ContextListener for web app-local IIO providers.

servlet/src/main/java/com/twelvemonkeys/servlet/image/IIOProviderContextListener.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2012, Harald Kuhr
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name "TwelveMonkeys" nor the
+ *       names of its contributors may be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package com.twelvemonkeys.servlet.image;
+
+import javax.imageio.ImageIO;
+import javax.imageio.spi.IIORegistry;
+import javax.imageio.spi.ServiceRegistry;
+import javax.servlet.ServletContextEvent;
+import javax.servlet.ServletContextListener;
+import java.util.Iterator;
+
+/**
+ * Takes care of registering and de-registering local ImageIO plugins (service providers) for the servlet context.
+ * <p/>
+ * Registers all available plugins on {@code contextInitialized} event, using {@code ImageIO.scanForPlugins()}, to make
+ * sure they are available to the current servlet context.
+ * De-registers all plugins which have the {@link Thread#getContextClassLoader() current thread's context class loader}
+ * as its class loader on {@code contextDestroyed} event, to avoid class/resource leak.
+ *
+ * @author <a href="mailto:harald.kuhr@gmail.com">Harald Kuhr</a>
+ * @author last modified by $Author: haraldk$
+ * @version $Id: IIOProviderContextListener.java,v 1.0 14.02.12 21:53 haraldk Exp$
+ * @see javax.imageio.ImageIO#scanForPlugins()
+ */
+public final class IIOProviderContextListener implements ServletContextListener {
+    
+    public void contextInitialized(final ServletContextEvent event) {
+        // Registers all locally available IIO plugins.
+        ImageIO.scanForPlugins();
+    }
+
+    public void contextDestroyed(final ServletContextEvent event) {
+        // De-register any locally registered IIO plugins. Relies on each web app having its own context class loader.
+        final IIORegistry registry = IIORegistry.getDefaultInstance();
+        final LocalFilter localFilter = new LocalFilter(Thread.currentThread().getContextClassLoader()); // scanForPlugins uses context class loader
+
+        Iterator<Class<?>> categories = registry.getCategories();
+        
+        while (categories.hasNext()) {
+            Class<?> category = categories.next();
+            Iterator<?> providers = registry.getServiceProviders(category, localFilter, false);
+
+            while (providers.hasNext()) {
+                Object provider = providers.next();
+                registry.deregisterServiceProvider(provider);
+                event.getServletContext().log(String.format("Unregistered locally installed provider class: %s", provider.getClass()));
+            }
+        }
+    }
+
+    static class LocalFilter implements ServiceRegistry.Filter {
+        private final ClassLoader loader;
+
+        public LocalFilter(ClassLoader loader) {
+            this.loader = loader;
+        }
+
+        public boolean filter(Object provider) {
+            return provider.getClass().getClassLoader() == loader;
+        }
+    }
+}