ServiceLoader.java revision 5419
609N/A * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved. 609N/A * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 609N/A * This code is free software; you can redistribute it and/or modify it 609N/A * under the terms of the GNU General Public License version 2 only, as 609N/A * published by the Free Software Foundation. Oracle designates this 609N/A * particular file as subject to the "Classpath" exception as provided 609N/A * by Oracle in the LICENSE file that accompanied this code. 609N/A * This code is distributed in the hope that it will be useful, but WITHOUT 609N/A * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 609N/A * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 609N/A * version 2 for more details (a copy is included in the LICENSE file that 609N/A * accompanied this code). 609N/A * You should have received a copy of the GNU General Public License version 609N/A * 2 along with this work; if not, write to the Free Software Foundation, 609N/A * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 609N/A * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 609N/A * or visit www.oracle.com if you need additional information or have any * A simple service-provider loading facility. * <p> 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 * can 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 can also be made available by adding them to the * application's class path or by some other platform-specific means. * <p> For the purpose of loading, a service is represented by a single type, * that is, a single interface or abstract class. (A concrete class can be * used, but this is not recommended.) A provider of a given service contains * one or more concrete classes that extend this <i>service type</i> with data * and code specific to the provider. The <i>provider class</i> is typically * not the entire provider itself but rather a proxy which 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 type is * defined here. The only requirement enforced by this facility is that * provider classes must have a zero-argument constructor so that they can be * instantiated during loading. * <p><a name="format"> A service provider is identified by placing a * <i>provider-configuration file</i> in the resource directory * The file contains a list of fully-qualified binary names of concrete * provider classes, one per line. Space and tab characters surrounding each * name, as well as blank lines, are ignored. The comment character is * <tt>'#'</tt> (<tt>'\u0023'</tt>, <font size="-1">NUMBER SIGN</font>); 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 are 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 from which the file was * <p> Providers are located and instantiated lazily, that is, on demand. A * service loader maintains a cache of the providers that have been loaded so * far. Each invocation of the {@link #iterator iterator} method returns an * iterator that first yields all of the elements of the cache, in * instantiation order, and then lazily locates and instantiates any remaining * providers, adding each one to the cache in turn. The cache can be cleared * via the {@link #reload reload} method. * <p> Service loaders always execute in the security context of the caller. * Trusted system code should typically invoke the methods in this class, and * the methods of the iterators which they return, from within a privileged * <p> Instances of this class are not safe for use by multiple concurrent * <p> Unless otherwise specified, passing a <tt>null</tt> argument to any * method in this class will cause a {@link NullPointerException} to be thrown. * <p><span style="font-weight: bold; padding-right: 1em">Example</span> * Suppose we have a service type <tt>com.example.CodecSet</tt> which is * intended to represent sets of encoder/decoder pairs for some protocol. In * this case it is an abstract class with two abstract methods: * public abstract Encoder getEncoder(String encodingName); * public abstract Decoder getDecoder(String encodingName);</pre></blockquote> * Each method returns an appropriate object or <tt>null</tt> if the provider * does not support the given encoding. Typical providers support more than * <p> If <tt>com.example.impl.StandardCodecs</tt> is an implementation of the * <tt>CodecSet</tt> service then its jar file also contains a file named * <p> This file contains the single line: * com.example.impl.StandardCodecs # Standard codecs</pre></blockquote> * <p> The <tt>CodecSet</tt> class creates and saves a single service instance * private static ServiceLoader<CodecSet> codecSetLoader * = ServiceLoader.load(CodecSet.class);</pre></blockquote> * <p> To locate an encoder for a given encoding name it defines a static * factory method which iterates through the known and available providers, * returning only when it has located a suitable encoder or has run out of * public static Encoder getEncoder(String encodingName) { * for (CodecSet cp : codecSetLoader) { * Encoder enc = cp.getEncoder(encodingName); * <p> A <tt>getDecoder</tt> method is defined similarly. * <p><span style="font-weight: bold; padding-right: 1em">Usage Note</span> If * the class path of a class loader that is used for provider loading includes * remote network URLs then those URLs will be dereferenced in the process of * searching for provider-configuration files. * <p> This activity is normal, although it may cause puzzling entries to be * created in web-server logs. If a web server is not configured correctly, * however, then this activity may cause the provider-loading algorithm to fail * <p> A web server should return an HTTP 404 (Not Found) response when a * requested resource does not exist. Sometimes, however, web servers are * erroneously configured to return an HTTP 200 (OK) response along with a * helpful HTML error page in such cases. This will cause a {@link * ServiceConfigurationError} to be thrown when this class attempts to parse * the HTML page as a provider-configuration file. The best solution to this * problem is to fix the misconfigured web server to return the correct * response code (HTTP 404) along with the HTML error page. * The type of the service to be loaded by this loader // The class or interface representing the service being loaded // The class loader used to locate, load, and instantiate providers // Cached providers, in instantiation order // The current lazy-lookup iterator * Clear this loader's provider cache so that all providers will be * <p> After invoking this method, subsequent invocations of the {@link * #iterator() iterator} method will lazily look up and instantiate * providers from scratch, just as is done by a newly-created loader. * <p> This method is intended for use in situations in which new providers * can be installed into a running Java virtual machine. // Parse a single line from the given configuration file, adding the name // on the line to the names list. // Parse the content of the given URL as a provider-configuration file. // The service type for which providers are being sought; // used to construct error detail strings // The URL naming the configuration file to be parsed // @return A (possibly empty) iterator that will yield the provider-class // names in the given configuration file that are not yet members // @throws ServiceConfigurationError // If an I/O error occurs while reading from the given URL, or // if a configuration-file format error is detected // Private inner class implementing fully-lazy provider lookup fail(
service,
"Error locating configuration files", x);
"Provider " +
cn +
" not found");
"Provider " +
cn +
" not a subtype");
"Provider " +
cn +
" could not be instantiated: " + x,
throw new Error();
// This cannot happen * Lazily loads the available providers of this loader's service. * <p> The iterator returned by this method first yields all of the * elements of the provider cache, in instantiation order. It then lazily * loads and instantiates any remaining providers, adding each one to the * <p> To achieve laziness the actual work of parsing the available * provider-configuration files and instantiating providers must be done by * the iterator itself. Its {@link java.util.Iterator#hasNext hasNext} and * {@link java.util.Iterator#next next} methods can therefore throw a * {@link ServiceConfigurationError} if a provider-configuration file * violates the specified format, or if it names a provider class that * cannot be found and instantiated, or if the result of instantiating the * class is not assignable to the service type, or if any other kind of * exception or error is thrown as the next provider is located and * instantiated. To write robust code it is only necessary to catch {@link * ServiceConfigurationError} when using a service iterator. * <p> If such an error is thrown then subsequent invocations of the * iterator will make a best effort to locate and instantiate the next * available provider, but in general such recovery cannot be guaranteed. * <blockquote style="font-size: smaller; line-height: 1.2"><span * style="padding-right: 1em; font-weight: bold">Design Note</span> * Throwing an error in these cases may seem extreme. The rationale for * this behavior is that a malformed provider-configuration file, like a * malformed class file, indicates a serious problem with the way the Java * virtual machine is configured or is being used. As such it is * preferable to throw an error rather than try to recover or, even worse, * fail silently.</blockquote> * <p> The iterator returned by this method does not support removal. * Invoking its {@link java.util.Iterator#remove() remove} method will * cause an {@link UnsupportedOperationException} to be thrown. * @return An iterator that lazily loads providers for this loader's * Creates a new service loader for the given service type and class * The interface or abstract class representing the service * The class loader to be used to load provider-configuration files * and provider classes, or <tt>null</tt> if the system class * loader (or, failing that, the bootstrap class loader) is to be * @return A new service loader * Creates a new service loader for the given service type, using the * current thread's {@linkplain java.lang.Thread#getContextClassLoader * <p> An invocation of this convenience method of the form * ServiceLoader.load(<i>service</i>)</pre></blockquote> * ServiceLoader.load(<i>service</i>, * Thread.currentThread().getContextClassLoader())</pre></blockquote> * The interface or abstract class representing the service * @return A new service loader * Creates a new service loader for the given service type, using the * extension class loader. * <p> This convenience method simply locates the extension class loader, * call it <tt><i>extClassLoader</i></tt>, and then returns * ServiceLoader.load(<i>service</i>, <i>extClassLoader</i>)</pre></blockquote> * <p> 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 * <p> This method is intended for use when only installed providers are * desired. The resulting service will only find and load providers that * have been installed into the current Java virtual machine; providers on * the application's class path will be ignored. * The interface or abstract class representing the service * @return A new service loader * Returns a string describing this service. * @return A descriptive string