Registering my own implementation of the ImageSessionContext for Apache XMLGraphics

Question

We use Apache FOP with its XMLGraphics library to generate documents for our customers based on their designed templates. Their templates often have images which are taken from a web based resource. It has become apparent however, that in some cases we are having difficulty when some older templates were designed with external images using the http protocol and the resource now sits behind the https protocol. This is because the xmlgraphics DefaultImageSessionContext does not support redirects when attempting to preload the image from the web service. This is due to it simply performing a URL.openStream() method.

What I would like to do is to create my own implementation to replace the default one. This should be possible the DefaultImageSessionContext extends a public abstract class AbstractImageSessionContext which in turn implements the ImageSessionContext.

Does anyone have any idea on how I can register my implementation with FOP?

Answer 1

This was my original answer:

---

Sorry if I give you some wrong information, it is being a long time without using FOP.

As indicated in the Apache XML Graphics Image Loading framework documentation, in order to preload an image, probably, in some part of your code, you have something like this:

ImageSessionContext sessionContext = new DefaultImageSessionContext(
  getImageManager().getImageContext(), null);

ImageInfo info = getImageManager().getImageInfo(url.toString(), sessionContext);

If you want to support redirects, as you suggest you can provide your custom implementation of ImageSessionContext.

Reviewing the source code of DefaultImageSessionContext, probably the best approach will be to create a new class that overrides the resolveURI method. Please, consider for example the following code, based on HttpURLConnection, which should follow redirects as required:

public class HttpRedirectsAwareImageSessionContext extends DefaultImageSessionContext {

  @Override
  protected Source resolveURI(String uri) {
    HttpURLConnection urlConnection = null;

    try {
      URL url = new URL(uri);

      urlConnection = (HttpURLConnection) url.openConnection();
 
      // Just read the InputStream, to a byte[] for instance. As in the example
      // you can use the convenient methods of Apache Commons IOUtils for the task
      // as well. See: https://commons.apache.org/proper/commons-io/apidocs/org/apache/commons/io/IOUtils.html#toBufferedInputStream-java.io.InputStream-
      InputStream in = IOUtils.toBufferedInputStream(urlConnection.getInputStream());

      return new StreamSource(in, url.toExternalForm());
    } catch (MalformedURLException e) {
      // Legacy DefaultImageSessionContext code
      File f = new File(baseDir, uri);
      if (f.isFile()) {
        return new StreamSource(f);
      } else {
        return null;
      }
    } catch (IOException ioe) {
      return null;
    } finally {
      if (urlConnection != null) {
        urlConnection.disconnect();
      }
    }
  }
}

Then, use the new created ImageSessionContext instead:

ImageSessionContext sessionContext = new HttpRedirectsAwareImageSessionContext(
  getImageManager().getImageContext(), null);

ImageInfo info = getImageManager().getImageInfo(url.toString(), sessionContext);

---

But I am afraid that I did not fully understand your requirements.

As you can see, for instance, in the FOPUserAgent source code, FOP provides its own underlying ImageSessionContext implementation:

   private final ImageSessionContext imageSessionContext;

   /**
     * Main constructor. <b>This constructor should not be called directly. Please use the
     * methods from FopFactory to construct FOUserAgent instances!</b>
     * @param factory the factory that provides environment-level information
     * @param resourceResolver the resolver used to acquire resources
     * @see org.apache.fop.apps.FopFactory
     */
    FOUserAgent(final FopFactory factory, InternalResourceResolver resourceResolver) {
        this.factory = factory;
        this.resourceResolver = resourceResolver;
        setTargetResolution(factory.getTargetResolution());
        setAccessibility(factory.isAccessibilityEnabled());
        setKeepEmptyTags(factory.isKeepEmptyTags());
        imageSessionContext = new AbstractImageSessionContext(factory.getFallbackResolver()) {

            public ImageContext getParentContext() {
                return factory;
            }

            public float getTargetResolution() {
                return FOUserAgent.this.getTargetResolution();
            }

            public Source resolveURI(String uri) {
                return FOUserAgent.this.resolveURI(uri);
            }
        };
    }


    /**
     * Attempts to resolve the given URI.
     * Will use the configured resolver and if not successful fall back
     * to the default resolver.
     * @param uri URI to access
     * @return A {@link javax.xml.transform.Source} object, or null if the URI
     * cannot be resolved.
     */
    public StreamSource resolveURI(String uri) {
        // TODO: What do we want to do when resources aren't found??? We also need to remove this
        // method entirely
        try {
            // Have to do this so we can resolve data URIs
            StreamSource src = new StreamSource(resourceResolver.getResource(uri));
            src.setSystemId(getResourceResolver().getBaseURI().toASCIIString());
            return src;
        } catch (URISyntaxException use) {
            return null;
        } catch (IOException ioe) {
            return null;
        }
    }

This implementation rely on the class InternalResourceResolver, which in turn rely on the ResourceResolver abstraction provided by the XML Graphics Image Loading Framework.

In order to solve your problem, you can provide your custom ResourceResolver implementation when initializing FOP via FopFactoryBuilder.

Although quite dated, the Apache FOP documentation provides some useful introduction here and in this and mainly this other articles. For example, based on the information provided in last link:

ResourceResolver resolver = new ResourceResolver() {
  public OutputStream getOutputStream(URI uri) throws IOException {
    URL url = uri.toURL();
    return url.openConnection().getOutputStream();
  }

  public Resource getResource(URI uri) throws IOException {
    // Based on the above-mentioned code
    HttpURLConnection urlConnection = null;

    try {
      URL url = uri.toURL();

      // Please, see the companion comment and SO answer if you need
      // http to https redirect
      // https://stackoverflow.com/questions/1884230/httpurlconnection-doesnt-follow-redirect-from-http-to-https/26046079#26046079
      urlConnection = (HttpURLConnection) url.openConnection();
 
      // Just read the InputStream, to a byte[] for instance. As in the example
      // you can use the convenient methods of Apache Commons IOUtils for the task
      // as well. See: https://commons.apache.org/proper/commons-io/apidocs/org/apache/commons/io/IOUtils.html#toBufferedInputStream-java.io.InputStream-
      InputStream in = IOUtils.toBufferedInputStream(urlConnection.getInputStream());

      return new StreamSource(in, url.toExternalForm());
    } catch (MalformedURLException e) {
      // Legacy DefaultImageSessionContext code
      File f = new File(baseDir, uri);
      if (f.isFile()) {
        return new StreamSource(f);
      } else {
        return null;
      }
    } catch (IOException ioe) {
      return null;
    } finally {
      if (urlConnection != null) {
        urlConnection.disconnect();
      }
    }
};

//Setting up the FOP factory
FopFactoryBuilder builder = new FopFactoryBuilder(new File(".").toURI(), resolver);
fopFactory = builder.build();

Please, be aware that it is a very simple example, the actual ResourceResolver, constructed by FOP in ResourceResolverFactory is quite complex, but maybe can give you some ideas about how to implement your own:

/*
 * 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.
 */

/* $Id$ */

package org.apache.fop.apps.io;

import java.io.File;
import java.io.FileOutputStream;
import java.io.FilterInputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.MalformedURLException;
import java.net.URI;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.xmlgraphics.io.Resource;
import org.apache.xmlgraphics.io.ResourceResolver;
import org.apache.xmlgraphics.io.TempResourceResolver;
import org.apache.xmlgraphics.io.TempResourceURIGenerator;

/**
 * A factory class for {@link ResourceResolver}s.
 */
public final class ResourceResolverFactory {

    private ResourceResolverFactory() {
    }

    /**
     * Returns the default resource resolver, this is most basic resolver which can be used when
     * no there are no I/O or file access restrictions.
     *
     * @return the default resource resolver
     */
    public static ResourceResolver createDefaultResourceResolver() {
        return DefaultResourceResolver.INSTANCE;
    }

    /**
     * A helper merthod that creates an internal resource resolver using the default resover:
     * {@link ResourceResolverFactory#createDefaultResourceResolver()}.
     *
     * @param baseURI the base URI from which to resolve URIs
     * @return the default internal resource resolver
     */
    public static InternalResourceResolver createDefaultInternalResourceResolver(URI baseURI) {
        return new InternalResourceResolver(baseURI, createDefaultResourceResolver());
    }

    /**
     * Creates an interal resource resolver given a base URI and a resource resolver.
     *
     * @param baseURI the base URI from which to resolve URIs
     * @param resolver the resource resolver
     * @return the internal resource resolver
     */
    public static InternalResourceResolver createInternalResourceResolver(URI baseURI,
            ResourceResolver resolver) {
        return new InternalResourceResolver(baseURI, resolver);
    }

    /**
     * Creates a temporary-resource-scheme aware resource resolver. Temporary resource URIs are
     * created by {@link TempResourceURIGenerator}.
     *
     * @param tempResourceResolver the temporary-resource-scheme resolver to use
     * @param defaultResourceResolver the default resource resolver to use
     * @return the ressource resolver
     */
    public static ResourceResolver createTempAwareResourceResolver(
            TempResourceResolver tempResourceResolver,
            ResourceResolver defaultResourceResolver) {
        return new TempAwareResourceResolver(tempResourceResolver, defaultResourceResolver);
    }

    /**
     * This creates the builder class for binding URI schemes to implementations of
     * {@link ResourceResolver}. This allows users to define their own URI schemes such that they
     * have finer control over the acquisition of resources.
     *
     * @param defaultResolver the default resource resolver that should be used in the event that
     * none of the other registered resolvers match the scheme
     * @return the scheme aware {@link ResourceResolver} builder
     */
    public static SchemeAwareResourceResolverBuilder createSchemeAwareResourceResolverBuilder(
            ResourceResolver defaultResolver) {
        return new SchemeAwareResourceResolverBuilderImpl(defaultResolver);
    }

    private static final class DefaultResourceResolver implements ResourceResolver {

        private static final ResourceResolver INSTANCE = new DefaultResourceResolver();

        private final TempAwareResourceResolver delegate;

        private DefaultResourceResolver() {
            delegate = new TempAwareResourceResolver(new DefaultTempResourceResolver(),
                    new NormalResourceResolver());
        }

        /** {@inheritDoc} */
        public Resource getResource(URI uri) throws IOException {
            return delegate.getResource(uri);
        }

        /** {@inheritDoc} */
        public OutputStream getOutputStream(URI uri) throws IOException {
            return delegate.getOutputStream(uri);
        }

    }

    private static final class TempAwareResourceResolver implements ResourceResolver {

        private final TempResourceResolver tempResourceResolver;

        private final ResourceResolver defaultResourceResolver;

        public TempAwareResourceResolver(TempResourceResolver tempResourceHandler,
                ResourceResolver defaultResourceResolver) {
            this.tempResourceResolver = tempResourceHandler;
            this.defaultResourceResolver = defaultResourceResolver;
        }

        private static boolean isTempURI(URI uri) {
            return TempResourceURIGenerator.isTempURI(uri);
        }

        /** {@inheritDoc} */
        public Resource getResource(URI uri) throws IOException {
            if (isTempURI(uri)) {
                return tempResourceResolver.getResource(uri.getPath());
            } else {
                return defaultResourceResolver.getResource(uri);
            }
        }

        /** {@inheritDoc} */
        public OutputStream getOutputStream(URI uri) throws IOException {
            if (isTempURI(uri)) {
                return tempResourceResolver.getOutputStream(uri.getPath());
            } else {
                return defaultResourceResolver.getOutputStream(uri);
            }
        }
    }

    private static class DefaultTempResourceResolver implements TempResourceResolver {

        private final ConcurrentHashMap<String, File> tempFiles = new ConcurrentHashMap<String, File>();

        private File getTempFile(String uri) throws IllegalStateException {
            File tempFile = tempFiles.remove(uri);
            if (tempFile == null) {
                throw new IllegalStateException(uri + " was never created or has been deleted");
            }
            return tempFile;
        }

        private File createTempFile(String path) throws IOException {
            File tempFile = File.createTempFile(path, ".fop.tmp");
            File oldFile = tempFiles.put(path, tempFile);
            if (oldFile != null) {
                String errorMsg = oldFile.getAbsolutePath() + " has been already created for " + path;
                boolean newTempDeleted = tempFile.delete();
                if (!newTempDeleted) {
                    errorMsg += ". " + tempFile.getAbsolutePath() + " was not deleted.";
                }
                throw new IOException(errorMsg);
            }
            return tempFile;
        }

        /** {@inheritDoc} */
        public Resource getResource(String id) throws IOException {
            return new Resource(new FileDeletingInputStream(getTempFile(id)));
        }

        /** {@inheritDoc} */
        public OutputStream getOutputStream(String id) throws IOException {
            return new FileOutputStream(createTempFile(id));
        }
    }

    private static class FileDeletingInputStream extends FilterInputStream {

        private final File file;

        protected FileDeletingInputStream(File file) throws MalformedURLException, IOException {
            super(file.toURI().toURL().openStream());
            this.file = file;
        }

        @Override
        public void close() throws IOException {
            try {
                super.close();
            } finally {
                file.delete();
            }
        }
    }

    private static class NormalResourceResolver implements ResourceResolver {
        public Resource getResource(URI uri) throws IOException {
            return new Resource(uri.toURL().openStream());
        }

        public OutputStream getOutputStream(URI uri) throws IOException {
            return new FileOutputStream(new File(uri));
        }
    }

    private static final class SchemeAwareResourceResolver implements ResourceResolver {

        private final Map<String, ResourceResolver> schemeHandlingResourceResolvers;

        private final ResourceResolver defaultResolver;

        private SchemeAwareResourceResolver(
                Map<String, ResourceResolver> schemEHandlingResourceResolvers,
                ResourceResolver defaultResolver) {
            this.schemeHandlingResourceResolvers = schemEHandlingResourceResolvers;
            this.defaultResolver = defaultResolver;
        }

        private ResourceResolver getResourceResolverForScheme(URI uri) {
            String scheme = uri.getScheme();
            if (schemeHandlingResourceResolvers.containsKey(scheme)) {
                return schemeHandlingResourceResolvers.get(scheme);
            } else {
                return defaultResolver;
            }
        }

        /** {@inheritDoc} */
        public Resource getResource(URI uri) throws IOException {
            return getResourceResolverForScheme(uri).getResource(uri);
        }

        /** {@inheritDoc} */
        public OutputStream getOutputStream(URI uri) throws IOException {
            return getResourceResolverForScheme(uri).getOutputStream(uri);
        }
    }

    /**
     * Implementations of this interface will be builders for {@link ResourceResolver}, they bind
     * URI schemes to their respective resolver. This gives users more control over the mechanisms
     * by which URIs are resolved.
     * <p>
     * Here is an example of how this could be used:
     * </p>
     * <p><code>
     * SchemeAwareResourceResolverBuilder builder
     *      = ResourceResolverFactory.createSchemeAwareResourceResolverBuilder(defaultResolver);
     * builder.registerResourceResolverForScheme("test", testResolver);
     * builder.registerResourceResolverForScheme("anotherTest", test2Resolver);
     * ResourceResolver resolver = builder.build();
     * </code></p>
     * This will result in all URIs for the form "test:///..." will be resolved using the
     * <code>testResolver</code> object; URIs of the form "anotherTest:///..." will be resolved
     * using <code>test2Resolver</code>; all other URIs will be resolved from the defaultResolver.
     */
    public interface SchemeAwareResourceResolverBuilder {

        /**
         * Register a scheme with its respective {@link ResourceResolver}. This resolver will be
         * used as the only resolver for the specified scheme.
         *
         * @param scheme the scheme to be used with the given resolver
         * @param resourceResolver the resource resolver
         */
        void registerResourceResolverForScheme(String scheme, ResourceResolver resourceResolver);

        /**
         * Builds a {@link ResourceResolver} that will delegate to the respective resource resolver
         * when a registered URI scheme is given
         *
         * @return a resolver that delegates to the appropriate scheme resolver
         */
        ResourceResolver build();
    }

    private static final class CompletedSchemeAwareResourceResolverBuilder
            implements SchemeAwareResourceResolverBuilder {

        private static final SchemeAwareResourceResolverBuilder INSTANCE
                = new CompletedSchemeAwareResourceResolverBuilder();

        /** {@inheritDoc} */
        public ResourceResolver build() {
            throw new IllegalStateException("Resource resolver already built");
        }

        /** {@inheritDoc} */
        public void registerResourceResolverForScheme(String scheme,
                ResourceResolver resourceResolver) {
            throw new IllegalStateException("Resource resolver already built");
        }
    }

    private static final class ActiveSchemeAwareResourceResolverBuilder
            implements SchemeAwareResourceResolverBuilder {

        private final Map<String, ResourceResolver> schemeHandlingResourceResolvers
                = new HashMap<String, ResourceResolver>();

        private final ResourceResolver defaultResolver;

        private ActiveSchemeAwareResourceResolverBuilder(ResourceResolver defaultResolver) {
            this.defaultResolver = defaultResolver;
        }

        /** {@inheritDoc} */
        public void registerResourceResolverForScheme(String scheme,
                ResourceResolver resourceResolver) {
            schemeHandlingResourceResolvers.put(scheme, resourceResolver);
        }

        /** {@inheritDoc} */
        public ResourceResolver build() {
            return new SchemeAwareResourceResolver(
                    Collections.unmodifiableMap(schemeHandlingResourceResolvers), defaultResolver);
        }

    }

    private static final class SchemeAwareResourceResolverBuilderImpl
            implements SchemeAwareResourceResolverBuilder {

        private SchemeAwareResourceResolverBuilder delegate;

        private SchemeAwareResourceResolverBuilderImpl(ResourceResolver defaultResolver) {
            this.delegate = new ActiveSchemeAwareResourceResolverBuilder(defaultResolver);
        }

        /** {@inheritDoc} */
        public void registerResourceResolverForScheme(String scheme,
                ResourceResolver resourceResolver) {
            delegate.registerResourceResolverForScheme(scheme, resourceResolver);
        }

        /** {@inheritDoc} */
        public ResourceResolver build() {
            ResourceResolver resourceResolver = delegate.build();
            delegate = CompletedSchemeAwareResourceResolverBuilder.INSTANCE;
            return resourceResolver;
        }
    }
}

Pay attention to NormalResourceResolver and the getResource method implementation.

All the source code mentioned is related with FOP version 2.6 downloadable from here.