/*
 * Copyright (c) 2016, 2018, 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.
 *
 * 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 jdk.vm.ci.services;

import static jdk.vm.ci.services.Services.IS_BUILDING_NATIVE_IMAGE;
import static jdk.vm.ci.services.Services.IS_IN_NATIVE_IMAGE;

import java.util.ArrayList;
import java.util.List;
import java.util.ServiceLoader;

/**
 * Service-provider class for the runtime to locate providers of JVMCI services where the latter are
 * not in packages exported by the JVMCI module. As part of instantiating a
 * {@link JVMCIServiceLocator}, all JVMCI packages will be opened to the module defining the class
 * of the instantiated object.
 *
 * While the {@link #getProvider(Class)} method can be used directly, it's usually easier to use
 * {@link #getProviders(Class)}.
 */
public abstract class JVMCIServiceLocator {

    private static Void checkPermission() {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new JVMCIPermission());
        }
        return null;
    }

    @SuppressWarnings("unused")
    private JVMCIServiceLocator(Void ignore) {
    }

    /**
     * Creates a capability for accessing JVMCI. Once successfully instantiated, JVMCI opens all its
     * packages to the module defining the type of this object.
     *
     * @throws SecurityException if a security manager has been installed and it denies
     *             {@link JVMCIPermission}
     */
    protected JVMCIServiceLocator() {
        this(checkPermission());
        Services.checkJVMCIEnabled();
        Services.openJVMCITo(getClass().getModule());
    }

    /**
     * Gets the provider of the service defined by {@code service} or {@code null} if this object
     * does not have a provider for {@code service}.
     */
    protected abstract <S> S getProvider(Class<S> service);

    private static volatile List<JVMCIServiceLocator> cachedLocators;

    private static Iterable<JVMCIServiceLocator> getJVMCIServiceLocators() {
        Iterable<JVMCIServiceLocator> result = cachedLocators;
        if (result != null) {
            return result;
        }
        result = ServiceLoader.load(JVMCIServiceLocator.class, ClassLoader.getSystemClassLoader());
        if (IS_BUILDING_NATIVE_IMAGE) {
            ArrayList<JVMCIServiceLocator> l = new ArrayList<>();
            for (JVMCIServiceLocator locator: result) {
                l.add(locator);
            }
            l.trimToSize();
            cachedLocators = l;
            return l;
        }
        return result;
    }

    /**
     * Gets the providers of the service defined by {@code service} by querying the available
     * {@link JVMCIServiceLocator} providers.
     *
     * @throws SecurityException if a security manager is present and it denies
     *             {@link JVMCIPermission}
     */
    public static <S> List<S> getProviders(Class<S> service) {
        Services.checkJVMCIEnabled();
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new JVMCIPermission());
        }
        List<S> providers = new ArrayList<>();
        for (JVMCIServiceLocator access : getJVMCIServiceLocators()) {
            S provider = access.getProvider(service);
            if (provider != null) {
                providers.add(provider);
            }
        }
        return providers;
    }
}