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 {@code 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 modules 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 {@code load} methods that locate * service providers using a class loader, then provider modules defined to * that class loader, or a class loader reachable using {@link * ClassLoader#getParent() 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 {@link #iterator 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 {@link #reload 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 {@link 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: * *
* * 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 After invoking this method, subsequent invocations of the {@link
* #iterator() 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 void reload() {
providers.clear();
assert layer == null || loader == null;
if (layer != null) {
layerLookupIterator = new LayerLookupIterator();
} else {
providerNames.clear();
moduleServicesIterator = new ModuleServicesIterator();
lazyClassPathIterator = new LazyClassPathIterator();
}
reloadCount++;
}
/**
* 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 that the
* caller's module does not declare that it uses 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 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 {@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 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 {@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 cache is 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.
*
* @implNote When adding providers to the cache, the {@link #iterator
* Iterator} processes resources in the order that the {@link
* java.lang.ClassLoader#getResources(java.lang.String)
* ClassLoader.getResources(String)} method finds the service configuration
* files.
*
* @return An iterator that lazily loads providers for this loader's
* service
*/
public Iterator 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 type of the service to be loaded by this loader
*
* @author Mark Reinhold
* @since 1.6
*/
public final class ServiceLoader
implements Iterable
{
private static final String PREFIX = "META-INF/services/";
// The class or interface representing the service being loaded
private final Class service;
// 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;
// Cached providers, in instantiation order
private List providers = new ArrayList<>();
// The class names of the cached providers, only used when locating
// service providers via a class loader
private Set svc) {
checkModule(caller.getModule(), svc);
this.service = svc;
this.layer = layer;
this.loader = null;
this.acc = (System.getSecurityManager() != null)
? AccessController.getContext()
: null;
reload();
}
/**
* 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 that the
* caller's module does not declare that it uses the service type.
*/
private ServiceLoader(Module callerModule, Class svc, ClassLoader cl) {
if (VM.isBooted()) {
checkModule(callerModule, 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 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.layer = null;
this.loader = cl;
this.acc = (System.getSecurityManager() != null)
? AccessController.getContext()
: null;
reload();
}
private ServiceLoader(Class caller, Class svc, ClassLoader cl) {
this(caller.getModule(), svc, cl);
}
/**
* 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 checkModule(Module module, Class svc) {
// Check that the service type is in a package that is
// exported to the caller.
if (!Reflection.verifyModuleAccess(module, svc)) {
fail(svc, "not accessible to " + module);
}
// If the caller is in a named module then it should "uses" the
// service type
if (!module.canUse(svc)) {
fail(svc, "use not declared in " + module);
}
}
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);
}
// Parse a single line from the given configuration file, adding the name
// on the line to the names list.
//
private int parseLine(Class service, URL u, BufferedReader r, int lc,
List
implements Iterator
{
/**
* Returns {@code true} if the iteration has more elements.
*/
abstract boolean hasNextService();
/**
* Returns the next element in the iteration
*/
abstract S nextService();
public final boolean hasNext() {
if (acc == null) {
return hasNextService();
} else {
PrivilegedAction action = new PrivilegedAction() {
public S run() { return nextService(); }
};
return AccessController.doPrivileged(action, acc);
}
}
}
/**
* Implements lazy service provider lookup of service providers that
* are provided by modules in a module Layer.
*
* For now, this iterator examines all modules in each Layer. This will
* be replaced once we decide on how the service-use graph is exposed
* in the module API.
*/
private class LayerLookupIterator
extends RestrictedIterator
{
final String serviceName;
Layer currentLayer;
Iterator
{
final JavaLangAccess langAccess = SharedSecrets.getJavaLangAccess();
ClassLoader currentLoader;
Iterator
{
EnumerationDesign 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() {
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();
}
public boolean hasNext() {
checkReloadCount();
if (index < providers.size())
return true;
if (layerLookupIterator != null) {
return layerLookupIterator.hasNext();
} else {
return moduleServicesIterator.hasNext() ||
lazyClassPathIterator.hasNext();
}
}
public S next() {
checkReloadCount();
S next;
if (index < providers.size()) {
next = providers.get(index);
} else {
if (layerLookupIterator != null) {
next = layerLookupIterator.next();
} else {
if (moduleServicesIterator.hasNext()) {
next = moduleServicesIterator.next();
} else {
next = lazyClassPathIterator.next();
}
}
}
index++;
return next;
}
};
}
/**
* Creates a new service loader for the given service type, class
* loader, and caller.
*
* @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 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 a named module and its module descriptor does
* not declare that it uses {@code service}
*/
@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}.
*
*
*
* is equivalent to
*
*
* ServiceLoader.load(service)
*
* @param
* ServiceLoader.load(service,
* Thread.currentThread().getContextClassLoader()) 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 a named module and its module descriptor does
* not declare that it uses {@code service}
*/
@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}.
*
*
*
*
* 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 a named module and its module descriptor does
* not declare that it uses {@code service}
*/
@CallerSensitive
public static ServiceLoader loadInstalled(Class service) {
ClassLoader cl = ClassLoader.getSystemClassLoader();
ClassLoader prev = null;
while (cl != null) {
prev = cl;
cl = cl.getParent();
}
return new ServiceLoader<>(Reflection.getCallerClass(), service, prev);
}
/**
* 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 a named module and its module descriptor does
* not declare that it uses {@code service}
*
* @since 9
*/
@CallerSensitive
public static ServiceLoader load(Layer layer, Class service) {
return new ServiceLoader<>(Reflection.getCallerClass(),
Objects.requireNonNull(layer),
Objects.requireNonNull(service));
}
/**
* Returns a string describing this service.
*
* @return A descriptive string
*/
public String toString() {
return "java.util.ServiceLoader[" + service.getName() + "]";
}
}