S
- The type of the service to be loaded by this loaderIterable<S>
public final class ServiceLoader<S> extends Object implements Iterable<S>
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.
Providers deployed as explicit modules on the module path are
instantiated by a provider factory or directly via the provider's
constructor. In the module declaration then the class name specified in the
provides clause is a provider factory if it is public and defines a
public static no-args method named "provider
". The return type of
the method must be assignable to the service type. If the class is
not a provider factory then it is public with a public zero-argument
constructor. The requirement that the provider factory or provider class
be public helps to document the intent that the provider will be
instantiated by the service-provider loading facility.
As an example, suppose a module declares the following:
provides com.example.CodecSet with com.example.impl.StandardCodecs;
provides com.example.CodecSet with com.example.impl.ExtendedCodecsFactory;
where com.example.CodecSet
is the service type,
com.example.impl.StandardCodecs
is a provider class that is public with a
public no-args constructor, com.example.impl.ExtendedCodecsFactory
is a public class that defines a public static no-args method named
"provider
" with a return type that is CodecSet
or a subtype
of. For this example then StandardCodecs
's no-arg constructor will
be used to instantiate StandardCodecs
. ExtendedCodecsFactory
will be treated as a provider factory and
ExtendedCodecsFactory.provider()
will be invoked to obtain the provider.
Providers deployed on the class path or as automatic-modules
on the
module path must have a public zero-argument constructor.
An application or library using this loading facility and developed and deployed as an explicit 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 an explicit 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 modules containing providers of the service.
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 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.
The load
methods locate providers using a class loader or module
layer
. When locating providers using a class loader then
providers in both named and unnamed modules may be located. When locating
providers using a module layer then only providers in named modules in
the layer (or parent layers) are located.
When locating providers using a class loader then any providers in named
modules defined to the class loader, or any class loader that is reachable
via parent delegation, are located. Additionally, providers in module layers
other than the boot
layer, where the module layer
contains modules defined to the class loader, or any class loader reachable
via parent delegation, are also located. For example, suppose there is a
module layer where each module is defined to its own class loader (see defineModulesWithManyLoaders
). If the
load
method is invoked to locate providers using any of these class
loaders for this layer then it will locate all of the providers in that
layer, irrespective of their defining class loader.
In the case of unnamed modules then the service configuration files are
located using the class loader's ClassLoader.getResources(String)
method. Any providers listed should be
visible via the class loader specified to the load
method. If a
provider in a named module is listed then it is ignored - this is to avoid
duplicates that would otherwise arise when a module has both a
provides clause and a service configuration file in
META-INF/services
that lists the same provider.
Service loaders created to locate providers using a ClassLoader
locate providers as follows:
ClassLoader
, or any class loader in the parent
delegation chain, defines modules in a custom module Layer
then
all providers in that layer are located, irrespective of their class
loader. The ordering of modules defined to the same class loader, or the
ordering of modules in a layer, is not defined. providers
list.
Providers added dynamically by instrumentation agents (redefineModule
)
are always located after providers declared by the module. ClassLoader.getResources(String)
method finds the service configuration files. Service loaders created to locate providers in a module Layer
will first locate providers in the layer, before locating providers in
parent layers. Traversal of parent layers is depth-first with each layer
visited at most once. For example, suppose L0 is the boot layer, L1 and
L2 are custom layers with L0 as their parent. Now suppose that L3 is
created with L1 and L2 as the parents (in that order). Using a service
loader to locate providers with L3 as the content will locate providers
in the following order: L3, L1, L0, L2. The ordering of modules in a layer
is not defined.
Selecting a provider or filtering providers will usually involve invoking
a provider method. Where selection or filtering based on the provider class is
needed then it can be done using a stream
. For example, the
following collects the providers that have a specific annotation:
Set<CodecSet> providers = ServiceLoader.load(CodecSet.class)
.stream()
.filter(p -> p.type().isAnnotationPresent(Managed.class))
.map(Provider::get)
.collect(Collectors.toSet());
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.
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.
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:
Each method returns an appropriate object or null if the provider does not support the given encoding. Typical providers support more than one encoding.public abstract Encoder getEncoder(String encodingName); public abstract Decoder getDecoder(String encodingName);
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.
Modifier and Type | Class | Description |
---|---|---|
static interface |
ServiceLoader.Provider<S> |
Represents a service provider located by
ServiceLoader . |
Modifier and Type | Method | Description |
---|---|---|
Optional<S> |
findFirst() |
Load the first available provider of this loader's service.
|
Iterator<S> |
iterator() |
Lazily load and instantiate 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.
|
Stream<ServiceLoader.Provider<S>> |
stream() |
Returns a stream that lazily loads the available providers of this
loader's service.
|
String |
toString() |
Returns a string describing this service.
|
forEach, spliterator
public Iterator<S> iterator()
The iterator returned by this method first yields all of the elements of the provider cache, in the order that they were loaded. 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 an appropriate static
factory method or 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 caches are 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.
public Stream<ServiceLoader.Provider<S>> stream()
Provider
, the Provider
's get
method
must be invoked to get or instantiate the provider.
When processing the stream then providers that were previously
loaded by stream operations are processed first, in load order. It then
lazily loads any remaining providers. If a provider class cannot be
loaded, can't be assigned to the service type, or some other error is
thrown when locating the provider then it is wrapped with a
ServiceConfigurationError
and thrown by whatever method caused the
provider to be loaded.
If this loader's provider caches are cleared by invoking the reload
method then existing streams for this service loader
should be discarded. The returned stream's source Spliterator
is
fail-fast and will throw ConcurrentModificationException
if the provider cache has been cleared.
The following examples demonstrate usage. The first example creates a stream of providers, the second example is the same except that it sorts the providers by provider class name (and so locate all providers).
Stream<CodecSet> providers = ServiceLoader.load(CodecSet.class)
.stream()
.map(Provider::get);
Stream<CodecSet> providers = ServiceLoader.load(CodecSet.class)
.stream()
.sorted(Comparator.comparing(p -> p.type().getName()))
.map(Provider::get);
public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader)
S
- the class of the service typeservice
- The interface or abstract class representing the serviceloader
- 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
usedServiceConfigurationError
- if the service type is not accessible to the caller or the
caller is in an explicit module and its module descriptor does
not declare that it uses service
public static <S> ServiceLoader<S> load(Class<S> service)
An invocation of this convenience method of the form
ServiceLoader.load(service)
is equivalent to
ServiceLoader.load(service, Thread.currentThread().getContextClassLoader())
S
- the class of the service typeservice
- The interface or abstract class representing the serviceServiceConfigurationError
- if the service type is not accessible to the caller or the
caller is in an explicit module and its module descriptor does
not declare that it uses service
public static <S> ServiceLoader<S> loadInstalled(Class<S> service)
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.
S
- the class of the service typeservice
- The interface or abstract class representing the serviceServiceConfigurationError
- if the service type is not accessible to the caller or the
caller is in an explicit module and its module descriptor does
not declare that it uses service
public static <S> ServiceLoader<S> load(Layer layer, Class<S> service)
Layer
and its
ancestors.load(S, null)
.S
- the class of the service typelayer
- The module Layerservice
- The interface or abstract class representing the serviceServiceConfigurationError
- if the service type is not accessible to the caller or the
caller is in an explicit module and its module descriptor does
not declare that it uses service
public Optional<S> findFirst()
iterator()
method and obtaining the first element. It therefore
returns the first element from the provider cache if possible, it
otherwise attempts to load and instantiate the first provider.
The following example loads the first available provider. If there are no providers deployed then it uses a default implementation.
CodecSet provider =
ServiceLoader.load(CodecSet.class).findFirst().orElse(DEFAULT_CODECSET);
Optional
if no providers
are locatedServiceConfigurationError
- If a provider class cannot be loaded, doesn't have the
appropriate static factory method or constructor, can't be
assigned to the service type, or if any other kind of exception
or error is thrown when locating or instantiating the provider.public void reload()
After invoking this method, subsequent invocations of the iterator
or stream
methods will lazily
look up providers (and instantiate in the case of iterator
)
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.
Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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DRAFT 9-internal+0-adhoc.mlchung.jdk9-jdeps