common/io/Resources.java

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * 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.common.io;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Charsets;
import com.google.common.base.MoreObjects;
import com.google.common.collect.Lists;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URL;
import java.nio.charset.Charset;
import java.util.List;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Provides utility methods for working with resources in the classpath. Note that even though these
 * methods use {@link URL} parameters, they are usually not appropriate for HTTP or other
 * non-classpath resources.
 *
 * <p>All method parameters must be non-null unless documented otherwise.
 *
 * @author Chris Nokleberg
 * @author Ben Yu
 * @author Colin Decker
 * @since 1.0
 */
@Beta
@GwtIncompatible
@ElementTypesAreNonnullByDefault
public final class Resources {
  private Resources() {}

  /**
   * Returns a {@link ByteSource} that reads from the given URL.
   *
   * @since 14.0
   */
  public static ByteSource asByteSource(URL url) {
    return new UrlByteSource(url);
  }

  /** A byte source that reads from a URL using {@link URL#openStream()}. */
  private static final class UrlByteSource extends ByteSource {

    private final URL url;

    private UrlByteSource(URL url) {
      this.url = checkNotNull(url);
    }

    @Override
    public InputStream openStream() throws IOException {
      return url.openStream();
    }

    @Override
    public String toString() {
      return "Resources.asByteSource(" + url + ")";
    }
  }

  /**
   * Returns a {@link CharSource} that reads from the given URL using the given character set.
   *
   * @since 14.0
   */
  public static CharSource asCharSource(URL url, Charset charset) {
    return asByteSource(url).asCharSource(charset);
  }

  /**
   * Reads all bytes from a URL into a byte array.
   *
   * @param url the URL to read from
   * @return a byte array containing all the bytes from the URL
   * @throws IOException if an I/O error occurs
   */
  public static byte[] toByteArray(URL url) throws IOException {
    return asByteSource(url).read();
  }

  /**
   * Reads all characters from a URL into a {@link String}, using the given character set.
   *
   * @param url the URL to read from
   * @param charset the charset used to decode the input stream; see {@link Charsets} for helpful
   *     predefined constants
   * @return a string containing all the characters from the URL
   * @throws IOException if an I/O error occurs.
   */
  public static String toString(URL url, Charset charset) throws IOException {
    return asCharSource(url, charset).read();
  }

  /**
   * Streams lines from a URL, stopping when our callback returns false, or we have read all of the
   * lines.
   *
   * @param url the URL to read from
   * @param charset the charset used to decode the input stream; see {@link Charsets} for helpful
   *     predefined constants
   * @param callback the LineProcessor to use to handle the lines
   * @return the output of processing the lines
   * @throws IOException if an I/O error occurs
   */
  @CanIgnoreReturnValue // some processors won't return a useful result
  @ParametricNullness
  public static <T extends @Nullable Object> T readLines(
      URL url, Charset charset, LineProcessor<T> callback) throws IOException {
    return asCharSource(url, charset).readLines(callback);
  }

  /**
   * Reads all of the lines from a URL. The lines do not include line-termination characters, but do
   * include other leading and trailing whitespace.
   *
   * <p>This method returns a mutable {@code List}. For an {@code ImmutableList}, use {@code
   * Resources.asCharSource(url, charset).readLines()}.
   *
   * @param url the URL to read from
   * @param charset the charset used to decode the input stream; see {@link Charsets} for helpful
   *     predefined constants
   * @return a mutable {@link List} containing all the lines
   * @throws IOException if an I/O error occurs
   */
  public static List<String> readLines(URL url, Charset charset) throws IOException {
    // don't use asCharSource(url, charset).readLines() because that returns
    // an immutable list, which would change the behavior of this method
    return readLines(
        url,
        charset,
        new LineProcessor<List<String>>() {
          final List<String> result = Lists.newArrayList();

          @Override
          public boolean processLine(String line) {
            result.add(line);
            return true;
          }

          @Override
          public List<String> getResult() {
            return result;
          }
        });
  }

  /**
   * Copies all bytes from a URL to an output stream.
   *
   * @param from the URL to read from
   * @param to the output stream
   * @throws IOException if an I/O error occurs
   */
  public static void copy(URL from, OutputStream to) throws IOException {
    asByteSource(from).copyTo(to);
  }

  /**
   * Returns a {@code URL} pointing to {@code resourceName} if the resource is found using the
   * {@linkplain Thread#getContextClassLoader() context class loader}. In simple environments, the
   * context class loader will find resources from the class path. In environments where different
   * threads can have different class loaders, for example app servers, the context class loader
   * will typically have been set to an appropriate loader for the current thread.
   *
   * <p>In the unusual case where the context class loader is null, the class loader that loaded
   * this class ({@code Resources}) will be used instead.
   *
   * @throws IllegalArgumentException if the resource is not found
   */
  @CanIgnoreReturnValue // being used to check if a resource exists
  // TODO(cgdecker): maybe add a better way to check if a resource exists
  // e.g. Optional<URL> tryGetResource or boolean resourceExists
  public static URL getResource(String resourceName) {
    ClassLoader loader =
        MoreObjects.firstNonNull(
            Thread.currentThread().getContextClassLoader(), Resources.class.getClassLoader());
    URL url = loader.getResource(resourceName);
    checkArgument(url != null, "resource %s not found.", resourceName);
    return url;
  }

  /**
   * Given a {@code resourceName} that is relative to {@code contextClass}, returns a {@code URL}
   * pointing to the named resource.
   *
   * @throws IllegalArgumentException if the resource is not found
   */
  @CanIgnoreReturnValue // being used to check if a resource exists
  public static URL getResource(Class<?> contextClass, String resourceName) {
    URL url = contextClass.getResource(resourceName);
    checkArgument(
        url != null, "resource %s relative to %s not found.", resourceName, contextClass.getName());
    return url;
  }
}