SPI.java

  /*
   * @(#)Service.java    1.16 06/10/10
   *
   * Copyright  1990-2006 Sun Microsystems, Inc. All Rights Reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER
   * 
   * This program is free software; you can redistribute it and/or
   * modify it under the terms of the GNU General Public License version
   * 2 only, as published by the Free Software Foundation. 
   * 
   * This program is distributed in the hope that it will be useful, but
   * WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
   * General Public License version 2 for more details (a copy is
   * included at /legal/license.txt). 
   * 
   * You should have received a copy of the GNU General Public License
   * version 2 along with this work; if not, write to the Free Software
   * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
   * 02110-1301 USA 
   * 
   * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa
   * Clara, CA 95054 or visit www.sun.com if you need additional
   * information or have any questions. 
   */
  
  package sun.misc;
  
  import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.InputStreamReader;
  import java.net.URL;
  import java.util.ArrayList;
  import java.util.Enumeration;
  import java.util.Iterator;
  import java.util.List;
  import java.util.NoSuchElementException;
  import java.util.Set;
  import java.util.TreeSet;
  
  /**
   * A simple service-provider lookup mechanism.  A <i>service</i> is a
   * well-known set of interfaces and (usually abstract) classes.  A <i>service
   * provider</i> is a specific implementation of a service.  The classes in a
   * provider typically implement the interfaces and subclass the classes defined
   * in the service itself.  Service providers may be installed in an
   * implementation of the Java platform in the form of extensions, that is, jar
   * files placed into any of the usual extension directories.  Providers may
   * also be made available by adding them to the applet or application class
   * path or by some other platform-specific means.
   *
   * <p> In this lookup mechanism a service is represented by an interface or an
   * abstract class.  (A concrete class may be used, but this is not
   * recommended.)  A provider of a given service contains one or more concrete
   * classes that extend this <i>service class</i> with data and code specific to
   * the provider.  This <i>provider class</i> will typically not be the entire
   * provider itself but rather a proxy that contains enough information to
   * decide whether the provider is able to satisfy a particular request together
   * with code that can create the actual provider on demand.  The details of
   * provider classes tend to be highly service-specific; no single class or
   * interface could possibly unify them, so no such class has been defined.  The
   * only requirement enforced here is that provider classes must have a
   * zero-argument constructor so that they may be instantiated during lookup.
   *
   * <p> A service provider identifies itself by placing a provider-configuration
   * file in the resource directory <tt>META-INF/services</tt>.  The file's name
   * should consist of the fully-qualified name of the abstract service class.
   * The file should contain a list of fully-qualified concrete provider-class
   * names, one per line.  Space and tab characters surrounding each name, as
   * well as blank lines, are ignored.  The comment character is <tt>'#'</tt>
   * (<tt>0x23</tt>); on each line all characters following the first comment
   * character are ignored.  The file must be encoded in UTF-8.
   *
   * <p> If a particular concrete provider class is named in more than one
   * configuration file, or is named in the same configuration file more than
   * once, then the duplicates will be ignored.  The configuration file naming a
   * particular provider need not be in the same jar file or other distribution
   * unit as the provider itself.  The provider must be accessible from the same
   * class loader that was initially queried to locate the configuration file;
   * note that this is not necessarily the class loader that found the file.
   *
   * <p> <b>Example:</b> Suppose we have a service class named
   * <tt>java.io.spi.CharCodec</tt>.  It has two abstract methods:
   *
   * <pre>
   *   public abstract CharEncoder getEncoder(String encodingName);
   *   public abstract CharDecoder getDecoder(String encodingName);
   * </pre>
   *
   * Each method returns an appropriate object or <tt>null</tt> if it cannot
   * translate the given encoding.  Typical <tt>CharCodec</tt> providers will
   * support more than one encoding.
   *
   * <p> If <tt>sun.io.StandardCodec</tt> is a provider of the <tt>CharCodec</tt>
   * service then its jar file would contain the file
   * <tt>META-INF/services/java.io.spi.CharCodec</tt>.  This file would contain
   * the single line:
   *
   * <pre>
   *   sun.io.StandardCodec    # Standard codecs for the platform
   * </pre>
   *
   * To locate an encoder for a given encoding name, the internal I/O code would
   * do something like this:
   *
   * <pre>
 	   *   CharEncoder getEncoder(String encodingName) {
  *       Iterator ps = Service.providers(CharCodec.class);
 	   *       while (ps.hasNext()) {
  *           CharCodec cc = (CharCodec)ps.next();
  *           CharEncoder ce = cc.getEncoder(encodingName);
  *           if (ce != null)
  *               return ce;
  *       }
  *       return null;
  *   }
  * </pre>
  *
  * The provider-lookup mechanism always executes in the security context of the
  * caller.  Trusted system code should typically invoke the methods in this
  * class from within a privileged security context.
  *
  * @author Mark Reinhold
  * @version 1.10, 02/08/19
  * @since 1.3
  */
 
 	  public final class Service {
     private static final String prefix = "META-INF/services/";
     private Service() {}
 
     private static void fail(Class service, String msg, Throwable cause)
     throws ServiceConfigurationError
 	      {
     ServiceConfigurationError sce
         = new ServiceConfigurationError(service.getName() + ": " + msg);
     sce.initCause(cause);
     throw sce;
     }
 
     private static void fail(Class service, String msg)
 	          throws ServiceConfigurationError {
         throw new ServiceConfigurationError(service.getName() + ": " + msg);
     }
 
     private static void fail(Class service, URL u, int line, String msg)
 	          throws ServiceConfigurationError {
         fail(service, u + ":" + line + ": " + msg);
     }
 
     /**
      * Parse a single line from the given configuration file, adding the name
      * on the line to both the names list and the returned set iff the name is
      * not already a member of the returned set.
      */
     private static int parseLine(Class service, URL u, BufferedReader r, int lc,
         List names, Set returned)
 	          throws IOException, ServiceConfigurationError {
         String ln = r.readLine();
 	          if (ln == null) {
             return -1;
         }
         int ci = ln.indexOf('#');
         if (ci >= 0) ln = ln.substring(0, ci);
         ln = ln.trim();
         int n = ln.length();
 	          if (n != 0) {
             if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))
                 fail(service, u, lc, "Illegal configuration-file syntax");
             if (!Character.isJavaIdentifierStart(ln.charAt(0)))
                 fail(service, u, lc, "Illegal provider-class name: " + ln);
 	              for (int i = 1; i < n; i++) {
                 char c = ln.charAt(i);
                 if (!Character.isJavaIdentifierPart(c) && (c != '.'))
                     fail(service, u, lc, "Illegal provider-class name: " + ln);
             }
 	              if (!returned.contains(ln)) {
                 names.add(ln);
                 returned.add(ln);
             }
         }
         return lc + 1;
     }
 
     /**
      * Parse the content of the given URL as a provider-configuration file.
      *
      * @param  service
      *         The service class for which providers are being sought;
      *         used to construct error detail strings
      *
      * @param  url
      *         The URL naming the configuration file to be parsed
      *
      * @param  returned
      *         A Set containing the names of provider classes that have already
      *         been returned.  This set will be updated to contain the names
      *         that will be yielded from the returned <tt>Iterator</tt>.
      *
      * @return A (possibly empty) <tt>Iterator</tt> that will yield the
      *         provider-class names in the given configuration file that are
      *         not yet members of the returned set
      *
      * @throws ServiceConfigurationError
      *         If an I/O error occurs while reading from the given URL, or
      *         if a configuration-file format error is detected
      */
     private static Iterator parse(Class service, URL u, Set returned)
 	          throws ServiceConfigurationError {
         InputStream in = null;
         BufferedReader r = null;
         ArrayList names = new ArrayList();
 	          try {
             in = u.openStream();
             r = new BufferedReader(new InputStreamReader(in, "utf-8"));
             int lc = 1;
             while ((lc = parseLine(service, u, r, lc, names, returned)) >= 0);
         } catch (IOException x) {
             fail(service, ": " + x);
         } finally {
 	              try {
                 if (r != null) r.close();
                 if (in != null) in.close();
             } catch (IOException y) {
                 fail(service, ": " + y);
             }
         }
         return names.iterator();
     }
     /**
      * Private inner class implementing fully-lazy provider lookup
      */
 	      private static class LazyIterator implements Iterator {
         Class service;
         ClassLoader loader;
         Enumeration configs = null;
         Iterator pending = null;
         Set returned = new TreeSet();
         String nextName = null;
 	          private LazyIterator(Class service, ClassLoader loader) {
             this.service = service;
             this.loader = loader;
         }
 
 	          public boolean hasNext() throws ServiceConfigurationError {
 	              if (nextName != null) {
                 return true;
             }
 	              if (configs == null) {
 	                  try {
                     String fullName = prefix + service.getName();
                     if (loader == null)
                         configs = ClassLoader.getSystemResources(fullName);
                     else
                         configs = loader.getResources(fullName);
                 } catch (IOException x) {
                     fail(service, ": " + x);
                 }
             }
 	              while ((pending == null) || !pending.hasNext()) {
 	                  if (!configs.hasMoreElements()) {
                     return false;
                 }
                 pending = parse(service, (URL) configs.nextElement(), returned);
             }
             nextName = (String) pending.next();
             return true;
         }
 
 	          public Object next() throws ServiceConfigurationError {
 	              if (!hasNext()) {
                 throw new NoSuchElementException();
             }
             String cn = nextName;
             nextName = null;
 	              try {
                 return Class.forName(cn, true, loader).newInstance();
             } catch (ClassNotFoundException x) {
                 fail(service,
                     "Provider " + cn + " not found");
             } catch (Exception x) {
                 fail(service,
                     "Provider " + cn + " could not be instantiated: " + x);
             }
             return null;    /* This cannot happen */
         }
 
 	      public void remove() {
         throw new UnsupportedOperationException();
     }
 
     }
     /**
      * Locates and incrementally instantiates the available providers of a
      * given service using the given class loader.
      *
      * <p> This method transforms the name of the given service class into a
      * provider-configuration filename as described above and then uses the
      * <tt>getResources</tt> method of the given class loader to find all
      * available files with that name.  These files are then read and parsed to
      * produce a list of provider-class names.  The iterator that is returned
      * uses the given class loader to lookup and then instantiate each element
      * of the list.
      *
      * <p> Because it is possible for extensions to be installed into a running
      * Java virtual machine, this method may return different results each time
      * it is invoked. <p>
      *
      * @param  service
      *         The service's abstract service class
      *
      * @param  loader
      *         The class loader to be used to load provider-configuration files
      *         and instantiate provider classes, or <tt>null</tt> if the system
      *         class loader (or, failing that the bootstrap class loader) is to
      *         be used
      * 
      * @return An <tt>Iterator</tt> that yields provider objects for the given
      *         service, in some arbitrary order.  The iterator will throw a
      *         <tt>ServiceConfigurationError</tt> if a provider-configuration
      *         file violates the specified format or if a provider class cannot
      *         be found and instantiated.
      *
      * @throws ServiceConfigurationError
      *         If a provider-configuration file violates the specified format
      *         or names a provider class that cannot be found and instantiated
      *
      * @see #providers(java.lang.Class)
      * @see #installedProviders(java.lang.Class)
      */
     public static Iterator providers(Class service, ClassLoader loader)
 	          throws ServiceConfigurationError {
         return new LazyIterator(service, loader);
     }
 
     /**
      * Locates and incrementally instantiates the available providers of a
      * given service using the context class loader.  This convenience method
      * is equivalent to
      *
      * <pre>
      *   ClassLoader cl = Thread.currentThread().getContextClassLoader();
      *   return Service.providers(service, cl);
      * </pre>
      *
      * @param  service
      *         The service's abstract service class
      *
      * @return An <tt>Iterator</tt> that yields provider objects for the given
      *         service, in some arbitrary order.  The iterator will throw a
      *         <tt>ServiceConfigurationError</tt> if a provider-configuration
      *         file violates the specified format or if a provider class cannot
      *         be found and instantiated.
      *
      * @throws ServiceConfigurationError
      *         If a provider-configuration file violates the specified format
      *         or names a provider class that cannot be found and instantiated
      *
      * @see #providers(java.lang.Class, java.lang.ClassLoader)
      */
     public static Iterator providers(Class service)
 ...	  
 
     /**
      * Locates and incrementally instantiates the available providers of a
      * given service using the extension class loader.  This convenience method
      * simply locates the extension class loader, call it
      * <tt>extClassLoader</tt>, and then does
      *
      * <pre>
      *   return Service.providers(service, extClassLoader);
      * </pre>
      *
      * If the extension class loader cannot be found then the system class
      * loader is used; if there is no system class loader then the bootstrap
      * class loader is used.
      *
      * @param  service
      *         The service's abstract service class
      *
      * @return An <tt>Iterator</tt> that yields provider objects for the given
      *         service, in some arbitrary order.  The iterator will throw a
      *         <tt>ServiceConfigurationError</tt> if a provider-configuration
      *         file violates the specified format or if a provider class cannot
      *         be found and instantiated.
      *
      * @throws ServiceConfigurationError
      *         If a provider-configuration file violates the specified format
      *         or names a provider class that cannot be found and instantiated
      *
      * @see #providers(java.lang.Class, java.lang.ClassLoader)
      */
     public static Iterator installedProviders(Class service)
     throws ServiceConfigurationError
 	      {
     ClassLoader cl = ClassLoader.getSystemClassLoader();
     ClassLoader prev = null;
 	      while (cl != null) {
         prev = cl;
         cl = cl.getParent();
     }
     return Service.providers(service, prev);
     }
 }