Java tutorial
/* * @(#)ServiceLoader.java 1.10 06/04/10 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package net.sf.ufsc; 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.LinkedHashMap; import java.util.List; import java.util.Map; import java.util.NoSuchElementException; import org.apache.commons.logging.Log; import org.apache.commons.logging.LogFactory; /** * 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 * <tt>META-INF/services</tt>. The file's name is the fully-qualified <a * href="../lang/ClassLoader.html#name">binary name</a> of the service's type. * 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 * actually loaded. * * <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 * security context. * * <p> Instances of this class are not safe for use by multiple concurrent * threads. * * <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: * * <blockquote><pre> * 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 * one encoding. * * <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 * * <blockquote><pre> * META-INF/services/com.example.CodecSet</pre></blockquote> * * <p> This file contains the single line: * * <blockquote><pre> * com.example.impl.StandardCodecs # Standard codecs</pre></blockquote> * * <p> The <tt>CodecSet</tt> class creates and saves a single service instance * at initialization: * * <blockquote><pre> * 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 * providers. * * <blockquote><pre> * public static Encoder getEncoder(String encodingName) { * for (CodecSet cp : codecSetLoader) { * Encoder enc = cp.getEncoder(encodingName); * if (enc != null) * return enc; * } * return null; * }</pre></blockquote> * * <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 * spuriously. * * <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. * * @param <S> * The type of the service to be loaded by this loader * * @author Mark Reinhold * @version 1.10, 06/04/10 * @since 1.6 */ public final class ServiceLoader<S> implements Iterable<S> { private Log logger = LogFactory.getLog(this.getClass()); private static final String PREFIX = "META-INF/services/"; // The class or interface representing the service being loaded private Class<S> service; // The class loader used to locate, load, and instantiate providers private ClassLoader loader; // Cached providers, in instantiation order private LinkedHashMap<String, S> providers = new LinkedHashMap<String, S>(); // The current lazy-lookup iterator private LazyIterator lookupIterator; /** * Clear this loader's provider cache so that all providers will be * reloaded. * * <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. */ public void reload() { providers.clear(); lookupIterator = new LazyIterator(service, loader); } private ServiceLoader(Class<S> svc, ClassLoader cl) { service = svc; loader = cl; reload(); } private static void fail(Class<?> service, String msg, Throwable cause) throws ServiceConfigurationError { throw new ServiceConfigurationError(service.getName() + ": " + msg, cause); } 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 the names list. // private int parseLine(Class<?> service, URL u, BufferedReader r, int lc, List<String> names) 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"); int cp = ln.codePointAt(0); if (!Character.isJavaIdentifierStart(cp)) fail(service, u, lc, "Illegal provider-class name: " + ln); for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) { cp = ln.codePointAt(i); if (!Character.isJavaIdentifierPart(cp) && (cp != '.')) fail(service, u, lc, "Illegal provider-class name: " + ln); } if (!providers.containsKey(ln) && !names.contains(ln)) names.add(ln); } return lc + 1; } // Parse the content of the given URL as a provider-configuration file. // // @param service // The service type for which providers are being sought; // used to construct error detail strings // // @param u // 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 // 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 Iterator<String> parse(Class<?> service, URL u) throws ServiceConfigurationError { InputStream in = null; BufferedReader r = null; ArrayList<String> names = new ArrayList<String>(); try { in = u.openStream(); r = new BufferedReader(new InputStreamReader(in, "utf-8")); int lc = 1; while ((lc = parseLine(service, u, r, lc, names)) >= 0) ; } catch (IOException x) { fail(service, "Error reading configuration file", x); } finally { try { if (r != null) r.close(); if (in != null) in.close(); } catch (IOException y) { fail(service, "Error closing configuration file", y); } } return names.iterator(); } // Private inner class implementing fully-lazy provider lookup // private class LazyIterator implements Iterator<S> { Class<S> service; ClassLoader loader; Enumeration<URL> configs = null; Iterator<String> pending = null; String nextName = null; private LazyIterator(Class<S> service, ClassLoader loader) { this.service = service; this.loader = loader; } public boolean hasNext() { if (nextName != null) { return true; } logger.info("loading " + configs.toString()); 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, "Error locating configuration files", x); } } while ((pending == null) || !pending.hasNext()) { if (!configs.hasMoreElements()) { return false; } pending = parse(service, configs.nextElement()); } nextName = pending.next(); return true; } public S next() { if (!hasNext()) { throw new NoSuchElementException(); } String cn = nextName; nextName = null; try { S p = service.cast(Class.forName(cn, true, loader).newInstance()); providers.put(cn, p); return p; } catch (ClassNotFoundException x) { fail(service, "Provider " + cn + " not found"); } catch (Throwable x) { fail(service, "Provider " + cn + " could not be instantiated: " + x, x); } throw new Error(); // This cannot happen } public void remove() { throw new UnsupportedOperationException(); } } /** * 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 * cache in turn. * * <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 * service */ public Iterator<S> iterator() { return new Iterator<S>() { Iterator<Map.Entry<String, S>> knownProviders = providers.entrySet().iterator(); public boolean hasNext() { if (knownProviders.hasNext()) return true; return lookupIterator.hasNext(); } public S next() { if (knownProviders.hasNext()) return knownProviders.next().getValue(); return lookupIterator.next(); } public void remove() { throw new UnsupportedOperationException(); } }; } /** * Creates a new service loader for the given service type and class * loader. * * @param service * The interface or abstract class representing the service * * @param loader * 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 * used * * @return A new service loader */ public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader) { return new ServiceLoader<S>(service, loader); } /** * Creates a new service loader for the given service type, using the * current thread's {@linkplain java.lang.Thread#getContextClassLoader * context class loader}. * * <p> An invocation of this convenience method of the form * * <blockquote><pre> * ServiceLoader.load(<i>service</i>)</pre></blockquote> * * is equivalent to * * <blockquote><pre> * ServiceLoader.load(<i>service</i>, * Thread.currentThread().getContextClassLoader())</pre></blockquote> * * @param service * The interface or abstract class representing the service * * @return A new service loader */ public static <S> ServiceLoader<S> load(Class<S> service) { ClassLoader cl = Thread.currentThread().getContextClassLoader(); Log log = LogFactory.getLog(service.getClass()); log.info("Loading ufsc service for " + service.getName()); return ServiceLoader.load(service, cl); } /** * 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 * * <blockquote><pre> * 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 * class loader is used. * * <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. * * @param service * The interface or abstract class representing the service * * @return A new service loader */ public static <S> ServiceLoader<S> loadInstalled(Class<S> service) { ClassLoader cl = ClassLoader.getSystemClassLoader(); ClassLoader prev = null; while (cl != null) { prev = cl; cl = cl.getParent(); } return ServiceLoader.load(service, prev); } /** * Returns a string describing this service. * * @return A descriptive string */ public String toString() { return "java.util.ServiceLoader[" + service.getName() + "]"; } }