1 /*

     2  * Copyright (c) 2005, 2013, Oracle and/or its affiliates. All rights reserved.

     3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

     4  *

     5  * This code is free software; you can redistribute it and/or modify it

     6  * under the terms of the GNU General Public License version 2 only, as

     7  * published by the Free Software Foundation.  Oracle designates this

     8  * particular file as subject to the "Classpath" exception as provided

     9  * by Oracle in the LICENSE file that accompanied this code.

    10  *

    11  * This code is distributed in the hope that it will be useful, but WITHOUT

    12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

    14  * version 2 for more details (a copy is included in the LICENSE file that

    15  * accompanied this code).

    16  *

    17  * You should have received a copy of the GNU General Public License version

    18  * 2 along with this work; if not, write to the Free Software Foundation,

    19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    20  *

    21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

    22  * or visit www.oracle.com if you need additional information or have any

    23  * questions.

    24  */

    25 

    26 package java.util;

    27 

    28 import java.io.BufferedReader;

    29 import java.io.IOException;

    30 import java.io.InputStream;

    31 import java.io.InputStreamReader;

    32 import java.net.URL;

    33 import java.security.AccessController;

    34 import java.security.AccessControlContext;

    35 import java.security.PrivilegedAction;

    36 import java.util.ArrayList;

    37 import java.util.Enumeration;

    38 import java.util.Iterator;

    39 import java.util.List;

    40 import java.util.NoSuchElementException;

    41 

    42 

    43 /**

    44  * A simple service-provider loading facility.

    45  *

    46  * <p> A <i>service</i> is a well-known set of interfaces and (usually

    47  * abstract) classes.  A <i>service provider</i> is a specific implementation

    48  * of a service.  The classes in a provider typically implement the interfaces

    49  * and subclass the classes defined in the service itself.  Service providers

    50  * can be installed in an implementation of the Java platform in the form of

    51  * extensions, that is, jar files placed into any of the usual extension

    52  * directories.  Providers can also be made available by adding them to the

    53  * application's class path or by some other platform-specific means.

    54  *

    55  * <p> For the purpose of loading, a service is represented by a single type,

    56  * that is, a single interface or abstract class.  (A concrete class can be

    57  * used, but this is not recommended.)  A provider of a given service contains

    58  * one or more concrete classes that extend this <i>service type</i> with data

    59  * and code specific to the provider.  The <i>provider class</i> is typically

    60  * not the entire provider itself but rather a proxy which contains enough

    61  * information to decide whether the provider is able to satisfy a particular

    62  * request together with code that can create the actual provider on demand.

    63  * The details of provider classes tend to be highly service-specific; no

    64  * single class or interface could possibly unify them, so no such type is

    65  * defined here.  The only requirement enforced by this facility is that

    66  * provider classes must have a zero-argument constructor so that they can be

    67  * instantiated during loading.

    68  *

    69  * <p><a name="format"> A service provider is identified by placing a

    70  * <i>provider-configuration file</i> in the resource directory

    71  * <tt>META-INF/services</tt>.</a>  The file's name is the fully-qualified <a

    72  * href="../lang/ClassLoader.html#name">binary name</a> of the service's type.

    73  * The file contains a list of fully-qualified binary names of concrete

    74  * provider classes, one per line.  Space and tab characters surrounding each

    75  * name, as well as blank lines, are ignored.  The comment character is

    76  * <tt>'#'</tt> (<tt>'&#92;u0023'</tt>,

    77  * <font style="font-size:smaller;">NUMBER SIGN</font>); on

    78  * each line all characters following the first comment character are ignored.

    79  * The file must be encoded in UTF-8.

    80  *

    81  * <p> If a particular concrete provider class is named in more than one

    82  * configuration file, or is named in the same configuration file more than

    83  * once, then the duplicates are ignored.  The configuration file naming a

    84  * particular provider need not be in the same jar file or other distribution

    85  * unit as the provider itself.  The provider must be accessible from the same

    86  * class loader that was initially queried to locate the configuration file;

    87  * note that this is not necessarily the class loader from which the file was

    88  * actually loaded.

    89  *

    90  * <p> Providers are located and instantiated lazily, that is, on demand.  A

    91  * service loader maintains a cache of the providers that have been loaded so

    92  * far.  Each invocation of the {@link #iterator iterator} method returns an

    93  * iterator that first yields all of the elements of the cache, in

    94  * instantiation order, and then lazily locates and instantiates any remaining

    95  * providers, adding each one to the cache in turn.  The cache can be cleared

    96  * via the {@link #reload reload} method.

    97  *

    98  * <p> Service loaders always execute in the security context of the caller.

    99  * Trusted system code should typically invoke the methods in this class, and

   100  * the methods of the iterators which they return, from within a privileged

   101  * security context.

   102  *

   103  * <p> Instances of this class are not safe for use by multiple concurrent

   104  * threads.

   105  *

   106  * <p> Unless otherwise specified, passing a <tt>null</tt> argument to any

   107  * method in this class will cause a {@link NullPointerException} to be thrown.

   108  *

   109  *

   110  * <p><span style="font-weight: bold; padding-right: 1em">Example</span>

   111  * Suppose we have a service type <tt>com.example.CodecSet</tt> which is

   112  * intended to represent sets of encoder/decoder pairs for some protocol.  In

   113  * this case it is an abstract class with two abstract methods:

   114  *

   115  * <blockquote><pre>

   116  * public abstract Encoder getEncoder(String encodingName);

   117  * public abstract Decoder getDecoder(String encodingName);</pre></blockquote>

   118  *

   119  * Each method returns an appropriate object or <tt>null</tt> if the provider

   120  * does not support the given encoding.  Typical providers support more than

   121  * one encoding.

   122  *

   123  * <p> If <tt>com.example.impl.StandardCodecs</tt> is an implementation of the

   124  * <tt>CodecSet</tt> service then its jar file also contains a file named

   125  *

   126  * <blockquote><pre>

   127  * META-INF/services/com.example.CodecSet</pre></blockquote>

   128  *

   129  * <p> This file contains the single line:

   130  *

   131  * <blockquote><pre>

   132  * com.example.impl.StandardCodecs    # Standard codecs</pre></blockquote>

   133  *

   134  * <p> The <tt>CodecSet</tt> class creates and saves a single service instance

   135  * at initialization:

   136  *

   137  * <blockquote><pre>

   138  * private static ServiceLoader<CodecSet> codecSetLoader

   139  *     = ServiceLoader.load(CodecSet.class);</pre></blockquote>

   140  *

   141  * <p> To locate an encoder for a given encoding name it defines a static

   142  * factory method which iterates through the known and available providers,

   143  * returning only when it has located a suitable encoder or has run out of

   144  * providers.

   145  *

   146  * <blockquote><pre>

   147  * public static Encoder getEncoder(String encodingName) {

   148  *     for (CodecSet cp : codecSetLoader) {

   149  *         Encoder enc = cp.getEncoder(encodingName);

   150  *         if (enc != null)

   151  *             return enc;

   152  *     }

   153  *     return null;

   154  * }</pre></blockquote>

   155  *

   156  * <p> A <tt>getDecoder</tt> method is defined similarly.

   157  *

   158  *

   159  * <p><span style="font-weight: bold; padding-right: 1em">Usage Note</span> If

   160  * the class path of a class loader that is used for provider loading includes

   161  * remote network URLs then those URLs will be dereferenced in the process of

   162  * searching for provider-configuration files.

   163  *

   164  * <p> This activity is normal, although it may cause puzzling entries to be

   165  * created in web-server logs.  If a web server is not configured correctly,

   166  * however, then this activity may cause the provider-loading algorithm to fail

   167  * spuriously.

   168  *

   169  * <p> A web server should return an HTTP 404 (Not Found) response when a

   170  * requested resource does not exist.  Sometimes, however, web servers are

   171  * erroneously configured to return an HTTP 200 (OK) response along with a

   172  * helpful HTML error page in such cases.  This will cause a {@link

   173  * ServiceConfigurationError} to be thrown when this class attempts to parse

   174  * the HTML page as a provider-configuration file.  The best solution to this

   175  * problem is to fix the misconfigured web server to return the correct

   176  * response code (HTTP 404) along with the HTML error page.

   177  *

   178  * @param  <S>

   179  *         The type of the service to be loaded by this loader

   180  *

   181  * @author Mark Reinhold

   182  * @since 1.6

   183  */

   184 

   185 public final class ServiceLoader<S>

   186     implements Iterable<S>

   187 {

   188 

   189     private static final String PREFIX = "META-INF/services/";

   190 

   191     // The class or interface representing the service being loaded

   192     private final Class<S> service;

   193 

   194     // The class loader used to locate, load, and instantiate providers

   195     private final ClassLoader loader;

   196 

   197     // The access control context taken when the ServiceLoader is created

   198     private final AccessControlContext acc;

   199 

   200     // Cached providers, in instantiation order

   201     private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

   202 

   203     // The current lazy-lookup iterator

   204     private LazyIterator lookupIterator;

   205 

   206     /**

   207      * Clear this loader's provider cache so that all providers will be

   208      * reloaded.

   209      *

   210      * <p> After invoking this method, subsequent invocations of the {@link

   211      * #iterator() iterator} method will lazily look up and instantiate

   212      * providers from scratch, just as is done by a newly-created loader.

   213      *

   214      * <p> This method is intended for use in situations in which new providers

   215      * can be installed into a running Java virtual machine.

   216      */

   217     public void reload() {

   218         providers.clear();

   219         lookupIterator = new LazyIterator(service, loader);

   220     }

   221 

   222     private ServiceLoader(Class<S> svc, ClassLoader cl) {

   223         service = Objects.requireNonNull(svc, "Service interface cannot be null");

   224         loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;

   225         acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;

   226         reload();

   227     }

   228 

   229     private static void fail(Class<?> service, String msg, Throwable cause)

   230         throws ServiceConfigurationError

   231     {

   232         throw new ServiceConfigurationError(service.getName() + ": " + msg,

   233                                             cause);

   234     }

   235 

   236     private static void fail(Class<?> service, String msg)

   237         throws ServiceConfigurationError

   238     {

   239         throw new ServiceConfigurationError(service.getName() + ": " + msg);

   240     }

   241 

   242     private static void fail(Class<?> service, URL u, int line, String msg)

   243         throws ServiceConfigurationError

   244     {

   245         fail(service, u + ":" + line + ": " + msg);

   246     }

   247 

   248     // Parse a single line from the given configuration file, adding the name

   249     // on the line to the names list.

   250     //

   251     private int parseLine(Class<?> service, URL u, BufferedReader r, int lc,

   252                           List<String> names)

   253         throws IOException, ServiceConfigurationError

   254     {

   255         String ln = r.readLine();

   256         if (ln == null) {

   257             return -1;

   258         }

   259         int ci = ln.indexOf('#');

   260         if (ci >= 0) ln = ln.substring(0, ci);

   261         ln = ln.trim();

   262         int n = ln.length();

   263         if (n != 0) {

   264             if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))

   265                 fail(service, u, lc, "Illegal configuration-file syntax");

   266             int cp = ln.codePointAt(0);

   267             if (!Character.isJavaIdentifierStart(cp))

   268                 fail(service, u, lc, "Illegal provider-class name: " + ln);

   269             for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {

   270                 cp = ln.codePointAt(i);

   271                 if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))

   272                     fail(service, u, lc, "Illegal provider-class name: " + ln);

   273             }

   274             if (!providers.containsKey(ln) && !names.contains(ln))

   275                 names.add(ln);

   276         }

   277         return lc + 1;

   278     }

   279 

   280     // Parse the content of the given URL as a provider-configuration file.

   281     //

   282     // @param  service

   283     //         The service type for which providers are being sought;

   284     //         used to construct error detail strings

   285     //

   286     // @param  u

   287     //         The URL naming the configuration file to be parsed

   288     //

   289     // @return A (possibly empty) iterator that will yield the provider-class

   290     //         names in the given configuration file that are not yet members

   291     //         of the returned set

   292     //

   293     // @throws ServiceConfigurationError

   294     //         If an I/O error occurs while reading from the given URL, or

   295     //         if a configuration-file format error is detected

   296     //

   297     private Iterator<String> parse(Class<?> service, URL u)

   298         throws ServiceConfigurationError

   299     {

   300         InputStream in = null;

   301         BufferedReader r = null;

   302         ArrayList<String> names = new ArrayList<>();

   303         try {

   304             in = u.openStream();

   305             r = new BufferedReader(new InputStreamReader(in, "utf-8"));

   306             int lc = 1;

   307             while ((lc = parseLine(service, u, r, lc, names)) >= 0);

   308         } catch (IOException x) {

   309             fail(service, "Error reading configuration file", x);

   310         } finally {

   311             try {

   312                 if (r != null) r.close();

   313                 if (in != null) in.close();

   314             } catch (IOException y) {

   315                 fail(service, "Error closing configuration file", y);

   316             }

   317         }

   318         return names.iterator();

   319     }

   320 

   321     // Private inner class implementing fully-lazy provider lookup

   322     //

   323     private class LazyIterator

   324         implements Iterator<S>

   325     {

   326 

   327         Class<S> service;

   328         ClassLoader loader;

   329         Enumeration<URL> configs = null;

   330         Iterator<String> pending = null;

   331         String nextName = null;

   332 

   333         private LazyIterator(Class<S> service, ClassLoader loader) {

   334             this.service = service;

   335             this.loader = loader;

   336         }

   337 

   338         private boolean hasNextService() {

   339             if (nextName != null) {

   340                 return true;

   341             }

   342             if (configs == null) {

   343                 try {

   344                     String fullName = PREFIX + service.getName();

   345                     if (loader == null)

   346                         configs = ClassLoader.getSystemResources(fullName);

   347                     else

   348                         configs = loader.getResources(fullName);

   349                 } catch (IOException x) {

   350                     fail(service, "Error locating configuration files", x);

   351                 }

   352             }

   353             while ((pending == null) || !pending.hasNext()) {

   354                 if (!configs.hasMoreElements()) {

   355                     return false;

   356                 }

   357                 pending = parse(service, configs.nextElement());

   358             }

   359             nextName = pending.next();

   360             return true;

   361         }

   362 

   363         private S nextService() {

   364             if (!hasNextService())

   365                 throw new NoSuchElementException();

   366             String cn = nextName;

   367             nextName = null;

   368             Class<?> c = null;

   369             try {

   370                 c = Class.forName(cn, false, loader);

   371             } catch (ClassNotFoundException x) {

   372                 fail(service,

   373                      "Provider " + cn + " not found");

   374             }

   375             if (!service.isAssignableFrom(c)) {

   376                 fail(service,

   377                      "Provider " + cn  + " not a subtype");

   378             }

   379             try {

   380                 S p = service.cast(c.newInstance());

   381                 providers.put(cn, p);

   382                 return p;

   383             } catch (Throwable x) {

   384                 fail(service,

   385                      "Provider " + cn + " could not be instantiated",

   386                      x);

   387             }

   388             throw new Error();          // This cannot happen

   389         }

   390 

   391         public boolean hasNext() {

   392             if (acc == null) {

   393                 return hasNextService();

   394             } else {

   395                 PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() {

   396                     public Boolean run() { return hasNextService(); }

   397                 };

   398                 return AccessController.doPrivileged(action, acc);

   399             }

   400         }

   401 

   402         public S next() {

   403             if (acc == null) {

   404                 return nextService();

   405             } else {

   406                 PrivilegedAction<S> action = new PrivilegedAction<S>() {

   407                     public S run() { return nextService(); }

   408                 };

   409                 return AccessController.doPrivileged(action, acc);

   410             }

   411         }

   412 

   413         public void remove() {

   414             throw new UnsupportedOperationException();

   415         }

   416 

   417     }

   418 

   419     /**

   420      * Lazily loads the available providers of this loader's service.

   421      *

   422      * <p> The iterator returned by this method first yields all of the

   423      * elements of the provider cache, in instantiation order.  It then lazily

   424      * loads and instantiates any remaining providers, adding each one to the

   425      * cache in turn.

   426      *

   427      * <p> To achieve laziness the actual work of parsing the available

   428      * provider-configuration files and instantiating providers must be done by

   429      * the iterator itself.  Its {@link java.util.Iterator#hasNext hasNext} and

   430      * {@link java.util.Iterator#next next} methods can therefore throw a

   431      * {@link ServiceConfigurationError} if a provider-configuration file

   432      * violates the specified format, or if it names a provider class that

   433      * cannot be found and instantiated, or if the result of instantiating the

   434      * class is not assignable to the service type, or if any other kind of

   435      * exception or error is thrown as the next provider is located and

   436      * instantiated.  To write robust code it is only necessary to catch {@link

   437      * ServiceConfigurationError} when using a service iterator.

   438      *

   439      * <p> If such an error is thrown then subsequent invocations of the

   440      * iterator will make a best effort to locate and instantiate the next

   441      * available provider, but in general such recovery cannot be guaranteed.

   442      *

   443      * <blockquote style="font-size: smaller; line-height: 1.2"><span

   444      * style="padding-right: 1em; font-weight: bold">Design Note</span>

   445      * Throwing an error in these cases may seem extreme.  The rationale for

   446      * this behavior is that a malformed provider-configuration file, like a

   447      * malformed class file, indicates a serious problem with the way the Java

   448      * virtual machine is configured or is being used.  As such it is

   449      * preferable to throw an error rather than try to recover or, even worse,

   450      * fail silently.</blockquote>

   451      *

   452      * <p> The iterator returned by this method does not support removal.

   453      * Invoking its {@link java.util.Iterator#remove() remove} method will

   454      * cause an {@link UnsupportedOperationException} to be thrown.

   455      *

   456      * @implNote When adding providers to the cache, the {@link #iterator

   457      * Iterator} processes resources in the order that the {@link

   458      * java.lang.ClassLoader#getResources(java.lang.String)

   459      * ClassLoader.getResources(String)} method finds the service configuration

   460      * files.

   461      *

   462      * @return  An iterator that lazily loads providers for this loader's

   463      *          service

   464      */

   465     public Iterator<S> iterator() {

   466         return new Iterator<S>() {

   467 

   468             Iterator<Map.Entry<String,S>> knownProviders

   469                 = providers.entrySet().iterator();

   470 

   471             public boolean hasNext() {

   472                 if (knownProviders.hasNext())

   473                     return true;

   474                 return lookupIterator.hasNext();

   475             }

   476 

   477             public S next() {

   478                 if (knownProviders.hasNext())

   479                     return knownProviders.next().getValue();

   480                 return lookupIterator.next();

   481             }

   482 

   483             public void remove() {

   484                 throw new UnsupportedOperationException();

   485             }

   486 

   487         };

   488     }

   489 

   490     /**

   491      * Creates a new service loader for the given service type and class

   492      * loader.

   493      *

   494      * @param  <S> the class of the service type

   495      *

   496      * @param  service

   497      *         The interface or abstract class representing the service

   498      *

   499      * @param  loader

   500      *         The class loader to be used to load provider-configuration files

   501      *         and provider classes, or <tt>null</tt> if the system class

   502      *         loader (or, failing that, the bootstrap class loader) is to be

   503      *         used

   504      *

   505      * @return A new service loader

   506      */

   507     public static <S> ServiceLoader<S> load(Class<S> service,

   508                                             ClassLoader loader)

   509     {

   510         return new ServiceLoader<>(service, loader);

   511     }

   512 

   513     /**

   514      * Creates a new service loader for the given service type, using the

   515      * current thread's {@linkplain java.lang.Thread#getContextClassLoader

   516      * context class loader}.

   517      *

   518      * <p> An invocation of this convenience method of the form

   519      *

   520      * <blockquote><pre>

   521      * ServiceLoader.load(<i>service</i>)</pre></blockquote>

   522      *

   523      * is equivalent to

   524      *

   525      * <blockquote><pre>

   526      * ServiceLoader.load(<i>service</i>,

   527      *                    Thread.currentThread().getContextClassLoader())</pre></blockquote>

   528      *

   529      * @param  <S> the class of the service type

   530      *

   531      * @param  service

   532      *         The interface or abstract class representing the service

   533      *

   534      * @return A new service loader

   535      */

   536     public static <S> ServiceLoader<S> load(Class<S> service) {

   537         ClassLoader cl = Thread.currentThread().getContextClassLoader();

   538         return ServiceLoader.load(service, cl);

   539     }

   540 

   541     /**

   542      * Creates a new service loader for the given service type, using the

   543      * extension class loader.

   544      *

   545      * <p> This convenience method simply locates the extension class loader,

   546      * call it <tt><i>extClassLoader</i></tt>, and then returns

   547      *

   548      * <blockquote><pre>

   549      * ServiceLoader.load(<i>service</i>, <i>extClassLoader</i>)</pre></blockquote>

   550      *

   551      * <p> If the extension class loader cannot be found then the system class

   552      * loader is used; if there is no system class loader then the bootstrap

   553      * class loader is used.

   554      *

   555      * <p> This method is intended for use when only installed providers are

   556      * desired.  The resulting service will only find and load providers that

   557      * have been installed into the current Java virtual machine; providers on

   558      * the application's class path will be ignored.

   559      *

   560      * @param  <S> the class of the service type

   561      *

   562      * @param  service

   563      *         The interface or abstract class representing the service

   564      *

   565      * @return A new service loader

   566      */

   567     public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {

   568         ClassLoader cl = ClassLoader.getSystemClassLoader();

   569         ClassLoader prev = null;

   570         while (cl != null) {

   571             prev = cl;

   572             cl = cl.getParent();

   573         }

   574         return ServiceLoader.load(service, prev);

   575     }

   576 

   577     /**

   578      * Returns a string describing this service.

   579      *

   580      * @return  A descriptive string

   581      */

   582     public String toString() {

   583         return "java.util.ServiceLoader[" + service.getName() + "]";

   584     }

   585 

   586 }