/* * Copyright (c) 1999, 2012, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code 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. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code 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 in the LICENSE file that * accompanied this code). * * 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.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 service is a * well-known set of interfaces and (usually abstract) classes. A service * provider 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. * *
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 service class with data and code specific to * the provider. This provider class 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. * *
A service provider identifies itself by placing a provider-configuration * file in the resource directory META-INF/services. 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 '#' * (0x23); on each line all characters following the first comment * character are ignored. The file must be encoded in UTF-8. * *
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. * *
Example: Suppose we have a service class named * java.io.spi.CharCodec. It has two abstract methods: * *
* public abstract CharEncoder getEncoder(String encodingName); * public abstract CharDecoder getDecoder(String encodingName); ** * Each method returns an appropriate object or null if it cannot * translate the given encoding. Typical CharCodec providers will * support more than one encoding. * *
If sun.io.StandardCodec is a provider of the CharCodec * service then its jar file would contain the file * META-INF/services/java.io.spi.CharCodec. This file would contain * the single line: * *
* sun.io.StandardCodec # Standard codecs for the platform ** * To locate an encoder for a given encoding name, the internal I/O code would * do something like this: * *
* 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; * } ** * 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 * @since 1.3 */ public final class Service
This method transforms the name of the given service class into a * provider-configuration filename as described above and then uses the * getResources 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. * *
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.
*
* @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 null if the system
* class loader (or, failing that the bootstrap class loader) is to
* be used
*
* @return An Iterator that yields provider objects for the given
* service, in some arbitrary order. The iterator will throw a
* ServiceConfigurationError 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
*
*
* ClassLoader cl = Thread.currentThread().getContextClassLoader();
* return Service.providers(service, cl);
*
*
* @param service
* The service's abstract service class
*
* @return An Iterator that yields provider objects for the given
* service, in some arbitrary order. The iterator will throw a
* ServiceConfigurationError 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)
throws ServiceConfigurationError
{
ClassLoader cl = Thread.currentThread().getContextClassLoader();
return Service.providers(service, cl);
}
/**
* 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
* extClassLoader, and then does
*
*
* return Service.providers(service, extClassLoader);
*
*
* 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 Iterator that yields provider objects for the given
* service, in some arbitrary order. The iterator will throw a
* ServiceConfigurationError 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);
}
}