opennlp.tools.util.ext.ExtensionLoader Maven / Gradle / Ivy
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You 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 opennlp.tools.util.ext;
import java.lang.reflect.Field;
/**
* The {@link ExtensionLoader} is responsible to load extensions to the OpenNLP library.
*
* Note: Do not use this class, internal use only!
*/
public class ExtensionLoader {
private static boolean isOsgiAvailable = false;
private ExtensionLoader() {
}
static boolean isOSGiAvailable() {
return isOsgiAvailable;
}
static void setOSGiAvailable() {
isOsgiAvailable = true;
}
// Pass in the type (interface) of the class to load
/**
* Instantiates an user provided extension to OpenNLP.
*
* The extension is either loaded from the class path or if running
* inside an OSGi environment via an OSGi service.
*
* Initially it tries using the public default
* constructor. If it is not found, it will check if the class follows the singleton
* pattern: a static field named INSTANCE that returns an object of the type
* T.
*
* @param clazz
* @param extensionClassName
*
* @return the instance of the extension class
*/
// TODO: Throw custom exception if loading fails ...
@SuppressWarnings("unchecked")
public static T instantiateExtension(Class clazz, String extensionClassName) {
// First try to load extension and instantiate extension from class path
try {
Class extClazz = Class.forName(extensionClassName);
if (clazz.isAssignableFrom(extClazz)) {
try {
return (T) extClazz.newInstance();
} catch (InstantiationException e) {
throw new ExtensionNotLoadedException(e);
} catch (IllegalAccessException e) {
// constructor is private. Try to load using INSTANCE
Field instanceField;
try {
instanceField = extClazz.getDeclaredField("INSTANCE");
} catch (NoSuchFieldException | SecurityException e1) {
throw new ExtensionNotLoadedException(e1);
}
if (instanceField != null) {
try {
return (T) instanceField.get(null);
} catch (IllegalArgumentException | IllegalAccessException e1) {
throw new ExtensionNotLoadedException(e1);
}
}
throw new ExtensionNotLoadedException(e);
}
}
else {
throw new ExtensionNotLoadedException("Extension class '" + extClazz.getName() +
"' needs to have type: " + clazz.getName());
}
} catch (ClassNotFoundException e) {
// Class is not on classpath
}
// Loading from class path failed
// Either something is wrong with the class name or OpenNLP is
// running in an OSGi environment. The extension classes are not
// on our classpath in this case.
// In OSGi we need to use services to get access to extensions.
// Determine if OSGi class is on class path
// Now load class which depends on OSGi API
if (isOsgiAvailable) {
// The OSGIExtensionLoader class will be loaded when the next line
// is executed, but not prior, and that is why it is safe to directly
// reference it here.
OSGiExtensionLoader extLoader = OSGiExtensionLoader.getInstance();
return extLoader.getExtension(clazz, extensionClassName);
}
throw new ExtensionNotLoadedException("Unable to find implementation for " +
clazz.getName() + ", the class or service " + extensionClassName +
" could not be located!");
}
}