src/main/java/org/apache/commons/configuration2/builder/BasicConfigurationBuilder.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
 *
 *     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
 * limitations under the License.
 */
package org.apache.commons.configuration2.builder;

import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

import org.apache.commons.configuration2.ConfigurationUtils;
import org.apache.commons.configuration2.ImmutableConfiguration;
import org.apache.commons.configuration2.Initializable;
import org.apache.commons.configuration2.beanutils.BeanDeclaration;
import org.apache.commons.configuration2.beanutils.BeanHelper;
import org.apache.commons.configuration2.beanutils.ConstructorArg;
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.EventListenerRegistrationData;
import org.apache.commons.configuration2.event.EventSource;
import org.apache.commons.configuration2.event.EventType;
import org.apache.commons.configuration2.ex.ConfigurationException;
import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
import org.apache.commons.configuration2.reloading.ReloadingController;

/**
 * <p>
 * An implementation of the {@code ConfigurationBuilder} interface which is able to create different concrete
 * {@code ImmutableConfiguration} implementations based on reflection.
 * </p>
 * <p>
 * When constructing an instance of this class the concrete {@code ImmutableConfiguration} implementation class has to
 * be provided. Then properties for the new {@code ImmutableConfiguration} instance can be set. The first call to
 * {@code getConfiguration()} creates and initializes the new {@code ImmutableConfiguration} object. It is cached and
 * returned by subsequent calls. This cache - and also the initialization properties set so far - can be flushed by
 * calling one of the {@code reset()} methods. That way other {@code ImmutableConfiguration} instances with different
 * properties can be created.
 * </p>
 * <p>
 * If the newly created {@code ImmutableConfiguration} object implements the {@code Initializable} interface, its
 * {@code initialize()} method is called after all initialization properties have been set. This way a concrete
 * implementation class can perform arbitrary initialization steps.
 * </p>
 * <p>
 * There are multiple options for setting up a {@code BasicConfigurationBuilder} instance:
 * </p>
 * <ul>
 * <li>All initialization properties can be set in one or multiple calls of the {@code configure()} method. In each call
 * an arbitrary number of {@link BuilderParameters} objects can be passed. The API allows method chaining and is
 * intended to be used from Java code.</li>
 * <li>If builder instances are created by other means - for example using a dependency injection framework -, the fluent API
 * approach may not be suitable. For those use cases it is also possible to pass in all initialization parameters as a
 * map. The keys of the map have to match initialization properties of the {@code ImmutableConfiguration} object to be
 * created, the values are the corresponding property values. For instance, the key <em>throwExceptionOnMissing</em> in
 * the map will cause the method {@code setThrowExceptionOnMissing()} on the {@code ImmutableConfiguration} object to be
 * called with the corresponding value as parameter.</li>
 * </ul>
 * <p>
 * A builder instance can be constructed with an <em>allowFailOnInit</em> flag. If set to <strong>true</strong>,
 * exceptions during initialization of the configuration are ignored; in such a case an empty configuration object is
 * returned. A use case for this flag is a scenario in which a configuration is optional and created on demand the first
 * time configuration data is to be stored. Consider an application that stores user-specific configuration data in the
 * user's home directory: When started for the first time by a new user there is no configuration file; so it makes
 * sense to start with an empty configuration object. On application exit, settings can be stored in this object and
 * written to the associated file. Then they are available on next application start.
 * </p>
 * <p>
 * This class is thread-safe. Multiple threads can modify initialization properties and call {@code getConfiguration()}.
 * However, the intended use case is that the builder is configured by a single thread first. Then
 * {@code getConfiguration()} can be called concurrently, and it is guaranteed that always the same
 * {@code ImmutableConfiguration} instance is returned until the builder is reset.
 * </p>
 *
 * @param <T> the concrete type of {@code ImmutableConfiguration} objects created by this builder
 * @since 2.0
 */
public class BasicConfigurationBuilder<T extends ImmutableConfiguration> implements ConfigurationBuilder<T> {

    /**
     * Registers an event listener at an event source object.
     *
     * @param evSrc the event source
     * @param regData the registration data object
     * @param <E> the type of the event listener
     */
    private static <E extends Event> void registerListener(final EventSource evSrc, final EventListenerRegistrationData<E> regData) {
        evSrc.addEventListener(regData.getEventType(), regData.getListener());
    }

    /**
     * Removes an event listener from an event source object.
     *
     * @param evSrc the event source
     * @param regData the registration data object
     * @param <E> the type of the event listener
     */
    private static <E extends Event> void removeListener(final EventSource evSrc, final EventListenerRegistrationData<E> regData) {
        evSrc.removeEventListener(regData.getEventType(), regData.getListener());
    }

    /** The class of the objects produced by this builder instance. */
    private final Class<? extends T> resultClass;

    /** An object managing the event listeners registered at this builder. */
    private final EventListenerList eventListeners;

    /** A flag whether exceptions on initializing configurations are allowed. */
    private final boolean allowFailOnInit;

    /** The map with current initialization parameters. */
    private Map<String, Object> parameters;

    /** The current bean declaration. */
    private BeanDeclaration resultDeclaration;

    /** The result object of this builder. */
    private volatile T result;

    /**
     * Creates a new instance of {@code BasicConfigurationBuilder} and initializes it with the given result class. No
     * initialization properties are set.
     *
     * @param resCls the result class (must not be <strong>null</strong>)
     * @throws IllegalArgumentException if the result class is <strong>null</strong>
     */
    public BasicConfigurationBuilder(final Class<? extends T> resCls) {
        this(resCls, null);
    }

    /**
     * Creates a new instance of {@code BasicConfigurationBuilder} and initializes it with the given result class and an
     * initial set of builder parameters. The <em>allowFailOnInit</em> flag is set to <strong>false</strong>.
     *
     * @param resCls the result class (must not be <strong>null</strong>)
     * @param params a map with initialization parameters
     * @throws IllegalArgumentException if the result class is <strong>null</strong>
     */
    public BasicConfigurationBuilder(final Class<? extends T> resCls, final Map<String, Object> params) {
        this(resCls, params, false);
    }

    /**
     * Creates a new instance of {@code BasicConfigurationBuilder} and initializes it with the given result class, an
     * initial set of builder parameters, and the <em>allowFailOnInit</em> flag. The map with parameters may be <strong>null</strong>,
     * in this case no initialization parameters are set.
     *
     * @param resCls the result class (must not be <strong>null</strong>)
     * @param params a map with initialization parameters
     * @param allowFailOnInit a flag whether exceptions on initializing a newly created {@code ImmutableConfiguration}
     *        object are allowed
     * @throws IllegalArgumentException if the result class is <strong>null</strong>
     */
    public BasicConfigurationBuilder(final Class<? extends T> resCls, final Map<String, Object> params, final boolean allowFailOnInit) {
        if (resCls == null) {
            throw new IllegalArgumentException("Result class must not be null!");
        }

        resultClass = resCls;
        this.allowFailOnInit = allowFailOnInit;
        eventListeners = new EventListenerList();
        updateParameters(params);
    }

    /**
     * {@inheritDoc} This implementation also takes care that the event listener is added to the managed configuration
     * object.
     *
     * @throws IllegalArgumentException if the event type or the listener is <strong>null</strong>
     */
    @Override
    public <E extends Event> void addEventListener(final EventType<E> eventType, final EventListener<? super E> listener) {
        installEventListener(eventType, listener);
    }

    /**
     * Adds the content of the given map to the already existing initialization parameters.
     *
     * @param params the map with additional initialization parameters; may be <strong>null</strong>, then this call has no effect
     * @return a reference to this builder for method chaining
     */
    public synchronized BasicConfigurationBuilder<T> addParameters(final Map<String, Object> params) {
        final Map<String, Object> newParams = new HashMap<>(getParameters());
        if (params != null) {
            newParams.putAll(params);
        }
        updateParameters(newParams);
        return this;
    }

    /**
     * Checks whether the class of the result configuration is compatible with this builder's result class. This is done to
     * ensure that only objects of the expected result class are created.
     *
     * @param inst the result instance to be checked
     * @throws ConfigurationRuntimeException if an invalid result class is detected
     */
    private void checkResultInstance(final Object inst) {
        if (!getResultClass().isInstance(inst)) {
            throw new ConfigurationRuntimeException("Incompatible result object: " + inst);
        }
    }

    /**
     * Appends the content of the specified {@code BuilderParameters} objects to the current initialization parameters.
     * Calling this method multiple times will create a union of the parameters provided.
     *
     * @param params an arbitrary number of objects with builder parameters
     * @return a reference to this builder for method chaining
     * @throws NullPointerException if a <strong>null</strong> array is passed
     */
    public BasicConfigurationBuilder<T> configure(final BuilderParameters... params) {
        final Map<String, Object> newParams = new HashMap<>();
        for (final BuilderParameters p : params) {
            newParams.putAll(p.getParameters());
            handleEventListenerProviders(p);
        }
        return setParameters(newParams);
    }

    /**
     * Connects this builder with a {@code ReloadingController}. With this method support for reloading can be added to an
     * arbitrary builder object. Event listeners are registered at the reloading controller and this builder with connect
     * both objects:
     * <ul>
     * <li>When the reloading controller detects that a reload is required, the builder's {@link #resetResult()} method is
     * called; so the managed result object is invalidated.</li>
     * <li>When a new result object has been created the controller's reloading state is reset, so that new changes can be
     * detected again.</li>
     * </ul>
     *
     * @param controller the {@code ReloadingController} to connect to (must not be <strong>null</strong>)
     * @throws IllegalArgumentException if the controller is <strong>null</strong>
     */
    public final void connectToReloadingController(final ReloadingController controller) {
        if (controller == null) {
            throw new IllegalArgumentException("ReloadingController must not be null!");
        }
        ReloadingBuilderSupportListener.connect(this, controller);
    }

    /**
     * Copies all {@code EventListener} objects registered at this builder to the specified target configuration builder.
     * This method is intended to be used by derived classes which support inheritance of their properties to other builder
     * objects.
     *
     * @param target the target configuration builder (must not be <strong>null</strong>)
     * @throws NullPointerException if the target builder is <strong>null</strong>
     */
    protected synchronized void copyEventListeners(final BasicConfigurationBuilder<?> target) {
        copyEventListeners(target, eventListeners);
    }

    /**
     * Copies all event listeners in the specified list to the specified target configuration builder. This method is
     * intended to be used by derived classes which have to deal with managed configuration builders that need to be
     * initialized with event listeners.
     *
     * @param target the target configuration builder (must not be <strong>null</strong>)
     * @param listeners the event listeners to be copied over
     * @throws NullPointerException if the target builder is <strong>null</strong>
     */
    protected void copyEventListeners(final BasicConfigurationBuilder<?> target, final EventListenerList listeners) {
        target.eventListeners.addAll(listeners);
    }

    /**
     * Creates a new, initialized result object. This method is called by {@code getConfiguration()} if no valid result
     * object exists. This base implementation performs two steps:
     * <ul>
     * <li>{@code createResultInstance()} is called to create a new, uninitialized result object.</li>
     * <li>{@code initResultInstance()} is called to process all initialization parameters.</li>
     * </ul>
     * It also evaluates the <em>allowFailOnInit</em> flag, i.e. if initialization causes an exception and this flag is set,
     * the exception is ignored, and the newly created, uninitialized configuration is returned. Note that this method is
     * called in a synchronized block.
     *
     * @return the newly created result object
     * @throws ConfigurationException if an error occurs
     */
    protected T createResult() throws ConfigurationException {
        final T resObj = createResultInstance();

        try {
            initResultInstance(resObj);
        } catch (final ConfigurationException cex) {
            if (!isAllowFailOnInit()) {
                throw cex;
            }
        }

        return resObj;
    }

    /**
     * Creates a new {@code BeanDeclaration} which is used for creating new result objects dynamically. This implementation
     * creates a specialized {@code BeanDeclaration} object that is initialized from the given map of initialization
     * parameters. The {@code BeanDeclaration} must be initialized with the result class of this builder, otherwise
     * exceptions will be thrown when the result object is created. Note: This method is invoked in a synchronized block.
     *
     * @param params a snapshot of the current initialization parameters
     * @return the {@code BeanDeclaration} for creating result objects
     * @throws ConfigurationException if an error occurs
     */
    protected BeanDeclaration createResultDeclaration(final Map<String, Object> params) throws ConfigurationException {
        return new BeanDeclaration() {
            @Override
            public String getBeanClassName() {
                return getResultClass().getName();
            }

            @Override
            public String getBeanFactoryName() {
                return null;
            }

            @Override
            public Object getBeanFactoryParameter() {
                return null;
            }

            @Override
            public Map<String, Object> getBeanProperties() {
                // the properties are equivalent to the parameters
                return params;
            }

            @Override
            public Collection<ConstructorArg> getConstructorArgs() {
                // no constructor arguments
                return Collections.emptySet();
            }

            @Override
            public Map<String, Object> getNestedBeanDeclarations() {
                // no nested beans
                return Collections.emptyMap();
            }
        };
    }

    /**
     * Creates the new, uninitialized result object. This is the first step of the process of producing a result object for
     * this builder. This implementation uses the {@link BeanHelper} class to create a new object based on the
     * {@link BeanDeclaration} returned by {@link #getResultDeclaration()}. Note: This method is invoked in a synchronized
     * block.
     *
     * @return the newly created, yet uninitialized result object
     * @throws ConfigurationException if an exception occurs
     */
    protected T createResultInstance() throws ConfigurationException {
        final Object bean = fetchBeanHelper().createBean(getResultDeclaration());
        checkResultInstance(bean);
        return getResultClass().cast(bean);
    }

    /**
     * Obtains the {@code BeanHelper} object to be used when dealing with bean declarations. This method checks whether this
     * builder was configured with a specific {@code BeanHelper} instance. If so, this instance is used. Otherwise, the
     * default {@code BeanHelper} is returned.
     *
     * @return the {@code BeanHelper} to be used
     */
    protected final BeanHelper fetchBeanHelper() {
        final BeanHelper helper = BasicBuilderParameters.fetchBeanHelper(getParameters());
        return helper != null ? helper : BeanHelper.INSTANCE;
    }

    /**
     * Returns an {@code EventSource} for the current result object. If there is no current result or if it does not extend
     * {@code EventSource}, a dummy event source is returned.
     *
     * @return the {@code EventSource} for the current result object
     */
    private EventSource fetchEventSource() {
        return ConfigurationUtils.asEventSource(result, true);
    }

    /**
     * Sends the specified builder event to all registered listeners.
     *
     * @param event the event to be fired
     */
    protected void fireBuilderEvent(final ConfigurationBuilderEvent event) {
        eventListeners.fire(event);
    }

    /**
     * {@inheritDoc} This implementation creates the result configuration on first access. Later invocations return the same
     * object until this builder is reset. The double-check idiom for lazy initialization is used (Bloch, Effective Java,
     * item 71).
     */
    @Override
    public T getConfiguration() throws ConfigurationException {
        fireBuilderEvent(new ConfigurationBuilderEvent(this, ConfigurationBuilderEvent.CONFIGURATION_REQUEST));

        T resObj = result;
        boolean created = false;
        if (resObj == null) {
            synchronized (this) {
                resObj = result;
                if (resObj == null) {
                    result = resObj = createResult();
                    created = true;
                }
            }
        }

        if (created) {
            fireBuilderEvent(new ConfigurationBuilderResultCreatedEvent(this, ConfigurationBuilderResultCreatedEvent.RESULT_CREATED, resObj));
        }
        return resObj;
    }

    /**
     * Gets a map with initialization parameters where all parameters starting with the reserved prefix have been
     * filtered out.
     *
     * @return the filtered parameters map
     */
    private Map<String, Object> getFilteredParameters() {
        final Map<String, Object> filteredMap = new HashMap<>(getParameters());
        filteredMap.keySet().removeIf(key -> key.startsWith(BuilderParameters.RESERVED_PARAMETER_PREFIX));
        return filteredMap;
    }

    /**
     * Gets a (unmodifiable) map with the current initialization parameters set for this builder. The map is populated
     * with the parameters set using the various configuration options.
     *
     * @return a map with the current set of initialization parameters
     */
    protected final synchronized Map<String, Object> getParameters() {
        if (parameters != null) {
            return parameters;
        }
        return Collections.emptyMap();
    }

    /**
     * Gets the result class of this builder. The objects produced by this builder have the class returned here.
     *
     * @return the result class of this builder
     */
    public Class<? extends T> getResultClass() {
        return resultClass;
    }

    /**
     * Gets the {@code BeanDeclaration} that is used to create and initialize result objects. The declaration is created
     * on first access (by invoking {@link #createResultDeclaration(Map)}) based on the current initialization parameters.
     *
     * @return the {@code BeanDeclaration} for dynamically creating a result object
     * @throws ConfigurationException if an error occurs
     */
    protected final synchronized BeanDeclaration getResultDeclaration() throws ConfigurationException {
        if (resultDeclaration == null) {
            resultDeclaration = createResultDeclaration(getFilteredParameters());
        }
        return resultDeclaration;
    }

    /**
     * Checks whether the specified parameters object implements the {@code EventListenerProvider} interface. If so, the
     * event listeners it provides are added to this builder.
     *
     * @param params the parameters object
     */
    private void handleEventListenerProviders(final BuilderParameters params) {
        if (params instanceof EventListenerProvider) {
            eventListeners.addAll(((EventListenerProvider) params).getListeners());
        }
    }

    /**
     * Performs special initialization of the result object. This method is called after parameters have been set on a newly
     * created result instance. If supported by the result class, the {@code initialize()} method is now called.
     *
     * @param obj the newly created result object
     */
    private void handleInitializable(final T obj) {
        if (obj instanceof Initializable) {
            ((Initializable) obj).initialize();
        }
    }

    /**
     * Initializes a newly created result object. This is the second step of the process of producing a result object for
     * this builder. This implementation uses the {@link BeanHelper} class to initialize the object's property based on the
     * {@link BeanDeclaration} returned by {@link #getResultDeclaration()}. Note: This method is invoked in a synchronized
     * block. This is required because internal state is accessed. Sub classes must not call this method without proper
     * synchronization.
     *
     * @param obj the object to be initialized
     * @throws ConfigurationException if an error occurs
     */
    protected void initResultInstance(final T obj) throws ConfigurationException {
        fetchBeanHelper().initBean(obj, getResultDeclaration());
        registerEventListeners(obj);
        handleInitializable(obj);
    }

    /**
     * Adds the specified event listener to this object. This method is called by {@code addEventListener()}, it does the
     * actual listener registration. Because it is final it can be called by sub classes in the constructor if there is
     * already the need to register an event listener.
     *
     * @param eventType the event type object
     * @param listener the listener to be registered
     * @param <E> the event type
     */
    protected final <E extends Event> void installEventListener(final EventType<E> eventType, final EventListener<? super E> listener) {
        fetchEventSource().addEventListener(eventType, listener);
        eventListeners.addEventListener(eventType, listener);
    }

    /**
     * Returns the <em>allowFailOnInit</em> flag. See the header comment for information about this flag.
     *
     * @return the <em>allowFailOnInit</em> flag
     */
    public boolean isAllowFailOnInit() {
        return allowFailOnInit;
    }

    /**
     * Registers the available event listeners at the given object. This method is called for each result object created by
     * the builder.
     *
     * @param obj the object to initialize
     */
    private void registerEventListeners(final T obj) {
        final EventSource evSrc = ConfigurationUtils.asEventSource(obj, true);
        eventListeners.getRegistrations().forEach(regData -> registerListener(evSrc, regData));
    }

    /**
     * {@inheritDoc} This implementation also takes care that the event listener is removed from the managed configuration
     * object.
     */
    @Override
    public <E extends Event> boolean removeEventListener(final EventType<E> eventType, final EventListener<? super E> listener) {
        fetchEventSource().removeEventListener(eventType, listener);
        return eventListeners.removeEventListener(eventType, listener);
    }

    /**
     * Removes all available event listeners from the given result object. This method is called when the result of this
     * builder is reset. Then the old managed configuration should no longer generate events.
     *
     * @param obj the affected result object
     */
    private void removeEventListeners(final T obj) {
        final EventSource evSrc = ConfigurationUtils.asEventSource(obj, true);
        eventListeners.getRegistrations().forEach(regData -> removeListener(evSrc, regData));
    }

    /**
     * Resets this builder. This is a convenience method which combines calls to {@link #resetResult()} and
     * {@link #resetParameters()}.
     */
    public synchronized void reset() {
        resetParameters();
        resetResult();
    }

    /**
     * Removes all initialization parameters of this builder. This method can be called if this builder is to be reused for
     * creating result objects with a different configuration.
     */
    public void resetParameters() {
        setParameters(null);
    }

    /**
     * Clears an existing result object. An invocation of this method causes a new {@code ImmutableConfiguration} object to
     * be created the next time {@link #getConfiguration()} is called.
     */
    public void resetResult() {
        final T oldResult;
        synchronized (this) {
            oldResult = result;
            result = null;
            resultDeclaration = null;
        }

        if (oldResult != null) {
            removeEventListeners(oldResult);
        }
        fireBuilderEvent(new ConfigurationBuilderEvent(this, ConfigurationBuilderEvent.RESET));
    }

    /**
     * Sets the initialization parameters of this builder. Already existing parameters are replaced by the content of the
     * given map.
     *
     * @param params the new initialization parameters of this builder; can be <strong>null</strong>, then all initialization
     *        parameters are removed
     * @return a reference to this builder for method chaining
     */
    public synchronized BasicConfigurationBuilder<T> setParameters(final Map<String, Object> params) {
        updateParameters(params);
        return this;
    }

    /**
     * Replaces the current map with parameters by a new one.
     *
     * @param newParams the map with new parameters (may be <strong>null</strong>)
     */
    private void updateParameters(final Map<String, Object> newParams) {
        final Map<String, Object> map = new HashMap<>();
        if (newParams != null) {
            map.putAll(newParams);
        }
        parameters = Collections.unmodifiableMap(map);
    }
}