examples/servicebased.host/src/main/java/org/apache/felix/example/servicebased/host/DefaultShape.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.felix.example.servicebased.host;

import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.Point;
import javax.swing.ImageIcon;
import org.apache.felix.example.servicebased.host.service.SimpleShape;
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceReference;

/**
 * This class is used as a proxy to defer object creation from shape
 * provider bundles and also as a placeholder shape when previously
 * used shapes are no longer available. These two purposes are
 * actually orthogonal, but were combined into a single class to
 * reduce the number of classes in the application. The proxy-related
 * functionality is introduced as a way to lazily create shape
 * objects in an effort to improve performance; this level of
 * indirection could be removed if eager creation of objects is not
 * a concern. Since this application uses the service-based extension
 * appraoch, lazy shape creation will only come into effect if
 * service providers register service factories instead of directly
 * registering <tt>SimpleShape</tt> or if they use a technology like
 * Declarative Services or iPOJO to register services. Since the
 * example providers register services instances directly there is
 * no laziness in the example, but the proxy approach is still used
 * to demonstrate how to make laziness possible and to keep it
 * similar to the extender-based approach.
**/
class DefaultShape implements SimpleShape
{
    private SimpleShape m_shape;
    private ImageIcon m_icon;
    private BundleContext m_context;
    private ServiceReference<SimpleShape> m_ref;

    /**
     * This constructs a placeholder shape that draws a default
     * icon. It is used when a previously drawn shape is no longer
     * available.
    **/
    public DefaultShape()
    {
        // Do nothing.
    }

    /**
     * This constructs a proxy shape that lazily gets the shape service.
     * @param context The bundle context to use for retrieving the shape service.
     * @param ref The service reference of the service.
    **/
    public DefaultShape(BundleContext context, ServiceReference<SimpleShape> ref)
    {
        m_context = context;
        m_ref = ref;
    }

    /**
     * This method tells the proxy to dispose of its service object; this
     * is called when the underlying service goes away.
    **/
    public void dispose()
    {
        if (m_shape != null)
        {
            m_context.ungetService(m_ref);
            m_context = null;
            m_ref = null;
            m_shape = null;
        }
    }

    /**
     * Implements the <tt>SimpleShape</tt> interface method. When acting as
     * a proxy, this method gets the shape service and then uses it to draw
     * the shape. When acting as a placeholder shape, this method draws the
     * default icon.
     * @param g2 The graphics object used for painting.
     * @param p The position to paint the triangle.
    **/
    @Override
    public void draw(Graphics2D g2, Point p)
    {
        // If this is a proxy shape, instantiate the shape class
        // and use it to draw the shape.
        if (m_context != null)
        {
            try
            {
                if (m_shape == null)
                {
                    // Get the shape service.
                    m_shape = m_context.getService(m_ref);
                }
                // Draw the shape.
                m_shape.draw(g2, p);
                // If everything was successful, then simply return.
                return;
            }
            catch (Exception ex)
            {
                // This generally should not happen, but if it does then
                // we can just fall through and paint the default icon.
            }
        }

        // If the proxied shape could not be drawn for any reason or if
        // this shape is simply a placeholder, then draw the default icon.
        if (m_icon == null)
        {
            try
            {
                m_icon = new ImageIcon(this.getClass().getResource("underc.png"));
            }
            catch (Exception ex)
            {
                ex.printStackTrace();
                g2.setColor(Color.red);
                g2.fillRect(0, 0, 60, 60);
                return;
            }
        }
        g2.drawImage(m_icon.getImage(), 0, 0, null);
    }
}