/*
 * Copyright (c) 2005, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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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.security.AccessController;
import java.security.AccessControlContext;
import java.security.PrivilegedAction;
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>.</a> 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 style="font-size:smaller;">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
 * @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 final Class<S> service;

 // The class loader used to locate, load, and instantiate providers
 private final ClassLoader loader;

 // The access control context taken when the ServiceLoader is created
 private final AccessControlContext acc;

 // Cached providers, in instantiation order
 private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

 // 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 = Objects.requireNonNull(svc, "Service interface cannot be null");
 loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
 acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
 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<>();
 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;
 }

 private boolean hasNextService() {
 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;
 }

 private S nextService() {
 if (!hasNextService())
 throw new NoSuchElementException();
 String cn = nextName;
 nextName = null;
 Class<?> c = null;
 try {
 c = Class.forName(cn, false, loader);
 } catch (ClassNotFoundException x) {
 fail(service,
 "Provider " + cn + " not found");
 }
 if (!service.isAssignableFrom(c)) {
 fail(service,
 "Provider " + cn + " not a subtype");
 }
 try {
 S p = service.cast(c.newInstance());
 providers.put(cn, p);
 return p;
 } catch (Throwable x) {
 fail(service,
 "Provider " + cn + " could not be instantiated",
 x);
 }
 throw new Error(); // This cannot happen
 }

 public boolean hasNext() {
 if (acc == null) {
 return hasNextService();
 } else {
 PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() {
 public Boolean run() { return hasNextService(); }
 };
 return AccessController.doPrivileged(action, acc);
 }
 }

 public S next() {
 if (acc == null) {
 return nextService();
 } else {
 PrivilegedAction<S> action = new PrivilegedAction<S>() {
 public S run() { return nextService(); }
 };
 return AccessController.doPrivileged(action, acc);
 }
 }

 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.
 *
 * @implNote When adding providers to the cache, the {@link #iterator
 * Iterator} processes resources in the order that the {@link
 * java.lang.ClassLoader#getResources(java.lang.String)
 * ClassLoader.getResources(String)} method finds the service configuration
 * files.
 *
 * @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 <S> the class of the service type
 *
 * @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<>(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 <S> the class of the service type
 *
 * @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 <S> the class of the service type
 *
 * @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() + "]";
 }

}