/**
* Copyright 1996-2014 FoxBPM ORG.
*
* 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.
*
* @author kenshin
* @author ych
*/
package org.foxbpm.engine.impl.util;
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;
import sun.misc.ServiceConfigurationError;
/**
* A simple service-provider loading facility.
*
* <p>
* A <i>service</i> 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 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</i> with data and code
* specific to the provider. The <i>provider class</i> 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 name="format"> A service provider is identified by placing a
* <i>provider-configuration file</i> in the resource directory
* <tt>META-INF/services</tt>. The file's name is the fully-qualified <a
* href="../lang/ClassLoader.html#name">binary name</a> 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>'#'</tt> (
* <tt>'\u0023'</tt>, <font size="-1">NUMBER SIGN</font>); 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 <tt>null</tt> argument to any method in
* this class will cause a {@link NullPointerException} to be thrown.
*
*
* <p>
* <span style="font-weight: bold; padding-right: 1em">Example</span> Suppose we
* have a service type <tt>com.example.CodecSet</tt> 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>
*
* <pre>
* public abstract Encoder getEncoder(String encodingName);
*
* public abstract Decoder getDecoder(String encodingName);
* </pre>
*
* </blockquote>
*
* Each method returns an appropriate object or <tt>null</tt> if the provider
* does not support the given encoding. Typical providers support more than one
* encoding.
*
* <p>
* If <tt>com.example.impl.StandardCodecs</tt> is an implementation of the
* <tt>CodecSet</tt> service then its jar file also contains a file named
*
* <blockquote>
*
* <pre>
* META - INF / services / com.example.CodecSet
* </pre>
*
* </blockquote>
*
* <p>
* This file contains the single line:
*
* <blockquote>
*
* <pre>
* com.example.impl.StandardCodecs # Standard codecs
* </pre>
*
* </blockquote>
*
* <p>
* The <tt>CodecSet</tt> class creates and saves a single service instance at
* initialization:
*
* <blockquote>
*
* <pre>
* private static ServiceLoader<CodecSet> codecSetLoader = ServiceLoader.load(CodecSet.class);
* </pre>
*
* </blockquote>
*
* <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>
*
* <pre>
* public static Encoder getEncoder(String encodingName) {
* for (CodecSet cp : codecSetLoader) {
* Encoder enc = cp.getEncoder(encodingName);
* if (enc != null)
* return enc;
* }
* return null;
* }
* </pre>
*
* </blockquote>
*
* <p>
* A <tt>getDecoder</tt> method is defined similarly.
*
*
* <p>
* <span style="font-weight: bold; padding-right: 1em">Usage Note</span> 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
* @version 1.10, 06/04/10
* @since 1.6
*
* 从1.6JDK中拷出，主要为了foxbpm引擎支持1.5版本JDK
*/
@SuppressWarnings("restriction")
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 Class<S> service;
// The class loader used to locate, load, and instantiate providers
private ClassLoader loader;
// Cached providers, in instantiation order
private LinkedHashMap<String, S> providers = new LinkedHashMap<String, S>();
// 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 = svc;
loader = cl;
reload();
}
@SuppressWarnings("rawtypes")
private static void fail(Class service, String msg, Throwable cause) throws ServiceConfigurationError {
throw new ServiceConfigurationError(cause);
}
@SuppressWarnings("rawtypes")
private static void fail(Class service, String msg) throws ServiceConfigurationError {
throw new ServiceConfigurationError(service.getName() + ": " + msg);
}
@SuppressWarnings("rawtypes")
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.
//
@SuppressWarnings("rawtypes")
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
//
@SuppressWarnings("rawtypes")
private Iterator<String> parse(Class service, URL u) throws ServiceConfigurationError {
InputStream in = null;
BufferedReader r = null;
ArrayList<String> names = new ArrayList<String>();
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;
}
public boolean hasNext() {
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;
}
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
}
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"><span
* style="padding-right: 1em; font-weight: bold">Design Note</span> 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.
*
* @return An iterator that lazily loads providers for this loader's service
*/
public Iterator<S> iterator() {
return new Iterator<S>() {
Iterator<Map.Entry<String, S>> 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 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</tt> 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<S> load(Class<S> service, ClassLoader loader) {
return new ServiceLoader<S>(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>
*
* <pre>
* ServiceLoader.load(<i>service</i>)
* </pre>
*
* </blockquote>
*
* is equivalent to
*
* <blockquote>
*
* <pre>
* ServiceLoader.load(<i>service</i>,
* Thread.currentThread().getContextClassLoader())
* </pre>
*
* </blockquote>
*
* @param service
* The interface or abstract class representing the service
*
* @return A new service loader
*/
public static <S> ServiceLoader<S> load(Class<S> 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><i>extClassLoader</i></tt>, and then returns
*
* <blockquote>
*
* <pre>
* ServiceLoader.load(<i>service</i>, <i>extClassLoader</i>)
* </pre>
*
* </blockquote>
*
* <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 service
* The interface or abstract class representing the service
*
* @return A new service loader
*/
public static <S> ServiceLoader<S> loadInstalled(Class<S> 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() + "]";
}
}