servicefactory.java

linux下建立JAVA虚拟机的源码KAFFE

  private static final class ServiceIterator
    implements Iterator
  {
    /**
     * The service provider interface (usually an interface, sometimes
     * an abstract class) which the services must implement.
     */
    private final Class spi;


    /**
     * An Enumeration<URL> over the URLs that contain a resource
     * <code>META-INF/services/&lt;org.foo.SomeService&gt;</code>,
     * as returned by {@link ClassLoader#getResources(String)}.
     */
    private final Enumeration urls;


    /**
     * The class loader used for loading service providers.
     */
    private final ClassLoader loader;


    /**
     * The security context used when loading and initializing service
     * providers. We want to load and initialize all plug-in service
     * providers under the same security context, namely the one that
     * was active when {@link #lookupProviders(Class, ClassLoader)} has
     * been called.
     */
    private final AccessControlContext securityContext;


    /**
     * A reader for the current file listing class names of service
     * implementors, or <code>null</code> when the last reader has
     * been fetched.
     */
    private BufferedReader reader;
    

    /**
     * The URL currently being processed. This is only used for
     * emitting error messages.
     */
    private URL currentURL;


    /**
     * The service provider that will be returned by the next call to
     * {@link #next()}, or <code>null</code> if the iterator has
     * already returned all service providers.
     */
    private Object nextProvider;


    /**
     * Constructs an Iterator that loads and initializes services on
     * demand.
     *
     * @param spi the service provider interface which the services
     * must implement. Usually, this is a Java interface type, but it
     * might also be an abstract class or even a concrete superclass.
     *
     * @param urls an Enumeration<URL> over the URLs that contain a
     * resource
     * <code>META-INF/services/&lt;org.foo.SomeService&gt;</code>, as
     * determined by {@link ClassLoader#getResources(String)}.
     *
     * @param loader the ClassLoader that gets used for loading
     * service providers.
     *
     * @param securityContext the security context to use when loading
     * and initializing service providers.
     */
    ServiceIterator(Class spi, Enumeration urls, ClassLoader loader,
                    AccessControlContext securityContext)
    {
      this.spi = spi;
      this.urls = urls;
      this.loader = loader;
      this.securityContext = securityContext;
      this.nextProvider = loadNextServiceProvider();
    }


    /**
     * @throws NoSuchElementException if {@link #hasNext} returns
     * <code>false</code>.
     */
    public Object next()
    {
      Object result;

      if (!hasNext())
        throw new NoSuchElementException();

      result = nextProvider;
      nextProvider = loadNextServiceProvider();
      return result;
    }


    public boolean hasNext()
    {
      return nextProvider != null;
    }


    public void remove()
    {
      throw new UnsupportedOperationException();
    }


    private Object loadNextServiceProvider()
    {
      String line;
      
      if (reader == null)
        advanceReader();

      for (;;)
        {
          /* If we have reached the last provider list, we cannot
           * retrieve any further lines.
           */
          if (reader == null)
            return null;

          try
            {
              line = reader.readLine();
            }
          catch (IOException readProblem)
            {
              log(Level.WARNING, "IOException upon reading {0}", currentURL,
                  readProblem);
              line = null;
            }

          /* When we are at the end of one list of services,
           * switch over to the next one.
           */
          if (line == null)
            {
              advanceReader();
              continue;
            }


          // Skip whitespace at the beginning and end of each line.
          line = line.trim();

          // Skip empty lines.
          if (line.length() == 0)
            continue;

          // Skip comment lines.
          if (line.charAt(0) == '#')
            continue;

          try
            {
              log(Level.FINE,
                  "Loading service provider \"{0}\", specified"
                  + " by \"META-INF/services/{1}\" in {2}.",
                  new Object[] { line, spi.getName(), currentURL },
                  null);

              /* Load the class in the security context that was
               * active when calling lookupProviders.
               */
              return AccessController.doPrivileged(
                new ServiceProviderLoadingAction(spi, line, loader),
                securityContext);
            }
          catch (Exception ex)
            {
              String msg = "Cannot load service provider class \"{0}\","
                + " specified by \"META-INF/services/{1}\" in {2}";
              if (ex instanceof PrivilegedActionException
                  && ex.getCause() instanceof ClassCastException)
                msg = "Service provider class \"{0}\" is not an instance"
                  + " of \"{1}\". Specified"
                  + " by \"META-INF/services/{1}\" in {2}.";

              log(Level.WARNING, msg,                  
                  new Object[] { line, spi.getName(), currentURL },
                  ex);
              continue;
            }
        }
    }


    private void advanceReader()
    {
      do
        {
          if (reader != null)
            {
              try
                {
                  reader.close();
                  log(Level.FINE, "closed {0}", currentURL, null);
                }
              catch (Exception ex)
                {
                  log(Level.WARNING, "cannot close {0}", currentURL, ex);
                }
              reader = null;
              currentURL = null;
            }

        if (!urls.hasMoreElements())
          return;

        currentURL = (URL) urls.nextElement();
        try
          {
            reader = new BufferedReader(new InputStreamReader(
              currentURL.openStream(), "UTF-8"));
            log(Level.FINE, "opened {0}", currentURL, null);
          }
        catch (Exception ex)
          {
            log(Level.WARNING, "cannot open {0}", currentURL, ex);
          }
        }
      while (reader == null);
    }
  }


  // Package-private to avoid a trampoline.
  /**
   * Passes a log message to the <code>java.util.logging</code>
   * framework. This call returns very quickly if no log message will
   * be produced, so there is not much overhead in the standard case.
   *
   * @param level the severity of the message, for instance {@link
   * Level#WARNING}.
   *
   * @param msg the log message, for instance <code>&#x201c;Could not
   * load {0}.&#x201d;</code>
   *
   * @param param the parameter(s) for the log message, or
   * <code>null</code> if <code>msg</code> does not specify any
   * parameters. If <code>param</code> is not an array, an array with
   * <code>param</code> as its single element gets passed to the
   * logging framework.
   *
   * @param t a Throwable that is associated with the log record, or
   * <code>null</code> if the log message is not associated with a
   * Throwable.
   */
  static void log(Level level, String msg, Object param, Throwable t)
  {
    LogRecord rec;

    // Return quickly if no log message will be produced.
    if (!LOGGER.isLoggable(level))
      return;

    rec = new LogRecord(level, msg);
    if (param != null && param.getClass().isArray())
      rec.setParameters((Object[]) param);
    else
      rec.setParameters(new Object[] { param });

    rec.setThrown(t);

    // While java.util.logging can sometimes infer the class and
    // method of the caller, this automatic inference is not reliable
    // on highly optimizing VMs. Also, log messages make more sense to
    // developers when they display a public method in a public class;
    // otherwise, they might feel tempted to figure out the internals
    // of ServiceFactory in order to understand the problem.
    rec.setSourceClassName(ServiceFactory.class.getName());
    rec.setSourceMethodName("lookupProviders");

    LOGGER.log(rec);
  }
}