBuilder(final Class<? extends T> resCls, final Map<String, Object> params) {
        super(resCls, params);
    }

    /**
     * Creates a new instance of {@code MultiFileConfigurationBuilder} and sets initialization parameters and a flag whether
     * initialization failures should be ignored.
     *
     * @param resCls the result configuration class
     * @param params a map with initialization parameters
     * @param allowFailOnInit a flag whether initialization errors should be ignored
     * @throws IllegalArgumentException if the result class is <b>null</b>
     */
    public MultiFileConfigurationBuilder(final Class<? extends T> resCls, final Map<String, Object> params, final boolean allowFailOnInit) {
        super(resCls, params, allowFailOnInit);
    }

    /**
     * {@inheritDoc} This implementation ensures that the listener is also added to managed configuration builders if
     * necessary. Listeners for the builder-related event types are excluded because otherwise they would be triggered by
     * the internally used configuration builders.
     */
    @Override
    public synchronized <E extends Event> void addEventListener(final EventType<E> eventType, final EventListener<? super E> l) {
        super.addEventListener(eventType, l);
        if (isEventTypeForManagedBuilders(eventType)) {
            getManagedBuilders().values().forEach(b -> b.addEventListener(eventType, l));
            configurationListeners.addEventListener(eventType, l);
        }
    }

    /**
     * {@inheritDoc} This method is overridden to adapt the return type.
     */
    @Override
    public MultiFileConfigurationBuilder<T> configure(final BuilderParameters... params) {
        super.configure(params);
        return this;
    }

    /**
     * Determines the file name of a configuration based on the file name pattern. This method is called on every access to
     * this builder's configuration. It obtains the {@link ConfigurationInterpolator} from this builder's parameters and
     * uses it to interpolate the file name pattern.
     *
     * @param multiParams the parameters object for this builder
     * @return the name of the configuration file to be loaded
     */
    protected String constructFileName(final MultiFileBuilderParametersImpl multiParams) {
        final ConfigurationInterpolator ci = getInterpolator();
        return String.valueOf(ci.interpolate(multiParams.getFilePattern()));
    }

    /**
     * Creates a new {@code ConfigurationBuilderEvent} based on the passed in event, but with the source changed to this
     * builder. This method is called when an event was received from a managed builder. In this case, the event has to be
     * passed to the builder listeners registered at this object, but with the correct source property.
     *
     * @param event the event received from a managed builder
     * @return the event to be propagated
     */
    private ConfigurationBuilderEvent createEventWithChangedSource(final ConfigurationBuilderEvent event) {
        if (ConfigurationBuilderResultCreatedEvent.RESULT_CREATED.equals(event.getEventType())) {
            return new ConfigurationBuilderResultCreatedEvent(this, ConfigurationBuilderResultCreatedEvent.RESULT_CREATED,
                ((ConfigurationBuilderResultCreatedEvent) event).getConfiguration());
        }
        @SuppressWarnings("unchecked")
        final
        // This is safe due to the constructor of ConfigurationBuilderEvent
        EventType<? extends ConfigurationBuilderEvent> type = (EventType<? extends ConfigurationBuilderEvent>) event.getEventType();
        return new ConfigurationBuilderEvent(this, type);
    }

    /**
     * Creates a fully initialized builder for a managed configuration. This method is called by {@code getConfiguration()}
     * whenever a configuration file is requested which has not yet been loaded. This implementation delegates to
     * {@code createManagedBuilder()} for actually creating the builder object. Then it sets the location to the
     * configuration file.
     *
     * @param fileName the name of the file to be loaded
     * @param params a map with initialization parameters for the new builder
     * @return the newly created and initialized builder instance
     * @throws ConfigurationException if an error occurs
     */
    protected FileBasedConfigurationBuilder<T> createInitializedManagedBuilder(final String fileName, final Map<String, Object> params)
        throws ConfigurationException {
        final FileBasedConfigurationBuilder<T> managedBuilder = createManagedBuilder(fileName, params);
        managedBuilder.getFileHandler().setFileName(fileName);
        return managedBuilder;
    }

    /**
     * Creates the {@code ConfigurationInterpolator} to be used by this instance. This method is called when a file name is
     * to be constructed, but no current {@code ConfigurationInterpolator} instance is available. It obtains an instance
     * from this builder's parameters. If no properties of the {@code ConfigurationInterpolator} are specified in the
     * parameters, a default instance without lookups is returned (which is probably not very helpful).
     *
     * @return the {@code ConfigurationInterpolator} to be used
     */
    protected ConfigurationInterpolator createInterpolator() {
        final InterpolatorSpecification spec = BasicBuilderParameters.fetchInterpolatorSpecification(getParameters());
        return ConfigurationInterpolator.fromSpecification(spec);
    }

    /**
     * Creates a builder for a managed configuration. This method is called whenever a configuration for a file name is
     * requested which has not yet been loaded. The passed in map with parameters is populated from this builder's
     * configuration (i.e. the basic parameters plus the optional parameters for managed builders). This base implementation
     * creates a standard builder for file-based configurations. Derived classes may override it to create special purpose
     * builders.
     *
     * @param fileName the name of the file to be loaded
     * @param params a map with initialization parameters for the new builder
     * @return the newly created builder instance
     * @throws ConfigurationException if an error occurs
     */
    protected FileBasedConfigurationBuilder<T> createManagedBuilder(final String fileName, final Map<String, Object> params) throws ConfigurationException {
        return new FileBasedConfigurationBuilder<>(getResultClass(), params, isAllowFailOnInit());
    }

    /**
     * Generates a file name for a managed builder based on the file name pattern. This method prevents infinite loops which
     * could happen if the file name pattern cannot be resolved and the {@code ConfigurationInterpolator} used by this
     * object causes a recursive lookup to this builder's configuration.
     *
     * @param multiParams the current builder parameters
     * @return the file name for a managed builder
     */
    private String fetchFileName(final MultiFileBuilderParametersImpl multiParams) {
        String fileName;
        final Boolean reentrant = inInterpolation.get();
        if (reentrant != null && reentrant.booleanValue()) {
            fileName = multiParams.getFilePattern();
        } else {
            inInterpolation.set(Boolean.TRUE);
            try {
                fileName = constructFileName(multiParams);
            } finally {
                inInterpolation.set(Boolean.FALSE);
            }
        }
        return fileName;
    }

    /**
     * {@inheritDoc} This implementation evaluates the file name pattern using the configured
     * {@code ConfigurationInterpolator}. If this file has already been loaded, the corresponding builder is accessed.
     * Otherwise, a new builder is created for loading this configuration file.
     */
    @Override
    public T getConfiguration() throws ConfigurationException {
        return getManagedBuilder().getConfiguration();
    }

    /**
     * Gets the {@code ConfigurationInterpolator} used by this instance. This is the object used for evaluating the file
     * name pattern. It is created on demand.
     *
     * @return the {@code ConfigurationInterpolator}
     */
    protected ConfigurationInterpolator getInterpolator() {
        ConfigurationInterpolator result;
        boolean done;

        // This might create multiple instances under high load,
        // however, always the same instance is returned.
        do {
            result = interpolator.get();
            if (result != null) {
                done = true;
            } else {
                result = createInterpolator();
                done = interpolator.compareAndSet(null, result);
            }
        } while (!done);

        return result;
    }

    /**
     * Gets the managed {@code FileBasedConfigurationBuilder} for the current file name pattern. It is determined based
     * on the evaluation of the file name pattern using the configured {@code ConfigurationInterpolator}. If this is the
     * first access to this configuration file, the builder is created.
     *
     * @return the configuration builder for the configuration corresponding to the current evaluation of the file name
     *         pattern
     * @throws ConfigurationException if the builder cannot be determined (e.g. due to missing initialization parameters)
     */
    public FileBasedConfigurationBuilder<T> getManagedBuilder() throws ConfigurationException {
        final Map<String, Object> params = getParameters();
        final MultiFileBuilderParametersImpl multiParams = MultiFileBuilderParametersImpl.fromParameters(params, true);
        if (multiParams.getFilePattern() == null) {
            throw new ConfigurationException("No file name pattern is set!");
        }
        final String fileName = fetchFileName(multiParams);

        FileBasedConfigurationBuilder<T> builder = getManagedBuilders().get(fileName);
        if (builder == null) {
            builder = createInitializedManagedBuilder(fileName, createManagedBuilderParameters(params, multiParams));
            final FileBasedConfigurationBuilder<T> newBuilder = ConcurrentUtils.putIfAbsent(getManagedBuilders(), fileName, builder);
            if (newBuilder == builder) {
                initListeners(newBuilder);
            } else {
                builder = newBuilder;
            }
        }
        return builder;
    }

    /**
     * Gets the map with the managed builders created so far by this {@code MultiFileConfigurationBuilder}. This map is
     * exposed to derived classes so they can access managed builders directly. However, derived classes are not expected to
     * manipulate this map.
     *
     * @return the map with the managed builders
     */
    protected ConcurrentMap<String, FileBasedConfigurationBuilder<T>> getManagedBuilders() {
        return managedBuilders;
    }

    /**
     * Handles events received from managed configuration builders. This method creates a new event with a source pointing
     * to this builder and propagates it to all registered listeners.
     *
     * @param event the event received from a managed builder
     */
    private void handleManagedBuilderEvent(final ConfigurationBuilderEvent event) {
        if (ConfigurationBuilderEvent.RESET.equals(event.getEventType())) {
            resetResult();
        } else {
            fireBuilderEvent(createEventWithChangedSource(event));
        }
    }

    /**
     * Registers event listeners at the passed in newly created managed builder. This method registers a special
     * {@code EventListener} which propagates builder events to listeners registered at this builder. In addition,
     * {@code ConfigurationListener} and {@code ConfigurationErrorListener} objects are registered at the new builder.
     *
     * @param newBuilder the builder to be initialized
     */
    private void initListeners(final FileBasedConfigurationBuilder<T> newBuilder) {
        copyEventListeners(newBuilder, configurationListeners);
        newBuilder.addEventListener(ConfigurationBuilderEvent.ANY, managedBuilderDelegationListener);
    }

    /**
     * {@inheritDoc} This implementation ensures that the listener is also removed from managed configuration builders if
     * necessary.
     */
    @Override
    public synchronized <E extends Event> boolean removeEventListener(final EventType<E> eventType, final EventListener<? super E> l) {
        final boolean result = super.removeEventListener(eventType, l);
        if (isEventTypeForManagedBuilders(eventType)) {
            getManagedBuilders().values().forEach(b -> b.removeEventListener(eventType, l));
            configurationListeners.removeEventListener(eventType, l);
        }
        return result;
    }

    /**
     * {@inheritDoc} This implementation clears the cache with all managed builders.
     */
    @Override
    public synchronized void resetParameters() {
        getManagedBuilders().values().forEach(b -> b.removeEventListener(ConfigurationBuilderEvent.ANY, managedBuilderDelegationListener));
        getManagedBuilders().clear();
        interpolator.set(null);
        super.resetParameters();
    }
}