org.jspresso.framework.util.resources.server.ResourceManager.java Source code

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Here is the source code for org.jspresso.framework.util.resources.server.ResourceManager.java

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/*
 * Copyright (c) 2005-2016 Vincent Vandenschrick. All rights reserved.
 *
 *  This file is part of the Jspresso framework.
 *
 *  Jspresso is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU Lesser General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  Jspresso is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public License
 *  along with Jspresso.  If not, see <http://www.gnu.org/licenses/>.
 */

package org.jspresso.framework.util.resources.server;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Map;

import org.apache.commons.collections4.map.AbstractReferenceMap;
import org.apache.commons.collections4.map.ReferenceMap;

import org.jspresso.framework.util.resources.IResourceBase;

/**
 * This class allows to register a resource provider by a unique id and to
 * display the resource provider's content for a given id. Once a resource has
 * been taken out of the resource manager, it is automatically unregistered. So
 * resource storage in resource manager is a one-time usage.
 */
public final class ResourceManager {

    private static final ResourceManager INSTANCE = new ResourceManager();

    private final SecureRandom random;
    private final Map<String, IResourceBase> resources;

    @SuppressWarnings("unchecked")
    private ResourceManager() {
        // resources = new HashMap<String, IResource>();
        resources = new ReferenceMap(AbstractReferenceMap.ReferenceStrength.SOFT,
                AbstractReferenceMap.ReferenceStrength.SOFT, true);
        random = new SecureRandom();
    }

    /**
     * Singleton pattern.
     *
     * @return the singleton instance.
     */
    public static ResourceManager getInstance() {
        return INSTANCE;
    }

    /**
     * Returns the registered resource or null.
     *
     * @param id
     *     the identifier under which the resource has been registered.
     * @return the registered resource or null.
     */
    public IResourceBase getRegistered(String id) {
        IResourceBase resource = resources.get(id);
        // Do not un-register resource once retrieved. There might be cases
        // when the resource must be retrieved multiple times.
        // un-register(id);
        return resource;
    }

    /**
     * Registers a resource.
     *
     * @param resource
     *     the resource to be registered.
     * @return the generated identifier under which the resource has been
     * registered.
     */
    public String register(IResourceBase resource) {
        try {
            String id = createId();
            resources.put(id, resource);
            return id;
        } catch (NoSuchAlgorithmException nsae) {
            throw new IllegalStateException("Could not generate random id: " + nsae.getLocalizedMessage());
        }
    }

    /**
     * Registers a resource.
     *
     * @param id
     *     the identifier under which the resource must be registered.
     * @param resource
     *     the resource to be registered.
     */
    public void register(String id, IResourceBase resource) {
        resources.put(id, resource);
    }

    /**
     * Un-registers a resource.
     *
     * @param id
     *     the identifier under which the resource is registered.
     */
    public void unregister(String id) {
        resources.remove(id);
    }

    private String createId() throws NoSuchAlgorithmException {
        byte[] bytes = new byte[24];
        random.nextBytes(bytes);
        bytes = MessageDigest.getInstance("MD5").digest(bytes);

        StringBuilder result = new StringBuilder();
        for (byte aByte : bytes) {
            byte b1 = (byte) ((aByte & 0xf0) >> 4);
            byte b2 = (byte) (aByte & 0x0f);

            result.append(toHex(b1));
            result.append(toHex(b2));
        }

        return result.toString();
    }

    private char toHex(byte b) {
        if (b < 10) {
            return (char) ('0' + b);
        }
        return (char) ('A' + b - 10);
    }
}