com.ovh.ws.jsonizer.api.base.ServiceLoader Maven / Gradle / Ivy
/**
* Copyright 2012, OVH. All rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and limitations under the License.
*/
package com.ovh.ws.jsonizer.api.base;
/*
* Copyright (c) 2005, 2006, 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.
*/
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.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
/**
* A simple service-provider loading facility.
*
* 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 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.
*
* 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. 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.
*
* A service provider 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
* 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.
*
* 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. 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:
*
*
* public abstract Encoder getEncoder(String encodingName);
*
* public abstract Decoder getDecoder(String encodingName);
*
*
*
Each method returns an appropriate object or null if the provider does not support the given
* encoding. Typical providers support more than one encoding.
*
* If com.example.impl.StandardCodecs is an implementation of the CodecSet service then its jar file
* also contains a file named
*
*
* META - INF / services / com.example.CodecSet
*
*
*
*
* This file contains the single line:
*
*
* com.example.impl.StandardCodecs # Standard codecs
*
*
*
*
* The CodecSet class creates and saves a single service instance at initialization:
*
*
* private static ServiceLoader<CodecSet> 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.
*
*
*
* public static Encoder getEncoder(String encodingName) {
* for (CodecSet cp : codecSetLoader) {
* Encoder enc = cp.getEncoder(encodingName);
* if (enc != null)
* return enc;
* }
* return null;
* }
*
*
*
*
* A getDecoder method is defined similarly.
*
* 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 RuntimeException} 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
* The type of the service to be loaded by this loader
* @author Mark Reinhold
* @since 1.6
*/
public final class ServiceLoader implements Iterable {
// The class or interface representing the service being loaded
private Class service;
// The class loader used to locate, load, and instantiate providers
private ClassLoader loader;
// Cached providers, in instantiation order
private LinkedHashMap providers = new LinkedHashMap();
// The current lazy-lookup iterator
private LazyIterator lookupIterator;
/**
* Clear this loader's provider cache so that all providers will be
* reloaded.
*
* 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();
lookupIterator = new LazyIterator(service, loader);
}
private ServiceLoader(Class svc, ClassLoader cl) {
service = svc;
loader = cl;
reload();
}
private static void fail(Class service, String msg, Throwable cause) {
throw new RuntimeException(service.getName() + ": " + msg, cause);
}
private static void fail(Class service, String msg) {
throw new RuntimeException(service.getName() + ": " + msg);
}
private static void fail(Class service, URL u, int line, String msg) {
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 names) throws IOException {
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 RuntimeException
// If an I/O error occurs while reading from the given URL, or
// if a configuration-file format error is detected
//
private Iterator parse(Class service, URL u) {
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;
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 {
Class service;
ClassLoader loader;
Enumeration configs = null;
Iterator pending = null;
String nextName = null;
private LazyIterator(Class service, ClassLoader loader) {
this.service = service;
this.loader = loader;
}
@Override
public boolean hasNext() {
if (nextName != null) {
return true;
}
if (configs == null) {
try {
String fullName = 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;
}
@Override
public S next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
String cn = nextName;
nextName = null;
try {
S p = service.cast(Class.forName(cn, true, loader).newInstance());
providers.put(cn, p);
return p;
} catch (ClassNotFoundException x) {
fail(service, "Provider " + cn + " not found");
} catch (Throwable x) {
fail(service, "Provider " + cn + " could not be instantiated: " + x, x);
}
throw new Error(); // This cannot happen
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
}
/**
* Lazily loads the available providers of this loader's service.
*
* 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 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 RuntimeException} 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 RuntimeException} 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.
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.
*
* 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
*/
@Override
public Iterator iterator() {
return new Iterator() {
Iterator> knownProviders = providers.entrySet().iterator();
@Override
public boolean hasNext() {
if (knownProviders.hasNext()) {
return true;
}
return lookupIterator.hasNext();
}
@Override
public S next() {
if (knownProviders.hasNext()) {
return knownProviders.next().getValue();
}
return lookupIterator.next();
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
};
}
/**
* Creates a new service loader for the given service type and class
* loader.
*
* @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
* @return A new service loader
*/
public static 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}.
*
* An invocation of this convenience method of the form
*
*
* ServiceLoader.load(service)
*
*
*
is equivalent to
*
*
* ServiceLoader.load(service,
* Thread.currentThread().getContextClassLoader())
*
*
*
*
* @param service
* The interface or abstract class representing the service
* @return A new service loader
*/
public static 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.
*
* This convenience method simply locates the extension class loader, call it extClassLoader, and
* then returns
*
*
* ServiceLoader.load(service, extClassLoader)
*
*
*
*
* 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.
*
* 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 service
* The interface or abstract class representing the service
* @return A new service loader
*/
public static 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
*/
@Override
public String toString() {
return "java.util.ServiceLoader[" + service.getName() + "]";
}
}