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 "{@code 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: * *
{@code
* provides com.example.CodecSet with com.example.impl.StandardCodecs;
* provides com.example.CodecSet with com.example.impl.ExtendedCodecsFactory;
* }
*
* where {@code com.example.CodecSet} is the service type, {@code * com.example.impl.StandardCodecs} is a provider class that is public with a * public no-args constructor, {@code com.example.impl.ExtendedCodecsFactory} * is a public class that defines a public static no-args method named * "{@code provider}" with a return type that is {@code CodecSet} or a subtype * of. For this example then {@code StandardCodecs}'s no-arg constructor will * be used to instantiate {@code StandardCodecs}. {@code ExtendedCodecsFactory} * will be treated as a provider factory and {@code * ExtendedCodecsFactory.provider()} will be invoked to obtain the provider. * *
Providers deployed on the class path or as {@link * java.lang.module.ModuleDescriptor#isAutomatic 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 {@link #iterator 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 {@link #stream 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 {@link #reload reload} method. * *
The {@code load} methods locate providers using a class loader or module * {@link Layer 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 {@link Layer#boot() 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 {@link * Layer#defineModulesWithManyLoaders defineModulesWithManyLoaders}). If the * {@code 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 {@link ClassLoader#getResources(String) * ClassLoader.getResources(String)} method. Any providers listed should be * visible via the class loader specified to the {@code 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 {@code * META-INF/services} that lists the same provider. * *
Service loaders created to locate providers using a {@code ClassLoader} * locate providers as follows: *
Service loaders created to locate providers in a module {@link 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 {@link #stream() stream}. For example, the * following collects the providers that have a specific annotation: *
{@code
* Set 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 {@code null} argument to any * method in this class will cause a {@link 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: * *
{@code
* private static ServiceLoader 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. * *
{@code
* public static Encoder getEncoder(String encodingName) {
* for (CodecSet cp : codecSetLoader) {
* Encoder enc = cp.getEncoder(encodingName);
* if (enc != null)
* return enc;
* }
* return null;
* }}
*
* A {@code 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: *
{@code uses com.example.CodecSet;}
*
* Now suppose that {@code com.example.impl.StandardCodecs} is an * implementation of the {@code 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: *
{@code provides com.example.CodecSet with com.example.impl.StandardCodecs;
* }
*
* On the other hand, suppose {@code com.example.impl.StandardCodecs} is * packaged in a JAR file for the class path then the JAR file will contain a * file named: *
{@code META-INF/services/com.example.CodecSet}
* that contains the single line:
* {@code 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 {@link
* 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.
*
* @param 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. 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 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, Layer layer, Class 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 {@link
* java.util.Iterator#hasNext hasNext} and {@link java.util.Iterator#next
* next} methods can therefore throw a {@link 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 {@link 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.
*
* If this loader's provider caches are cleared by invoking the {@link
* #reload() reload} method then existing iterators for this service
* loader should be discarded.
* The {@link java.util.Iterator#hasNext() hasNext} and {@link
* java.util.Iterator#next() next} methods of the iterator throw {@link
* java.util.ConcurrentModificationException ConcurrentModificationException}
* if used after the provider cache has been cleared.
*
* The iterator returned by this method does not support removal.
* Invoking its {@link java.util.Iterator#remove() remove} method will
* cause an {@link UnsupportedOperationException} to be thrown.
*
* @return An iterator that lazily loads providers for this loader's
* service
*
* @revised 9
* @spec JPMS
*/
public Iterator 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 {@code
* ServiceConfigurationError} and thrown by whatever method caused the
* provider to be loaded. If this loader's provider caches are cleared by invoking the {@link
* #reload() reload} method then existing streams for this service loader
* should be discarded. The returned stream's source {@code Spliterator} is
* fail-fast and will throw {@link 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).
* An invocation of this convenience method of the form
* This convenience method is equivalent to: 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.
*
* @param The following example loads the first available provider. If there
* are no providers deployed then it uses a default implementation.
* After invoking this method, subsequent invocations of the {@link
* #iterator() iterator} or {@link #stream() stream} methods will lazily
* look up providers (and instantiate in the case of {@code 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.
*/
public void reload() {
lookupIterator1 = null;
instantiatedProviders.clear();
lookupIterator2 = null;
loadedProviders.clear();
loadedAllProviders = false;
// increment count to allow CME be thrown
reloadCount++;
}
/**
* Returns a string describing this service.
*
* @return A descriptive string
*/
public String toString() {
return "java.util.ServiceLoader[" + service.getName() + "]";
}
}
* 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
implements Iterable
{
// The class or interface representing the service being loaded
private final Class 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 Layer 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 instantiatedProviders = new ArrayList<>();
// The lazy-lookup iterator for stream operations
private Iterator The service type
* @since 9
* @spec JPMS
*/
public static interface Provider extends Supplier {
/**
* 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.
*
* 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;
}
/**
* Initializes a new instance of this class for locating service providers
* via a class loader.
*
* @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, Class svc, ClassLoader cl) {
Objects.requireNonNull(svc);
if (VM.isBooted()) {
checkCaller(caller, svc);
if (cl == null) {
cl = ClassLoader.getSystemClassLoader();
}
} else {
// if we get here then it means that ServiceLoader is being used
// before the VM initialization has completed. At this point then
// only code in the java.base should be executing.
Module callerModule = caller.getModule();
Module base = Object.class.getModule();
Module svcModule = svc.getModule();
if (callerModule != base || svcModule != base) {
fail(svc, "not accessible to " + callerModule + " during VM init");
}
// restricted to boot loader during startup
cl = null;
}
this.service = svc;
this.serviceName = svc.getName();
this.layer = null;
this.loader = cl;
this.acc = (System.getSecurityManager() != null)
? AccessController.getContext()
: null;
}
/**
* Initializes a new instance of this class for locating service providers
* via a class loader.
*
* @apiNote For use by ResourceBundle
*
* @throws ServiceConfigurationError
* If the caller module does not use the service type.
*/
private ServiceLoader(Module callerModule, Class svc, ClassLoader cl) {
if (!callerModule.canUse(svc)) {
fail(svc, callerModule + " does not declare `uses`");
}
this.service = Objects.requireNonNull(svc);
this.serviceName = svc.getName();
this.layer = null;
this.loader = cl;
this.acc = (System.getSecurityManager() != null)
? AccessController.getContext()
: null;
}
/**
* Checks that the given service type is accessible to types in the given
* module, and check that the module declare that it uses the service type. ??
*/
private static void checkCaller(Class caller, Class svc) {
Module callerModule = caller.getModule();
// Check access to the service type
int mods = svc.getModifiers();
if (!Reflection.verifyMemberAccess(caller, svc, null, mods)) {
fail(svc, "service type not accessible to " + callerModule);
}
// If the caller is in a named module then it should "uses" the
// service type
if (!callerModule.canUse(svc)) {
fail(svc, callerModule + " does not declare `uses`");
}
}
private static void fail(Class service, String msg, Throwable cause)
throws ServiceConfigurationError
{
throw new ServiceConfigurationError(service.getName() + ": " + msg,
cause);
}
private static void fail(Class service, String msg)
throws ServiceConfigurationError
{
throw new ServiceConfigurationError(service.getName() + ": " + msg);
}
private static void fail(Class service, URL u, int line, String msg)
throws ServiceConfigurationError
{
fail(service, u + ":" + line + ": " + msg);
}
/**
* Uses Class.forName to load a provider class in a module.
*
* @throws ServiceConfigurationError
* If the class cannot be loaded
*/
private Class loadProviderInModule(Module module, String cn) {
Class clazz = null;
if (acc == null) {
try {
clazz = Class.forName(module, cn);
} catch (LinkageError e) {
fail(service, "Unable to load " + cn, e);
}
} else {
PrivilegedExceptionAction implements Provider {
final Class service;
final AccessControlContext acc;
final Method factoryMethod; // factory method or null
final Class type;
final Constructor ctor; // public no-args constructor or null
/**
* Creates a Provider.
*
* @param service
* The service type
* @param clazz
* The provider (or provider factory) class
* @param acc
* The access control context when running with security manager
*
* @throws ServiceConfigurationError
* If the class is not public; If the class defines a public
* static provider() method with a return type that is assignable
* to the service type or the class is not a provider class with
* a public no-args constructor.
*/
@SuppressWarnings("unchecked")
ProviderImpl(Class service, Class clazz, AccessControlContext acc) {
this.service = (Class) service;
this.acc = acc;
int mods = clazz.getModifiers();
if (!Modifier.isPublic(mods)) {
fail(service, clazz + " is not public");
}
// if the class is in an explicit module then see if it is
// a provider factory class
Method factoryMethod = null;
if (inExplicitModule(clazz)) {
factoryMethod = findStaticProviderMethod(clazz);
if (factoryMethod != null) {
Class returnType = factoryMethod.getReturnType();
if (!service.isAssignableFrom(returnType)) {
fail(service, factoryMethod + " return type not a subtype");
}
}
}
this.factoryMethod = factoryMethod;
if (factoryMethod == null) {
// no factory method so must have a public no-args constructor
if (!service.isAssignableFrom(clazz)) {
fail(service, clazz.getName() + " not a subtype");
}
this.type = (Class) clazz;
this.ctor = (Constructor) getConstructor(clazz);
} else {
this.type = (Class) factoryMethod.getReturnType();
this.ctor = null;
}
}
@Override
public Class type() {
return type;
}
@Override
public S get() {
if (factoryMethod != null) {
return invokeFactoryMethod();
} else {
return newInstance();
}
}
/**
* Returns {@code true} if the provider is in an explicit module
*/
private boolean inExplicitModule(Class clazz) {
Module module = clazz.getModule();
return module.isNamed() && !module.getDescriptor().isAutomatic();
}
/**
* Returns the public static provider method if found.
*
* @throws ServiceConfigurationError if there is an error finding the
* provider method
*/
private Method findStaticProviderMethod(Class clazz) {
Method method = null;
try {
method = LANG_ACCESS.getMethodOrNull(clazz, "provider");
} catch (Throwable x) {
fail(service, "Unable to get public provider() method", x);
}
if (method != null) {
int mods = method.getModifiers();
if (Modifier.isStatic(mods)) {
assert Modifier.isPublic(mods);
Method m = method;
PrivilegedAction pa = new PrivilegedExceptionAction<>() {
@Override
public S run() throws Exception {
return ctor.newInstance();
}
};
// invoke constructor with permissions restricted by acc
try {
p = AccessController.doPrivileged(pa, acc);
} catch (PrivilegedActionException pae) {
exc = pae.getCause();
}
}
if (exc != null) {
if (exc instanceof InvocationTargetException)
exc = exc.getCause();
String cn = ctor.getDeclaringClass().getName();
fail(service,
"Provider " + cn + " could not be instantiated", exc);
}
return p;
}
// For now, equals/hashCode uses the access control context to ensure
// that two Providers created with different contexts are not equal
// when running with a security manager.
@Override
public int hashCode() {
return Objects.hash(type, acc);
}
@Override
public boolean equals(Object ob) {
if (!(ob instanceof ProviderImpl))
return false;
@SuppressWarnings("unchecked")
ProviderImpl that = (ProviderImpl)ob;
return this.type == that.type
&& Objects.equals(this.acc, that.acc);
}
}
/**
* Implements lazy service provider lookup of service providers that
* are provided by modules in a module Layer (or parent layers)
*/
private final class LayerLookupIterator next() {
if (first.hasNext()) {
return first.next();
} else if (second.hasNext()) {
return second.next();
} else {
throw new NoSuchElementException();
}
}
};
}
}
/**
* Lazily load and instantiate the available providers of this loader's
* service.
*
* 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.
*
* iterator() {
// create lookup iterator if needed
if (lookupIterator1 == null) {
lookupIterator1 = newLookupIterator();
}
return new Iterator() {
// record reload count
final int expectedReloadCount = ServiceLoader.this.reloadCount;
// index into the cached providers list
int index;
/**
* Throws ConcurrentModificationException if the list of cached
* providers has been cleared by reload.
*/
private void checkReloadCount() {
if (ServiceLoader.this.reloadCount != expectedReloadCount)
throw new ConcurrentModificationException();
}
@Override
public boolean hasNext() {
checkReloadCount();
if (index < instantiatedProviders.size())
return true;
return lookupIterator1.hasNext();
}
@Override
public S next() {
checkReloadCount();
S next;
if (index < instantiatedProviders.size()) {
next = instantiatedProviders.get(index);
} else {
next = lookupIterator1.next().get();
instantiatedProviders.add(next);
}
index++;
return next;
}
};
}
/**
* Returns a stream that lazily loads the available providers of this
* loader's service. The stream elements are of type {@link Provider
* Provider}, the {@code Provider}'s {@link Provider#get() get} method
* must be invoked to get or instantiate the provider.
*
* {@code
* Stream
*
* @return A stream that lazily loads providers for this loader's service
*
* @since 9
* @spec JPMS
*/
public Stream the class of the service type
*
* @param service
* The interface or abstract class representing the service
*
* @param 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
*
* @param callerModule
* The caller's module for which a new service loader is created
*
* @return A new service loader
*/
static ServiceLoader load(Class service,
ClassLoader loader,
Module callerModule)
{
return new ServiceLoader<>(callerModule, service, loader);
}
/**
* Creates a new service loader for the given service type and class
* loader.
*
* @param the class of the service type
*
* @param service
* The interface or abstract class representing the service
*
* @param loader
* The class loader to be used to load provider-configuration files
* and provider classes, or {@code null} if the system class
* loader (or, failing that, the bootstrap class loader) is to be
* used
*
* @return A new service loader
*
* @throws ServiceConfigurationError
* 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 {@code service}
*
* @revised 9
* @spec JPMS
*/
@CallerSensitive
public static ServiceLoader load(Class service,
ClassLoader loader)
{
return new ServiceLoader<>(Reflection.getCallerClass(), service, loader);
}
/**
* Creates a new service loader for the given service type, using the
* current thread's {@linkplain java.lang.Thread#getContextClassLoader
* context class loader}.
*
* {@code
* ServiceLoader.load(service)
* }
*
* is equivalent to
*
* {@code
* ServiceLoader.load(service, Thread.currentThread().getContextClassLoader())
* }
*
* @apiNote Service loader objects obtained with this method should not be
* cached VM-wide. For example, different applications in the same VM may
* have different thread context class loaders. A lookup by one application
* may locate a service provider that is only visible via its thread
* context class loader and so is not suitable to be located by the other
* application. Memory leaks can also arise. A thread local may be suited
* to some applications.
*
* @param the class of the service type
*
* @param service
* The interface or abstract class representing the service
*
* @return A new service loader
*
* @throws ServiceConfigurationError
* 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 {@code service}
*
* @revised 9
* @spec JPMS
*/
@CallerSensitive
public static ServiceLoader load(Class service) {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
return new ServiceLoader<>(Reflection.getCallerClass(), service, cl);
}
/**
* Creates a new service loader for the given service type, using the
* {@linkplain ClassLoader#getPlatformClassLoader() platform class loader}.
*
* {@code
* ServiceLoader.load(service, ClassLoader.getPlatformClassLoader())
* }
*
* the class of the service type
*
* @param service
* The interface or abstract class representing the service
*
* @return A new service loader
*
* @throws ServiceConfigurationError
* 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 {@code service}
*
* @revised 9
* @spec JPMS
*/
@CallerSensitive
public static ServiceLoader loadInstalled(Class service) {
ClassLoader cl = ClassLoader.getPlatformClassLoader();
return new ServiceLoader<>(Reflection.getCallerClass(), service, cl);
}
/**
* Creates a new service loader for the given service type that loads
* service providers from modules in the given {@code Layer} and its
* ancestors.
*
* @apiNote 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 {@code load(S, null)}.
*
* @param the class of the service type
*
* @param layer
* The module Layer
*
* @param service
* The interface or abstract class representing the service
*
* @return A new service loader
*
* @throws ServiceConfigurationError
* 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 {@code service}
*
* @since 9
* @spec JPMS
*/
@CallerSensitive
public static ServiceLoader load(Layer layer, Class service) {
return new ServiceLoader<>(Reflection.getCallerClass(), layer, service);
}
/**
* Load the first available provider of this loader's service. This
* convenience method is equivalent to invoking the {@link #iterator()
* 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.
*
* {@code
* CodecSet provider =
* ServiceLoader.load(CodecSet.class).findFirst().orElse(DEFAULT_CODECSET);
* }
* @return The first provider or empty {@code Optional} if no providers
* are located
*
* @throws ServiceConfigurationError
* 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.
*
* @since 9
* @spec JPMS
*/
public Optional findFirst() {
Iterator iterator = iterator();
if (iterator.hasNext()) {
return Optional.of(iterator.next());
} else {
return Optional.empty();
}
}
/**
* Clear this loader's provider cache so that all providers will be
* reloaded.
*
*