src/main/java/com/google/devtools/build/lib/util/ResourceFileLoader.java

// Copyright 2014 The Bazel Authors. All rights reserved.
//
// Licensed 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 com.google.devtools.build.lib.util;

import static java.nio.charset.StandardCharsets.UTF_8;

import com.google.common.io.ByteStreams;
import java.io.IOException;
import java.io.InputStream;

/**
 * A little utility to load resources (property files) from jars or the classpath. Recommended for
 * longer texts that do not fit nicely into a piece of Java code - e.g. a template for a lengthy
 * email.
 */
public final class ResourceFileLoader {

  private ResourceFileLoader() {}

  public static boolean resourceExists(Class<?> relativeToClass, String resourceName) {
    try (InputStream resourceStream = getResourceAsStream(relativeToClass, resourceName)) {
      return resourceStream != null;
    } catch (IOException e) {
      return false;
    }
  }

  /**
   * Loads a text resource that is located in a directory on the Java classpath that corresponds to
   * the package of <code>relativeToClass</code> using UTF8 encoding. E.g. <code>
   * loadResource(Class.forName("com.google.foo.Foo", "bar.txt"))</code> will look for <code>
   * com/google/foo/bar.txt</code> in the classpath.
   */
  public static String loadResource(Class<?> relativeToClass, String resourceName)
      throws IOException {
    try (InputStream stream = getResourceAsStream(relativeToClass, resourceName)) {
      if (stream == null) {
        throw new IOException(resourceName + " not found.");
      }
      return new String(ByteStreams.toByteArray(stream), UTF_8);
    }
  }

  private static InputStream getResourceAsStream(Class<?> relativeToClass, String resourceName) {
    ClassLoader loader = relativeToClass.getClassLoader();
    String resource = resolveResource(relativeToClass, resourceName);
    return loader.getResourceAsStream(resource);
  }

  /**
   * Converts a relative resource name and Java class to a full resource path, using the same logic
   * as {@link #loadResource}.
   */
  public static String resolveResource(Class<?> relativeToClass, String resourceName) {
    // TODO(bazel-team): use relativeToClass.getPackage().getName().
    String className = relativeToClass.getName();
    String packageName = className.substring(0, className.lastIndexOf('.'));
    String path = packageName.replace('.', '/');
    return path + '/' + resourceName;
  }
}