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/*
 * Copyright 2012-2016 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
 *
 *      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.springframework.data.util;

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;

import org.springframework.core.annotation.AnnotatedElementUtils;
import org.springframework.util.Assert;
import org.springframework.util.ReflectionUtils;
import org.springframework.util.ReflectionUtils.FieldCallback;

/**
 * A {@link FieldCallback} that will inspect each field for a given annotation. This field's type can then be accessed
 * afterwards.
 * 
 * @author Oliver Gierke
 * @author Christoph Strobl
 */
public class AnnotationDetectionFieldCallback implements FieldCallback {

	private final Class annotationType;
	private Field field;

	/**
	 * Creates a new {@link AnnotationDetectionFieldCallback} scanning for an annotation of the given type.
	 * 
	 * @param annotationType must not be {@literal null}.
	 */
	public AnnotationDetectionFieldCallback(Class annotationType) {

		Assert.notNull(annotationType);
		this.annotationType = annotationType;
	}

	/*
	 * (non-Javadoc)
	 * @see org.springframework.util.ReflectionUtils.FieldCallback#doWith(java.lang.reflect.Field)
	 */
	public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {

		if (this.field != null) {
			return;
		}

		Annotation annotation = AnnotatedElementUtils.findMergedAnnotation(field, annotationType);

		if (annotation != null) {

			this.field = field;
			ReflectionUtils.makeAccessible(this.field);
		}
	}

	/**
	 * Returns the type of the field.
	 * 
	 * @return
	 */
	public Class getType() {
		return field == null ? null : field.getType();
	}

	/**
	 * Retrieves the value of the field by reflection.
	 * 
	 * @param source must not be {@literal null}.
	 * @return
	 */
	@SuppressWarnings("unchecked")
	public  T getValue(Object source) {

		Assert.notNull(source);
		return field == null ? null : (T) ReflectionUtils.getField(field, source);
	}
}