/*
 * 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
 *
 *      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.beans.factory.config;

import java.lang.reflect.Field;

import org.springframework.beans.factory.BeanClassLoaderAware;
import org.springframework.beans.factory.BeanFactoryUtils;
import org.springframework.beans.factory.BeanNameAware;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.beans.factory.FactoryBeanNotInitializedException;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.ReflectionUtils;
import org.springframework.util.StringUtils;

FactoryBean which retrieves a static or non-static field value. 
Typically used for retrieving public static final constants. Usage example:
// standard definition for exposing a static field, specifying the "staticField" property
<bean id="myField" class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean">
  <property name="staticField" value="java.sql.Connection.TRANSACTION_SERIALIZABLE"/>
</bean>
// convenience version that specifies a static field pattern as bean name
<bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE"
      class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean"/>

If you are using Spring 2.0, you can also use the following style of configuration for
public static fields.
<util:constant static-field="java.sql.Connection.TRANSACTION_SERIALIZABLE"/>
Author:
Juergen Hoeller
See Also:
setStaticField
Since:
1.1/**
 * {@link FactoryBean} which retrieves a static or non-static field value.
 *
 * <p>Typically used for retrieving public static final constants. Usage example:
 *
 * <pre class="code">
 * // standard definition for exposing a static field, specifying the "staticField" property
 * &lt;bean id="myField" class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean"&gt;
 *   &lt;property name="staticField" value="java.sql.Connection.TRANSACTION_SERIALIZABLE"/&gt;
 * &lt;/bean&gt;
 *
 * // convenience version that specifies a static field pattern as bean name
 * &lt;bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE"
 *       class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean"/&gt;
 * </pre>
 *
 * <p>If you are using Spring 2.0, you can also use the following style of configuration for
 * public static fields.
 *
 * <pre class="code">&lt;util:constant static-field="java.sql.Connection.TRANSACTION_SERIALIZABLE"/&gt;</pre>
 *
 * @author Juergen Hoeller
 * @since 1.1
 * @see #setStaticField
 */
public class FieldRetrievingFactoryBean
		implements FactoryBean<Object>, BeanNameAware, BeanClassLoaderAware, InitializingBean {

	@Nullable
	private Class<?> targetClass;

	@Nullable
	private Object targetObject;

	@Nullable
	private String targetField;

	@Nullable
	private String staticField;

	@Nullable
	private String beanName;

	@Nullable
	private ClassLoader beanClassLoader = ClassUtils.getDefaultClassLoader();

	// the field we will retrieve
	@Nullable
	private Field fieldObject;


	
Set the target class on which the field is defined.
Only necessary when the target field is static; else,
a target object needs to be specified anyway.
See Also:
setTargetObject
setTargetField/**
	 * Set the target class on which the field is defined.
	 * Only necessary when the target field is static; else,
	 * a target object needs to be specified anyway.
	 * @see #setTargetObject
	 * @see #setTargetField
	 */
	public void setTargetClass(@Nullable Class<?> targetClass) {
		this.targetClass = targetClass;
	}

	
Return the target class on which the field is defined.
/**
	 * Return the target class on which the field is defined.
	 */
	@Nullable
	public Class<?> getTargetClass() {
		return this.targetClass;
	}

	
Set the target object on which the field is defined.
Only necessary when the target field is not static;
else, a target class is sufficient.
See Also:
setTargetClass
setTargetField/**
	 * Set the target object on which the field is defined.
	 * Only necessary when the target field is not static;
	 * else, a target class is sufficient.
	 * @see #setTargetClass
	 * @see #setTargetField
	 */
	public void setTargetObject(@Nullable Object targetObject) {
		this.targetObject = targetObject;
	}

	
Return the target object on which the field is defined.
/**
	 * Return the target object on which the field is defined.
	 */
	@Nullable
	public Object getTargetObject() {
		return this.targetObject;
	}

	
Set the name of the field to be retrieved.
Refers to either a static field or a non-static field,
depending on a target object being set.
See Also:
setTargetClass
setTargetObject/**
	 * Set the name of the field to be retrieved.
	 * Refers to either a static field or a non-static field,
	 * depending on a target object being set.
	 * @see #setTargetClass
	 * @see #setTargetObject
	 */
	public void setTargetField(@Nullable String targetField) {
		this.targetField = (targetField != null ? StringUtils.trimAllWhitespace(targetField) : null);
	}

	
Return the name of the field to be retrieved.
/**
	 * Return the name of the field to be retrieved.
	 */
	@Nullable
	public String getTargetField() {
		return this.targetField;
	}

	
Set a fully qualified static field name to retrieve,
e.g. "example.MyExampleClass.MY_EXAMPLE_FIELD".
Convenient alternative to specifying targetClass and targetField.
See Also:
setTargetClass
setTargetField/**
	 * Set a fully qualified static field name to retrieve,
	 * e.g. "example.MyExampleClass.MY_EXAMPLE_FIELD".
	 * Convenient alternative to specifying targetClass and targetField.
	 * @see #setTargetClass
	 * @see #setTargetField
	 */
	public void setStaticField(String staticField) {
		this.staticField = StringUtils.trimAllWhitespace(staticField);
	}

	
The bean name of this FieldRetrievingFactoryBean will be interpreted
as "staticField" pattern, if neither "targetClass" nor "targetObject"
nor "targetField" have been specified.
This allows for concise bean definitions with just an id/name.
/**
	 * The bean name of this FieldRetrievingFactoryBean will be interpreted
	 * as "staticField" pattern, if neither "targetClass" nor "targetObject"
	 * nor "targetField" have been specified.
	 * This allows for concise bean definitions with just an id/name.
	 */
	@Override
	public void setBeanName(String beanName) {
		this.beanName = StringUtils.trimAllWhitespace(BeanFactoryUtils.originalBeanName(beanName));
	}

	@Override
	public void setBeanClassLoader(ClassLoader classLoader) {
		this.beanClassLoader = classLoader;
	}


	@Override
	public void afterPropertiesSet() throws ClassNotFoundException, NoSuchFieldException {
		if (this.targetClass != null && this.targetObject != null) {
			throw new IllegalArgumentException("Specify either targetClass or targetObject, not both");
		}

		if (this.targetClass == null && this.targetObject == null) {
			if (this.targetField != null) {
				throw new IllegalArgumentException(
						"Specify targetClass or targetObject in combination with targetField");
			}

			// If no other property specified, consider bean name as static field expression.
			if (this.staticField == null) {
				this.staticField = this.beanName;
				Assert.state(this.staticField != null, "No target field specified");
			}

			// Try to parse static field into class and field.
			int lastDotIndex = this.staticField.lastIndexOf('.');
			if (lastDotIndex == -1 || lastDotIndex == this.staticField.length()) {
				throw new IllegalArgumentException(
						"staticField must be a fully qualified class plus static field name: " +
						"e.g. 'example.MyExampleClass.MY_EXAMPLE_FIELD'");
			}
			String className = this.staticField.substring(0, lastDotIndex);
			String fieldName = this.staticField.substring(lastDotIndex + 1);
			this.targetClass = ClassUtils.forName(className, this.beanClassLoader);
			this.targetField = fieldName;
		}

		else if (this.targetField == null) {
			// Either targetClass or targetObject specified.
			throw new IllegalArgumentException("targetField is required");
		}

		// Try to get the exact method first.
		Class<?> targetClass = (this.targetObject != null ? this.targetObject.getClass() : this.targetClass);
		this.fieldObject = targetClass.getField(this.targetField);
	}


	@Override
	@Nullable
	public Object getObject() throws IllegalAccessException {
		if (this.fieldObject == null) {
			throw new FactoryBeanNotInitializedException();
		}
		ReflectionUtils.makeAccessible(this.fieldObject);
		if (this.targetObject != null) {
			// instance field
			return this.fieldObject.get(this.targetObject);
		}
		else {
			// class field
			return this.fieldObject.get(null);
		}
	}

	@Override
	public Class<?> getObjectType() {
		return (this.fieldObject != null ? this.fieldObject.getType() : null);
	}

	@Override
	public boolean isSingleton() {
		return false;
	}

}