Module java.base

Package java.util

Class ServiceLoader<S>

java.lang.Object

java.util.ServiceLoader<S>

Type Parameters:

S - The type of the service to be loaded by this loader

All Implemented Interfaces:

Iterable<S>

public final class ServiceLoader<S>
extends Object
implements Iterable<S>

A simple service-provider loading facility.

A service is a well-known set of interfaces and (usually abstract) classes. A service provider is a specific implementation of a service. The classes in a provider typically implement the interfaces and subclass the classes defined in the service itself. Providers may be developed and deployed as modules and made available using the application module path. Providers may alternatively be packaged as JAR files and made available by adding them to the application class path. The advantage of developing a provider as a module is that the provider can be fully encapsulated to hide all details of its implementation.

For the purpose of loading, a service is represented by a single type, that is, a single interface or abstract class. (A concrete class can be used, but this is not recommended.) A provider of a given service contains one or more concrete classes that extend this service type with data and code specific to the provider. The provider class is typically not the entire provider itself but rather a proxy which contains enough information to decide whether the provider is able to satisfy a particular request together with code that can create the actual provider on demand. The details of provider classes tend to be highly service-specific; no single class or interface could possibly unify them, so no such type is defined here. A requirement enforced by this facility is that each provider class must have a public zero-argument constructor.

An application or library using this loading facility and developed and deployed as a named module must have an appropriate uses clause in its module descriptor to declare that the module uses implementations of the service. A corresponding requirement is that a provider deployed as a named module must have an appropriate provides clause in its module descriptor to declare that the module provides an implementation of the service. The uses and provides allow consumers of a service to be linked to providers of the service. In the case of load methods that locate service providers using a class loader, then provider modules defined to that class loader, or a class loader reachable using parent delegation, will be located.

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 file's name is the fully-qualified binary name of the service's type. The file contains a list of fully-qualified binary names of concrete provider classes, one per line. Space and tab characters surrounding each name, as well as blank lines, are ignored. The comment character is '#' ('\u0023', NUMBER SIGN); on each line all characters following the first comment character are ignored. The file must be encoded in UTF-8. If a particular concrete provider class is named in more than one configuration file, or is named in the same configuration file more than once, then the duplicates are ignored. The configuration file naming a particular provider need not be in the same JAR file or other distribution unit as the provider itself. The provider must be visible from the same class loader that was initially queried to locate the configuration file; note that this is not necessarily the class loader from which the file was actually loaded.

Providers are located 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 of the cache, in instantiation order, and then lazily locates and instantiates any remaining providers, adding each one to the cache in turn. The cache can be cleared via the reload method.

Service loaders always execute in the security context of the caller of the iterator 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.

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

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

Example Suppose we have a service type com.example.CodecSet which is intended to represent sets of encoder/decoder pairs for some protocol. In this case it is an abstract class with two abstract methods:

 public abstract Encoder getEncoder(String encodingName);
 public abstract Decoder getDecoder(String encodingName);

Each method returns an appropriate object or null if the provider does not support the given encoding. Typical providers support more than one encoding.

The CodecSet class creates and saves a single service instance at initialization:

 private static ServiceLoader<CodecSet> codecSetLoader
     = ServiceLoader.load(CodecSet.class);
 

To locate an encoder for a given encoding name it defines a static factory method which iterates through the known and available providers, returning only when it has located a suitable encoder or has run out of providers.

 public static Encoder getEncoder(String encodingName) {
     for (CodecSet cp : codecSetLoader) {
         Encoder enc = cp.getEncoder(encodingName);
         if (enc != null)
             return enc;
     }
     return null;
 }

A getDecoder method is defined similarly.

If the code creating and using the service loader is developed as a module then its module descriptor will declare the usage with:

uses com.example.CodecSet;

Now suppose that com.example.impl.StandardCodecs is an implementation of the CodecSet service and developed as a module. In that case then the module with the service provider module will declare this in its module descriptor:

provides com.example.CodecSet with com.example.impl.StandardCodecs;
 

On the other hand, suppose com.example.impl.StandardCodecs is packaged in a JAR file for the class path then the JAR file will contain a file named:

META-INF/services/com.example.CodecSet

that contains the single line:

com.example.impl.StandardCodecs    # Standard codecs

Usage Note If the class path of a class loader that is used for provider loading includes remote network URLs then those URLs will be dereferenced in the process of searching for provider-configuration files.

This activity is normal, although it may cause puzzling entries to be created in web-server logs. If a web server is not configured correctly, however, then this activity may cause the provider-loading algorithm to fail spuriously.

A web server should return an HTTP 404 (Not Found) response when a requested resource does not exist. Sometimes, however, web servers are erroneously configured to return an HTTP 200 (OK) response along with a helpful HTML error page in such cases. This will cause a ServiceConfigurationError to be thrown when this class attempts to parse the HTML page as a provider-configuration file. The best solution to this problem is to fix the misconfigured web server to return the correct response code (HTTP 404) along with the HTML error page.

Since:

1.6

Method Summary

Modifier and Type	Method	Description
Iterator<S>	iterator()	Lazily loads the available providers of this loader's service.
static <S> ServiceLoader<S>	load(Class<S> service)	Creates a new service loader for the given service type, using the current thread's context class loader.
static <S> ServiceLoader<S>	load(Class<S> service, ClassLoader loader)	Creates a new service loader for the given service type and class loader.
static <S> ServiceLoader<S>	load(Layer layer, Class<S> service)	Creates a new service loader for the given service type that loads service providers from modules in the given Layer and its ancestors.
static <S> ServiceLoader<S>	loadInstalled(Class<S> service)	Creates a new service loader for the given service type, using the platform class loader.
void	reload()	Clear this loader's provider cache so that all providers will be reloaded.
String	toString()	Returns a string describing this service.

Methods inherited from interface java.lang.Iterable

forEach, spliterator

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Method Detail

reload

public void reload()

Clear this loader's provider cache so that all providers will be reloaded.

After invoking this method, subsequent invocations of the iterator method will lazily look up and instantiate providers from scratch, just as is done by a newly-created loader.

This method is intended for use in situations in which new providers can be installed into a running Java virtual machine.

iterator

public Iterator<S> iterator()

Lazily loads the available providers of this loader's service.

The iterator returned by this method first yields all of the elements of the provider cache, in instantiation order. It then lazily loads and instantiates any remaining providers, adding each one to the cache in turn.

To achieve laziness the actual work of locating and instantiating providers must be done by the iterator itself. Its hasNext and next methods can therefore throw a ServiceConfigurationError if a provider class cannot be loaded, doesn't have the appropriate constructor, can't be assigned to the service type or if any other kind of exception or error is thrown as the next provider is located and instantiated. To write robust code it is only necessary to catch ServiceConfigurationError when using a service iterator.

If such an error is thrown then subsequent invocations of the iterator will make a best effort to locate and instantiate the next available provider, but in general such recovery cannot be guaranteed.

Design Note Throwing an error in these cases may seem extreme. The rationale for this behavior is that a malformed provider-configuration file, like a malformed class file, indicates a serious problem with the way the Java virtual machine is configured or is being used. As such it is preferable to throw an error rather than try to recover or, even worse, fail silently.

If this loader's provider cache is cleared by invoking the reload method then existing iterators for this service loader should be discarded. The hasNext and next methods of the iterator throw ConcurrentModificationException if used after the provider cache has been cleared.

The iterator returned by this method does not support removal. Invoking its remove method will cause an UnsupportedOperationException to be thrown.

Specified by:

iterator in interface Iterable<S>

Implementation Note:

When adding providers to the cache, the Iterator processes resources in the order that the ClassLoader.getResources(String) method finds the service configuration files.

Returns:

An iterator that lazily loads providers for this loader's service

load

public static <S> ServiceLoader<S> load(Class<S> service,
                                        ClassLoader loader)

Creates a new service loader for the given service type and class loader.

Type Parameters:

S - the class of the service type

Parameters:

service - The interface or abstract class representing the service

loader - The class loader to be used to load provider-configuration files and provider classes, or null if the system class loader (or, failing that, the bootstrap class loader) is to be used

Returns:

A new service loader

Throws:

ServiceConfigurationError - if the service type is not accessible to the caller or the caller is in a named module and its module descriptor does not declare that it uses service

load

public static <S> ServiceLoader<S> load(Class<S> service)

Creates a new service loader for the given service type, using the current thread's context class loader.

An invocation of this convenience method of the form

 ServiceLoader.load(service)

is equivalent to

 ServiceLoader.load(service,
                    Thread.currentThread().getContextClassLoader())

Type Parameters:

S - the class of the service type

Parameters:

service - The interface or abstract class representing the service

Returns:

A new service loader

Throws:

ServiceConfigurationError - if the service type is not accessible to the caller or the caller is in a named module and its module descriptor does not declare that it uses service

loadInstalled

public static <S> ServiceLoader<S> loadInstalled(Class<S> service)

Creates a new service loader for the given service type, using the platform class loader.

This convenience method is equivalent to:

 ServiceLoader.load(service, ClassLoader.getPlatformClassLoader())
 

This method is intended for use when only installed providers are desired. The resulting service will only find and load providers that have been installed into the current Java virtual machine; providers on the application's module path or class path will be ignored.

Type Parameters:

S - the class of the service type

Parameters:

service - The interface or abstract class representing the service

Returns:

A new service loader

Throws:

ServiceConfigurationError - if the service type is not accessible to the caller or the caller is in a named module and its module descriptor does not declare that it uses service

load

public static <S> ServiceLoader<S> load(Layer layer,
                                        Class<S> service)

Creates a new service loader for the given service type that loads service providers from modules in the given Layer and its ancestors.

API Note:

Unlike the other load methods defined here, the service type is the second parameter. The reason for this is to avoid source compatibility issues for code that uses load(S, null).

Type Parameters:

S - the class of the service type

Parameters:

layer - The module Layer

service - The interface or abstract class representing the service

Returns:

A new service loader

Throws:

ServiceConfigurationError - if the service type is not accessible to the caller or the caller is in a named module and its module descriptor does not declare that it uses service

Since:

9

toString

public String toString()

Returns a string describing this service.

Overrides:

toString in class Object

Returns:

A descriptive string