org.apache.shindig.gadgets.preload.ConcurrentPreloads.java Source code

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Here is the source code for org.apache.shindig.gadgets.preload.ConcurrentPreloads.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.shindig.gadgets.preload;

import java.util.Collection;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;

import com.google.common.base.Function;
import com.google.common.collect.ForwardingCollection;
import com.google.common.collect.Lists;

/**
 * Preloads data by evaluating Futures for PreloadedData.
 * This class is not, however, thread-safe - tasks must be
 * added and read from a single thread..
 */
class ConcurrentPreloads extends ForwardingCollection<PreloadedData> {
    private final List<Future<PreloadedData>> tasks;
    private Collection<PreloadedData> loaded;

    ConcurrentPreloads() {
        tasks = Lists.newArrayList();
    }

    ConcurrentPreloads(int size) {
        tasks = Lists.newArrayListWithCapacity(size);
    }

    /**
     * Add an active preloading process.
     *
     * @param futureData A future that will return the preloaded data.
     */
    ConcurrentPreloads add(Future<PreloadedData> futureData) {
        tasks.add(futureData);
        return this;
    }

    @Override
    protected Collection<PreloadedData> delegate() {
        if (loaded == null) {
            loaded = getData();
        }

        return loaded;
    }

    private Collection<PreloadedData> getData() {
        return Lists.transform(tasks, new Function<Future<PreloadedData>, PreloadedData>() {
            public PreloadedData apply(Future<PreloadedData> preloadedDataFuture) {
                return getPreloadedData(preloadedDataFuture);
            }
        });
    }

    /**
     * Gets the preloaded data, handling any exceptions from Future processing.
     */
    protected PreloadedData getPreloadedData(Future<PreloadedData> preloadedDataFuture) {
        try {
            return preloadedDataFuture.get();
        } catch (ExecutionException ee) {
            return new FailedPreload(ee.getCause());
        } catch (InterruptedException ie) {
            // Do NOT Propagate the interrupt
            throw new RuntimeException("Preloading was interrupted by thread termination", ie);
        }
    }

    /** PreloadData implementation that reports failure */
    private static class FailedPreload implements PreloadedData {
        private final Throwable t;

        public FailedPreload(Throwable t) {
            this.t = t;
        }

        public Collection<Object> toJson() throws PreloadException {
            if (t instanceof PreloadException) {
                throw (PreloadException) t;
            }

            throw new PreloadException(t);
        }
    }
}