/*
 * Copyright (c) 2014, 2016, 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 java.lang.reflect.Module;
import java.util.Formatter;
import java.util.Iterator;
import java.util.ServiceConfigurationError;
import java.util.ServiceLoader;

/**
 * A mechanism for accessing service providers via JVMCI.
 */
public final class Services {

    private Services() {
    }

    /**
     * Performs any required security checks and dynamic reconfiguration to allow the module of a
     * given class to access the classes in the JVMCI module.
     *
     * Note: This API uses {@link Class} instead of {@link Module} to provide backwards
     * compatibility for JVMCI clients compiled against a JDK release earlier than 9.
     *
     * @param requestor a class requesting access to the JVMCI module for its module
     * @throws SecurityException if a security manager is present and it denies
     *             {@link JVMCIPermission}
     */
    public static void exportJVMCITo(Class<?> requestor) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new JVMCIPermission());
        }
        Module jvmci = Services.class.getModule();
        Module requestorModule = requestor.getModule();
        if (jvmci != requestorModule) {
            for (String pkg : jvmci.getPackages()) {
                // Export all JVMCI packages dynamically instead
                // of requiring a long list of --add-exports
                // options on the JVM command line.
                if (!jvmci.isExported(pkg, requestorModule)) {
                    jvmci.addExports(pkg, requestorModule);
                }
            }
        }
    }

    /**
     * Gets an {@link Iterable} of the JVMCI providers available for a given service.
     *
     * @throws SecurityException if a security manager is present and it denies
     *             {@link JVMCIPermission}
     */
    public static <S> Iterable<S> load(Class<S> service) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new JVMCIPermission());
        }
        Module jvmci = Services.class.getModule();
        jvmci.addUses(service);

        // Restrict JVMCI clients to be on the class path or module path
        return ServiceLoader.load(service, ClassLoader.getSystemClassLoader());
    }

    /**
     * Gets the JVMCI provider for a given service for which at most one provider must be available.
     *
     * @param service the service whose provider is being requested
     * @param required specifies if an {@link InternalError} should be thrown if no provider of
     *            {@code service} is available
     * @throws SecurityException if a security manager is present and it denies
     *             {@link JVMCIPermission}
     */
    public static <S> S loadSingle(Class<S> service, boolean required) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new JVMCIPermission());
        }
        Module jvmci = Services.class.getModule();
        jvmci.addUses(service);
        // Restrict JVMCI clients to be on the class path or module path
        Iterable<S> providers = ServiceLoader.load(service, ClassLoader.getSystemClassLoader());
        S singleProvider = null;
        try {
            for (Iterator<S> it = providers.iterator(); it.hasNext();) {
                singleProvider = it.next();
                if (it.hasNext()) {
                    throw new InternalError(String.format("Multiple %s providers found", service.getName()));
                }
            }
        } catch (ServiceConfigurationError e) {
            // If the service is required we will bail out below.
        }
        if (singleProvider == null && required) {
            String javaHome = System.getProperty("java.home");
            String vmName = System.getProperty("java.vm.name");
            Formatter errorMessage = new Formatter();
            errorMessage.format("The VM does not expose required service %s.%n", service.getName());
            errorMessage.format("Currently used Java home directory is %s.%n", javaHome);
            errorMessage.format("Currently used VM configuration is: %s", vmName);
            throw new UnsupportedOperationException(errorMessage.toString());
        }
        return singleProvider;
    }
}