org.apache.wicket.model.LoadableDetachableModel.java Source code

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Here is the source code for org.apache.wicket.model.LoadableDetachableModel.java

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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
 *
 *      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.wicket.model;

import java.util.Locale;

import org.danekja.java.util.function.serializable.SerializableSupplier;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Model that makes working with detachable models a breeze. LoadableDetachableModel holds a
 * temporary, transient model object, that is set when {@link #getObject()} is called by calling
 * abstract method 'load', and that will be reset/ set to null on {@link #detach()}.
 * 
 * A usage example:
 * 
 * <pre>
 * LoadableDetachableModel venueListModel = new LoadableDetachableModel()
 * {
 *    protected Object load()
 *    {
 *       return getVenueDao().findVenues();
 *    }
 * };
 * </pre>
 * 
 * <p>
 * Though you can override methods {@link #onAttach()} and {@link #onDetach()} for additional
 * attach/ detach behavior, the point of this class is to hide as much of the attaching/ detaching
 * as possible. So you should rarely need to override those methods, if ever.
 * </p>
 * 
 * @author Eelco Hillenius
 * @author Igor Vaynberg
 * 
 * @param <T>
 *            The Model Object type
 */
public abstract class LoadableDetachableModel<T> implements IModel<T> {
    private static final long serialVersionUID = 1L;

    private static final Logger log = LoggerFactory.getLogger(LoadableDetachableModel.class);

    /** Internal state of the LoadableDetachableModel. */
    private enum InternalState {
        DETACHED, ATTACHING, ATTACHED;

        @Override
        public String toString() {
            return name().toLowerCase(Locale.ROOT);
        }
    }

    /** Keeps track of whether this model is attached or detached */
    private transient InternalState state = InternalState.DETACHED;

    /** temporary, transient object. */
    private transient T transientModelObject;

    /**
     * Default constructor, constructs the model in detached state with no data associated with the
     * model.
     */
    public LoadableDetachableModel() {
    }

    /**
     * This constructor is used if you already have the object retrieved and want to wrap it with a
     * detachable model. Constructs the model in attached state. Calls to {@link #getObject()} will
     * return {@code object} until {@link #detach()} is called.
     * 
     * @param object
     *            retrieved instance of the detachable object
     */
    public LoadableDetachableModel(T object) {
        this.transientModelObject = object;
        state = InternalState.ATTACHED;
    }

    @Override
    public void detach() {
        // even if LDM is in partial attached state (ATTACHING) it should be detached
        if (state != null && state != InternalState.DETACHED) {
            try {
                onDetach();
            } finally {
                state = InternalState.DETACHED;
                transientModelObject = null;

                log.debug("removed transient object for '{}'", this);
            }
        }
    }

    @Override
    public final T getObject() {
        if (state == null || state == InternalState.DETACHED) {
            // prevent infinite attachment loops
            state = InternalState.ATTACHING;

            transientModelObject = load();

            if (log.isDebugEnabled()) {
                log.debug("loaded transient object '{}' for '{}'", transientModelObject, this);
            }

            state = InternalState.ATTACHED;
            onAttach();
        }
        return transientModelObject;
    }

    /**
     * Gets the attached status of this model instance
     * 
     * @return true if the model is attached, false otherwise
     */
    public final boolean isAttached() {
        return state == InternalState.ATTACHED;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder(super.toString());
        sb.append(":attached=").append(isAttached()).append(":tempModelObject=[").append(this.transientModelObject)
                .append(']');
        return sb.toString();
    }

    /**
     * Loads and returns the (temporary) model object.
     * 
     * @return the (temporary) model object
     */
    protected abstract T load();

    /**
     * Attaches to the current request. Implement this method with custom behavior, such as loading
     * the model object.
     */
    protected void onAttach() {
    }

    /**
     * Detaches from the current request. Implement this method with custom behavior, such as
     * setting the model object to null.
     */
    protected void onDetach() {
    }

    /**
     * Manually loads the model with the specified object. Subsequent calls to {@link #getObject()}
     * will return {@code object} until {@link #detach()} is called.
     * 
     * @param object
     *            The object to set into the model
     */
    @Override
    public void setObject(final T object) {
        state = InternalState.ATTACHED;
        transientModelObject = object;
    }

    /**
     * Create a {@link LoadableDetachableModel} for the given supplier.
     *
     * @param <T>
     * @param getter Used for the getObject() method.
     * @return the model
     */
    public static <T> LoadableDetachableModel<T> of(SerializableSupplier<T> getter) {
        return new LoadableDetachableModel<T>() {
            private static final long serialVersionUID = 1L;

            @Override
            protected T load() {
                return getter.get();
            }
        };
    }
}