org.glassfish.grizzly.utils.ServiceFinder Maven / Gradle / Ivy
/*
* Copyright (c) 2010, 2020 Oracle and/or its affiliates. All rights reserved.
*
* This program and the accompanying materials are made available under the
* terms of the Eclipse Public License v. 2.0, which is available at
* http://www.eclipse.org/legal/epl-2.0.
*
* This Source Code may also be made available under the following Secondary
* Licenses when the conditions for such availability set forth in the
* Eclipse Public License v. 2.0 are satisfied: GNU General Public License,
* version 2 with the GNU Classpath Exception, which is available at
* https://www.gnu.org/software/classpath/license.html.
*
* SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
*/
package org.glassfish.grizzly.utils;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.lang.reflect.Array;
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 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. 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.
*
*
* 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 service class with data and code specific to the provider. This 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.
*
*
* A service provider identifies itself by placing a provider-configuration file in the resource directory
* 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 '#'
* (0x23); 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 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.
*
*
* Example: Suppose we have a service class named java.io.spi.CharCodec. It has two abstract methods:
*
*
*
* public abstract CharEncoder getEncoder(String encodingName);
*
* public abstract CharDecoder getDecoder(String encodingName);
*
*
* Each method returns an appropriate object or null if it cannot translate the given encoding. Typical
* CharCodec providers will support more than one encoding.
*
*
* If glassfish.io.StandardCodec is a provider of the CharCodec service then its jar file would
* contain the file META-INF/services/java.io.spi.CharCodec. This file would contain the single line:
*
*
*
* glassfish.io.StandardCodec # Standard codecs for the platform
*
*
* To locate an codec for a given encoding name, the internal I/O code would do something like this:
*
*
*
* CharEncoder getEncoder(String encodingName) {
* for (CharCodec cc : ServiceFinder.find(CharCodec.class)) {
* CharEncoder ce = cc.getEncoder(encodingName);
* if (ce != null)
* return ce;
* }
* return null;
* }
*
*
* 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
* @version 1.11, 03/12/19
* @since 1.3
*/
public final class ServiceFinder implements Iterable {
private static final String prefix = "META-INF/services/";
private final Class serviceClass;
private final ClassLoader classLoader;
/**
* Locates and incrementally instantiates the available providers of a given service using the given class loader.
*
*
* This method transforms the name of the given service class into a provider-configuration filename as described above
* and then uses the 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.
*
*
* 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.
*
*
* @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
* null if the system class loader (or, failing that the bootstrap class loader) is to be used
* @throws ServiceConfigurationError If a provider-configuration file violates the specified format or names a provider
* class that cannot be found and instantiated
* @see #find(Class)
*/
public static ServiceFinder find(Class service, ClassLoader loader) {
return new ServiceFinder<>(service, loader);
}
/**
* Locates and incrementally instantiates the available providers of a given service using the context class loader.
* This convenience method is equivalent to
*
*
*
* ClassLoader cl = Thread.currentThread().getContextClassLoader();
* return Service.providers(service, cl);
*
*
* @param service The service's abstract service class
*
* @throws ServiceConfigurationError If a provider-configuration file violates the specified format or names a provider
* class that cannot be found and instantiated
* @see #find(Class, ClassLoader)
*/
public static ServiceFinder find(Class service) {
return find(service, Thread.currentThread().getContextClassLoader());
}
private ServiceFinder(Class service, ClassLoader loader) {
this.serviceClass = service;
this.classLoader = loader;
}
/**
* Returns discovered objects incrementally.
*
* @return An Iterator that yields provider objects for the given service, in some arbitrary order. The
* iterator will throw a ServiceConfigurationError if a provider-configuration file violates the specified
* format or if a provider class cannot be found and instantiated.
*/
@Override
public Iterator iterator() {
return new LazyIterator<>(serviceClass, classLoader);
}
/**
* Returns discovered objects all at once.
*
* @return can be empty but never null.
*
* @throws ServiceConfigurationError
*/
@SuppressWarnings("unchecked")
public T[] toArray() {
List result = new ArrayList<>();
for (T t : this) {
result.add(t);
}
return result.toArray((T[]) Array.newInstance(serviceClass, result.size()));
}
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 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 u 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 Iterator.
* @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
*/
@SuppressWarnings({ "empty-statement" })
private static Iterator parse(Class service, URL u, Set returned) throws ServiceConfigurationError {
InputStream in = null;
BufferedReader r = null;
ArrayList names = new ArrayList<>();
try {
in = u.openStream();
r = new BufferedReader(new InputStreamReader(in, "utf-8"));
int lc = 1;
// noinspection StatementWithEmptyBody
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 implements Iterator {
final Class service;
final ClassLoader loader;
Enumeration configs = null;
Iterator pending = null;
final Set returned = new TreeSet<>();
String nextName = null;
URL currentConfig = null;
private LazyIterator(Class service, ClassLoader loader) {
this.service = service;
this.loader = loader;
}
@Override
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;
}
currentConfig = configs.nextElement();
pending = parse(service, currentConfig, returned);
}
nextName = pending.next();
return true;
}
@Override
public T next() throws ServiceConfigurationError {
if (!hasNext()) {
throw new NoSuchElementException();
}
String cn = nextName;
nextName = null;
try {
return service.cast(Class.forName(cn, true, loader).newInstance());
} catch (ClassNotFoundException x) {
fail(service, "Provider " + cn + " is specified in " + currentConfig + " but not found");
} catch (Exception x) {
fail(service, "Provider " + cn + " is specified in " + currentConfig + "but could not be instantiated: " + x, x);
}
return null; /* This cannot happen */
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
}
}