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Dropwizard bundle to make working with Webjars (http://webjars.org) easier.
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package io.dropwizard.bundles.webjars;

import com.google.common.base.Charsets;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.common.io.ByteStreams;
import com.google.common.io.Resources;
import com.google.common.net.MediaType;
import java.net.URL;
import java.nio.charset.Charset;
import org.eclipse.jetty.http.MimeTypes;

/** Locates an loads a particular WebJar asset from the classpath. */
class AssetLoader extends CacheLoader {
  public static final Asset NOT_FOUND = new Asset(null, null);

  // For determining content type and content encoding
  private static final MimeTypes MIME_TYPES = new MimeTypes();
  private static final MediaType DEFAULT_MEDIA_TYPE = MediaType.HTML_UTF_8;
  private static final Charset DEFAULT_CHARSET = Charsets.UTF_8;

  private final LoadingCache versionCache;

  AssetLoader(CacheLoader versionLoader) {
    versionCache = CacheBuilder.newBuilder()
        .maximumSize(10)
        .build(versionLoader);
  }

  @Override
  public Asset load(AssetId id) throws Exception {
    String version = versionCache.getUnchecked(id.library);
    if (VersionLoader.NOT_FOUND.equals(version)) {
      return NOT_FOUND;
    }

    // Sometimes the WebJar has multiple releases which gets represented by a -# suffix to the
    // version number inside of pom.properties.  When this happens, the files inside of the jar
    // don't include the WebJar release number as part of the file path.  For example, the angularjs
    // 1.1.1 WebJar has a version inside of pom.properties of 1.1.1-1.  But the path to angular.js
    // inside of the jar is META-INF/resources/webjars/angularjs/1.1.1/angular.js.
    //
    // Alternatively sometimes the developer of the library includes a -suffix in the true library
    // version.  In these cases the WebJar pom.properties will include that suffix in the version
    // number, and the file paths inside of the jar will also include the suffix.  For example, the
    // backbone-marionette 1.0.0-beta6 WebJar has a version inside of pom.properties of 1.0.0-beta6.
    // The path to backbone.marionette.js is also
    // META-INF/resources/webjars/backbone-marionette/1.0.0-beta6/backbone.marionette.js.
    //
    // So based on the data inside of pom.properties it's going to be impossible to determine
    // whether a -suffix should be stripped off or not.  A reasonable heuristic however is going to
    // be to just keep trying over and over starting with the most specific version number, then
    // stripping a suffix off at a time until there are no more suffixes and the right version
    // number is determined.
    do {
      final String pathPattern = "META-INF/resources/webjars/%s/%s/%s";
      final String path = String.format(pathPattern, id.library, version, id.resource);

      try {
        URL resource = Resources.getResource(path);

        // Determine the media type of this resource
        MediaType mediaType = getMediaType(path);

        // We know that this version was valid.  Update the version cache to make sure that we
        // remember it for next time around.
        versionCache.put(id.library, version);

        return new Asset(ByteStreams.toByteArray(resource.openStream()), mediaType);
      } catch (IllegalArgumentException e) {
        // ignored
      }

      // Trim a suffix off of the version number
      int hyphen = version.lastIndexOf('-');
      if (hyphen == -1) {
        return NOT_FOUND;
      }

      version = version.substring(0, hyphen);
    }
    while (true);
  }

  private MediaType getMediaType(String path) {
    String mimeType = MIME_TYPES.getMimeByExtension(path);
    if (mimeType == null) {
      return DEFAULT_MEDIA_TYPE;
    }

    MediaType mediaType;
    try {
      mediaType = MediaType.parse(mimeType);

      if (mediaType.is(MediaType.ANY_TEXT_TYPE)) {
        mediaType = mediaType.withCharset(DEFAULT_CHARSET);
      }
    } catch (IllegalArgumentException e) {
      return DEFAULT_MEDIA_TYPE;
    }

    return mediaType;
  }
}