com.phloc.commons.lang.ServiceLoaderBackport Maven / Gradle / Ivy
/**
* Copyright (C) 2006-2014 phloc systems
* http://www.phloc.com
* office[at]phloc[dot]com
*
* 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.phloc.commons.lang;
import java.io.IOException;
import java.io.InputStream;
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 org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import com.phloc.commons.annotations.DevelopersNote;
import com.phloc.commons.annotations.UnsupportedOperation;
import com.phloc.commons.charset.CCharset;
import com.phloc.commons.io.streams.NonBlockingBufferedReader;
import com.phloc.commons.io.streams.StreamUtils;
import com.phloc.commons.string.ToStringGenerator;
import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
/**
* 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 ServiceLoaderException} 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
* @version 1.10, 06/04/10
* @since 1.6
*/
// IFJDK5
// ELSE
@Deprecated
// ENDIF
@DevelopersNote ("Required only in JDK5. Use the regular ServiceLoader from JDK 1.6 otherwise.")
public final class ServiceLoaderBackport implements Iterable
{
private static final Logger s_aLogger = LoggerFactory.getLogger (ServiceLoaderBackport.class);
static final String PREFIX = "META-INF/services/";
// The class or interface representing the service being loaded
private final Class m_aService;
// The class loader used to locate, load, and instantiate providers
private final ClassLoader m_aClassLoader;
// Cached providers, in instantiation order
private final Map m_aProviders = new LinkedHashMap ();
// The current lazy-lookup iterator
private LazyIterator m_aLookupIterator;
/**
* 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 ()
{
m_aProviders.clear ();
m_aLookupIterator = new LazyIterator (m_aService, m_aClassLoader);
}
private ServiceLoaderBackport (final Class svc, final ClassLoader cl)
{
if (s_aLogger.isDebugEnabled ())
s_aLogger.debug ("Loading all SPI implementations of " + svc.toString ());
m_aService = svc;
m_aClassLoader = cl;
reload ();
}
private static void _fail (final Class service, final String msg, final Throwable cause)
{
throw new ServiceLoaderException (service.getName () + ": " + msg, cause);
}
private static void _fail (final Class service, final String msg)
{
throw new ServiceLoaderException (service.getName () + ": " + msg);
}
private static void _fail (final Class service, final URL u, final int line, final String msg)
{
throw new ServiceLoaderException (service.getName () + ": " + 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 (final Class aService,
final URL u,
final NonBlockingBufferedReader r,
final int lc,
final List names) throws IOException
{
String ln = r.readLine ();
if (ln == null)
return -1;
final int ci = ln.indexOf ('#');
if (ci >= 0)
ln = ln.substring (0, ci);
ln = ln.trim ();
final int n = ln.length ();
if (n != 0)
{
if ((ln.indexOf (' ') >= 0) || (ln.indexOf ('\t') >= 0))
_fail (aService, u, lc, "Illegal configuration-file syntax");
int cp = ln.codePointAt (0);
if (!Character.isJavaIdentifierStart (cp))
_fail (aService, 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 (aService, u, lc, "Illegal provider-class name: " + ln);
}
if (!m_aProviders.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
//
// @
// If an I/O error occurs while reading from the given URL, or
// if a configuration-file format error is detected
//
private Iterator _parse (final Class aService, final URL u)
{
InputStream in = null;
NonBlockingBufferedReader r = null;
final ArrayList names = new ArrayList ();
try
{
in = u.openStream ();
r = new NonBlockingBufferedReader (StreamUtils.createReader (in, CCharset.CHARSET_SERVICE_LOADER_OBJ));
int lc = 1;
while ((lc = _parseLine (aService, u, r, lc, names)) >= 0)
{// NOPMD
// do nothing
}
}
catch (final IOException x)
{
_fail (aService, "Error reading configuration file", x);
}
finally
{
try
{
if (r != null)
r.close ();
if (in != null)
in.close ();
}
catch (final IOException y)
{
_fail (aService, "Error closing configuration file", y);
}
}
return names.iterator ();
}
// Private inner class implementing fully-lazy provider lookup
@SuppressFBWarnings ("UWF_NULL_FIELD")
private final class LazyIterator implements Iterator
{
private final Class m_aLIService;
private final ClassLoader m_aLILoader;
private Enumeration m_aLIConfigs = null;
private Iterator m_aLIPending = null;
private String m_sLINextName = null;
public LazyIterator (final Class aService, final ClassLoader aLoader)
{
m_aLIService = aService;
m_aLILoader = aLoader;
}
public boolean hasNext ()
{
if (m_sLINextName != null)
return true;
if (m_aLIConfigs == null)
{
try
{
final String fullName = PREFIX + m_aLIService.getName ();
if (m_aLILoader == null)
m_aLIConfigs = ClassLoader.getSystemResources (fullName);
else
m_aLIConfigs = m_aLILoader.getResources (fullName);
}
catch (final IOException ex)
{
_fail (m_aLIService, "Error locating configuration files", ex);
}
}
while ((m_aLIPending == null) || !m_aLIPending.hasNext ())
{
if (!m_aLIConfigs.hasMoreElements ())
{
return false;
}
m_aLIPending = _parse (m_aLIService, m_aLIConfigs.nextElement ());
}
m_sLINextName = m_aLIPending.next ();
return true;
}
public SPITYPE next ()
{
if (!hasNext ())
throw new NoSuchElementException ();
final String sClassName = m_sLINextName;
m_sLINextName = null;
try
{
final SPITYPE p = m_aLIService.cast (Class.forName (sClassName, true, m_aLILoader).newInstance ());
m_aProviders.put (sClassName, p);
return p;
}
catch (final ClassNotFoundException x)
{
_fail (m_aLIService, "Provider " + sClassName + " not found");
}
catch (final Throwable x)
{
_fail (m_aLIService, "Provider " + sClassName + " could not be instantiated: " + x, x);
}
throw new Error (); // This cannot happen
}
@UnsupportedOperation
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 ServiceLoaderException} 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 ServiceLoaderException} 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
*/
public Iterator iterator ()
{
return new Iterator ()
{
private final Iterator > m_aKnownProviders = m_aProviders.entrySet ().iterator ();
public boolean hasNext ()
{
return m_aKnownProviders.hasNext () || m_aLookupIterator.hasNext ();
}
public SPITYPE next ()
{
if (m_aKnownProviders.hasNext ())
return m_aKnownProviders.next ().getValue ();
return m_aLookupIterator.next ();
}
@UnsupportedOperation
public void remove ()
{
throw new UnsupportedOperationException ();
}
};
}
/**
* Returns a string describing this service.
*
* @return A descriptive string
*/
@Override
public String toString ()
{
return new ToStringGenerator (this).append ("serviceName", m_aService.getName ()).toString ();
}
/**
* 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 ServiceLoaderBackport load (final Class service, final ClassLoader loader)
{
return new ServiceLoaderBackport (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(<i>service</i>)
*
*
*
is equivalent to
*
*
* ServiceLoader.load(<i>service</i>,
* Thread.currentThread().getContextClassLoader())
*
*
*
*
* @param service
* The interface or abstract class representing the service
* @return A new service loader
*/
public static ServiceLoaderBackport load (final Class service)
{
return load (service, ClassHelper.getDefaultClassLoader ());
}
/**
* 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(<i>service</i>, <i>extClassLoader</i>)
*
*
*
*
* 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 ServiceLoaderBackport loadInstalled (final Class service)
{
ClassLoader cl = ClassLoader.getSystemClassLoader ();
ClassLoader prev = null;
while (cl != null)
{
prev = cl;
cl = cl.getParent ();
}
return ServiceLoaderBackport.load (service, prev);
}
}