/*

 * Copyright (c) 2005, 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.  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 java.util;

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStream;

import java.io.InputStreamReader;

import java.lang.reflect.Constructor;

import java.lang.reflect.InvocationTargetException;

import java.lang.reflect.Method;

import java.lang.reflect.Modifier;

import java.net.URL;

import java.net.URLConnection;

import java.security.AccessControlContext;

import java.security.AccessController;

import java.security.PrivilegedAction;

import java.security.PrivilegedActionException;

import java.security.PrivilegedExceptionAction;

import java.util.function.Consumer;

import java.util.function.Supplier;

import java.util.stream.Stream;

import java.util.stream.StreamSupport;

import jdk.internal.loader.BootLoader;

import jdk.internal.loader.ClassLoaders;

import jdk.internal.misc.JavaLangAccess;

import jdk.internal.misc.SharedSecrets;

import jdk.internal.misc.VM;

import jdk.internal.module.ServicesCatalog;

import jdk.internal.module.ServicesCatalog.ServiceProvider;

import jdk.internal.reflect.CallerSensitive;

import jdk.internal.reflect.Reflection;

/**

 * A facility to load implementations of a service.

 *

 * <p> A <i>service</i> is a well-known interface or class for which zero, one,

 * or many service providers exist. A <i>service provider</i> (or just

 * <i>provider</i>) is a class that implements or subclasses the well-known

 * interface or class. A {@code ServiceLoader} is an object that locates and

 * loads service providers deployed in the run time environment at a time of an

 * application's choosing. Application code refers only to the service, not to

 * service providers, and is assumed to be capable of choosing between multiple

 * service providers (based on the functionality they expose through the service),

 * and handling the possibility that no service providers are located.

 *

 * <h3> Obtaining a service loader </h3>

 *

 * <p> An application obtains a service loader for a given service by invoking

 * one of the static {@code load} methods of {@code ServiceLoader}. If the

 * application is a module, then its module declaration must have a <i>uses</i>

 * directive that specifies the service; this helps to locate providers and ensure

 * they will execute reliably. In addition, if the application module does not

 * contain the service, then its module declaration must have a <i>requires</i>

 * directive that specifies the module which exports the service. It is strongly

 * recommended that the application module does <b>not</b> require modules which

 * contain providers of the service.

 *

 * <p> A service loader can be used to locate and instantiate providers of the

 * service by means of the {@link #iterator() iterator} method. {@code ServiceLoader}

 * also defines the {@link #stream() stream} method to obtain a stream of providers

 * that can be inspected and filtered without instantiating them.

 *

 * <p> As an example, suppose the service is {@code com.example.CodecFactory}, an

 * interface that defines methods for producing encoders and decoders:

 *

 * <pre>{@code

 *     package com.example;

 *     public interface CodecFactory {

 *         Encoder getEncoder(String encodingName);

 *         Decoder getDecoder(String encodingName);

 *     }

 * }</pre>

 *

 * <p> The following code obtains a service loader for the {@code CodecFactory}

 * service, then uses its iterator (created automatically by the enhanced-for

 * loop) to yield instances of the service providers that are located:

 *

 * <pre>{@code

 *     ServiceLoader<CodecFactory> loader = ServiceLoader.load(CodecFactory.class);

 *     for (CodecFactory factory : loader) {

 *         Encoder enc = factory.getEncoder("PNG");

 *         if (enc != null)

 *             ... use enc to encode a PNG file

 *             break;

 *         }

 * }</pre>

 *

 * <p> If this code resides in a module, then in order to refer to the

 * {@code com.example.CodecFactory} interface, the module declaration would

 * require the module which exports the interface. The module declaration would

 * also specify use of {@code com.example.CodecFactory}:

 * <pre>{@code

 *     requires com.example.codec.core;

 *     uses com.example.CodecFactory;

 * }</pre>

 *

 * <p> Sometimes an application may wish to inspect a service provider before

 * instantiating it, in order to determine if an instance of that service

 * provider would be useful. For example, a service provider for {@code

 * CodecFactory} that is capable of producing a "PNG" encoder may be annotated

 * with {@code @PNG}. The following code uses service loader's {@code stream}

 * method to yield instances of {@code Provider<CodecFactory>} in contrast to

 * how the iterator yields instances of {@code CodecFactory}:

 * <pre>{@code

 *     ServiceLoader<CodecFactory> loader = ServiceLoader.load(CodecFactory.class);

 *     Set<CodecFactory> pngFactories = loader

 *            .stream()                                              // Note a below

 *            .filter(p -> p.type().isAnnotationPresent(PNG.class))  // Note b

 *            .map(Provider::get)                                    // Note c

 *            .collect(Collectors.toSet());

 * }</pre>

 * <ol type="a">

 *   <li> A stream of {@code Provider<CodecFactory>} objects </li>

 *   <li> {@code p.type()} yields a {@code Class<CodecFactory>} </li>

 *   <li> {@code get()} yields an instance of {@code CodecFactory} </li>

 * </ol>

 *

 * <h3> Designing services </h3>

 *

 * <p> A service is a single type, usually an interface or abstract class. A

 * concrete class can be used, but this is not recommended. The type may have

 * any accessibility. The methods of a service are highly domain-specific, so

 * this API specification cannot give concrete advice about their form or

 * function. However, there are two general guidelines:

 * <ol>

 *   <li><p> A service should declare as many methods as needed to allow service

 *   providers to communicate their domain-specific properties and other

 *   quality-of-implementation factors. An application which obtains a service

 *   loader for the service may then invoke these methods on each instance of

 *   a service provider, in order to choose the best provider for the

 *   application. </p></li>

 *   <li><p> A service should express whether its service providers are intended

 *   to be direct implementations of the service or to be an indirection

 *   mechanism such as a "proxy" or a "factory". Service providers tend to be

 *   indirection mechanisms when domain-specific objects are relatively

 *   expensive to instantiate; in this case, the service should be designed

 *   so that service providers are abstractions which create the "real"

 *   implementation on demand. For example, the {@code CodecFactory} service

 *   expresses through its name that its service providers are factories

 *   for codecs, rather than codecs themselves, because it may be expensive

 *   or complicated to produce certain codecs. </p></li>

 * </ol>

 *

 * <h3> <a id="developing-service-providers">Developing service providers</a> </h3>

 *

 * <p> A service provider is a single type, usually a concrete class. An

 * interface or abstract class is permitted because it may declare a static

 * provider method, discussed later. The type must be public and must not be

 * an inner class.

 *

 * <p> A service provider and its supporting code may be developed in a module,

 * which is then deployed on the application module path or in a modular

 * image. Alternatively, a service provider and its supporting code may be

 * packaged as a JAR file and deployed on the application class path. The

 * advantage of developing a service provider in a module is that the provider

 * can be fully encapsulated to hide all details of its implementation.

 *

 * <p> An application that obtains a service loader for a given service is

 * indifferent to whether providers of the service are deployed in modules or

 * packaged as JAR files. The application instantiates service providers via

 * the service loader's iterator, or via {@link Provider Provider} objects in

 * the service loader's stream, without knowledge of the service providers'

 * locations.

 *

 * <h3> Deploying service providers as modules </h3>

 *

 * <p> A service provider that is developed in a module must be specified in a

 * <i>provides</i> directive in the module declaration. The provides directive

 * specifies both the service and the service provider; this helps to locate the

 * provider when another module, with a <i>uses</i> directive for the service,

 * obtains a service loader for the service. It is strongly recommended that the

 * module does not export the package containing the service provider. There is

 * no support for a module specifying, in a <i>provides</i> directive, a service

 * provider in another module.

 * <p> A service provider that is developed in a module has no control over when

 * it is instantiated, since that occurs at the behest of the application, but it

 * does have control over how it is instantiated:

 *

 * <ul>

 *

 *   <li> If the service provider declares a provider method, then the service

 *   loader invokes that method to obtain an instance of the service provider. A

 *   provider method is a public static method named "provider" with no formal

 *   parameters and a return type that is assignable to the service's interface

 *   or class.

 *   <p> In this case, the service provider itself need not be assignable to the

 *   service's interface or class. </li>

 *

 *   <li> If the service provider does not declare a provider method, then the

 *   service provider is instantiated directly, via its provider constructor. A

 *   provider constructor is a public constructor with no formal parameters.

 *   <p> In this case, the service provider must be assignable to the service's

 *   interface or class </li>

 *

 * </ul>

 *

 * <p> A service provider that is deployed as an

 * {@linkplain java.lang.module.ModuleDescriptor#isAutomatic automatic module} on

 * the application module path must have a provider constructor. There is no

 * support for a provider method in this case.

 *

 * <p> As an example, suppose a module specifies the following directives:

 * <pre>{@code

 *     provides com.example.CodecFactory with com.example.impl.StandardCodecs;

 *     provides com.example.CodecFactory with com.example.impl.ExtendedCodecsFactory;

 * }</pre>

 *

 * <p> where

 *

 * <ul>

 *   <li> {@code com.example.CodecFactory} is the two-method service from

 *   earlier. </li>

 *

 *   <li> {@code com.example.impl.StandardCodecs} is a public class that implements

 *   {@code CodecFactory} and has a public no-args constructor. </li>

 *

 *   <li> {@code com.example.impl.ExtendedCodecsFactory} is a public class that

 *   does not implement CodecFactory, but it declares a public static no-args

 *   method named "provider" with a return type of {@code CodecFactory}. </li>

 * </ul>

 *

 * <p> A service loader will instantiate {@code StandardCodecs} via its

 * constructor, and will instantiate {@code ExtendedCodecsFactory} by invoking

 * its {@code provider} method. The requirement that the provider constructor or

 * provider method is public helps to document the intent that the class (that is,

 * the service provider) will be instantiated by an entity (that is, a service

 * loader) which is outside the class's package.

 *

 * <h3> Deploying service providers on the class path </h3>

 *

 * A service provider that is packaged as a JAR file for the class path is

 * identified by placing a <i>provider-configuration file</i> in the resource

 * directory {@code META-INF/services}. The name of the provider-configuration

 * file is the fully qualified binary name of the service. The provider-configuration

 * file contains a list of fully qualified binary names of service providers, one

 * per line.

 *

 * <p> For example, suppose the service provider

 * {@code com.example.impl.StandardCodecs} is packaged in a JAR file for the

 * class path. The JAR file will contain a provider-configuration file named:

 *

 * <blockquote>{@code

 *     META-INF/services/com.example.CodecFactory

 * }</blockquote>

 *

 * that contains the line:

 *

 * <blockquote>{@code

 *     com.example.impl.StandardCodecs # Standard codecs

 * }</blockquote>

 *

 * <p><a id="format">The provider-configuration file must be encoded in UTF-8. </a>

 * Space and tab characters surrounding each service provider's name, as well as

 * blank lines, are ignored. The comment character is {@code '#'}

 * ({@code '&#92;u0023'} <span style="font-size:smaller;">NUMBER SIGN</span>);

 * on each line all characters following the first comment character are ignored.

 * If a service provider class name is listed more than once in a

 * provider-configuration file then the duplicate is ignored. If a service

 * provider class is named in more than one configuration file then the duplicate

 * is ignored.

 *

 * <p> A service provider that is mentioned in a provider-configuration file may

 * be located in the same JAR file as the provider-configuration file or in a

 * different JAR file. The service provider must be visible from the class loader

 * that is initially queried to locate the provider-configuration file; this is

 * not necessarily the class loader which ultimately locates the

 * provider-configuration file.

 *

 * <h3> Timing of provider discovery </h3>

 *

 * <p> Service providers are loaded and instantiated lazily, that is, on demand.

 * A service loader maintains a cache of the providers that have been loaded so

 * far. Each invocation of the {@code iterator} method returns an {@code Iterator}

 * that first yields all of the elements cached from previous iteration, in

 * instantiation order, and then lazily locates and instantiates any remaining

 * providers, adding each one to the cache in turn. Similarly, each invocation

 * of the stream method returns a {@code Stream} that first processes all

 * providers loaded by previous stream operations, in load order, and then lazily

 * locates any remaining providers. Caches are cleared via the {@link #reload

 * reload} method.

 *

 * <h3> <a id="errors">Errors</a> </h3>

 *

 * <p> When using the service loader's {@code iterator}, the {@link

 * Iterator#hasNext() hasNext} and {@link Iterator#next() next} methods will

 * fail with {@link ServiceConfigurationError} if an error occurs locating,

 * loading or instantiating a service provider. When processing the service

 * loader's stream then {@code ServiceConfigurationError} may be thrown by any

 * method that causes a service provider to be located or loaded.

 *

 * <p> When loading or instantiating a service provider in a module, {@code

 * ServiceConfigurationError} can be thrown for the following reasons:

 *

 * <ul>

 *

 *   <li> The service provider cannot be loaded. </li>

 *

 *   <li> The service provider does not declare a provider method, and either

 *   it is not assignable to the service's interface/class or does not have a

 *   provider constructor. </li>

 *

 *   <li> The service provider declares a public static no-args method named

 *   "provider" with a return type that is not assignable to the service's

 *   interface or class. </li>

 *

 *   <li> The service provider class file has more than one public static

 *   no-args method named "{@code provider}". </li>

 *

 *   <li> The service provider declares a provider method and it fails by

 *   returning {@code null} or throwing an exception. </li>

 *

 *   <li> The service provider does not declare a provider method, and its

 *   provider constructor fails by throwing an exception. </li>

 *

 * </ul>

 *

 * <p> When reading a provider-configuration file, or loading or instantiating

 * a provider class named in a provider-configuration file, then {@code

 * ServiceConfigurationError} can be thrown for the following reasons:

 *

 * <ul>

 *

 *   <li> The format of the provider-configuration file violates the <a

 *   href="ServiceLoader.html#format">format</a> specified above; </li>

 *

 *   <li> An {@link IOException IOException} occurs while reading the

 *   provider-configuration file; </li>

 *

 *   <li> A service provider cannot be loaded; </li>

 *

 *   <li> A service provider is not assignable to the service's interface or

 *   class, or does not define a provider constructor, or cannot be

 *   instantiated. </li>

 *

 * </ul>

 *

 * <h3> Security </h3>

 *

 * <p> Service loaders always execute in the security context of the caller

 * of the iterator or stream methods and may also be restricted by the security

 * context of the caller that created the service loader.

 * Trusted system code should typically invoke the methods in this class, and

 * the methods of the iterators which they return, from within a privileged

 * security context.

 *

 * <h3> Concurrency </h3>

 *

 * <p> Instances of this class are not safe for use by multiple concurrent

 * threads.

 *

 * <h3> Null handling </h3>

 *

 * <p> Unless otherwise specified, passing a {@code null} argument to any

 * method in this class will cause a {@link NullPointerException} to be thrown.

 *

 * @param  <S>

 *         The type of the service to be loaded by this loader

 *

 * @author Mark Reinhold

 * @since 1.6

 * @revised 9

 * @spec JPMS

 */

public final class ServiceLoader<S>

    implements Iterable<S>

{

    // The class or interface representing the service being loaded

    private final Class<S> service;

    // The class of the service type

    private final String serviceName;

    // The module layer used to locate providers; null when locating

    // providers using a class loader

    private final ModuleLayer layer;

    // The class loader used to locate, load, and instantiate providers;

    // null when locating provider using a module layer

    private final ClassLoader loader;

    // The access control context taken when the ServiceLoader is created

    private final AccessControlContext acc;

    // The lazy-lookup iterator for iterator operations

    private Iterator<Provider<S>> lookupIterator1;

    private final List<S> instantiatedProviders = new ArrayList<>();

    // The lazy-lookup iterator for stream operations

    private Iterator<Provider<S>> lookupIterator2;

    private final List<Provider<S>> loadedProviders = new ArrayList<>();

    private boolean loadedAllProviders; // true when all providers loaded

    // Incremented when reload is called

    private int reloadCount;

    private static JavaLangAccess LANG_ACCESS;

    static {

        LANG_ACCESS = SharedSecrets.getJavaLangAccess();

    }

    /**

     * Represents a service provider located by {@code ServiceLoader}.

     *

     * <p> When using a loader's {@link ServiceLoader#stream() stream()} method

     * then the elements are of type {@code Provider}. This allows processing

     * to select or filter on the provider class without instantiating the

     * provider. </p>

     *

     * @param  <S> The service type

     * @since 9

     * @spec JPMS

     */

    public static interface Provider<S> extends Supplier<S> {

        /**

         * Returns the provider type. There is no guarantee that this type is

         * accessible or that it has a public no-args constructor. The {@link

         * #get() get()} method should be used to obtain the provider instance.

         *

         * <p> When a module declares that the provider class is created by a

         * provider factory then this method returns the return type of its

         * public static "{@code provider()}" method.

         *

         * @return The provider type

         */

        Class<? extends S> type();

        /**

         * Returns an instance of the provider.

         *

         * @return An instance of the provider.

         *

         * @throws ServiceConfigurationError

         *         If the service provider cannot be instantiated, or in the

         *         case of a provider factory, the public static

         *         "{@code provider()}" method returns {@code null} or throws

         *         an error or exception. The {@code ServiceConfigurationError}

         *         will carry an appropriate cause where possible.

         */

        @Override S get();

    }

    /**

     * Initializes a new instance of this class for locating service providers

     * in a module layer.

     *

     * @throws ServiceConfigurationError

     *         If {@code svc} is not accessible to {@code caller} or the caller

     *         module does not use the service type.

     */

    private ServiceLoader(Class<?> caller, ModuleLayer layer, Class<S> svc) {

        Objects.requireNonNull(caller);

        Objects.requireNonNull(layer);

        Objects.requireNonNull(svc);

        checkCaller(caller, svc);

        this.service = svc;

        this.serviceName = svc.getName();

        this.layer = layer;

        this.loader = null;

        this.acc = (System.getSecurityManager() != null)

                ? AccessController.getContext()

                : null;

    }