Java tutorial
/* * 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 org.apache.lucene.util; import java.lang.invoke.MethodHandle; import java.lang.invoke.MethodHandles; import java.lang.invoke.MethodType; import java.lang.reflect.UndeclaredThrowableException; /** * An AttributeFactory creates instances of {@link AttributeImpl}s. */ public abstract class AttributeFactory { /** * Returns an {@link AttributeImpl} for the supplied {@link Attribute} interface class. * * @throws UndeclaredThrowableException A wrapper runtime exception thrown if the * constructor of the attribute class throws a checked exception. * Note that attributes should not throw or declare * checked exceptions; this may be verified and fail early in the future. */ public abstract AttributeImpl createAttributeInstance(Class<? extends Attribute> attClass) throws UndeclaredThrowableException; /** * Returns a correctly typed {@link MethodHandle} for the no-arg ctor of the given class. */ static final MethodHandle findAttributeImplCtor(Class<? extends AttributeImpl> clazz) { try { return lookup.findConstructor(clazz, NO_ARG_CTOR).asType(NO_ARG_RETURNING_ATTRIBUTEIMPL); } catch (NoSuchMethodException | IllegalAccessException e) { throw new IllegalArgumentException( "Cannot lookup accessible no-arg constructor for: " + clazz.getName(), e); } } private static final MethodHandles.Lookup lookup = MethodHandles.publicLookup(); private static final MethodType NO_ARG_CTOR = MethodType.methodType(void.class); private static final MethodType NO_ARG_RETURNING_ATTRIBUTEIMPL = MethodType.methodType(AttributeImpl.class); /** * This is the default factory that creates {@link AttributeImpl}s using the * class name of the supplied {@link Attribute} interface class by appending <code>Impl</code> to it. */ public static final AttributeFactory DEFAULT_ATTRIBUTE_FACTORY = new DefaultAttributeFactory(); private static final class DefaultAttributeFactory extends AttributeFactory { private final ClassValue<MethodHandle> constructors = new ClassValue<MethodHandle>() { @Override protected MethodHandle computeValue(Class<?> attClass) { return findAttributeImplCtor(findImplClass(attClass.asSubclass(Attribute.class))); } }; DefaultAttributeFactory() { } @Override public AttributeImpl createAttributeInstance(Class<? extends Attribute> attClass) { try { return (AttributeImpl) constructors.get(attClass).invokeExact(); } catch (Error | RuntimeException e) { throw e; } catch (Throwable e) { throw new UndeclaredThrowableException(e); } } private Class<? extends AttributeImpl> findImplClass(Class<? extends Attribute> attClass) { try { return Class.forName(attClass.getName() + "Impl", true, attClass.getClassLoader()) .asSubclass(AttributeImpl.class); } catch (ClassNotFoundException cnfe) { throw new IllegalArgumentException("Cannot find implementing class for: " + attClass.getName()); } } } /** <b>Expert</b>: AttributeFactory returning an instance of the given {@code clazz} for the * attributes it implements. For all other attributes it calls the given delegate factory * as fallback. This class can be used to prefer a specific {@code AttributeImpl} which * combines multiple attributes over separate classes. * @lucene.internal */ public abstract static class StaticImplementationAttributeFactory<A extends AttributeImpl> extends AttributeFactory { private final AttributeFactory delegate; private final Class<A> clazz; /** <b>Expert</b>: Creates an AttributeFactory returning {@code clazz} as instance for the * attributes it implements and for all other attributes calls the given delegate factory. */ public StaticImplementationAttributeFactory(AttributeFactory delegate, Class<A> clazz) { this.delegate = delegate; this.clazz = clazz; } @Override public final AttributeImpl createAttributeInstance(Class<? extends Attribute> attClass) { return attClass.isAssignableFrom(clazz) ? createInstance() : delegate.createAttributeInstance(attClass); } /** Creates an instance of {@code A}. */ protected abstract A createInstance(); @Override public boolean equals(Object other) { if (this == other) return true; if (other == null || other.getClass() != this.getClass()) return false; @SuppressWarnings("rawtypes") final StaticImplementationAttributeFactory af = (StaticImplementationAttributeFactory) other; return this.delegate.equals(af.delegate) && this.clazz == af.clazz; } @Override public int hashCode() { return 31 * delegate.hashCode() + clazz.hashCode(); } } /** Returns an AttributeFactory returning an instance of the given {@code clazz} for the * attributes it implements. The given {@code clazz} must have a public no-arg constructor. * For all other attributes it calls the given delegate factory as fallback. * This method can be used to prefer a specific {@code AttributeImpl} which combines * multiple attributes over separate classes. * <p>Please save instances created by this method in a static final field, because * on each call, this does reflection for creating a {@link MethodHandle}. */ public static <A extends AttributeImpl> AttributeFactory getStaticImplementation(AttributeFactory delegate, Class<A> clazz) { final MethodHandle constr = findAttributeImplCtor(clazz); return new StaticImplementationAttributeFactory<A>(delegate, clazz) { @Override protected A createInstance() { try { return (A) constr.invokeExact(); } catch (Error | RuntimeException e) { throw e; } catch (Throwable e) { throw new UndeclaredThrowableException(e); } } }; } }