A facility to load implementations of a service.

A service is a well-known interface or class for which zero, one, or many service providers exist. A service provider (or just provider) is a class that implements or subclasses the well-known interface or class. A 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.

Obtaining a service loader

An application obtains a service loader for a given service by invoking one of the static load methods of ServiceLoader. If the application is a module, then its module declaration must have a uses 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 requires directive that specifies the module which exports the service. It is strongly recommended that the application module does not require modules which contain providers of the service.

A service loader can be used to locate and instantiate providers of the service by means of the iterator method. ServiceLoader also defines the stream method to obtain a stream of providers that can be inspected and filtered without instantiating them.

As an example, suppose the service is com.example.CodecFactory, an interface that defines methods for producing encoders and decoders:

package com.example;
    public interface CodecFactory {
        Encoder getEncoder(String encodingName);
        Decoder getDecoder(String encodingName);
    }

The following code obtains a service loader for the CodecFactory service, then uses its iterator (created automatically by the enhanced-for loop) to yield instances of the service providers that are located:

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;
        }

If this code resides in a module, then in order to refer to the com.example.CodecFactory interface, the module declaration would require the module which exports the interface. The module declaration would also specify use of com.example.CodecFactory:

requires com.example.codec.core;
    uses com.example.CodecFactory;

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 CodecFactory that is capable of producing a "PNG" encoder may be annotated with @PNG. The following code uses service loader's stream method to yield instances of Provider<CodecFactory> in contrast to how the iterator yields instances of CodecFactory:

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());

1. A stream of Provider<CodecFactory> objects
2. p.type() yields a Class<CodecFactory>
3. get() yields an instance of CodecFactory

Designing services

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:

1. 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.

2. 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 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.

Developing service providers

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.

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.

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 Provider objects in the service loader's stream, without knowledge of the service providers' locations.

Deploying service providers as modules

A service provider that is developed in a module must be specified in a provides 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 uses 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 provides directive, a service provider in another module.

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:

• 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.

  In this case, the service provider itself need not be assignable to the service's interface or class.

• 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.

  In this case, the service provider must be assignable to the service's interface or class

A service provider that is deployed as an automatic module on the application module path must have a provider constructor. There is no support for a provider method in this case.

As an example, suppose a module specifies the following directive:

provides com.example.CodecFactory with com.example.impl.StandardCodecs,
             com.example.impl.ExtendedCodecsFactory;

where

• com.example.CodecFactory is the two-method service from earlier.
• com.example.impl.StandardCodecs is a public class that implements CodecFactory and has a public no-args constructor.
• 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 CodecFactory.

A service loader will instantiate StandardCodecs via its constructor, and will instantiate ExtendedCodecsFactory by invoking its 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.

Deploying service providers on the class path

A service provider that is packaged as a JAR file for the class path is identified by placing a provider-configuration file in the resource directory 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.

For example, suppose the service provider com.example.impl.StandardCodecs is packaged in a JAR file for the class path. The JAR file will contain a provider-configuration file named:

META-INF/services/com.example.CodecFactory

that contains the line:

com.example.impl.StandardCodecs # Standard codecs

The provider-configuration file must be encoded in UTF-8. Space and tab characters surrounding each service provider's name, as well as blank lines, are ignored. The comment character is '#' (U+0023 NUMBER SIGN); 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.

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.

Timing of provider discovery

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 iterator method returns an 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 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 reload method.

Errors

When using the service loader's iterator, the hasNext and next methods will fail with ServiceConfigurationError if an error occurs locating, loading or instantiating a service provider. When processing the service loader's stream then ServiceConfigurationError may be thrown by any method that causes a service provider to be located or loaded.

When loading or instantiating a service provider in a module, ServiceConfigurationError can be thrown for the following reasons:

• The service provider cannot be loaded.
• 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.
• 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.
• The service provider class file has more than one public static no-args method named "provider".
• The service provider declares a provider method and it fails by returning null or throwing an exception.
• The service provider does not declare a provider method, and its provider constructor fails by throwing an exception.

When reading a provider-configuration file, or loading or instantiating a provider class named in a provider-configuration file, then ServiceConfigurationError can be thrown for the following reasons:

• The format of the provider-configuration file violates the format specified above;
• An IOException occurs while reading the provider-configuration file;
• A service provider cannot be loaded;
• A service provider is not assignable to the service's interface or class, or does not define a provider constructor, or cannot be instantiated.

Security

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.

Concurrency

Instances of this class are not safe for use by multiple concurrent threads.

Null handling

Unless otherwise specified, passing a null argument to any method in this class will cause a NullPointerException to be thrown.