为什么在 Android 世界里 ClassLoader.getResourceAsStream 这么慢？

@Override
protected URL findResource(String name) {
    return pathList.findResource(name);
}

public class BaseDexClassLoader extends ClassLoader {
  private final DexPathList pathList;

  /**
   * Constructs an instance.
   *
   * @param dexPath the list of jar/apk files containing classes and
   * resources, delimited by {@code File.pathSeparator}, which
   * defaults to {@code ":"} on Android
   * @param optimizedDirectory directory where optimized dex files
   * should be written; may be {@code null}
   * @param libraryPath the list of directories containing native
   * libraries, delimited by {@code File.pathSeparator}; may be
   * {@code null}
   * @param parent the parent class loader
   */
  public BaseDexClassLoader(String dexPath, File optimizedDirectory,
          String libraryPath, ClassLoader parent) {
      super(parent);
      this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
  }

/**
 * A pair of lists of entries, associated with a {@code ClassLoader}.
 * One of the lists is a dex/resource path — typically referred
 * to as a "class path" — list, and the other names directories
 * containing native code libraries. Class path entries may be any of:
 * a {@code .jar} or {@code .zip} file containing an optional
 * top-level {@code classes.dex} file as well as arbitrary resources,
 * or a plain {@code .dex} file (with no possibility of associated
 * resources).
 *
 * 
This class also contains methods to use these lists to look up
 * classes and resources.
 */
/*package*/ final class DexPathList {

/**
 * Finds the named resource in one of the zip/jar files pointed at
 * by this instance. This will find the one in the earliest listed
 * path element.
 *
 * @return a URL to the named resource or {@code null} if the
 * resource is not found in any of the zip/jar files
 */
public URL findResource(String name) {
    for (Element element : dexElements) {
        URL url = element.findResource(name);
        if (url != null) {
            return url;
        }
    }

    return null;
}

public URL findResource(String name) {
  maybeInit();

  // We support directories so we can run tests and/or legacy code
  // that uses Class.getResource.
  if (isDirectory) {
      File resourceFile = new File(dir, name);
      if (resourceFile.exists()) {
          try {
              return resourceFile.toURI().toURL();
          } catch (MalformedURLException ex) {
              throw new RuntimeException(ex);
          }
      }
  }

  if (zipFile == null || zipFile.getEntry(name) == null) {
      /*
       * Either this element has no zip/jar file (first
       * clause), or the zip/jar file doesn't have an entry
       * for the given name (second clause).
       */
      return null;
  }

  try {
      /*
       * File.toURL() is compliant with RFC 1738 in
       * always creating absolute path names. If we
       * construct the URL by concatenating strings, we
       * might end up with illegal URLs for relative
       * names.
       */
      return new URL("jar:" + zip.toURL() + "!/" + name);
  } catch (MalformedURLException ex) {
      throw new RuntimeException(ex);
  }
}

if (zipFile == null || zipFile.getEntry(name) == null) {

...
  private final LinkedHashMap entries = new LinkedHashMap();  
...
  public ZipEntry getEntry(String entryName) {
      checkNotClosed();
      if (entryName == null) {
          throw new NullPointerException("entryName == null");
      }

      ZipEntry ze = entries.get(entryName);
      if (ze == null) {
          ze = entries.get(entryName + "/");
      }
      return ze;
  }

public synchronized void maybeInit() {
  if (initialized) {
      return;
  }

  initialized = true;

  if (isDirectory || zip == null) {
      return;
  }

  try {
      zipFile = new ZipFile(zip);
  } catch (IOException ioe) {
      /*
       * Note: ZipException (a subclass of IOException)
       * might get thrown by the ZipFile constructor
       * (e.g. if the file isn't actually a zip/jar
       * file).
       */
      System.logE("Unable to open zip file: " + zip, ioe);
      zipFile = null;
  }
}

zipFile = new ZipFile(zip);

public ZipFile(File file) throws ZipException, IOException {
    this(file, OPEN_READ);
}

  public InputStream getResourceAsStream(String resName) {
    try {
      long start; long end;

      start = System.currentTimeMillis();
      URL url = getResource(resName);
      end = System.currentTimeMillis();
      Logger.getLogger("NimbleDroid RESEARCH").info("getResource: " + (end - start));

      if (url != null) {
          start = System.currentTimeMillis();
          InputStream inputStream = url.openStream();
          end = System.currentTimeMillis();
          Logger.getLogger("NimbleDroid RESEARCH").info("url.openStream: " + (end - start));

          return inputStream;
      }
      ...

@Override
public InputStream getInputStream() throws IOException {
    if (closed) {
        throw new IllegalStateException("JarURLConnection InputStream has been closed");
    }
    connect();
    if (jarInput != null) {
        return jarInput;
    }
    if (jarEntry == null) {
        throw new IOException("Jar entry not specified");
    }
    return jarInput = new JarURLConnectionInputStream(jarFile
            .getInputStream(jarEntry), jarFile);
}

@Override
public void connect() throws IOException {
    if (!connected) {
        findJarFile(); // ensure the file can be found
        findJarEntry(); // ensure the entry, if any, can be found
        connected = true;
    }
}

private void findJarFile() throws IOException {
    if (getUseCaches()) {
        synchronized (jarCache) {
            jarFile = jarCache.get(jarFileURL);
        }
        if (jarFile == null) {
            JarFile jar = openJarFile();
            synchronized (jarCache) {
                jarFile = jarCache.get(jarFileURL);
                if (jarFile == null) {
                    jarCache.put(jarFileURL, jar);
                    jarFile = jar;
                } else {
                    jar.close();
                }
            }
        }
    } else {
        jarFile = openJarFile();
    }

    if (jarFile == null) {
        throw new IOException();
    }
}

public abstract class URLConnection {
...
  private static boolean defaultUseCaches = true;
  ...

private JarFile openJarFile() throws IOException {
  if (jarFileURL.getProtocol().equals("file")) {
      String decodedFile = UriCodec.decode(jarFileURL.getFile());
      return new JarFile(new File(decodedFile), true, ZipFile.OPEN_READ);
  } else {
    ...

/**
 * Create a new {@code JarFile} using the contents of file.
 *
 * @param file
 *            the JAR file as {@link File}.
 * @param verify
 *            if this JAR filed is signed whether it must be verified.
 * @param mode
 *            the mode to use, either {@link ZipFile#OPEN_READ OPEN_READ} or
 *            {@link ZipFile#OPEN_DELETE OPEN_DELETE}.
 * @throws IOException
 *             If the file cannot be read.
 */
public JarFile(File file, boolean verify, int mode) throws IOException {
    super(file, mode);

    // Step 1: Scan the central directory for meta entries (MANIFEST.mf
    // & possibly the signature files) and read them fully.
    HashMap metaEntries = readMetaEntries(this, verify);

    // Step 2: Construct a verifier with the information we have.
    // Verification is possible *only* if the JAR file contains a manifest
    // *AND* it contains signing related information (signature block
    // files and the signature files).
    //
    // TODO: Is this really the behaviour we want if verify == true ?
    // We silently skip verification for files that have no manifest or
    // no signatures.
    if (verify && metaEntries.containsKey(MANIFEST_NAME) &&
            metaEntries.size() > 1) {
        // We create the manifest straight away, so that we can create
        // the jar verifier as well.
        manifest = new Manifest(metaEntries.get(MANIFEST_NAME), true);
        verifier = new JarVerifier(getName(), manifest, metaEntries);
    } else {
        verifier = null;
        manifestBytes = metaEntries.get(MANIFEST_NAME);
    }
}

public Class findClass(String name, List suppressed) {
  for (Element element : dexElements) {
      DexFile dex = element.dexFile;

      if (dex != null) {
          Class clazz = dex.loadClassBinaryName(name, definingContext, suppressed);
          if (clazz != null) {
              return clazz;
          }
      }
  }
  if (dexElementsSuppressedExceptions != null) {
      suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));
  }
  return null;
}