/*

 * Copyright (c) 2005, 2013, 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 com.sun.tools.javac.util;

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStream;

import java.io.InputStreamReader;

import java.net.URL;

import java.net.URLConnection;

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 java.util.Objects;

import java.util.ServiceConfigurationError;

/**

 * This is a temporary, modified copy of java.util.ServiceLoader, for use by

 * javac, to work around bug JDK-8004082.

 *

 * The bug describes problems in the interaction between ServiceLoader and

 * URLClassLoader, such that references to a jar file passed to URLClassLoader

 * may be retained after calling URLClassLoader.close(), preventing the jar

 * file from being deleted on Windows.

 *

 *  <p><b>This is NOT part of any supported API.

 *  If you write code that depends on this, you do so at your own risk.

 *  This code and its internal interfaces are subject to change or

 *  deletion without notice.</b>

 */

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<>();

    // 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;

        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 {

            // The problem is that by default, streams opened with

            // u.openInputStream use a cached reference to a JarFile, which

            // is separate from the reference used by URLClassLoader, and

            // which is not closed by URLClassLoader.close().

            // The workaround is to disable caching for this specific jar file,

            // so that the reference to the jar file can be closed when the

            // file has been read.

            // Original code:

            // in = u.openStream();

            // Workaround ...

            URLConnection uc = u.openConnection();

            uc.setUseCaches(false);

            in = uc.getInputStream();

            // ... end of workaround.

            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;

            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,

                     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<>(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() + "]";

    }

}