athenz.shade.zts.org.glassfish.hk2.osgiresourcelocator.ServiceLoader Maven / Gradle / Ivy
/*
* Copyright (c) 2009, 2018 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 athenz.shade.zts.athenz.shade.zts.org.glassfish.hk2.osgiresourcelocator;
/**
* This is a gateway to OSGi bundles as far as META-INF/services files are concerned.
* Since META-INF/services files are not exportable, clients relying on Java SPI mechanism
* can't discover all providers. This utility helps in such a situation. It provides a utility method
* {@link #lookupProviderInstances} which can find META-INF/services being part of OSGi bundles.
* This class has been carefully coded to avoid any reference to OSGi classes so that
* it can be called in a non-OSGi environment as well. In such an environment,
* it simply returns null.
* In an OSGi environment, we expect the class to be initialized by the bundle activator.
*
* @see {@link #lookupProviderInstances}
*
* @author [email protected]
*/
public abstract class ServiceLoader {
/**
* Singleton
*/
private static ServiceLoader _me;
/*package*/ ServiceLoader() {}
public static synchronized void initialize(ServiceLoader singleton) {
if (singleton == null) throw new NullPointerException("Did you intend to call reset()?");
if (_me != null) throw new IllegalStateException("Already initialzed with [" + _me + "]");
_me = singleton;
}
public static synchronized void reset() {
if (_me == null) {
throw new IllegalStateException("Not yet initialized");
}
_me = null;
}
public interface ProviderFactory {
T make(Class providerClass, Class serviceClass) throws Exception;
}
/**
* Calling this method is equivalent to calling {@link #lookupProviderInstances(Class, ProviderFactory)}
* with a null factory object.
*
* @see #lookupProviderInstances(Class, athenz.shade.zts.athenz.shade.zts.org.glassfish.hk2.osgiresourcelocator.ServiceLoader.ProviderFactory)
*/
public static Iterable lookupProviderInstances(Class serviceClass) {
return lookupProviderInstances(serviceClass, null);
}
/**
* @param serviceClass type of service requested
* @param factory ProviderFactory used to instantiate provider instance from a provider class. If null is supplied,
* it calls Class.newInstance to obtain a provider instance from provider class.
* @param
* @return provider instances implementing the given service class.
*/
public static Iterable lookupProviderInstances(Class serviceClass, ProviderFactory factory) {
if (_me == null) return null;
return _me.lookupProviderInstances1(serviceClass, factory);
}
/**
* It is not clear why one needs this method, but it is provided just in case one needs it.
* Returns classes found in META-INF/services/serviceClass.getName() in OSGi bundles. This method searches for
* such named resources in every OSGi bundle. For every resource found, it assumes that the file contains
* a class name. It loads the class name mentioned in that file using the bundle containing the resource.
* It does not check if the class mentioned in provider file actually implements/extends service class,
* because there are cases where it does not. JAXB is an example. JAXBContext provider file contains
* a class that's used as a factory class for JAXBContext. To handle such issues, yet not return classes
* that are not class loader compatible, this method will only return those classes which see the same service
* class as supplied in the parameter.
* Let's take an example. We have JAXB classes exported by JRE and there is a bundle A which contains JAXB API
* and implementation. The bundle B is wired to itself for JAXB classes. Assume a user supplied bundle
* B is wired to JRE's JAXB. When bundle B calls JAXBContext.createContext(JAXBContext.class),
* JRE's JAXBContext will use this API. The implementation will look for META-INF/services/JAXBContext files and
* find a file in JAXB bundle. Let's say that file contains com.acme.jaxb.JAXBContextFactory.
* com.acme.jaxb.ContextFactory does not implement JAXBContext.class. Instead, as per JAXB spec,
* it must provide some factory methods to create JAXBContext. If our implementation simply returns that class
* without doing any further tests, a nasty class cast exception is going to be resulted, because user's bundle B
* uses JAXB from JRE, which means JAXBContext.class is loaded by JRE, yet com.acme.jaxb.JAXBContextFactory
* uses JAXBContext from bundle A. To avoid such problem, we will try to load JAXBContext using class loader of
* com.acme.jaxb.JAXBContextFactory and see if that's same as supplied JAXBContext.class. If they are same,
* we return com.acme.jaxb.JAXBContextFactory, else we don't. In this example, we won't.
*
* @param serviceClass type of service requested
* @param
* @return classes corresponding to entries in META-INF/services file for the service class.
*/
public static Iterable lookupProviderClasses(Class serviceClass) {
return _me.lookupProviderClasses1(serviceClass);
}
/*package*/ abstract Iterable lookupProviderInstances1(Class serviceType, ProviderFactory factory);
/*package*/ abstract Iterable lookupProviderClasses1(Class serviceType);
}