indexer-reader/src/test/java/org/apache/maven/index/reader/CachingResourceHandler.java

package org.apache.maven.index.reader;

/*
 * 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.
 */

import org.apache.maven.index.reader.WritableResourceHandler.WritableResource;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.HashSet;
import java.util.Set;

/**
 * A trivial caching {@link ResourceHandler} that caches forever during single session (existence of the instance).
 */
public class CachingResourceHandler
    implements ResourceHandler
{
    private static final Resource NOT_EXISTING_RESOURCE = new Resource()
    {
        public InputStream read()
            throws IOException
        {
            return null;
        }
    };

    private final WritableResourceHandler local;

    private final ResourceHandler remote;

    private final Set<String> notFoundResources;

    public CachingResourceHandler( final WritableResourceHandler local, final ResourceHandler remote )
    {
        if ( local == null || remote == null )
        {
            throw new NullPointerException( "null resource handler" );
        }
        this.local = local;
        this.remote = remote;
        this.notFoundResources = new HashSet<>();
    }

    public Resource locate( final String name )
        throws IOException
    {
        if ( notFoundResources.contains( name ) )
        {
            return NOT_EXISTING_RESOURCE;
        }
        else
        {
            return new CachingResource( name );
        }
    }

    private class CachingResource
        implements Resource
    {
        private final String name;

        private CachingResource( final String name )
        {
            this.name = name;
        }

        public InputStream read()
            throws IOException
        {
            InputStream inputStream = local.locate( name ).read();
            if ( inputStream != null )
            {
                return inputStream;
            }
            if ( cacheLocally( name ) )
            {
                return local.locate( name ).read();
            }
            notFoundResources.add( name );
            return null;
        }

        private boolean cacheLocally( final String name )
            throws IOException
        {
            final Resource remoteResource = remote.locate( name );
            final WritableResource localResource = local.locate( name );
            try (final InputStream inputStream = remoteResource.read(); //
                 final OutputStream outputStream = localResource.write())
            {
                if ( inputStream != null )
                {
                    int read;
                    byte[] bytes = new byte[8192];
                    while ( ( read = inputStream.read( bytes ) ) != -1 )
                    {
                        outputStream.write( bytes, 0, read );
                    }
                    outputStream.flush();
                    return true;
                }
                return false;
            }
            finally
            {
                localResource.close();
            }
        }
    }

    public void close()
        throws IOException
    {
        remote.close();
        local.close();
    }
}