/*
 * @(#)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 java.util;

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;


/**
 * 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>'&#92;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&lt;CodecSet&gt; 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 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;
	    }
	    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();
	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() + "]";
    }

}