/*

 * Copyright (c) 2009, 2021, 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 sun.net.ftp;

import java.security.AccessController;

import java.security.PrivilegedAction;

import java.util.ServiceConfigurationError;

//import java.util.ServiceLoader;

/**

 * Service provider class for FtpClient.

 * Sub-classes of FtpClientProvider provide an implementation of {@link FtpClient}

 * and associated classes. Applications do not normally use this class directly.

 * See {@link #provider() } for how providers are found and loaded.

 *

 * @since 1.7

 */

public abstract class FtpClientProvider {

    /**

     * Creates a FtpClient from this provider.

     *

     * @return The created {@link FtpClient}.

     */

    public abstract FtpClient createFtpClient();

    private static final Object lock = new Object();

    private static FtpClientProvider provider = null;

    /**

     * Initializes a new instance of this class.

     *

     * @throws SecurityException if a security manager is installed and it denies

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

     */

    protected FtpClientProvider() {

        @SuppressWarnings("removal")

        SecurityManager sm = System.getSecurityManager();

        if (sm != null) {

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

        }

    }

    private static boolean loadProviderFromProperty() {

        String cm = System.getProperty("sun.net.ftpClientProvider");

        if (cm == null) {

            return false;

        }

        try {

            @SuppressWarnings("deprecation")

            Object o = Class.forName(cm, true, null).newInstance();

            provider = (FtpClientProvider)o;

            return true;

        } catch (ClassNotFoundException |

                 IllegalAccessException |

                 InstantiationException |

                 SecurityException x) {

            throw new ServiceConfigurationError(x.toString());

        }

    }

    private static boolean loadProviderAsService() {

//        Iterator<FtpClientProvider> i =

//                ServiceLoader.load(FtpClientProvider.class,

//                                   ClassLoader.getSystemClassLoader()).iterator();

//

//        while (i.hasNext()) {

//            try {

//                provider = i.next();

//                return true;

//            } catch (ServiceConfigurationError sce) {

//                if (sce.getCause() instanceof SecurityException) {

//                    // Ignore, try next provider, if any

//                    continue;

//                }

//                throw sce;

//            }

//        }

        return false;

    }

    /**

     * Returns the system wide default FtpClientProvider 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

     *   {@code java.net.FtpClientProvider} 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 unchecked error or exception 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.net.FtpClientProvider} 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 unchecked error or exception 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 FtpClientProvider

     */

    @SuppressWarnings("removal")

    public static FtpClientProvider provider() {

        synchronized (lock) {

            if (provider != null) {

                return provider;

            }

            return (FtpClientProvider) AccessController.doPrivileged(

                    new PrivilegedAction<Object>() {

                        public Object run() {

                            if (loadProviderFromProperty()) {

                                return provider;

                            }

                            if (loadProviderAsService()) {

                                return provider;

                            }

                            provider = new sun.net.ftp.impl.DefaultFtpClientProvider();

                            return provider;

                        }

                    });

        }

    }

}