The ServiceLoader.java Java example source code
/*
* Copyright (c) 2005, 2013, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.util;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
import java.security.AccessController;
import java.security.AccessControlContext;
import java.security.PrivilegedAction;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
/**
* A simple service-provider loading facility.
*
* <p> A service is a well-known set of interfaces and (usually
* abstract) classes. A <i>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.
*
* <p> 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 <i>service type with data
* and code specific to the provider. The <i>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.
*
* <p> A service provider is identified by placing a
* <i>provider-configuration file in the resource directory
* <tt>META-INF/services. The file's name is the fully-qualified 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
* <tt>'#' ('\u0023',
* <font style="font-size:smaller;">NUMBER SIGN); on
* each line all characters following the first comment character are ignored.
* The file must be encoded in UTF-8.
*
* <p> 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.
*
* <p> 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.
*
* <p> 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.
*
* <p> Instances of this class are not safe for use by multiple concurrent
* threads.
*
* <p> Unless otherwise specified, passing a null argument to any
* method in this class will cause a {@link NullPointerException} to be thrown.
*
*
* <p>Example
* Suppose we have a service type <tt>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:
*
* <blockquote> * public abstract Encoder getEncoder(String encodingName);
* public abstract Decoder getDecoder(String encodingName);</pre>
*
* Each method returns an appropriate object or <tt>null if the provider
* does not support the given encoding. Typical providers support more than
* one encoding.
*
* <p> If com.example.impl.StandardCodecs is an implementation of the
* <tt>CodecSet service then its jar file also contains a file named
*
* <blockquote> * META-INF/services/com.example.CodecSet</pre>
*
* <p> This file contains the single line:
*
* <blockquote> * com.example.impl.StandardCodecs # Standard codecs</pre>
*
* <p> The CodecSet class creates and saves a single service instance
* at initialization:
*
* <blockquote> * private static ServiceLoader<CodecSet> codecSetLoader
* = ServiceLoader.load(CodecSet.class);</pre>
*
* <p> 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.
*
* <blockquote> * public static Encoder getEncoder(String encodingName) {
* for (CodecSet cp : codecSetLoader) {
* Encoder enc = cp.getEncoder(encodingName);
* if (enc != null)
* return enc;
* }
* return null;
* }</pre>
*
* <p> A getDecoder method is defined similarly.
*
*
* <p>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.
*
* <p> 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.
*
* <p> 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 <S>
* The type of the service to be loaded by this loader
*
* @author Mark Reinhold
* @since 1.6
*/
public final class ServiceLoader<S>
implements Iterable<S>
{
private static final String PREFIX = "META-INF/services/";
// The class or interface representing the service being loaded
private final Class<S> service;
// The class loader used to locate, load, and instantiate providers
private final ClassLoader loader;
// The access control context taken when the ServiceLoader is created
private final AccessControlContext acc;
// Cached providers, in instantiation order
private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
// The current lazy-lookup iterator
private LazyIterator lookupIterator;
/**
* Clear this loader's provider cache so that all providers will be
* reloaded.
*
* <p> 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.
*
* <p> 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();
lookupIterator = new LazyIterator(service, loader);
}
private ServiceLoader(Class<S> svc, ClassLoader cl) {
service = Objects.requireNonNull(svc, "Service interface cannot be null");
loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
reload();
}
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<String> names)
throws IOException, ServiceConfigurationError
{
String ln = r.readLine();
if (ln == null) {
return -1;
}
int ci = ln.indexOf('#');
if (ci >= 0) ln = ln.substring(0, ci);
ln = ln.trim();
int n = ln.length();
if (n != 0) {
if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))
fail(service, u, lc, "Illegal configuration-file syntax");
int cp = ln.codePointAt(0);
if (!Character.isJavaIdentifierStart(cp))
fail(service, u, lc, "Illegal provider-class name: " + ln);
for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
cp = ln.codePointAt(i);
if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))
fail(service, u, lc, "Illegal provider-class name: " + ln);
}
if (!providers.containsKey(ln) && !names.contains(ln))
names.add(ln);
}
return lc + 1;
}
// Parse the content of the given URL as a provider-configuration file.
//
// @param service
// The service type for which providers are being sought;
// used to construct error detail strings
//
// @param u
// The URL naming the configuration file to be parsed
//
// @return A (possibly empty) iterator that will yield the provider-class
// names in the given configuration file that are not yet members
// of the returned set
//
// @throws ServiceConfigurationError
// If an I/O error occurs while reading from the given URL, or
// if a configuration-file format error is detected
//
private Iterator<String> parse(Class service, URL u)
throws ServiceConfigurationError
{
InputStream in = null;
BufferedReader r = null;
ArrayList<String> names = new ArrayList<>();
try {
in = u.openStream();
r = new BufferedReader(new InputStreamReader(in, "utf-8"));
int lc = 1;
while ((lc = parseLine(service, u, r, lc, names)) >= 0);
} catch (IOException x) {
fail(service, "Error reading configuration file", x);
} finally {
try {
if (r != null) r.close();
if (in != null) in.close();
} catch (IOException y) {
fail(service, "Error closing configuration file", y);
}
}
return names.iterator();
}
// Private inner class implementing fully-lazy provider lookup
//
private class LazyIterator
implements Iterator<S>
{
Class<S> service;
ClassLoader loader;
Enumeration<URL> configs = null;
Iterator<String> pending = null;
String nextName = null;
private LazyIterator(Class<S> service, ClassLoader loader) {
this.service = service;
this.loader = loader;
}
private boolean hasNextService() {
if (nextName != null) {
return true;
}
if (configs == null) {
try {
String fullName = PREFIX + service.getName();
if (loader == null)
configs = ClassLoader.getSystemResources(fullName);
else
configs = loader.getResources(fullName);
} catch (IOException x) {
fail(service, "Error locating configuration files", x);
}
}
while ((pending == null) || !pending.hasNext()) {
if (!configs.hasMoreElements()) {
return false;
}
pending = parse(service, configs.nextElement());
}
nextName = pending.next();
return true;
}
private S nextService() {
if (!hasNextService())
throw new NoSuchElementException();
String cn = nextName;
nextName = null;
Class<?> c = null;
try {
c = Class.forName(cn, false, loader);
} catch (ClassNotFoundException x) {
fail(service,
"Provider " + cn + " not found");
}
if (!service.isAssignableFrom(c)) {
fail(service,
"Provider " + cn + " not a subtype");
}
try {
S p = service.cast(c.newInstance());
providers.put(cn, p);
return p;
} catch (Throwable x) {
fail(service,
"Provider " + cn + " could not be instantiated: " + x,
x);
}
throw new Error(); // This cannot happen
}
public boolean hasNext() {
if (acc == null) {
return hasNextService();
} else {
PrivilegedAction<Boolean> action = new PrivilegedAction() {
public Boolean run() { return hasNextService(); }
};
return AccessController.doPrivileged(action, acc);
}
}
public S next() {
if (acc == null) {
return nextService();
} else {
PrivilegedAction<S> action = new PrivilegedAction() {
public S run() { return nextService(); }
};
return AccessController.doPrivileged(action, acc);
}
}
public void remove() {
throw new UnsupportedOperationException();
}
}
/**
* Lazily loads the available providers of this loader's service.
*
* <p> 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.
*
* <p> To achieve laziness the actual work of parsing the available
* provider-configuration files 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-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 {@link
* ServiceConfigurationError} when using a service iterator.
*
* <p> 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.
*
* <blockquote style="font-size: smaller; line-height: 1.2">
* 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.</blockquote>
*
* <p> 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<S> iterator() {
return new Iterator<S>() {
Iterator<Map.Entry knownProviders
= providers.entrySet().iterator();
public boolean hasNext() {
if (knownProviders.hasNext())
return true;
return lookupIterator.hasNext();
}
public S next() {
if (knownProviders.hasNext())
return knownProviders.next().getValue();
return lookupIterator.next();
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
/**
* Creates a new service loader for the given service type and class
* loader.
*
* @param <S> 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 <tt>null if the system class
* loader (or, failing that, the bootstrap class loader) is to be
* used
*
* @return A new service loader
*/
public static <S> ServiceLoader load(Class service,
ClassLoader loader)
{
return new ServiceLoader<>(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}.
*
* <p> An invocation of this convenience method of the form
*
* <blockquote> * ServiceLoader.load(<i>service)
*
* is equivalent to
*
* <blockquote> * ServiceLoader.load(<i>service,
* Thread.currentThread().getContextClassLoader())</pre>
*
* @param <S> the class of the service type
*
* @param service
* The interface or abstract class representing the service
*
* @return A new service loader
*/
public static <S> ServiceLoader load(Class service) {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
return ServiceLoader.load(service, cl);
}
/**
* Creates a new service loader for the given service type, using the
* extension class loader.
*
* <p> This convenience method simply locates the extension class loader,
* call it <tt>extClassLoader, and then returns
*
* <blockquote> * ServiceLoader.load(<i>service, extClassLoader)
*
* <p> 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.
*
* <p> 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.
*
* @param <S> the class of the service type
*
* @param service
* The interface or abstract class representing the service
*
* @return A new service loader
*/
public static <S> ServiceLoader loadInstalled(Class service) {
ClassLoader cl = ClassLoader.getSystemClassLoader();
ClassLoader prev = null;
while (cl != null) {
prev = cl;
cl = cl.getParent();
}
return ServiceLoader.load(service, prev);
}
/**
* Returns a string describing this service.
*
* @return A descriptive string
*/
public String toString() {
return "java.util.ServiceLoader[" + service.getName() + "]";
}
}
The search page
Other Java source code examples at this package level
Click here to learn more about this project