ServiceLoader

AI-generated Key Takeaways

• ServiceLoader is a facility for discovering and loading service providers at runtime.

• Service providers are declared in provider-configuration files located in META-INF/services/.

• ServiceLoader.load() is used to obtain an iterator to access available providers.

• The iterator loads providers lazily and caches them for subsequent use.

• ServiceLoader facilitates loose coupling between services and their implementations.

public final class ServiceLoader 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. Service providers can be installed in an implementation of the Java platform in the form of extensions, that is, jar files placed into any of the usual extension directories. Providers can also be made available by adding them to the application's class path or by some other platform-specific means.

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. The only requirement enforced by this facility is that provider classes must have a zero-argument constructor so that they can be instantiated during loading.

A service provider 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 accessible 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. 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.

If com.example.impl.StandardCodecs is an implementation of the CodecSet service then its jar file also contains a file named

 META-INF/services/com.example.CodecSet

This file contains the single line:

 com.example.impl.StandardCodecs    # Standard codecs

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.

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.

Public Method Summary

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 {@linkplain java.lang.Thread#getContextClassLoader 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>	loadInstalled(Class<S> service) Creates a new service loader for the given service type, using the extension 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.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

From interface java.lang.Iterable

void	forEach(Consumer<? super T> action) Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
abstract Iterator<S>	iterator() Returns an iterator over elements of type T.
Spliterator<S>	spliterator() Creates a Spliterator over the elements described by this Iterable.

Public Methods

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 parsing the available provider-configuration files and instantiating providers must be done by the iterator itself. Its hasNext and next methods can therefore throw a ServiceConfigurationError if a provider-configuration file violates the specified format, or if it names a provider class that cannot be found and instantiated, or if the result of instantiating the class is not assignable 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.

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

Returns

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

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

Creates a new service loader for the given service type, using the current thread's {@linkplain java.lang.Thread#getContextClassLoader context class loader}.

An invocation of this convenience method of the form

 ServiceLoader.load(service)

is equivalent to

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

Parameters

service	The interface or abstract class representing the service

Returns

• A new service loader

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

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

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

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

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

This convenience method simply locates the extension class loader, call it extClassLoader, and then returns

 ServiceLoader.load(service, extClassLoader)

If the extension class loader cannot be found then the system class loader is used; if there is no system class loader then the bootstrap class loader is used.

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 class path will be ignored.

Parameters

service	The interface or abstract class representing the service

Returns

• A new service loader

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.

public String toString ()

Returns a string describing this service.

Returns

• A descriptive string