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;
import java.lang.reflect.InvocationTargetException;
import opennlp.tools.commons.Internal;
/**
* The {@link ExtensionLoader} is responsible to load extensions to the OpenNLP library.
*
* Note:
* Do not use this class, internal use only!
*/
@Internal
public class ExtensionLoader {
private ExtensionLoader() {
}
// Pass in the type (interface) of the class to load
/**
* Instantiates a user provided extension to OpenNLP.
*
* The extension is loaded from the class path.
*
* Initially, the load is conducted using the public no-arg constructor.
* If no such constructor is not found, it is checked if the class follows the
* {@code Singleton} pattern: a static field named {@code INSTANCE} that
* returns an object of the type {@link T}.
*
* @param clazz A reference to {@link Class}.
* @param extensionClassName The (fully-qualified) name of the class
* by which the extension shall be loaded.
*
* @return the instance of the extension class
*
* @throws ExtensionNotLoadedException Thrown if the load operation failed.
*/
@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.getDeclaredConstructor().newInstance();
} catch (InstantiationException | NoSuchMethodException 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);
} catch (InvocationTargetException e) {
throw new RuntimeException(e);
}
}
else {
throw new ExtensionNotLoadedException("Extension class '" + extClazz.getName() +
"' needs to have type: " + clazz.getName());
}
} catch (ClassNotFoundException e) {
// Class is not on classpath
}
throw new ExtensionNotLoadedException("Unable to find implementation for " +
clazz.getName() + ", the class or service " + extensionClassName +
" could not be located!");
}
}