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Java example source code file (ServiceLoader.java)
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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 com.sun.tools.javac.util;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;
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 java.util.Objects;
import java.util.ServiceConfigurationError;
/**
* This is a temporary, modified copy of java.util.ServiceLoader, for use by
* javac, to work around bug JDK-8004082.
*
* The bug describes problems in the interaction between ServiceLoader and
* URLClassLoader, such that references to a jar file passed to URLClassLoader
* may be retained after calling URLClassLoader.close(), preventing the jar
* file from being deleted on Windows.
*
* <p>This is NOT part of any supported API.
* If you write code that depends on this, you do so at your own risk.
* This code and its internal interfaces are subject to change or
* deletion without notice.</b>
*/
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<>();
// 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;
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 {
// The problem is that by default, streams opened with
// u.openInputStream use a cached reference to a JarFile, which
// is separate from the reference used by URLClassLoader, and
// which is not closed by URLClassLoader.close().
// The workaround is to disable caching for this specific jar file,
// so that the reference to the jar file can be closed when the
// file has been read.
// Original code:
// in = u.openStream();
// Workaround ...
URLConnection uc = u.openConnection();
uc.setUseCaches(false);
in = uc.getInputStream();
// ... end of workaround.
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;
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 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.
*
* @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 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 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 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() + "]";
}
}
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