ServiceLoader.java

/*
 * 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
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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() + "]";
    }

}