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 * Copyright (c) 2000, 2019, 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 java.nio.channels.spi;

import java.io.IOException;

import java.net.ProtocolFamily;

import java.nio.channels.*;

import java.security.AccessController;

import java.security.PrivilegedAction;

import java.util.Iterator;

import java.util.ServiceLoader;

import java.util.ServiceConfigurationError;

import sun.security.action.GetPropertyAction;

/**

 * Service-provider class for selectors and selectable channels.

 *

 * <p> A selector provider is a concrete subclass of this class that has a

 * zero-argument constructor and implements the abstract methods specified

 * below.  A given invocation of the Java virtual machine maintains a single

 * system-wide default provider instance, which is returned by the {@link

 * #provider() provider} method.  The first invocation of that method will locate

 * the default provider as specified below.

 *

 * <p> The system-wide default provider is used by the static {@code open}

 * methods of the {@link java.nio.channels.DatagramChannel#open

 * DatagramChannel}, {@link java.nio.channels.Pipe#open Pipe}, {@link

 * java.nio.channels.Selector#open Selector}, {@link

 * java.nio.channels.ServerSocketChannel#open ServerSocketChannel}, and {@link

 * java.nio.channels.SocketChannel#open SocketChannel} classes.  It is also

 * used by the {@link java.lang.System#inheritedChannel System.inheritedChannel()}

 * method. A program may make use of a provider other than the default provider

 * by instantiating that provider and then directly invoking the {@code open}

 * methods defined in this class.

 *

 * <p> All of the methods in this class are safe for use by multiple concurrent

 * threads.  </p>

 *

 *

 * @author Mark Reinhold

 * @author JSR-51 Expert Group

 * @since 1.4

 */

public abstract class SelectorProvider {

    private static final Object lock = new Object();

    private static SelectorProvider provider = null;

    private static Void checkPermission() {

        SecurityManager sm = System.getSecurityManager();

        if (sm != null)

            sm.checkPermission(new RuntimePermission("selectorProvider"));

        return null;

    }

    private SelectorProvider(Void ignore) { }

    /**

     * Initializes a new instance of this class.

     *

     * @throws  SecurityException

     *          If a security manager has been installed and it denies

     *          {@link RuntimePermission}{@code ("selectorProvider")}

     */

    protected SelectorProvider() {

        this(checkPermission());

    }

    private static boolean loadProviderFromProperty() {

        String cn = System.getProperty("java.nio.channels.spi.SelectorProvider");

        if (cn == null)

            return false;

        try {

            @SuppressWarnings("deprecation")

            Object tmp = Class.forName(cn, true,

                                       ClassLoader.getSystemClassLoader()).newInstance();

            provider = (SelectorProvider)tmp;

            return true;

        } catch (ClassNotFoundException x) {

            throw new ServiceConfigurationError(null, x);

        } catch (IllegalAccessException x) {

            throw new ServiceConfigurationError(null, x);

        } catch (InstantiationException x) {

            throw new ServiceConfigurationError(null, x);

        } catch (SecurityException x) {

            throw new ServiceConfigurationError(null, x);

        }

    }

    private static boolean loadProviderAsService() {

        ServiceLoader<SelectorProvider> sl =

            ServiceLoader.load(SelectorProvider.class,

                               ClassLoader.getSystemClassLoader());

        Iterator<SelectorProvider> i = sl.iterator();

        for (;;) {

            try {

                if (!i.hasNext())

                    return false;

                provider = i.next();

                return true;

            } catch (ServiceConfigurationError sce) {

                if (sce.getCause() instanceof SecurityException) {

                    // Ignore the security exception, try the next provider

                    continue;

                }

                throw sce;

            }

        }

    }

    /**

     * Returns the system-wide default selector provider for this invocation of

     * the Java virtual machine.

     *

     * <p> The first invocation of this method locates the default provider

     * object as follows: </p>

     *

     * <ol>

     *

     *   <li><p> If the system property

     *   {@systemProperty java.nio.channels.spi.SelectorProvider} is defined

     *   then it is taken to be the fully-qualified name of a concrete provider

     *   class. The class is loaded and instantiated; if this process fails then

     *   an unspecified error is thrown.  </p></li>

     *

     *   <li><p> If a provider class has been installed in a jar file that is

     *   visible to the system class loader, and that jar file contains a

     *   provider-configuration file named

     *   {@code java.nio.channels.spi.SelectorProvider} in the resource

     *   directory {@code META-INF/services}, then the first class name

     *   specified in that file is taken.  The class is loaded and

     *   instantiated; if this process fails then an unspecified error is

     *   thrown.  </p></li>

     *

     *   <li><p> Finally, if no provider has been specified by any of the above

     *   means then the system-default provider class is instantiated and the

     *   result is returned.  </p></li>

     *

     * </ol>

     *

     * <p> Subsequent invocations of this method return the provider that was

     * returned by the first invocation.  </p>

     *

     * @return  The system-wide default selector provider

     */

    public static SelectorProvider provider() {

        synchronized (lock) {

            if (provider != null)

                return provider;

            return AccessController.doPrivileged(

                new PrivilegedAction<>() {

                    public SelectorProvider run() {

                            if (loadProviderFromProperty())

                                return provider;

                            if (loadProviderAsService())

                                return provider;

                            provider = sun.nio.ch.DefaultSelectorProvider.create();

                            return provider;

                        }

                    });

        }

    }

    /**

     * Opens a datagram channel.

     *

     * @return  The new channel

     *

     * @throws  IOException

     *          If an I/O error occurs

     */

    public abstract DatagramChannel openDatagramChannel()

        throws IOException;

    /**

     * Opens a datagram channel.

     *

     * @param   family

     *          The protocol family

     *

     * @return  A new datagram channel

     *

     * @throws  UnsupportedOperationException

     *          If the specified protocol family is not supported

     * @throws  IOException

     *          If an I/O error occurs

     *

     * @since 1.7

     */

    public abstract DatagramChannel openDatagramChannel(ProtocolFamily family)

        throws IOException;

    /**

     * Opens a pipe.

     *

     * @return  The new pipe

     *

     * @throws  IOException

     *          If an I/O error occurs

     */

    public abstract Pipe openPipe()

        throws IOException;

    /**

     * Opens a selector.

     *

     * @return  The new selector

     *

     * @throws  IOException

     *          If an I/O error occurs

     */

    public abstract AbstractSelector openSelector()

        throws IOException;

    /**

     * Opens a server-socket channel.

     *

     * @return  The new channel

     *

     * @throws  IOException

     *          If an I/O error occurs

     */

    public abstract ServerSocketChannel openServerSocketChannel()

        throws IOException;

    /**

     * Opens a socket channel.

     *

     * @return  The new channel

     *

     * @throws  IOException

     *          If an I/O error occurs

     */

    public abstract SocketChannel openSocketChannel()

        throws IOException;

    /**

     * Returns the channel inherited from the entity that created this

     * Java virtual machine.

     *

     * <p> On many operating systems a process, such as a Java virtual

     * machine, can be started in a manner that allows the process to

     * inherit a channel from the entity that created the process. The

     * manner in which this is done is system dependent, as are the

     * possible entities to which the channel may be connected. For example,

     * on UNIX systems, the Internet services daemon (<i>inetd</i>) is used to

     * start programs to service requests when a request arrives on an

     * associated network port. In this example, the process that is started,

     * inherits a channel representing a network socket.

     *

     * <p> In cases where the inherited channel represents a network socket

     * then the {@link java.nio.channels.Channel Channel} type returned

     * by this method is determined as follows:

     *

     * <ul>

     *

     *  <li><p> If the inherited channel represents a stream-oriented connected

     *  socket then a {@link java.nio.channels.SocketChannel SocketChannel} is

     *  returned. The socket channel is, at least initially, in blocking

     *  mode, bound to a socket address, and connected to a peer.

     *  </p></li>

     *

     *  <li><p> If the inherited channel represents a stream-oriented listening

     *  socket then a {@link java.nio.channels.ServerSocketChannel

     *  ServerSocketChannel} is returned. The server-socket channel is, at

     *  least initially, in blocking mode, and bound to a socket address.

     *  </p></li>

     *

     *  <li><p> If the inherited channel is a datagram-oriented socket

     *  then a {@link java.nio.channels.DatagramChannel DatagramChannel} is

     *  returned. The datagram channel is, at least initially, in blocking

     *  mode, and bound to a socket address.

     *  </p></li>

     *

     * </ul>

     *

     * <p> In addition to the network-oriented channels described, this method

     * may return other kinds of channels in the future.

     *

     * <p> The first invocation of this method creates the channel that is

     * returned. Subsequent invocations of this method return the same

     * channel. </p>

     *

     * @return  The inherited channel, if any, otherwise {@code null}.

     *

     * @throws  IOException

     *          If an I/O error occurs

     *

     * @throws  SecurityException

     *          If a security manager has been installed and it denies

     *          {@link RuntimePermission}{@code ("inheritedChannel")}

     *

     * @since 1.5

     */

        return null;

}