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The Service.java Java example source code
/*
* Copyright (c) 1999, 2012, 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 sun.misc;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.TreeSet;
/**
* A simple service-provider lookup mechanism. A <i>service is a
* well-known set of interfaces and (usually abstract) classes. A <i>service
* provider</i> 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 may 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 may
* also be made available by adding them to the applet or application class
* path or by some other platform-specific means.
*
* <p> In this lookup mechanism a service is represented by an interface or an
* abstract class. (A concrete class may be used, but this is not
* recommended.) A provider of a given service contains one or more concrete
* classes that extend this <i>service class with data and code specific to
* the provider. This <i>provider class will typically not be the entire
* provider itself but rather a proxy that 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 class has been defined. The
* only requirement enforced here is that provider classes must have a
* zero-argument constructor so that they may be instantiated during lookup.
*
* <p> A service provider identifies itself by placing a provider-configuration
* file in the resource directory <tt>META-INF/services. The file's name
* should consist of the fully-qualified name of the abstract service class.
* The file should contain a list of fully-qualified concrete provider-class
* names, one per line. Space and tab characters surrounding each name, as
* well as blank lines, are ignored. The comment character is <tt>'#'
* (<tt>0x23); 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 will be 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 that found the file.
*
* <p> Example: Suppose we have a service class named
* <tt>java.io.spi.CharCodec. It has two abstract methods:
*
* <pre>
* public abstract CharEncoder getEncoder(String encodingName);
* public abstract CharDecoder getDecoder(String encodingName);
* </pre>
*
* Each method returns an appropriate object or <tt>null if it cannot
* translate the given encoding. Typical <tt>CharCodec providers will
* support more than one encoding.
*
* <p> If sun.io.StandardCodec is a provider of the CharCodec
* service then its jar file would contain the file
* <tt>META-INF/services/java.io.spi.CharCodec. This file would contain
* the single line:
*
* <pre>
* sun.io.StandardCodec # Standard codecs for the platform
* </pre>
*
* To locate an encoder for a given encoding name, the internal I/O code would
* do something like this:
*
* <pre>
* CharEncoder getEncoder(String encodingName) {
* Iterator ps = Service.providers(CharCodec.class);
* while (ps.hasNext()) {
* CharCodec cc = (CharCodec)ps.next();
* CharEncoder ce = cc.getEncoder(encodingName);
* if (ce != null)
* return ce;
* }
* return null;
* }
* </pre>
*
* The provider-lookup mechanism always executes in the security context of the
* caller. Trusted system code should typically invoke the methods in this
* class from within a privileged security context.
*
* @author Mark Reinhold
* @since 1.3
*/
public final class Service<S> {
private static final String prefix = "META-INF/services/";
private Service() { }
private static void fail(Class<?> service, String msg, Throwable cause)
throws ServiceConfigurationError
{
ServiceConfigurationError sce
= new ServiceConfigurationError(service.getName() + ": " + msg);
sce.initCause(cause);
throw sce;
}
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 both the names list and the returned set iff the name is
* not already a member of the returned set.
*/
private static int parseLine(Class<?> service, URL u, BufferedReader r, int lc,
List<String> names, Set returned)
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 (!returned.contains(ln)) {
names.add(ln);
returned.add(ln);
}
}
return lc + 1;
}
/**
* Parse the content of the given URL as a provider-configuration file.
*
* @param service
* The service class for which providers are being sought;
* used to construct error detail strings
*
* @param url
* The URL naming the configuration file to be parsed
*
* @param returned
* A Set containing the names of provider classes that have already
* been returned. This set will be updated to contain the names
* that will be yielded from the returned <tt>Iterator.
*
* @return A (possibly empty) <tt>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 static Iterator<String> parse(Class service, URL u, Set returned)
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, returned)) >= 0);
} catch (IOException x) {
fail(service, ": " + x);
} finally {
try {
if (r != null) r.close();
if (in != null) in.close();
} catch (IOException y) {
fail(service, ": " + y);
}
}
return names.iterator();
}
/**
* Private inner class implementing fully-lazy provider lookup
*/
private static class LazyIterator<S> implements Iterator {
Class<S> service;
ClassLoader loader;
Enumeration<URL> configs = null;
Iterator<String> pending = null;
Set<String> returned = new TreeSet<>();
String nextName = null;
private LazyIterator(Class<S> service, ClassLoader loader) {
this.service = service;
this.loader = loader;
}
public boolean hasNext() throws ServiceConfigurationError {
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, ": " + x);
}
}
while ((pending == null) || !pending.hasNext()) {
if (!configs.hasMoreElements()) {
return false;
}
pending = parse(service, configs.nextElement(), returned);
}
nextName = pending.next();
return true;
}
public S next() throws ServiceConfigurationError {
if (!hasNext()) {
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 {
return service.cast(c.newInstance());
} catch (Throwable x) {
fail(service,
"Provider " + cn + " could not be instantiated: " + x,
x);
}
return null; /* This cannot happen */
}
public void remove() {
throw new UnsupportedOperationException();
}
}
/**
* Locates and incrementally instantiates the available providers of a
* given service using the given class loader.
*
* <p> This method transforms the name of the given service class into a
* provider-configuration filename as described above and then uses the
* <tt>getResources method of the given class loader to find all
* available files with that name. These files are then read and parsed to
* produce a list of provider-class names. The iterator that is returned
* uses the given class loader to lookup and then instantiate each element
* of the list.
*
* <p> Because it is possible for extensions to be installed into a running
* Java virtual machine, this method may return different results each time
* it is invoked. <p>
*
* @param service
* The service's abstract service class
*
* @param loader
* The class loader to be used to load provider-configuration files
* and instantiate provider classes, or <tt>null if the system
* class loader (or, failing that the bootstrap class loader) is to
* be used
*
* @return An <tt>Iterator that yields provider objects for the given
* service, in some arbitrary order. The iterator will throw a
* <tt>ServiceConfigurationError if a provider-configuration
* file violates the specified format or if a provider class cannot
* be found and instantiated.
*
* @throws ServiceConfigurationError
* If a provider-configuration file violates the specified format
* or names a provider class that cannot be found and instantiated
*
* @see #providers(java.lang.Class)
* @see #installedProviders(java.lang.Class)
*/
public static <S> Iterator providers(Class service, ClassLoader loader)
throws ServiceConfigurationError
{
return new LazyIterator<S>(service, loader);
}
/**
* Locates and incrementally instantiates the available providers of a
* given service using the context class loader. This convenience method
* is equivalent to
*
* <pre>
* ClassLoader cl = Thread.currentThread().getContextClassLoader();
* return Service.providers(service, cl);
* </pre>
*
* @param service
* The service's abstract service class
*
* @return An <tt>Iterator that yields provider objects for the given
* service, in some arbitrary order. The iterator will throw a
* <tt>ServiceConfigurationError if a provider-configuration
* file violates the specified format or if a provider class cannot
* be found and instantiated.
*
* @throws ServiceConfigurationError
* If a provider-configuration file violates the specified format
* or names a provider class that cannot be found and instantiated
*
* @see #providers(java.lang.Class, java.lang.ClassLoader)
*/
public static <S> Iterator providers(Class service)
throws ServiceConfigurationError
{
ClassLoader cl = Thread.currentThread().getContextClassLoader();
return Service.providers(service, cl);
}
/**
* Locates and incrementally instantiates the available providers of a
* given service using the extension class loader. This convenience method
* simply locates the extension class loader, call it
* <tt>extClassLoader, and then does
*
* <pre>
* return Service.providers(service, extClassLoader);
* </pre>
*
* 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.
*
* @param service
* The service's abstract service class
*
* @return An <tt>Iterator that yields provider objects for the given
* service, in some arbitrary order. The iterator will throw a
* <tt>ServiceConfigurationError if a provider-configuration
* file violates the specified format or if a provider class cannot
* be found and instantiated.
*
* @throws ServiceConfigurationError
* If a provider-configuration file violates the specified format
* or names a provider class that cannot be found and instantiated
*
* @see #providers(java.lang.Class, java.lang.ClassLoader)
*/
public static <S> Iterator installedProviders(Class service)
throws ServiceConfigurationError
{
ClassLoader cl = ClassLoader.getSystemClassLoader();
ClassLoader prev = null;
while (cl != null) {
prev = cl;
cl = cl.getParent();
}
return Service.providers(service, prev);
}
}
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