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 * Copyright 2002-2018 the original author or authors.
 *
 * 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
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
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package org.springframework.context.annotation;

import java.lang.annotation.Annotation;

import org.springframework.core.GenericTypeResolver;
import org.springframework.core.annotation.AnnotationAttributes;
import org.springframework.core.type.AnnotationMetadata;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;

Convenient base class for ImportSelector implementations that select imports based on an AdviceMode value from an annotation (such as the @Enable* annotations). 
Author:
Chris Beams
Type parameters:
<A> – annotation containing AdviceMode attribute
Since:
3.1/**
 * Convenient base class for {@link ImportSelector} implementations that select imports
 * based on an {@link AdviceMode} value from an annotation (such as the {@code @Enable*}
 * annotations).
 *
 * @author Chris Beams
 * @since 3.1
 * @param <A> annotation containing {@linkplain #getAdviceModeAttributeName() AdviceMode attribute}
 */
public abstract class AdviceModeImportSelector<A extends Annotation> implements ImportSelector {

	
The default advice mode attribute name.
/**
	 * The default advice mode attribute name.
	 */
	public static final String DEFAULT_ADVICE_MODE_ATTRIBUTE_NAME = "mode";


	
The name of the AdviceMode attribute for the annotation specified by the generic type A. The default is "mode", but subclasses may override in order to customize. /**
	 * The name of the {@link AdviceMode} attribute for the annotation specified by the
	 * generic type {@code A}. The default is {@value #DEFAULT_ADVICE_MODE_ATTRIBUTE_NAME},
	 * but subclasses may override in order to customize.
	 */
	protected String getAdviceModeAttributeName() {
		return DEFAULT_ADVICE_MODE_ATTRIBUTE_NAME;
	}

	
This implementation resolves the type of annotation from generic metadata and validates that (a) the annotation is in fact present on the importing @Configuration class and (b) that the given annotation has an advice mode attribute of type AdviceMode. 
The selectImports(AdviceMode) method is then invoked, allowing the concrete implementation to choose imports in a safe and convenient fashion. 
Throws:
IllegalArgumentException – if expected annotation A is not present on the importing @Configuration class or if selectImports(AdviceMode) returns null/**
	 * This implementation resolves the type of annotation from generic metadata and
	 * validates that (a) the annotation is in fact present on the importing
	 * {@code @Configuration} class and (b) that the given annotation has an
	 * {@linkplain #getAdviceModeAttributeName() advice mode attribute} of type
	 * {@link AdviceMode}.
	 * <p>The {@link #selectImports(AdviceMode)} method is then invoked, allowing the
	 * concrete implementation to choose imports in a safe and convenient fashion.
	 * @throws IllegalArgumentException if expected annotation {@code A} is not present
	 * on the importing {@code @Configuration} class or if {@link #selectImports(AdviceMode)}
	 * returns {@code null}
	 */
	@Override
	public final String[] selectImports(AnnotationMetadata importingClassMetadata) {
		Class<?> annType = GenericTypeResolver.resolveTypeArgument(getClass(), AdviceModeImportSelector.class);
		Assert.state(annType != null, "Unresolvable type argument for AdviceModeImportSelector");

		AnnotationAttributes attributes = AnnotationConfigUtils.attributesFor(importingClassMetadata, annType);
		if (attributes == null) {
			throw new IllegalArgumentException(String.format(
					"@%s is not present on importing class '%s' as expected",
					annType.getSimpleName(), importingClassMetadata.getClassName()));
		}

		AdviceMode adviceMode = attributes.getEnum(getAdviceModeAttributeName());
		String[] imports = selectImports(adviceMode);
		if (imports == null) {
			throw new IllegalArgumentException("Unknown AdviceMode: " + adviceMode);
		}
		return imports;
	}

	
Determine which classes should be imported based on the given AdviceMode. 
Returning null from this method indicates that the AdviceMode could not be handled or was unknown and that an IllegalArgumentException should be thrown. 
Params:
adviceMode – the value of the 
advice mode attribute for the annotation specified via generics.
Returns:
array containing classes to import (empty array if none; null if the given AdviceMode is unknown)/**
	 * Determine which classes should be imported based on the given {@code AdviceMode}.
	 * <p>Returning {@code null} from this method indicates that the {@code AdviceMode}
	 * could not be handled or was unknown and that an {@code IllegalArgumentException}
	 * should be thrown.
	 * @param adviceMode the value of the {@linkplain #getAdviceModeAttributeName()
	 * advice mode attribute} for the annotation specified via generics.
	 * @return array containing classes to import (empty array if none;
	 * {@code null} if the given {@code AdviceMode} is unknown)
	 */
	@Nullable
	protected abstract String[] selectImports(AdviceMode adviceMode);

}